microsoft word - molding notes rev 03

7/28/2019 Microsoft Word - Molding Notes Rev 03

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Reference: Prepare by Panir / Afendi

MTM P1 EngInternet source

Engineering forum

Moulding handbook

A. General Information

Complete cycle of injection molding

a. Closingb. Fillingc. Packingd. Coolinge. Mould Openingf. Ejection

Series configuration


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Temperature

a. Flow history of plastic material includes traveling from hopper into heating cylinderof injection unit. The material is augured in heating cylinder and through machine

nozzle and then pushed into mold.

b. The temperature of the melt must be controlled all along pathc. Heating bands for 3 zones

i. Rear zoneii. Center zone (10F-20F hotter)

iii. Front Zone (10F-20F hotter)d. Fourth zone is the nozzle


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Mould temperature

a. Mold Temperature is measured directly from molding surface with data acquisition orsurface pyrometer.

b. Object of the cooling process is to lower the temperature of the molded plastic to thepoint at which it solidifies.

Shear Heat

a. During mold filling, the event will take place in a quick time with high pressuresupport. This will cause heat as the material forced or shear along. The heat is not

spread uniformly throughout the material but is highest where the shear rate is highest

as well. It may cause overheating with material.

Pressure

a. Two areas require pressure and pressure controli. Injection unit

ii. Clamp unit

b. Initial injection unit - Applied to the molten plastic and resulting from the mainhydraulic pressure pushing against the back end of the injection screw.c. Injection pressure - Lower than hold and pack pressure which be between 10,000psi

and 20,000 psi


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Hold Pressure

a. Used to finish the filling of the mold and pack the partb. Rule of thumb: Hold pressure = 50% of injection pressurec. Applied at the end of the initial injection stroke and is intended to complete the final

filling of the mold and hold pressure to solidify while staying dense or packed

d. The purpose of the injection hold pressure phase of the process is to maintain theplastic pressure inside of the cavity until the gate has frozen off, which then acts as a

barrier to any further pressure loss in the cavity. The amount of hold pressure needs

to be just high enough to maintain the cavity pressure during this time, without anyloss of pressure or discharge of plastic resin back out of the gate. Assuming the part

has been designed correctly, this should give you a part of acceptable quality.

Back pressure

a. Applied after the injection phases are completeb. When holding pressure is complete the screw begins to turn in order to bring new

material to the front of the barrel in preparation for next shot. In other word, the

material has filled and the screw is pushed back.

c. Back pressure is small compared to injection pressure (between 50 psi and 500 psi(screw may not turn if exceeded).

d. Procedure is to start with small amount of back pressure and steadily increase inincrements of 10 psi.

e. Back pressure is a tool to help provide a consistent and homogenized melt stream. Ifyou applied zero back pressure to your screw during the process, the movement of

plastic down the screw flights during rotation would create an uncontrolled and

inconsistent shot density during each cycle.

f. This would contribute to shot-to-shot variations in the entire molding process,because of the direct effect it has on shot size consistency.

g. Back pressure also adds a frictional component to the melting process, taking someamount of the load off the barrel heaters as well. These settings will often be a low as

50 PSI and as high as 500 PSI, with 100 to 200 PSI being somewhere nears the

common middle of the range.


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h. The use of back pressure is vital to our melt consistency, and also to the removal orprevention of cold pellets in the melt stream. It improves the consistency and

homogenization of the melt, and insures that shot after shot it will be the same.

i. Back pressure ensures consistency in part weight, density, and material appearance.Squeezes out any trapped air or moisture. Minimizes voids in molded parts.

Clamp Unit

a. The purpose of developing clamp pressure is to keep the mold clamped shut againstthe forces developed when the injection pressure pushes plastic into the closed mold.

Fill Time

a. How long it takes to fill part. Faster filling rate = shorter fill timeb. Pressure is a function of the flow rate. Faster flow rate = higher pressures, except at

very slow fill which causes larger core and smaller flow channel and then higher

pressures

Cushion

a. Cushion (0.125 to 0.25in) of material should be left in barrel for the hold pressure tobe applied against.

b. Cushion is created by creating a total shot size that is slightly larger than that requiredto fill the mold.

i. Example,ii. Amount of material required to fill mold is 2.9 oz (82.2g), then the

total shot size would be 3 oz.

c. Thickness of cushion is critical

i. Minimum is 1/8 because anything less would be difficult to controland there is a chance it could go to zero from the inconsistencies of

the density.

ii. Maximum of 1/4 thick because any more than this and the cushionmight solidify and block the nozzle.

