Processing guidelines for injection molding of thermoplastic

Processing guidelines for injection

molding of thermoplastic vulcanizates

Santoprene

™

TPVs

TABLE OF CONTENTS - 3

INTRODUCTION AND SAFETY GUIDELINES ............................................................................................................. 5

EQUIPMENT

.............................................................................................................................................................................................................. 6

GENERAL PROCESSING

Start up

Mold filling, packing and screw rotation

............................................................................................................................ 8

Estimating molding cycle time

....................................................................................................................................................... 9

Drying of the material

............................................................................................................................................................................... 10

Coloring

Regrinding

Shrinkage

MOLD DESIGN

Mold steel selection

.................................................................................................................................................................................... 12

Mold surfaces

Mold plating and coating

..................................................................................................................................................................... 13

Mold venting

Sprue, runner and gate design

...................................................................................................................................................... 14

Ejection

Undercut design guidelines

............................................................................................................................................................... 19

Mold temperature control

................................................................................................................................................................... 19

HOT RUNNER SYSTEMS

System selection

............................................................................................................................................................................................. 20

Mold design

Gate selection

.................................................................................................................................................................................................... 21

MULTI-MATERIAL MOLDING

Overmolding

Insert- molding

................................................................................................................................................................................................... 23

Two- shot molding

........................................................................................................................................................................................ 23

Co- injection molding

................................................................................................................................................................................ 23

QUICK REFERENCE NOTES

................................................................................................................................................................ 24

TROUBLESHOOTING

................................................................................................................................................................................... 24

INTRODUCTION - 5

INTRODUCTION AND SAFETY

GUIDELINES

Santoprene™ thermoplastic vulcanizates (TPVs) from

ExxonMobil Chemical combine the processability of thermo

plastic materials with many of the performance characteristics

of thermoset rubber. Santoprene TPVs are commonly employed

in the plastics industry for efficient and easy processing with

conventional injection molding equipment. They are suitable for

a broad range of systems that require excellent part definition.

TPVs are categorized as a type of thermoplastic elastomer

(TPE). They contain very small fully vulcanized rubber particles

dispersed in a plastic matrix. This fully vulcanized structure gives

a rheo logy similar to a composite and a flow behavior that is

pseudoplastic. Typically TPEs and TPVs have a melt viscosity

that is more dependent on shear than on melt temperature. To

make handling easier, we supply our Santoprene TPVs in pellet

form. Most TPEs and TPVs absorb moisture, and pre drying is

recommended (see Drying section for more information.)

The processing guidelines included in this document are

intended only for general purpose and molding grades of

Santoprene TPVs from ExxonMobil Chemical. Other specialty

TPV grades may require different processing procedures.

Safety guidelines

ExxonMobil Chemical is committed to continually improving the

safety of operations so they are free from injury or incident.

Remember, the typical processing operation uses powerful and

potentially dangerous electrical, hydraulic and mechanical

systems. Material temperatures can be as high as 290 °C (550

°F). Hot and cold water and hot machine oil can be

encountered. Industry best practices strongly recommend the

following when processing TPVs:

• Check all safety systems (electrical, hydraulic and mechanical)

at least once daily to ensure they are fully operational.

• Fix all leaks on or around mac ; avoid oil and water spills on the

floor.

• Purge all previous materials from the press using an LDPE or

PP (some of our materials are not compatible with other

thermoplastic materials please refer to each material’s MSDS

and product data sheet prior to use).

• Wear the proper personal protective equipment while in any

processing area.

• Follow all the safety recommendations from the machinery

and material manufacturers.

For more information, contact the AnswerPerson

at:

tpe.answerperson@exxonmobil.com or visit us at

www.santoprene.com

6 - EQUIPMENT

clamping

unit

control

drive

mold

unit

injection

system

heaters

barrel

reciprocating

screw

nozzle

motor and gears

for screw rotation

nonreturn

valve

feed hopper

cylinder for

screw-ram

stationary

platen

movable

platen

tie rods (4)

clamping

cylinder

injection unit

clamping unit

mold

Figure 1

Exterior view of a typical thermoplastic injection molding machine

Figure 2

Cross section of a typical thermoplastic injection molding machine

EQUIPMENT - 7

EQUIPMENT

Machine selection

Santoprene TPVs from ExxonMobil Chemical have been

processed successfully on many makes and sizes of standard

reciprocating screw injection molding machines. The quality of

a final molded part depends on the ability of a machine to

deliver the optimum amount of plasticized material for the

intended application.

Screw design

In most cases, a general purpose polyolefinic screw with a

compression ratio of 2:1 to 2.5:1 and a length to diameter ratio

between 16:1 and 22:1 is sufficient. A low shear screw, such as

those used in PVC processing, is not recommended. Unlike

many thermoplastic materials, Santoprene TPVs demonstrate

better flow characteristics during the injection filling phase

when subjected to high levels of shear during fill.

Barrel capacity

Avoid under utilization of the barrel wherever possible, since it

can lead to long residence times. Small shots run on a large

capacity barrel complicate processing. The best practice for any

injection molding is to utilize 20 to 80% of the barrel capacity

for each shot. This typically translates to 1.3 to 5 shots in the

barrel.

Nozzle

Most common TPVs do not typically drool or degrade at normal

processing temperatures and therefore do not require a special

nozzle. A conventional general purpose nozzle is commonly

suited to most applications (see Figure 3).

The nozzle orifice is usually 10 to 15% smaller than the “O”

diameter of the sprue bushing. Hot runner manufacturers

recommend that hot sprues should run at the same

temperature as the nozzle setting or slightly higher.

Recommended clamping force

Follow typical TPV recommendations for clamping force of 4.0

to 6.9 kN/cm

(3 to 5 tons/in

) of the projected area.

Figure 3

Nozzle types

General purpose taper

Full internal taper

Nylon reverse taper

8 - GENERAL PROCESSING

GENERAL PROCESSING

Start-up

General conditions

Most of our TPVs process with medium fast to fast injection

rates. Fill times are typically between 0.5 and 2.0 seconds. The

melt temperatures for most materials are approximately 205 °C

(400 °F); injection pressures are moderate. Starting conditions

can be found in the Product Data Sheet for each grade and/or

the Quick Processing Reference. These contain detailed

information on processing setup and unique features. Both

documents are available on www.santoprene.com

Parts made with Santoprene TPVs ordinarily release freely from

the mold when following general industry guidelines for part

and mold design and processing recommendations. The use of

mold release sprays or powders is not recommended, except

under special circumstances.

Injection molding pre-start-up

1. Purge barrel using a general purpose poly propylene or other

suitable purging compound.

2. Set heat zones to obtain melt temperature near the middle

of the recommended range for the TPV being used.

Set mold temperature to approximately 20 to 30 °C (70 to 80 °F).

4. Set screw speed to 100 to 200 rpm.

5. Check the clamp setting.

6. Check the ejector stroke and return.

7. Check to ensure that low pressure mold pro tection is set at

the proper distance.

Ensure that the machine water supply is on and available to the

heat exchanger, feed throat, mold and other machine area

Quick process start-up

The following procedure works well for any velocity controlled

injection molding machine. The following conditions, although

generally applicable, are not necessarily optimum for a specific

application.

1. Set pack and hold timers to 0.

2. Set pack and hold pressures to 0.

3. Set injection pressure to the maximum setting.

4. Set transfer position to 6 to 7 mm (0.25 to 0.30”).

5. Set injection time long enough to reach transfer position.

Set injection speed to achieve a fill time of 0.5 to 1.5 seconds.

7. Set shot size to achieve a short shot.

8. Increase shot size until part is 95% filled.

9. Note the injection pressure required to maintain 95% fill of

the part.

10. Set injection pressure to 21 to 28 bar (300 to 400 psi) above

the injection pressure deter mi ned in the previous step; this

insures a velocity controlled filling. The molded part should

still be 95% filled at this setting.

11. Set the pack hold pressure to 50% of the injection pressure

setting.

12. Set the pack hold time to 2 to 3 seconds to ensure that the

remaining 5% of the part is filled and that there is enough

additional material to compensate for shrinkage.

13. Fine tune the pack hold time by running a series of test

molding cycles. Weigh each part after each test cycle.

Continue to increase the pack hold time as required until the

part weight does not increase. This ensures that gate

freeze off has occurred and that parts have a repeatable,

correct weight. Watch out for over packing, usually indicated

by a series of rings around a gate or evidenced by a

“punched in” gate.

Mold filling, packing and screw

rotation

The majority of our TPVs are pseudoplastic shear dependent

materials. They require medium fast to fast mold filling rates via

high injection speeds and pressure to increase shear through

the system, which reduces their viscosity. This gives maximum

flow length to wall thickness ratios, which produces a well filled

part. We recommend the injection speed be medium fast to

fast. This should generate a shear rate of approximately 10

sec 1 through the gates.

Mold packing takes place immediately after the filling stage. This

compensates for the loss in volume as the material returns to

solid density from the melt. Hold pressure then keeps the

molten material under pressure until the gate freezes. Some

press systems have controls for both of these phases, and some

just have hold phase controls. The user must then configure the

single control to accomplish both actions within this constraint.

Cushion requirements

We recommend a small cushion, typically 3 to 6 mm (0.125 to

0.250”) for good cavity packing.

Injection pressures

The actual injection pressure depends on many variables such

as melt temperature, mold temperature, part geometry, wall

thickness, flow length and other considerations associated with

the mold and equipment. Remember that injection pressure is

an output from the above mechanical and thermal parameters

of the system. The correct injection molding pressure, as

discussed earlier, is to obtain sufficient excess injection pressure

to allow a velocity controlled filling cycle.

GENERAL PROCESSING - 9

Mold packing and holding

Once a mold is 95 to 99% filled, pack and hold time may be

optimized. To allow for ample packing, increase the hold time

until the gate freezes. Next, adjust hold or pack pressure to

ensure full part packing before gate freeze off. In general,

holding pressure should be about 1/2 to 3/4 of the actual

injection pressure. See the Quick Process Setup or Scientific

Molding Setup Procedure in the Process Start up section for

more information.

Screw rotation and back pressure

A high screw RPM (100 to 200) is recommended. Back pressure

is not always needed, however, a back pressure of 3.5 to 7.0 bar

(50 to 100 psi) may be used to insure a homogeneous melt and

maintain a consistent shot size. A higher back pressure is

normally employed when using masterbatches.

Estimating molding cycle time

The following guidelines can help to estimate and minimize

molding cycle time. These guidelines assume the use of

recommended temperatures and processing procedures, and a

mold that has a fixed number of cavities plus adequate knock-

out area to permit easy part removal.

Molding cycle

Santoprene TPVs from ExxonMobil Chemical allow the use of

automatic molding cycles, which eliminates costly manual part

removal and reduces the overall molding cycle time by

minimizing cooling time. Typically, molded parts form a solid

skin and become semi rigid, even while their interiors are still

molten. This allows parts to be de molded quickly with minimal

deformation problems. Obviously, reducing molding time

means more cost effective processing.

Oversized runners can be the biggest time wasters in a system.

Mold fill usually takes 0.5 to 2.0 seconds, depending on part

volume, runner gate style and size, cavity location and injection

pressure. Pack (hold time) is usually 2 to 3 times the mold fill

times, depending on gate size. If larger gates are used, expect a

longer molding cycle time. The speed of an injection molding

machine and the part geometry determine the times of other

molding cycle phases such as mold close, mold open, ejection

and part removal.

Cooling time

Cooling time can be estimated using the equa tion in Table 1.

Table 1

Cooling time (tc)

tc = (s

/a) x T factor

tc = (s

/a) x (1/π

) x ln ((8/π

) x ((Tm Tw ) / (Te Tw)))

Symbol Description Unit

tc Cooling time s

s section = wall thickness m

π Pi = 3.14159

ln logarithm base e

Tm Temp. melt °C

Tw Temp tool wall °C

Te Temp ejection °C

a Thermal diffusivity = ratio of thermal

conductivity to volumetric heat capacity

a =

thermal conductivity

W/m °C

(density x specific heat) ((kg/m

) x (J/kg °C)) s

Source: How to make injection molds:

Hanser-ISBN: 3-466-16305-0 Chapter 8.1 / pg 272

Cooling time can be estimated by use of the Moldflow Plastics

Insight (MPI™) or Moldflow Plastics Advisers (MPA™) programs

from Moldflow Pty Ltd. Also, the equation above can be iterated

manually.

Cooling complex and thicker parts

If a part has deep undercuts that measure more than 1.6 mm

(0.0625”), increase the cooling time to ensure de molding

without deforming the part. If a mold has untapered cores as

long as 100 mm (4”), decrease the cooling time ; otherwise, the

part may shrink tightly on the core, which could cause defor-

mation when the core is pulled.

10 - GENERAL PROCESSING

Drying of the material

Most of our general purpose and injection molding grades are

hygroscopic. More details are available from the Product Data

Sheet for each grade and/or the Quick Processing Reference.

This includes detailed information on processing setup and

unique features. Both documents are available on

www.santoprene.com

Effective moisture control ensures high quality parts that have

an appealing surface appearance. It is recommended for these

grades that the moisture be reduced to 0.08% or less before

processing. Specialized grades may have different drying

requirements. These will be spelled out on the Quick Processing

Reference for that grade. With any TPV, moisture impacts

processing, physical properties, product performance,

appearance, or a combination of these properties. Specifically,

moisture can cause poor melt quality, splay, voids, porosity,

rough surfaces, uneven appearance and/or silver streaking. For

some of the specialty bonding grades, drying is essential to

prevent material degradation.

Recommended best practices for drying

1. If possible, tumble blend colorants with the thermo plastic

pellets before drying in a desiccant/dehumidified hopper

dryer or vacuum oven. If colorants or additives are metered

at the hopper, pre dry colorants separately, unless not

recommended by the colorant supplier.

2. Open the drying hopper and make sure it is free of material.

Clean the hopper as needed. Be sure to follow standard

safety guidelines. Close the hopper and power it up.

3. Set the temperature control to ensure that air into the

hopper inlet is at the set point. We recommend using 80° C

(180 ° F) for most general purpose and injection molding

grades. Some specialty bonding grades have different drying

require ments. Please refer to the Product Data Sheet for

each grade or Quick Processing Reference for detailed

recommendations.