Injection Speed

a. Refer to the speed of mold filling. When molding thin sectioned component, highinjection speeds are essential in order to fill the molding before freezing occurs. A

better surface finish is obtained on moldings with thicker sections by using a slower

speed. Range of speed may avoid jetting and air trap which will lead to mold fault.

Injection Distance

a. Set to ensure 95% of the intended material is injected.b. Ideal shot size is 50% of barrel capacity.

Injection-hold Distance

a. After filling 95% of the required material, the machine switches to holding pressure.b. This finishes filling and holds pressure against material previously injected

Screw-return Distance


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a. Prepare for the next shot. The set point is such that slightly more material in the barrelthan is required to fill the mold.

b. RPM should fall within 30 to 160 RPM

Velocity Pressure Transfer (VPT)

a. A sensor is used to track the screw position and the position of screw with respect totime can be plotted. The information from the sensor is used to a constant injection

molding rate.

b. The pressure progressively increases because the resistance to flow progressivelyincreases as the mold fills. Example; when the mold is full or the gate is freeze, the

resistance to flow becomes very high and it becomes unrealistic to expect the screw

to maintain the desire rate. At this point, control is shifted from velocity controlled to

pressure controlled.

c. Changeover at VPT may set to trigger;i. Screw positionii. Hydraulic pressure

iii. Cavity pressureiv. Nozzle pressure (melting pressure)v. Mold opening force

vi. Mold opening positiond. Molder must select a critical cavity pressure at which to change from filling pressure

to hold. This critical pressure is selected by observing what peak cavity pressure is

associated with an acceptable molding without cavity pressure control.

e. VPT point is capable of being set very precisely. If this condition cannot be met thenmolding with various properties may result.

f. Pressure measuring system should check periodically to ensure the preset valuesbeing obtained.

A = the un-pressurized melt at the injection temperature

B = the pressurized melt when the melt front reaches the end of the cavityC = the fully packed melt at the time the gate freezes off


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D = the solidified polymer

E = the polymer part when it is ejected from the mold

Further away from the gate, pressure rises slowly and it decays quicker than at the pointscloser to the gate.

Mould Venting

a. At the start of the injection cycle, the mould cavity contains air which must be ventedas it is displaced by the incoming melt. Inadequate venting can result in considerable

compression and heating of the trapped air, resulting in slow filling, poor welds and

possible burning of the resin.

b. Vents are located at the end of flow paths, most often in the parting line but alsoaround ejector pins.

c. Vents must be large enough to allow free passage of air, but prevent the passage ofmelt.

Mould Cooling

a. It is necessary to remove heat from the part to freeze it and cool it below its softeningtemperature before it can be ejected. It is important to remove the heat rapidly, for

fast cycle time, and evenly to prevent warpage related to uneven crystallization.

b. In multi cavity moulds, cooling must be uniform in all parts.c. Mould temperature is controlled by circulating cooling water through conduits in the

mould. Cooling conduits must be carefully placed in the mould to ensure even

cooling of the part.

d. "Hot spots" may be encountered in areas around gates, due to shear heating, or inareas of the mould which are difficult to reach with cooling conduits.

e. Special metal inserts may be used in these areas, made from material with high heattransfer rates, such as beryllium copper.

f. Separate cooling channels can also be installed in these areas, operating with higherflow rates or lower temperature cooling water than the rest of the mould.

Ejector Systems

a. Some force must be applied to the moulded parts to eject them from the mould.

Mechanical knock-out devices such as pins or sleeves can be used, driven by themovement of the mould as it opens.


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b. With mechanical ejectors, the area of the knock out device in contact with the partmust be large enough to avoid damaging or stressing the part.

B. General Moulding Process Flow


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C. CYCLE TIME

a. The sequence of events during the injection mold of a plastic part is called theinjection moulding cycle. The cycle begins when the mold closes, followed by the

injection of the polymer into the mold cavity. Once the cavity is filled, a holding

pressure is maintained to compensate for material shrinkage. In the next step, the

screw turns, feeding the next shot to the front screw. This causes the screw to retract

as the next shot is prepared. Once the part is sufficiently cool, the mold opens and thepart is ejected.

b. The total cycle time can be calculated using tcycle = tclosing + tcooling + tejectionc. The closing and ejection times, can last from a fraction of a second to a few seconds,

depending on the size of the mold and machine. The cooling times, which dominate

the process, depend on the maximum thickness of the part

D. Gates

Sub Gate - (May Also called a Tunnel Gate, Cashew or Banana)


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a. Gating away from the parting line can be accomplished by using a Sub Gate. TheSub Gate also provides for automatic De-Gating of the Runner and Part within

the mold.

b. Cashew, Banana gates require split inserted steels. Split steels are required tofacilitate machining. Standard inserts are readily available. The diameter at the

gate is .030-.090 for unfilled materials and .100-.125 for filled materials. The

angle is typically at 30 to 45 degrees from vertical. Ejector Pins are required to

ensure automatic de-gating.