4. Once the dryer temperature is steady, check the dew point

indicator to ensure that a dew point temperature of 18 °C (0

°F) or lower exists. Fill the dryer hopper. Be sure to record

the material, grade, lot number, date and drying start up

time. As a suggestion, duplicate the same information on an

index or note card and attach it to the side of the dryer.

5. Dry virgin TPV pellets at least three hours, or dry blends of

virgin and regrind for four hours.

6. Once the drying cycle is complete, record the finished drying

time and move the TPV material to the processing

equipment. A best practice would be to have the drying

hopper integrated with the injection molding machine. This

insures that moisture cannot return to the dried material.

7. To eliminate the chance of an undried material “dead zone”

in the dryer, recycle some material from the bottom to the

top of the hopper.

8. A best practice would be to measure and confirm the

moisture level with a moisture analyzer. For best results, the

moisture level should not exceed 0.08%.

9. After the dryer is empty, power it down and again clean the

hopper.

Coloring

Parts molded with Santoprene TPV from ExxonMobil Chemical

can be produced on conventional molding equipment in any

color to match or com plement a finished product. Simply blend

neutral Santoprene TPV pellets with the appropriate colorant to

achieve nearly any color or hue desired. Pre colored TPV pellets

are available from several sources.

Solid color concentrates

Solid color concentrates are a versatile, pelletized form of a

colorant. These are widely used in the plastics industry due to

their stability, compatibility and dispersibility in almost any resin.

Color concentrates commonly consist of a pigment

compounded with a carrier resin such as polypro pylene or

polyethylene. Additionally these are well known for allowing

good control of color intensity in day to day consistency. They

are also dust free and easily used. Color concentrates can be

either tumble blended with the base resin or accurately metered

into a batch prior to processing. Color changes normally are

quick while minimizing cleanup.

The addition of any carrier resin can affect material properties,

including hardness, and it may slightly affect the material

processing and can affect part shrinkage. Some pigments also

affect these properties. Carrier less color concentrates are also

available to minimize property and processing effect.

Please follow the individual manufacture’s recom mendations

for loading. Typically this is about 1 to 5 weight percent,

depending on the application and the final color required.

Colorant carriers

We recommend using polyolefinic carriers such as

polypropylene with most colorable grades. The specialty

bonding grades may require specific or unique carrier or carrier-

less systems. Please refer to the Quick Processing Reference for

these recom men dations. Never use incompatible carriers. An

incompatible carrier can cause problems with melt quality,

which is evidenced by delamination in high shear regions or by

discoloration. Also, do not use polyvinyl chloride (PVC) as it is

not stable at normal TPV processing conditions.

As noted in the drying section above, most colorants absorb

moisture, so always follow proper drying procedures unless it

not recommended by the colorant supplier.

GENERAL PROCESSING - 11

Carrier-less color concentrates

Carrier less color concentrates are similar to standard color

concentrates except there is no plastic resin used to disperse

the pigment. A small quantity of binding agent is used. Typically,

there is a higher loading of pigment in these systems, and thus

less concentrate is used to achieve the same final color as

standard concentrates. Also, as no carrier resin is used there is

virtually no alteration of physical properties or processing

except as caused by the pigment itself.

Regrinding

Santoprene TPVs allow for regrinding and reprocessing of clean

runners, sprues and scrap parts with minimal variation in

material properties. Our materials can be exposed to multiple

heat histories with minimal change in tensile strength, modulus,

elongation and other properties. Although regrind levels up to

100% can be successfully employed. We recom mend using a

consistent percentage of regrind, i.e., 10 to 60%. The ability to

use higher regrind levels depends on factors such as machine-

part configuration, uniformity of the virgin regrind blend and

adherence to good house keeping practices. Higher

percentages of regrind (above 40%) tend to cause feeding

problems. It is industry best practice to use hopper or feed-

throat magnets to capture ferrous metal contami nates from

other manufacturing operations.

The following figure (see Figure 4) shows an example of

properties retention. This chart is based on Santoprene 201 80

TPV processed 6 times through a typical injection molding

machine. Virgin material was processed in the first pass. Each

subsequent pass contained 75% virgin material and 25%

regrind from the previous shot. Thus, the last pass contained

regrind in proportion from every previous pass. Injection

molded plaques of 2 mm were molded and test specimens

re taken. Tensile strength at break (psi), ultimate elongation

(%) and tensile modulus at 100% elongation (psi) were

measured using an ASTM Die C specimen using Test Method

TPE 0153 (derived from ASTM D 638). Also tested was 15

second delay Shore A Hardness per Test Method TPE 0189 (see

Table 2) and viscosity, Linear Capillary Rheometer (LCR) at 1200

sec 1 per Test Method TPE 0200 (see Figure 5).

Always dry regrind before processing. Please refer to the

Product Data Sheet for each grade or Quick Processing

Reference for these recommendations.

Figure 4

Tensile properties of Santoprene 201 80 TPV

(25% regrind material / 75% virgin material)

Table 2

Hardness change of Santoprene 201 80 TPV after multiple

molding cycles (25% regrind material / 75% virgin material)

Virgin 1

pass

pass 3

pass 4

pass 5

pass

Hardness,

Shore A

84.9 84.7 84.9 85.3 85.6 85.1

Santoprene 201-80 TPV, lot #PGF1369, per Test Method TPE-0189

Figure 5

LCR Viscosity at 1200 sec 1

(25% regrind material / 75% virgin material)

Shrinkage

For data on shrinkage, please contact the Answer person

tpe.answerperson@exxonmobil.com

1500

1000

500

Tensite strengh at break, psi

Ultimate elongation, %

Tensile modulus at 100% elongation, psi

Virgin 1

pass 2

pass 3

pass 4

pass 5

pass

Santoprene 201-80 TPV, lot #PGF1369, per Test Method TPE-0153

Virgin

LCR Viscosity

(Pa* sec)

LCR of Santoprene 201-80 TPV through multiple cycles per Test Method TPE-0200

pass 2

pass 3

pass 4

pass 5

pass

12 - MOLD DESIGN

MOLD DESIGN

Mold steel selection

The specifics of the considered injection molding application

determines the type of steel to use in a mold. Table 3 matches

certain applications with an appropriate steel grade.

Usually, structural mold sections are made from medium carbon

SAE 1030 or AISI 4130 steel. Since Santoprene TPVs are

generally non corrosive and non abrasive, we recommend P 20

steel for base plates.

Some TPVs, however, can corrode untreated tool steel after

long term contact. In this case, specify H 13 steel or use a

protective mold coating. Alternately, a corrosion resistant steel

like H13 or AISI 420 can be used.

Mold surfaces

The mold cavity finish determines the surface finish of a part.

Typically, an SPI B 3 to SPI D 1 finish (VDI 30 36 / Ra 3 6), provides

good mold release for TPVs ; however, if a high polish, SPI A 1 or

A 2 finish (<3Ra / 30 VDI) is used, an air assisted ejection is

required for easy part removal and minimal distortion. When

texturing a surface, we suggest a minimum draft of ½° per side,

plus ½° per each 0.025 mm (0.001”) depth of texture used.

For mold construction, obtain more information about mold

steel selection from steel suppliers and mold base

manufacturers. They can provide more detailed information

about the steels listed in table 4.

Type of steel Typical uses in injection molds

SAE 1030 or AISA 4130 general mold base plates

P -20 high grade mold base plates not recommended for cavities, cores, slides and interlocks

420 Stainless steel best grade mold base plates (no plating required), large cavities, cores and inserts

H- 13 (nitrided) cavities, cores, inserts, ejector pins and sleeves (nitrided)

S -7 cavities, cores, inserts and stripper rings

A- 2 small inserts in high wear areas

D -2 cavities, cores, runner and gate inserts for abrasive plastic materials

Table 3

Typical high grade mold steels from applications for typical mold steels

Process Material Applied to Purpose

Coating by impingement

molecularly bond

Tungsten

disulfide

Any metal Reduce friction in metal to metal wear with a dry film, which is

non migrating

Coating by impingement

organically bond

Graphite

Any metal Reduce sticking of plastics to mold surface - migrates

Electroplating Hard chrome

Steel, nickel and

copper alloys

Protect polish and reduce wear and corrosion

Electroplating Nickel

Steel and copper

alloys

Resist corrosion, improve bond under chrome, build up and repair

worn or undersized molds

Electroless plating Nickel

Steel Protect non molding surfaces from oxidation

Electroless plating

Phosphor nickel

Steel and copper Resist wear and corrosion

Nitriding Nitrogen gas

or ammonia

Certain steel

alloys

Improve corrosion resistance, reduce wear and galling; alternative

to chrome and nickel plating

Liquid nitriding Patented bath All ferrous alloys I mprove lubricity and minimize galling

Table 4

Mold surface plating and treatments

MOLD DESIGN - 13

Mold plating and coating

The working area of a mold can/should be plated, coated and/

or heat treated to resist wear, corrosion and release problems.

Table 4: Mold surface plating and treatmens lists treatments

that can help reduce wear on gates, runners, ejector pins, core

pins, inserts and particularly cavities opposite a gate.

Note: Plating and coating protects only the surfaces of a mold ;

heat-treating affects the physical properties of the entire mold.

Mold venting

When molten Santoprene TPV enters a mold quickly under high

pressure, enclosed air must exit just as quickly. Sufficient mold

venting is essential for efficient processing of TPVs. Even if

proper conditions exist throughout a molding cycle, inadequate

venting can cause “short shots” parts with burn marks, sink

marks, poor surface appearance and low weld line strength.

Additionally, without the proper venting, the following can

occur:

• Trapped air

• Slow fill time

• Voids

• Shorts Shots

• Poor weld lines

Santoprene TPVs require generous venting as the filling speed is

usually very fast (between 0.5 2 seconds). Vents should be

positioned at the final fill points or on the periphery of a mold

cavity. Long runner segments should be vented separately from

the cavity. Soft grades of Santorpene TPV are more susceptible

to vent related problems. Additional best practices would

include:

• All split lines should have peripheral vents.

• All blind grooves or pockets should be inserted.

• Vent and or air blast on ejector pins.

Venting dimensions

Mold venting should provide fast, efficient air remo val without

interfering with the molding process. Vents usually extend from

the cavity to the mold plate’s outer edge. Vents should be 0.02

to 0.04 mm (0.0008 to 0.0015”) deep.

Figure 6

Typical vent design

CAVITY VENT DETAIL

Vents

Section A-A

0.020 - 0.025 mm

vent depth

(0.0008 - 0.0010”)

Cavity

Depth

increased

to 1.3 mm

(0.05”)

vent to

atmosphere

RUNNER VENT DETAIL

Primary

runner

Secondary

runner

Vent

Mold

Section B-B

2.54 - 3.81 mm land

length (0.100 - 0.150”)

Depth increased

to 1.3 mm (0.050”)

vent to atmosphere

Parting

line

Runner

0.020 - 0.025 mm

vent depth

(0.0008 - 0.0010”)

14 - MOLD DESIGN

Vent locations

Always place a vent on the mold parting line at a point farthest

from a gate to ensure complete mold filling. Additional vents

may be needed, depending on the specific configuration of a

part. For simpler, general purpose molds, locate vents at every

spot where knit lines occur. When necessary, use full peripheral

vents.

For improved mold filling, vent the runner system as well. For

ribs or other restricted areas, drill a hole through each rib top

on the mold and plug it with a flat sided pin. This technique is

less than ideal since these pins are not self cleaning and could

clog. For deep mold cavities, use strategically located vent pins

or ejector sleeves. You can flatten an ejector pin by grinding off

opposing sides. This type of vent pin is also self cleaning during

the ejection stroke (see Figure 7).

Figure 7

Flattened vent pin

Sprue, runner and gate design

Sprues

They promote a smooth melt transition to the runner and

permit easy extraction while providing an optimum pressure

drop.

A standard sprue bushing with a minimum taper per side

approximately a 2.5 to 3.5 ° included angle (see Figure 8).

Figure 8

Typical sprue extractors

Runner types

Full round runners have the smallest surface to volume ratio and

are the most efficient cross section for pressure transmission

and heat retention. This type of runner is cut identically into the

2 mold halves so machining must be precise.

Trapezoidal runners also deliver efficient melt flow. This type of

runner system is machined into a single mold plate. Runner

walls should have a 5° taper with a round runner bottom. A

modified trapezoidal runners mold is shown in the Runner

Types figure (see Figure 9).

Half round runners are not recommended since they give the

lowest flow and are the most easily over cooled of all runner

types in common use.

Grind away from

opposing ides

0.05 mm (0.002”) 0.05 mm (0.002”)

Tapered extractor Z-pin extractor

Undercut extractor Sucker-pin extractor

Tapered extractor

Sucker

SprueRunner

MOLD DESIGN - 15

Figure 9

Runner patterns

HerringboneRadialH-type

Not recommendedNot idealRecommended

Figure 10

Cold slug well size and location

Vent to atmosphere

L = 1.0 to 1.5 D

Cold slug wells

Cold slug wells (see Figure 10) capture the cooled material

found in the nozzle tip and the material that is cooled during

the runner filling. If injected into a part, a cold slug can lead to

surface imperfections. A cold slug well captures solidified

material and prevents it from interfering inside that cavity.

Additional wells should be used at the end of each runner

section (turn/split).

Gating

Gates play a number of crucial roles in injection molding.

Santoprene TPVs from ExxonMobil Chemical have a relatively

high melt viscosity at low shear rates. Viscosity decreases as the

shear rate increases.

Increasing temperature has little effect on TPV melt viscosity.

Smaller gates and higher shear rates keep melt viscosity low

and improve melt flow.

As a general rule, minimize the number of gates per cavity. One

gate per cavity is usually best to avoid weld lines and cross flow

patterns ; however, some applications may require multiple

gating. For more information contact the Answerperson

at:

tpe.answerperson@exxonmobil.com or visit us at

www.santoprene.com

16 - MOLD DESIGN

Gate location

Gate location(s) affects the mold filling pattern and flow orientation

; gate location is just as important as gate size and type. Locate

gates to ensure rapid and uniform mold filling. As a general

guideline and recommendation:

1. Gates should be located to feed the thickest section of the part

first. Material will flow from thick to thin sections. This

promotes uniform flow and allows good packing.

2. Optimum gate locations will direct the melt flow through the

length of the part with nearly equal flow distances to all its

edges.

3. Place the gate in an unstressed (nonfunctional) area of the part.

Gate areas often contain high residual stresses from the filling

process.