A Sub Gate will leave a Pin sized Scar on the part.

Flash Gate


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a. The Flash gate is typically used on flat acrylic parts, where flatness and non warpingis to be kept to a minimum. The runner adjacent to the gate usually runs parallel with

the edge of part. Flash gates typically exceed 25% of the width of the part at the

gating location.

b. The Flash gate requires post processing to remove the extensive scar.

Sprue Gate

a. The Sprue gate is used when single cavity cylindrical parts need to be balanced andconcentric. Sprue gated pars have very good weld-line strength (if any), and typically

are lower stressed, and are of high strength.

b. A Sprue Gate will leave a significant Scar equal to the size of the sprue diameter at

the point of contact of the part.


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Ring Gate

a. A Ring Gate will produce a Scar around the entire part, the height is equal to the gateheight.

Pin Gates

a. Pin Gates are used in three-plate molds. The actual gate diameter is from .030 - .100diameter


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b. A Pin Gate will leave a small Scar that is the size of the gate

Fan Gate

a. Fan Gates deliver plastic to a wide area of the part. This minimizes backfilling, andprovides for better part surfaces, and reduces stress as well as imperfections.


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b. A Fan Gate will leave a Scar the size of the cross section of the gate, and requires(typically) manual trimming from the runner.

Edge Gates

a. Edge Gates are the most commonly used of all gating options.b. The height of the gate should equal 75-100% of the wall thickness up to .125 in.c. The width should equal 2 times the depth, as it would appear in a mold

d. An Edge Gate will leave a Scar at the Parting Line equal to the cross section of thesize of the gate


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E. Runner

a. A Full Round Runner is the most efficient shape for reducing the cooling effect onthe material as it flows in the runner.

b. The Half Round runner is simply a runner system machined with a ball nose cutter

into one plate of the mold.

c. Trapezoidal Runners are very common in three plate molds. While not as efficient inchilling effect of a full round runner, the ease of cutting the runner shape, and the

elimination of the need to mate two runner plates together, makes the trapezoidal

runner a good second choice of runner shape.


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Cold Slug Well

a. Cold Slug Wells are highly desirable in an Injection Mold. The Cold Slug Wellprovides a small reservoir (well) to trap air and impurities before they enter the

Runner, Gate and Cavity.

b. A Cold Slug Well is located above the Sprue Puller Pin. Typically, as the runnerchanges from a primary to secondary and secondary to tertiary there is also a cold

slug well at each intersection.

c. A Hot Runner Mold is similar to a hot glue gun. Material is heated to a molten state,and then it is dispensed at the tip to the desired area.

d. Parts can be small single gated, or large and multi-gated. Hot Runner Molds havemany unique advantages over "Cold Runner" molds.

e. Hot Runner Molds are typically more expensive than Cold Runner" molds, the cost ofthe mold can be offset in other ways. Thermoplastic Hot Runner Molds can reduce

costs due to :


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i. No scraping of the runner: As the term implies, the runner in a HotRunner mold stays in a molten state at all times (no regrind).

ii. Reducing the cycle time: In a Cold Runner mold the runner typicallyhas the largest cross sectional area; therefore, the runner takes longer

to solidify. Eliminating the runner reduces the overall cycle time.

Furthermore, injection time is reduced due to the shot size being reduced

by the elimination of the runner.

a. Hot Runner Molds have the ability to improve both part andmold design with flexibility of gating locations, which provides

options for cavity orientation.

b. Pressure drops are greatly reduced due to the balanced melt flowas the temperature is consistent from the machine nozzle to the

gate. Precise material temperature control is critical to successful

Hot Runner processing

F. Quick Reference on common moulding defect

Poor cavity filling

Cause:

Barrel and mold temperature are too low

Injection pressure and/or speed are too high

Material flow is too high, i.e. material is too soft

Injection starts before clamping pressure has been applied

Solution:

Increase temperature of Barrel and mold

Adjust injection pressure and flow control to fill cavity slowly

Use harder material

Delay injection starting time

Black Spots, Brown streaks

Description

Black spots and brown streaks appear as dark spots or streaks in the molded part and are

usually caused by thermal damage to the melt.

Cause:

Particles on the tool surface, contaminated material or foreign debris in the barrel, ortoo much shear heat burning the material prior to injection

Possible Solutions


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Check the material for contamination.

Decrease the melt temperature.

Decrease the overall cycle time.

Purge and/or clean the screw and barrel.

Decrease the screw speed. High screw speeds may cause the material to degrade.

Material may have too much regrind content.