4. Gates must be located to direct trapped air towards the vents.

Locate venting opposite the gate at the farthest flow point (see

Mold venting section for more information).

5. Avoid gating locations that will cause melt fronts to converge

or backfill (race track), thus entrapping air.

6. Locate a gate so that it directs material towards the wall or a

pin instead of into a free area. This will cause the material to

disperse and continue to flow instead of jetting.

7. Position the gate to minimize weld line problems, or to move

weld lines to preferred areas.

8. To reduce the number of multiple weld lines and venting

problems, use as few gates as possible to fill the part.

9. Use center gating for round or cylindrical parts to maintain

concentricity.

10. Gate land lengths should be kept short as possible.

11. Gate size should be kept small. A gate shear rate in the range of

to 10

sec

is recommended.

Gate design

Virtually any of the gating styles typically used in the injection

molding industry can be used to success fully process Santoprene

TPVs. As with the gate style, it is important to select a gate type

that is appropriate for the specific part geometry.

Direct sprue gates

Direct or sprue gates (see Figure 11) may be used with single cavity

mold where the sprue feeds material directly into the cavity. If

possible, provide a cold slug well opposite the sprue gate.

Figure 11

Direct sprue gate

Sprue

Activity/part

Pin gates

Pin gates (see Figure 12) are actually small gates located on the

parting line to minimize gate vestige/blemish. They are especially

popular for automatic de gating in three plate molds. Another

advantage of pin gates is that they can easily provide multiple

gating to a cavity (for thin walled parts).

Figure 12

Pin gate

Radial flow path

Sucker pin

Breakpoint

Zero land length

3 deg taper per side

1.3 mm (0.05’’) minimum

Part

90°

MOLD DESIGN - 17

Submarine gates

Submarine gates provide automatic de gating of a part from the

runner system during part ejection. Submarine gates enter the

mold below the parting line. To reduce pressure loss and prevent

premature gate freeze off, submarine gates should be as short as

possible. The typical size range is 0.80 to 1.5 mm (0.03 to 0.06”). A

sharp angle should range from 20 to 30 °, with a 60 ° angle from

the runner. Figure 13 illustrates a suggested gate configuration.

Unless special drilling is used, the gate will be oval due to the angle

of entry.

Figure 13

Submarine (tunnel) gate

Ejector pin

Gates diameter

45°

30°

60°

Curved submarine gates

Cashew gates move the gate vestige from the side of a part (often

an appearance surface) to somewhere underneath the part where

it can be more tolerated. The following figure shows a schematic of

this type of gate.

Figure 14

Curved submarine (cashew) gate

Parting line

Ring and diaphragm (disc) gates

A diaphragm or disc gate is used for cylindrical parts that require

good concentricity and elimination of weld lines (for high strength).

The gate land or membrane should be significantly thinner in

cross section than the runner ring or central disk. The thickness

differential forces the ring or disk to fill completely before the melt

fills the membrane. This type of gate requires a post mold de gating

operation.

Edge gates

Edge gates, both rectangular and round, must be large enough to

avoid melt overheating due to frictional heating. The preceding

figure details typical depth to width ratios for edge gates.

Figure 15

Edge (rectangular) gate

D:W Ratio

Smaller parts: 1:1

Medium parts: 1:2

Large parts: 1:3

L = Land length

D = Gate depth

W = Gate width

18 - MOLD DESIGN

Fan gates

For flat mold sections, a modified fan gate may be used, which

minimizes jetting and significantly reduces the high stresses that

occur during mold packing. The plastics industry suggests

smooth radii and transitions between runners and gates. Fan

gates can be balanced so they distribute the melt evenly across

the land before the melt enters the mold cavity. Fan gates

require a post molding de gating operation and may leave

noticeable vestige.

Figure 16

Fan gate

L = Land length

T = 40 - 60% of t

W = 2 D min.

Note: use full

round runner

Tab gates

Tab gates provide a uniform melt orientation when an

application requires a large volume for mold filling. Tab gates

help reduce the effect of residual stresses, gate blush and

jetting in the gate area. They are used where flatness is critical

or in large surface areas that may have a tendency to warp. Tab

gates require a post molding de gating operation and may leave

noticeable vestige.

Figure 17

Tab gate

Runner

Tab

Tab gate

Part

Flash gates

A flash gate is used to minimize warping in flat or very large

parts. Flash gates extend across a part from 0.38 to 0.76 mm

(0.015 to 0.030”) deep and have a chisel taper across the gate

land length. Flash gates require a post molding de gating

opera tion and may leave noticeable vestige.

Figure 18

Flash gate

Runner

Part Flash gate

MOLD DESIGN - 19

Ejection

The ejector system design and method should be based on a

particular product and its hardness. Ejector system design

method and selection recom mendations follow:

1. To de mold soft grades of Santoprene TPV, a combination of

stripper plates and integrated air blast is required.This is

essential when it comes to parts with deep draw and heavily

undercuts.

2. To de mold hard grades of Santoprene TPV, ejector pins are

required. The pins should always be as large as possible and

push against supported sections to avoid part deformation.

3. To make part de molding easier, sand or vapor blast the mold

cavity walls with a matte or satin surface finish SPI #3 or VDI

33 36 reference 3400.

4. Use draft to ensure trouble free part ejection. On cavity and

core sides, an angle of 0.25 to 1° per side is normally

sufficient for the majority of injection molded parts. Bosses

and ribs should be molded with a minimum of 3° draft. Refer

to the Quick Processing Reference for any special draft

considerations.

The inherent elastic properties of Santoprene TPVs make it

possible to de mold parts of diverse design and complexity

without damaging them when they are stretched or flexed.

Parts with undercuts that would normally require side cores,

slides, collapsible cores and other mold design

accommodations associated with hard thermoplastic materials

can be de molded easily using a more simply constructed mold.

Undercut design guidelines

Proper part and tool designs are even more critical if a molded

part has undercuts. The following recom men dations identify

key undercut design conside ra tions:

1. Fillet and taper an undercut well to allow for expansion and

slippage ; this avoids tearing at sharp corners during part

ejection.

2. The undercut part must be free to stretch or compress ; that

is, the part wall opposite an undercut must clear the mold or

core before part ejection starts. Use a matte, bead blasted

steel finish (SPI #3 VDI 27 or coarser) for cavity and core

inserts. This finish eases part release during ejection.

3.Air breaks the vacuum that exists between a part and the

undercut core before the mechanical ejector takes over.

Mold temperature control

The cooling time for a molded part is typically 60 to 80% of the

total cycle time. Uniform and efficient mold temperature control

is critical for minimizing cooling time, maximizing production

rate and controlling part tolerances. Adequate cooling is one of

the most important aspects of mold design. The cooling system

should have the ability to rapidly and uniformly cool a mold. It

also should be able to maintain mold and system temperature

settings in a production environment from cycle to cycle.

Uniform cooling not only ensures a shorter molding cycle, but

also minimizes differential shrinkage and reduces internal

stresses.

20 - HOT RUNNER SYSTEMS

HOT RUNNER SYSTEMS

Hot runner tooling techniques can be used to efficiently process

Santoprene TPVs in a variety of commercial applications. In fact,

in most cases, hot runner processing can actually improve part

quality and reduce molding cycle time.