Material may be over dried. Decrease drying time/temperature. Refer to dryinginstructions provided by the material supplier.

Material may be prone to thermal degradation. It may be necessary to use a morethermally stable material.

Dead spots may be occurring, ensure that the alignment between the machine nozzleand mold sprue is correct.

Residence time may be too long, or the shot size may be too small for the machine. It

may be necessary to move the mold to a machine with less injection capacity.

Blisters (Air Entrapment)

Description

Blisters are hollows created on or in the molded part. In contrast to a void (vacuum) this

entrapped gas can also appear near the walls.

Cause:

Tool or material is too hot, often caused by a lack of cooling around the tool or afaulty heater

Possible Solutions

Decrease melts temperature.

Decrease screw speed.

Dry material.

Increase back pressure.

Increase mold temperature. Ensure regrind is not too coarse.

Provide additional mold vents.

Relocate gate.

Brittleness

Description

Brittleness is a condition where the part cracks or breaks at a much lower stress level than

would normally be expected based on the virgin material properties.


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Possible Solutions

Check for material contamination.

Decrease amount of regrind use.

Decrease back pressure.

Decrease injection pressure.

Decrease screw speed.

Increase melts temperature.

Dry material. Refer to the drying instructions provided by the material supplier.

Bubbles

Description

Bubbles are similar to blisters in that there is air entrapped in the molded part.

Possible Solutions

Decrease injection speed.

Decrease injection temperature.

Dry material further.

Increase injection pressure.

Increase number and/or size of vents.

Increase shot size.

Burn Marks, Dieseling

Cause:

Tool lacks venting, injection speed is too high

Description

Burn Marks or Dieseling show up on the finish molded parts as charred or dark plastic caused

by trapped gas and is usually accompanied by a distinctive burnt smell.Note: If this problem is allowed to continue without fixing the root cause it will very quickly

cause damage to the molding surface.

Possible Solutions

Alter gate position and/or increase gate size.

Check for heater malfunction.

Decrease booster time.




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Decrease melt and/or mold temperature.

Improve mold cavity venting. Vents may become smaller over time due to wear andthey will need to be brought back to their original depth.

Reduce clamp force to improve venting. Vents may become smaller because they arebeing crushed by the clamping force. If it is possible to reduce the clamping force

without causing flash then this should be done. Note: This is always good practice to

minimize wear on the mold and machine.

Improve venting at the burn location. Burn marks often occur on deep ribs that haveno venting. If possible it may be helpful to put an ejector pin or sleeve at the burnt

area to allow the trapped gas to escape to atmosphere.

Cracking, Crazing

Description

Cracking or Crazing is caused by high internal molded in stress or by an external forceimposed upon the part. They can also be caused by an incompatible external chemical being

applied to the finished parts the cracks often don't appear until days or weeks after the parts

have been molded.

Possible Solutions


Dry material.

Increase cylinder temperature.

Increase mold temperature.

Increase nozzle temperature.

Modify injection speed.

If the material is partially crystalline then it may help to reduce the mold and/or melttemperature.

If the material is amorphous then it may help to increase the mold and/or melttemperature.

De-lamination

Description

De-lamination occurs when single surface layers start flaking off the molded part.

Cause:

Contamination of the material e.g. PP mixed with ABS, very dangerous if the part isbeing used for a safety critical application as the material has very little strength when

delaminated as the materials cannot bond

Possible Solutions


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Adjust injection speed.

Check for material contamination. Incompatible resins or colorants may have beenaccidently mixed causing this condition to be seen.

Dry material.

Increase melts temperature. Increase mold temperature.

Insufficient Blending. Check melts homogeneity and plasticizing performance.

Discoloration

Description

Discoloration is similar to burn marks or brown streaks but generally not as dark or severe. It

may cause the part to be a darker shade than the virgin pellets and is often found nearest the

gate area, however it can also appear asdark streaks throughout the part.

Possible Solutions

Check hopper and feed zone for contamination.



Decrease nozzle temperature.

Move mold to smaller shot-size press.

Provide additional vents in mold.

Purge heating cylinder.

Shorten overall cycle.

Excessive Flash

Description

Excessive Flash is often seen near sealing faces, out of vent grooves, or down ejector pins. It

appears as thin or sometimes thick sections of plastic where it would not be on a normal part.Note: Flash can very quickly (within a few cycles) damage the parting line surfaces.

Cause:

Mold is over packed or parting line on the tool is damaged, too much injection speed/material

injected, clamping force too low. Also be caused by dirt and contaminants around tooling

surfaces.

Possible Solutions

Decrease back pressure. Decrease cylinder temperature.


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Decrease injection hold time.



Decrease mold temperature.

Increase clamp pressure.

Check mold venting. Vents may have been ground too deep for the material beingused.

Check sealing surfaces to ensure that they seal off properly by "blueing" them inunder clamp tonnage.

Check ejector pin bore diameter to pin diameter tolerances. The tolerances may betoo large allowing plastic to flash down the opening. The tolerances may be too large

for the material being used and can occur due to wear over time.

Flow, Halo, Blush Marks

Description

Flow, Halo, Blush Marks are marks seen on the part due to flow of the molten plastic across

the moulding surface.

Cause:

Injection speeds too slow (the plastic has cooled down too much during injection,injection speeds must be set as fast as you can get away with at all times)

Possible Solutions


Increase cold slug area in size or number.




Increase nozzle temperature.

Increase size of sprue/runner/gate.

Gate Stringing, Drooling

Description

The part does not break cleanly from the gate area.

Possible Solutions

Insufficient cooling time during the cycle.

Excessive heat in the gate area. Check thermocouple in the nozzle or decrease the

temperature of the hot runner manifold and nozzle.


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Increase cooling at the gate area. Ensure that you have controllable turbulent flow inthe gate area.

Gels

Description

Gels are bubbles or blisters seen on or in the part due to poor melt quality.

Possible Solutions

Change screw speed.



Increase overall cycle time.

Increase plasticizing capacity of machine or use machine with large plasticizingcapacity.

Jetting

Description

Jetting is caused by an undeveloped frontal flow of melt in the cavity. The uninterrupted

plastic flows or "snakes" into the cavity and cools off enough so that it does not fuse

homogeneously with the material that follows.

Cause:

Poor tool design, gate position or runner. Injection speed set too high.

Possible Solutions


Change the melt temperature, up or down.

Use higher compression screw.

Increase the gate diameter.

Move the gate so that when the plastic first enters the cavity it hits an obstructionsuch as a rib or wall.

Material Leakage

Description

Material Leakage is usually caused by material forces overcoming the structural strength of

the mold.

NOTE: One side indicates that material has leaked, inspect the manifold reaches processingtemperature.


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Possible Solutions

Manifold locator is oversize.

Processing temperature may be too low causing increased pressure in the manifold.

Manifold locator may be hobbled into the mold. Decrease the force applied to thenozzle pad and repairs the damaged area. If necessary replaces the locator.

Insufficient number of mold assembly screws. Ensure that the quantity, type of screw,and the location of the screws correspond to the general assembly drawing.

Nozzle may have overheated causing damage to the seal or gate. Check/replace thethermocouple in the nozzle, then check and if necessary repair the nozzle well area.

Manifold may have overheated. Check and replace if necessary the followingcomponents; nozzle well area, thermocouple, valve disks, sprue disks, or pressure

disks.

Oversized Part

Description

Part is too large when compared to the drawing specifications.

Possible Solutions


Decrease cylinder temperature.

Decrease holding pressure.



Decrease overall cycle time.

Increase gate size and/or change gate location.


Part Sticking

Description

Part is getting not pulling out of the cavity and in rarer circumstances cannot be ejected off

the core.

Possible Solutions

Check mold for undercuts and/or insufficient draft.


Decrease cylinder and nozzle temperature.


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Decrease injection-hold.

Decrease mold cavity temperature.

Increase clamp pressure.

Increase mold-close time.

Texturing on part is too deep. The parts may stick in the cavity if a new texture or aretexturing has been performed on the cavity half of the mold.

If possible add undercuts to the core to allow the part to pull out of the cavity.

Short Shot (Incomplete Filled Parts)

Description

Short Shots occur when the part does not completely fill.

Cause:

Lack of material, injection speed or pressure too low, mold too cold

Possible Solutions



Increase injection speed.



Increase nozzle temperature. Ensure that the manifold and nozzles have reached theset temperature.

Increase shot size and confirm cushion.

Make sure mold is vented correctly and vents are clear.

Confirm that the non-return valve used is not leaking excessively.

Increase the switch over pressure, distance, or time (whichever method is being used)

point from fill to hold so the fill stage is used longer. Change part design. Thin areas of the mold may not fill completely, especially if

there is a thick to thin transition, or there is a long rib that cannot be vented very well.

If the part design allows it, change in these areas can

improve the situation.

Sink Marks

Description

Sink Marks occur during the cooling process if certain areas of the part are not cooled

sufficiently causing them to contract.


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Moisture in the material, usually when hygroscopic resins are dried improperly.Trapping of gas in "rib" areas due to excessive injection velocity in these areas.

Material too hot.

Possible Solutions

Check for contamination.



Dry resin pellets before use. As per the manufacturers recommendations.

Incorrect storage of pellets. Moisture on the pellets could be transferred into the melt,especially if the resin is not normally pre-dried.