Hot runner systems have runners that are internal to the mold,

and the system maintains material in the runner at about the

same temperature as the barrel. The benefits of hot runner

systems are that there are no runners or gates to be trimmed

and recycled, and cooling the runner does not require additional

cycle time. In addition, smaller gates can be used to improve

molded part appearance.

System selection

For successful processing, a hot runner system should keep the

material melt temperature at or near nozzle temperature as the

material passes through the gate and into the mold cavity.

These four steps are recommended:

1. Minimize the time material remains in a hot runner system. If

possible, it should contain no more than one shot.

2. Design melt channels for uniform pressure drop throughout

the system.

3. Prevent or eliminate possible “hang ups” (i.e. burrs, sharp

corners and heating rods) and other melt flow interruptions.

4. Maintain precise temperature control and make sure heat is

evenly distributed throughout the system, including drops

and manifolds.

Externally heated manifold

An externally heated manifold contains a manifold block with a

hollow interior that channels the main runner flow. Heaters

parallel the runner flow channel and heat the entire manifold

block. Drops or auxiliary channels are usually heated as well and

feed the melt into the mold cavity.

We recommend using this type of manifold system for hot

runner tooling since it provides a “straight through” melt flow

path. It minimizes pressure drop and reduces the chance of

material “hang up,” discoloration and burning. In addition, the

entire manifold block is heated, making it easier to maintain

uniform temperature. An externally heated manifold system

requires a sufficient number of heaters and temperature

sensors for precise temperature control.

Insulated runner molds

We do not recommend insulated runner molds. Insulated

runner molds incorporate an oversized runner and create a skin

of cooled material that insulates the melt. As a result, degraded

or unmelted material can enter the melt flow. During long cycle

interruptions, the entire unheated runner can freeze off and

require disassembling the mold for cleaning.

Mold design

Manifolds

In general, hot runner manifolds for most TPVs have reduced

flow channel diameters compared to those used for rigid

thermoplastics. For most TPV applica tions requiring the use of

hot runners, we recommend small bore, full round channels with

a 6.4 mm (0.25”) diameter. This channel size provides a uniform

pres sure drop that minimizes material residence time within the

manifold. Make sure all material passages are smooth and

streamlined without protrusions, sharp corners or blind spots.

To take advantage of the melt flow properties of Santoprene

TPVs, flow channels should be cylindrical rather than annular.

A balanced manifold system is essential for efficient, multi cavity

hot runner processing. In each section of a mold, match the

length and diameter of the primary and secondary flow

channels. Since flow channels in a manifold function as full-

round runners, the melt flow must be balanced to uniformly fill

all cavities.

Shot size verses manifold volume

Shot size verses the manifold volume determines the residence

time. If the diameter of a manifold is too large, the material

tends to hang and form channels, which results in an

insufficient shear rate and an excessive residence time possibly

leading to degradation. Ideally, the manifold volume (runners

plus probes) should be significantly less than the shot volume ;

however, high residence times with small parts cannot be

avoided.

Probes

Gates are an integral part of a heated probe. The probe is

attached to the downstream end of the melt channel. The

probe fits down the melt channel center, forming an annular

flow path for the material. Manufacturers of probes offer

various models according to the type of gate: hot tip, edge,

valve or core. Remember all probes need to be insulated using

DuPont™ Vespel ® or equivalent. An end cap or bushing will

restrict the amount of dead material coming into contact with

the relatively cold mold steel. This forms a cold slug.

It is essential in any gating system to maintain the melt

temperature with minimum temperature loss between the flow

channel and the mold cavity. Excessive temperature loss could

cause a cold slug to form at a gate. The cold could cause a

defect on the surface of a molded part. Probe recommen-

dations are summarized below:

HOT RUNNER SYSTEMS - 21

1. The probe channel must be provided with an external heater.

This can be achieved most efficiently by embedding cartridge

heaters in the probe metal. Other methods include heating

with a socket (continuous heating) or a coil (discontinuous

heating). The choice depends on the type of probe selected.

2. The probe channel should be small enough to achieve a high

shear rate.

3. The tip of the probe is often the critical point and any heat

loss at this juncture must be avoided. The cartridge heater

should be entirely embedded in the metal of the probe.

Copper alloys enable an adequate temperature to be

maintained through to the gate. In most critical cases,

adding a thermoplastic insulator, i.e., DuPont™ Vespel ®,

around the tip of the probe further assists flow at this point

and should improve surface finish in the gate area.

4. Effective gate diameter at the end of the probe should be

approximately 1.0 mm (0.040”) for parts with a maximum

weight of 50 grams (1.76 oz) ; for parts that weigh 10 grams

(0.35 oz) or less, a diameter of 0.8 mm (0.032”) is

acceptable.

5. A minimum probe clearance between the gate and cavity will

help optimize appearance at the injection point (see Figure

19). Thus, a clearance (at ambient temperature) of about 0.2

mm (0.0080”) results in a barely visible injection point (gate

ves tige). For additional details, consult the manu facturer’s

specifications for hot runners and probes.

Figure 19

Probe clearance

Probe

Melt

channel

Gate size

Probe tip

setting

Gate selection

For individual hot runner applications, carefully evaluate part

design and gate styles to determine the following:

1. Optimum location for gating

2. Number of gates needed to fill a part as quickly as possible

(typically 0.5 to 2.0 seconds)

3. Ensure the air and gas is directed to an area of the part

where it can be effectively collected and vented during the

extremely short filling phase of the process

Hot runner processing requires relatively smaller gates than

those used in cold runner processing. In general, hot runner

gates range in size from 0.8 to 1.5 mm (0.032 to 0.060”). A

gating system should maintain melt temperature with a

minimum of heat loss between the flow channel and the cooler

mold cavity.

22 - HOT RUNNER SYSTEMS

Open / torpedo-tip gates

A torpedo tip, or open, gate has a heated torpedo tip or probe

which provides a high pressure seal from the hot nozzle to the

cooled gate insert and minimizes heat loss. Due to the flow

characteristics of most TPVs, clearances can be a problem and

occasionally the torpedo tip support fins can also create flow

problems. The gate diameter should range from 0.8 to 1.5 mm

(0.032 to 0.060”), and the probe or the torpedo should be 0.02

mm (0.001”) behind the gate and the heat source. If needed,

the torpedo can be moved forward into the gate to eliminate

vestige. As this gate is always open, there is always a tenancy to

drool. This probe design is only recommended for large

non aesthetic parts where gate vestige is not important. For a

fully controlled runner system and good gate aspect, valve gate

systems are recommended.

Figure 20

Open / torpedo tip gates

DuPont

™

Vespel

Valve gates

Valve gates are the only gate types recommended for use with

Santoprene TPVs. They are the only way to effectively control

nozzle / gate drool as well as give a good gate aspect. Valve

gates feature a small double acting piston in their center that

usually is powered by hydraulic or pneumatic pressure. This

piston opens during filling and packing, then closes to minimize

gating vestige. To avoid damaging the gate area, the piston

stem should have a positive stop for closing. Good shutoff

virtually eliminates gate vestige on a molded part, thus

improving appearance and preventing nozzle drool when the

mold opens. They are suitable for both small and large molded

components.

For additional details, consult the manufacturer’s design

requirements for valve gate probes and their installation.