Raise mold temperature. This will prevent condensation on the mold walls from beingcarried into the melt.

Ensure the mold is not leaking water onto the cores or cavities. Again this willprevent condensation on the mold walls from being carried into the melt.

Relocate gates on or as near as possible to thick sections.

Shorten overall cycle.

Sprue Sticking

Description

Sprue Sticking generally occurs in a cold runner mold when the sprue is staying in the mold.

Possible Solutions

Check mold for undercuts and/or insufficient draft.




Decrease injection-hold.

Decrease mold close time.


Increase core temperature.

Open the gates.

Ensure that the correct design of nozzle tip for the material is being used.

Surface Finish (Low Gloss)

Description

Surface Finish (Low Gloss), Gloss is the appearance of the surface of the molded part when

light is reflected off of it. Molds that are textured or resins that are filled have an inherently

reduced level of gloss when compared to highly polished mold surfaces.


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Possible Solutions

Clean mold surface.

If the part design allows increase the polish of the molding surface.

Increase cylinder temperature. This applies to molds that have a polished surface.

Increase injection pressure. This applies to molds that have a polished surface.

Increase injection speed. This applies to molds that have a polished surface.

Increase mold temperature. This applies to molds that have a polished surface.

Decrease cylinder temperature. This applies to molds that have a textured surface.

Decrease injection pressure. This applies to molds that have a textured surface.

Decrease injection speed. This applies to molds that have a textured surface.

Decrease mold temperature. This applies to molds that have a textured surface.


Make sure venting is adequate.

Surface Finish (Scars, Wrinkles)

Description

Surface Finish (Scars, Wrinkles), is the appearance of the ripples or wrinkles on the surface of

the molded part.

Possible Solutions



Increase booster time.

Increase the melt temperature.



Increase overall cycle time.

Increase shot size.

Inspect mold for surface defects.

Undersized Part

Description

Part is too small when compared to the drawing specifications.

Possible Solutions

Decrease mold temperature.

Increase booster time.


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Increase hold-time.

Increase holding pressure.



Inspect mold for surface defects.

Valve Pin Does Not Close

Description

Valve pin does not close properly. This will leave the gate protruding from the part. This may

also occur if the valve pin is too hot; the material may stick to the valve pin.

Possible Solutions

Valve pin is too short. Check and replace if necessary.

Valve pin fit. Ensure that the valve pin is lapped to the gate steel when appropriate.

Damaged gate. Check if valve pin is too long, rework if necessary. Also check toensure that the valve pin is concentric with the gate, if not replace it.

Hydraulic / Pneumatic seals may be worn. Replace as necessary.

Insufficient pin/land area in the gate area of the mold. Increase the gate area cooling,or increase the valve pin land contact.

Insufficient hydraulic or air pressure. Increase the pressure up to but not beyond the

maximum rating of the unit being used.

Excessive hold time. Decrease the hold time.

Voids

Description

Voids are hollows created in the part. They are normally found in thick sectioned parts caused

by material being pulled away from the hot center section towards cold mold walls leaving a

void in the center.

Cause:

Lack of holding pressure (holding pressure is used to pack out the part during theholding time). Filling to fast, not allowing the edges of the part to set up. Also mold

may be out of registration (when the two halves don't center properly and part walls

are not the same thickness).

Possible Solutions

Clean vents.



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Dry material.


Increase injection-hold.


Increase shot length.

Increase size of gate.

Increase size of sprue and/or runners and/or gates.

Warping, Part Distortion

Description

Warping, Part Distortion is shows up as parts being bowed, warped, bent or twisted beyond

the normal specification outlined on the drawing.

Cause:

Cooling is too short, material is too hot, lack of cooling around the tool, incorrectwater temperatures (the parts bow inwards towards the hot side of the tool) Uneven

shrinking between areas of the part

Possible Solutions Adjust melt Temperature (increase to relieve molded-in stress, decrease to avoid over

packing). Stress, decrease to avoid over packing). Stress, decrease to avoid over

packing).

Check gates for proper location and adequate size.

Check mold knockout mechanism for proper design and operation.

Equalize/balance mold temperature of both halves.

Increase injection-hold.

Increase mold cooling time.

Relocate gates on or as near as possible to thick sections.

Try increasing or decreasing injection pressure.

Weld Lines

Description

Weld Lines are created when two or more melt flow fronts meet possibly causing a

cosmetically visible line. It can also create a weakened area in the finished molded part

especially with filled resins.


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Cause:

Caused by the melt-front flowing around an object standing proud in a plastic part aswell as at the end of fill where the melt-front comes together again. Can be

minimized or eliminated with a mold-flow study when the mold is in design phase.

Once the mold is made and the gate is placed, one can minimize this flaw only by

changing the melt and the mold temperature.

Mold/material temperatures set too low (the material is cold when they meet, so theydon't bond). Point between injection and transfer (to packing and holding) too early.

Possible Solutions



Increase injection hold.



Make sure part contains no sharp variation in cross-sections.

Vent cavity in the weld area.

Cold Flow

Wavy or streaked appearance on the part surface Looks like a fingerprint or small waves like

you would see on the surface of water.

Cause:Low melt temperature, low injection speed or low injection pressure.

Cold Slug

Cold piece of plastic that has been forced into the part along with the melt

Cause:

Plastic from last shot left in nozzle solidifies between shots. The tool designer usually is able

to allow for a "cold slug well" in the runner to catch this piece. 2. Cold slug effects can also

occur at the end of a long runner.

Solution:

Add a cold slug well at each intersection in the runner. Addition of a shortened ejector pin on

the runner very close to the gate may divert the cold slug. For direct sprue gating try to make

a feature in the part to catch the slug or use a heated nozzle


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Drag

Fine, straight lines scraped in the line of draw.

Cause

Depends upon location; Cavity side happens during mold opening and is usually from

insufficient draft for the texture used or from over packing. Core side drag happens during

ejection and is usually from inadequate draft, rough core, or over packing.

Solution

Solve over packing problem. Cavity side drag, tone down the texture by stoning then bead

blast. Core side drag, polish core, adds draft.

Mismatch (parting)

The cavity side of the tool does not fall in perfect registry with the core side resulting in a stepat parting line. It may look like flash if it is slight. If it is smooth as your finger runs across

one way and feels sharp the other way it is mismatch. If you can feel it both ways it is flash.

Cause:

Uneven pressure in the mold cavity can push the cavity one direction and the core the other.

This usually happens in very asymmetrical parts or parts with a parting surface that slopes

only one way. Mold maker did not properly position the cavity relative to the core. In older

tools mismatch may occur as locking faces wear.

Solution

Straight locks at parting line. The best are those made by Progressive Components (higher

tonnages).

Pin push

Circular or semicircular white stress rings on the side of the part opposite an ejector pin. May

even be raised circular bumps. In serious cases pins may push right through the part

Cause:

Over packing, sticking on the core and inadequate ejection.

Solution

Solve over packing problem. Polish core or increase draft on core. Add more ejector pins.More small pins are better than a few

Plate out

A change of mold texture over time that is not due to wear.

Cause

Build up of chemical residue from out gassing and Build up of mold release.

Solution

Have the mold cleared


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Pulling

Deformed, twisted and smeared plastic on the part

Cause

Cavity side: part sticking to the cavity when tool opening. Listen to the mold as it opens to see

if you can hear it pop free. Core side: Uneven part ejection is not pushing the part out straight.

The part gets skewed as it ejects, the resulting damage is called pulling.

Solution

Cavity side pulling, add undercuts or texture on core side so part pulls cleanly from the

cavity. Core side pulling, add ejection. More small pins are better than a few big ones

G. Injection Molding Glossary

Air Burn: A patch or streak of brown or black material on the component caused by air or

gases that have not been properly vented from the mold and caused the material to overheat

and burnt.

Antioxidant: Additive used to help protect plastics from degradation through sources such as

heat, age, chemicals, stress, etc.

Antistatic Agent: Additive used to help eliminate or lessen static electricity from the surface

of the plastic part.

Aspect Ratio: Ratio of total flow length to average wall thickness.

Back Pressure: The pressure applied to the plastic during screw recovery. By increasing back

pressure, mixing and plasticizing are improved; however, screw recovery rates are reduced.

Backing Plate: Plate use to support the mold cavity blocks, guide pins, bushings and etc.

Blistering: A raise or layered patch of material on the surface of the component.

Boss: Protuberance on a plastic part designed to add strength, facilitate alignment, provide

fastening, etc.

Broken Mold Marks: Filled in areas not per drawing specification due to mold damage.

Bubbles: Air pockets that have formed in the material of the component. Bubbles may vary in

size.

Cavity: The space inside a mold into which material is injected.

Charge: The measurement or weight of material necessary to fill a mold during one cycle.

Clamp: The part of an injection molding machine incorporating the platens that provides the

force necessary to hold the mold closed during injection of the molten resin and open themold to eject the molded part.


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Clamping Plate: A plate fitted to a mold and used to fasten the mold to a platen.

Clamping Pressure: The pressure applied to the mold to keep it closed during a cycle,

usually expressed in tons.

Clarifiers: Additive used in polypropylene random copolymers to improve clarity.