Figure 21

Valve gates

Insulation

Melt stream

MULTI-MATERIAL MOLDING - 23

Overmolding

For an even wider variety of part applications, polymers can be

combined through several different types of multi shot injection

molding processes. These include insert molding, two shot

molding/2K or 2 component molding, and co injection molding.

We offer TPVs that can bond directly to a variety of substrates.

Standard grades of Santoprene TPV bond to polypropylene,

some polyethylenes and some TPOs. Nylon bondable grades of

the PA Series of Santoprene TPV bond to nylon 6, nylon 6 with

a variety of fillers, and to some blends of nylon 6/6. Only

two shot molding is recommended. Other special bondable

grades bond to a variety of engineered thermoplastic (ETP)

substrates such as PC, ABS, ASA, PC/ABS, PET, PBT, PS and

HIPS. Please use the Advanced Product Data Search tool on

the website.

Insert molding

Insert molding (sometimes called over molding) consists of

placing a solid insert (or preform) into the mold and injecting

around it. If the insert and the over molded material are

compatible, a melt bond occurs between the two materials at

their interface. The strength of this bond is affected by several

factors, including interface temperature, cleanliness of the insert

and the melt temperature. Bonding can be improved by

pre heating the insert and ensuring that the surface of the insert

is free from dirt, grease, hand oil from an operator or any other

contaminant. Non compatible insert materials such as metals

also can be used. This requires that an adhesive be applied to

the insert so that it bonds with the over molded material.

Otherwise, a design utilizing mechanical locks is required.

Insert molding is the simplest of the multi shot processes since it

is most similar to standard injection molding. For low volume

applications, the inserts can be hand loaded, but for higher

volumes, robotic pick and place methods may be a cost effective

choice.

Two-shot molding

Two shot (also known as 2K or 2 component) molding consists

of a machine having two independent injection units shooting

two different materials into the same mold through separate

runners and gates. The first material is shot through a primary

runner system as in normal molding, while the runner system

for the second material is shut off. After the first shot, the mold

slides or rotates to shut off the first runner system and opens

the second for the next shot of material. After the second shot,

the part is cooled and ejected normally.

Two shot molding requires that the two materials be compatible

or no bonding occurs. The bond strength of a two shot molded

part is generally better than that of an insert molded part due to

the higher temperature at the melt interface.

Co-injection molding

Co injection or sandwich molding differs from two shot molding

because co injection molding uses a common runner and gate

for both materials. This allows for a soft touch material to be on

the outside while having a hard, compatible core material to

provide mechanical properties to the system. This construction

allows the designer to take advantage of the individual

properties of each material. For example, consider a heavy-

walled part. Cycle time can be substantially reduced by

processing the core material at a lower melt temperature (to

decrease cooling time) while running the skin material at a

higher melt temperature to achieve a smooth surface.

Efficient co injection requires a separate injection unit for each

material. Also, materials can be co injected into specially

designed rotary or shuttle molds.

24 - QUICK REFERENCE NOTES

QUICK REFERENCE NOTES

Screw design

Special equipment is not required use a screw L/D ratio of

approximately 20:1 and a compression ratio of 2.5:1.

Barrel capacity

Barrel capacity should be no less than 1.3 shots and no more

than 4 shots.

Clamp pressure

Clamp pressure should be 29 to 49 MPa (3 to 5 T/in2) of

projected area.

Screw RPM

Screw RPM should be 100 to 200.

Screw cushion

Screw cushion should be 3 to 6 mm (0.120 to 0.250”) Refer

to the processing section on page 7 of this manual.

TROUBLESHOOTING

Injection molding requires controlling and fine tuning many

machine and process variables. Problems can arise that

result in molded parts that are unsatisfactory. Thus,

troubleshooting involves control of the molding process by

maintaining the correct adjustment and balance of these

major variables.

Varying machine conditions in a logical manner during the

molding cycle is an effective approach for trouble shooting

and correcting most processing problems. Troubleshooting

recommendations in this section frequently contain more

than one recommended corrective action, so change only

one condition at a time and be sure to allow sufficient time

for the system to reach equilibrium at the new condition. If

the trouble is not corrected, proceed to the next

recommendation. Keep a careful record of each change,

noting how it affects subsequently molded parts.

TROUBLESHOOTING - 25

Troubleshooting guide

Problem Probable cause Corrective action

BURN SPOTS

ON PART

1. Mold is insufficiently vented. A. Make sure vents are clear of obstructions.

B. Insure vents are deep.

C. Add vents, if necessary, to assure good operation.

2. Injection rate is too high. A. Reduce injection speed.

3. Screw RPM is too high. A. Decrease screw RPM.

4. Back pressure is too high.

A. Reduce back pressure. Minimum is 0.3 MPa (50 psi).

5. Clamp pressure is too high. A. Reduce clamp pressure.

BURN MARKS

AT GATE

1. Melt flow across restricted

gate area causes excess shear

heat.

A. Reduce injection speed.

B. Reduce back pressure.

C. Decrease nozzle temperature.

D. Decrease front barrel zone temperature.

E. Decrease mold temperature in gate area.

F. Check for resin contamination.

DISCOLORED

STREAKS OR

CHUNKS

1. Additives or colorants have

incompatible carriers.

A. Change to an additive or colorant material which has a polyolefin base,

such as polyethylene or polypropylene.

Never use additive or colorant materials which have polyvinyl chloride carriers.

2. Prior runs have contaminated

the plastication system.

A. Physically clean the entire system - hopper, barrel, screw, shut off valve,

nozzle and mold.

B. Purge the system with polyethylene or polypropylene.

3. Melt has insufficient color

concentrate blending.

A. Increase back pressure.

B. Increase barrel temperature.

C.Reduce rear zone temperature.

D.Change screw RPM.

E. Decrease nozzle orifice diameter.

F. Replace screw or nozzle with one which has a mixing design.

DISTORTED

PARTS

1. Material is oriented during

injection.

A. Decrease injection velocity for slower filling.

B. Increase mold temperature.

C. Adjust velocity profile to provide homogeneous melt.

2. There is a difference in

packing density.

A. Increase hold pressure.

B. Increase injection fill speed to fill faster.

C. Increase screw RPM.

3. Molded part has stresses. A. Increase melt temperature.

Prevent overpacking at gate by reducing hold pressure or reducing hold time.

C. Lengthen cooling time.

D. Delay knockout actuation.

E. Increase injection forward time.

F. Reduce temperature differential in mold.

4. Material flow into mold is not

uniform.

A. With multi-cavity molds, make sure flow is balanced.

5. Part is too hot when ejected. A. Reduce mold temperature.

B. Increase cooling time.

C. Reduce melt temperature.

6 Knockout is operating

improperly.

A. Knockout engaging too fast, slow down ejection.

B. Non-uniform operation of pins.

C. Bearing area of knockout pins is too small. Increase diameter and number

of knockout pins.

D. Relocate knockout pins to strong area of part.

E. With delicate parts, change to air blow-off.

7. Part design is incorrect. A. Make sure part contains no sharp variations in cross sections.

B. Correct taper or draft insufficiencies.

26 - TROUBLESHOOTING

Problem Probable cause Corrective action

FLASH ON

PARTING LINE OF

PART

1. Excessive injection pressure

causes mold to part.

A. Verify injection fill time.

B. Decrease final injection speed.

C. Reduce back pressure.

D. Decrease hold pressure.

E. Increase clamp pressure.

F. Move transfer point back.

2. Melt temperature is too high. A. Reduce nozzle temperature.

B. Reduce front barrel zone temperature.

C. Reduce back pressure.

D. Decrease mold temperature.

E. Reduce screw RPM.

3. Mold function is incorrect. A. Make sure mold faces are in correct alignment.

B. Remove foreign matter from mold faces.

C. Decrease size of vents.

D. Decrease mold temperature.

E. Check platens for square.

4. Mold release agent causing

flash on part.

A. Mold surface lubricant should not be used.

JETTING 1. Mold design has insufficient

impingement

A. Relocation of gate to create impingement.

MATERIAL

DROOLS FROM

NOZZLE

1. Pressure build up. A. Add decompression/suck-back.

2. Barrel front zone is too hot. A. Reduce front barrel zone temperature.

3. Nozzle is too hot. A. Reduce nozzle temperature.

PART

DIMENSIONS ARE

TOO LARGE

1. Mold is over packed. A. Verify injection fill time.

B. Decrease hold pressure.

C. Decrease overall injection speed.

D. Decrease final injection speed.

E. Decrease back pressure.

F. Decrease front barrel zone temperature.

G. Increase mold temperature.

2. Mold designed incorrectly. A. Verify mold shrinkage factor used; adjust mold design if necessary.

3. Material is wet. A. Pre-dry the material.

PART

DIMENSIONS ARE

TOO SMALL

1. Mold is underpacked. A. Verify injection fill time.

B. Increase overall injection speed.

C. Increase hold pressure and hold time.

D. Reduce cushion in cylinder to 6.0 to 3.2 mm (0.250 to 0.125”).

E. Increase back pressure.

F. Decrease mold temperature.

G. Increase melt temperature.

H. Increase cavity venting.

I. Increase cooling time.

2. Mold design is incorrect. A. Verify mold shrinkage factor used; adjust mold design if necessary.

B. Add additional gates.

PARTS STICK IN

MOLD CAVITY OR

SPRUE

1. Part is not sufficiently

cooled.

A. Reduce stock temperature.

B. Decrease second stage injection time.

C. Reduce injection pressure.

D. Reduce screw RPM.

E. Add decompression.

F. Increase overall cycle time.

G. Decrease mold Temperature.

2. Mold design is incorrect. A. Seal nozzle correctly in sprue bushing.

B. Reduce length of sprue.

C. Increase draft on side walls of part to assure smooth release.

D. Draw polish cavity surface in direction of ejection. (Consult the

Answerperson

before implementing this corrective action.)

TROUBLESHOOTING - 27

Problem Probable cause Corrective action

PARTS STICK IN

MOLD CAVITY

OR SPRUE (cont.)

3. Cores are too slender. A. Make cores stronger to prevent misalignment by injection pressure.

4. There is an overpack problem. A. Reduce injection speed.

B. Back off transfer point.

C. Reduce pack/hold pressure.

POOR FINISH 1. Mold fill is too slow. A. Increase injection pressure.

B. Increase injection speed.

C. Increase melt temperature.

D. Increase gate size.

2. Mold function is incorrect. A. Make sure venting is adequate.

B. Increase mold temperature.

C. Do not use mold lubricants.

D. Clean cavity surfaces.

3. Part design is incorrect. A. Reduce abrupt changes in section thicknesses.

B. Avoid ribbing that is too thick.

POOR SURFACE

DETAIL

1. Mold is filling improperly. A. Increase shot size.

B. Increase overall injection speed.

C. Increase back pressure.

D. Increase mold temperature.

2. Packing conditions are

improper.

A. Increase injection fill time.

B. Increase mold temperature.

C. Increase second stage pressure and Time.

POOR WELDS

ON PART

1. Material flow is cooling in

cavity too quickly.

A. Increase melt and mold temperature.

B. Increase injection pressure.

C. Increase ram speed.

D. Increase nozzle temperature.

E. Lengthen cycle time.

2. Mold design and/or function

are improper.

A. Make sure vents are not plugged.

B. Increase gate size; relocate for most efficient melt flow.

C. Raise mold temperature.

D. Increase size of runner.

E. Decrease fill speed.

SHORT SHOTS

PREVENT

COMPLETE

MOLD FILLING

1. Available melt volume is

insufficient.

A. Increase shot size; confirm a cushion of 3.2 mm (1/8”).

B. Increase injection fill time.

C. Make sure nozzle and runners are clear of obstructions.

D. Increase front barrel zone temperature.

E. Increase nozzle temperature.

F. Verify operation of non-return check valve on screw.

2. Mold has high pressure drop. A. Increase overall injection speed.

B. Increase back pressure.

C. Increase hold pressure.

D. Try a higher flow grade.

E. Add gates.

3. Mold function is incorrect. A. Increase mold temperature.

B. In multi-cavity molds, be sure all gate sizes are uniform to prevent unbalanced

melt flow.

C. Check part design to make sure there are no restrictions.

D. Make sure mold is vented correctly and vents are clear.

SILVER

STREAKING

1. Material has trapped

moisture.

A. Pre-dry material according to instructions.

B. Make sure mold is vented properly.

2. Melt temperature is too high. A. Reduce barrel heating to prevent decomposition.

B. Reduce screw RPM to stop excessive shear heating.

3. Cold mold causes moisture

condensing.

A. Increase mold temperature; make sure cavity is dry.

SINK MARKS 1. Mold is underpacked. A. Increase overall injection speed.

B. Increase boost cut off set point.

C. Increase hold pressure and temperature.

D. Increase cushion in cylinder.

28 - TROUBLESHOOTING

Problem Probable cause Corrective action

SINK MARKS

(CONT.)

1. Mold is underpacked.

(Continued)

E. Increase shot size.

F. Increase barrel heating.

G. Increase back pressure.

H. Add or increase nozzle heating.

2. Mold function is incorrect. A. Be sure gates are large enough and are correctly located.

B. Reduce length of sprue bushing, increase bushing diameter.

C. Increase mold temperature.

D. Make sure vents are large enough and are clear.

E. Increase mold cooling cycle time.

3. Part design is incorrect. A. Redesign part to remove sharp thickness changes in section.

SPLAY MARKS IN

GATE AREA OF

PART

1. Last part of mold fill has slow

pressurization.

A. Increase mold fill speed.

B. Increase back pressure.

C. Increase second stage injection time.

D. Increase barrel temperature.

E. Increase mold temperature.

F. Increase screw RPM.

G. Remove decompression.

2. Mold set up is not correct. A. Make sure sprue and runner system are direct and sized correctly to ensure

fast, efficient flow.

B. Shorten land length of gate.

VOIDS IN

MOLDED PART

1. Material in core sets up more

slowly than surface material,

especially in thick cross

sections.

A. Increase injection pressure and time.

B. Increase back pressure.

C. Increase mold temperature.

D. Make sure sprue, runner and gates are large enough to assure free flow.

E. Increase cooling time.

2. There is moisture in pellets. A. Pre-dry pellets according to instructions.

WARPING 1. See Distorted Parts above. A. See DISTORTED PARTS problem above for corrective action.

TROUBLESHOOTING - 29

S0817-030E49

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