Closed-loop Control: System for monitoring complete, injection molding- process conditions

of temperature, pressure and time, and automatically making any changes required to keep

part production within preset tolerances.

Cold Flow/Orange Peel/Lumps: Any material that has not cooled uniformly causing the

appearance of either a speck of material lighter than what was used or a rippling effect on the

surface of the component.

Cooling Channels: Channels located within the body of a mold through which a cooling

medium is circulated to control the mold surface temperature.

Crack/Splits/Chips: A physical separation or tearing of the part.

Cushion: Extra material left in barrel during cycle to try and ensure that the part is packed out

during the hold time.

Cycle: The complete sequence of operations in a process to complete one set of moldings.

The cycle is taken at a point in the operation and ends when this point is again reached and

moving platens of the clamp unit in the fully open position.

Cycle Time: The time required by an injection molding system to mold a part.

De-lamination: When the surface of a finished part separates or appears to be composed of

layers. Strata or fish-scale-type appearance where the layers may be separated.

Diaphragm Gate: Used in symmetrical cavity filling to reduce weld-line formations and

improve filling rates.

Dimensional Problems: Parts not made to drawing dimensional specifications due to internal

part stress warping, mold damage, incorrect mold manufacturing, etc.

Direct Gate: The sprue that feeds directly into the mold cavity.

Discoloration: Any change from the designated color of the material or component. Incorrect

color of the component.

Drag Marks: A form of deep scratch or scratches on the surface of the component that have

no visible signs of loose chips or material.

Dwell: A pause in the applied pressure to a mold during the injection cycle just before the

mold is completely closed. This dwell allows any gases formed or present to escape from the

molding material.

Ejection Pin Marks: See Raised Ejector Site.


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Ejector Pins: Pins that are pushed into a mold cavity from the rear as the mold opens to force

the finished part out of the mold. Also called knockout pins.

Ejector Return Pins: Projections that push the ejector assembly back as the mold closes.

Also called surface pins or return pins.

Ejector Rod: A bar that actuates the ejector assembly when the mold opens.

Fan Gate: A gate used to help reduce stress concentrations in the gate area by spreading the

opening over a wider area. Less warping of parts can usually be expected by the use of this

type of gate.

Fill: The packing of the cavity or cavities of the mold as required to give a complete part or

parts that are free of flash.

Fin: The web of material remaining in holes or openings in a molded part which must be

removed for final assembly.

Flash: Any excess material that is formed with and attached to the component along a seam

or mold parting line.

Flow: A qualitative description of the fluidity of a plastic material during the process of

molding. A measure of its mold ability generally expressed as melt flow rate or melt index.

Flow Line: Marks visible on the finished items that indicate the direction of the flow of the

melt into the mold.

Flow Marks: Wavy surface appearances on a molded part caused by improper flow of the

melt into the mold.

Gas-Assisted Injection Molding: In the gas assisted process, an inert gas is injected into the

center of the flow of plastic. This method provides parts which combine thick and thin walls,

parts with hollow sections and elongated shapes, and more complex parts replacing multipart

assemblies.

Gate: An orifice through which the melt enters the mold cavity.

Gate Trim: Remnant of plastic left over from cutting the component from the runner or

sprue, usually to be cut flush with the edge of the component.

Hob: A master model in hardened steel. The hob is used to sink the shape of a mold into a

soft metal block.

Hopper Dryers: Auxiliary equipment that removes moisture from resin pellets.

Hopper Loader: Auxiliary equipment for automatically loading resin pellets into machine

hopper.

Hot-Runner Mold: A mold in which the runners are insulated from the chilled cavities and

are kept hot. Hot-runner molds make parts that have no scrap.

Injection Pressure: The pressure on the face of the injection screw or ram when injectingmaterial into the mold, usually expressed in PSI.


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Inmold Decoration (IMD): A molding process where films, coatings or printing are placed

in the mold prior to injection to become a more permanent part of the molded item.

Inmold Labeling (IML): A molding process where a label is placed in the mold prior to

injection to become a more permanently attached to the part.

Insert Molding: Insert molding is the process of molding plastic around preformed metal

inserts. This process is compatible with both thermoplastic and thermoset materials.

Jetting: A turbulent flow in the melt caused by an undersized gate or where a thin section

rapidly becomes thicker.

Knit Lines: Where melted material flows together to form a line or lines that may cause

weakening or breaking of the component.

Knockout Pins: A rod or device for knocking a finished part out of a mold.

Melt Flow Rate: A measure of the molten viscosity of a polymer determined by the weight of

polymer extruded through an orifice under specified conditions of pressure and temperature.

Particular conditions are dependent upon the type of polymer being tested. MFR usually is

reported in grams per 10 minutes. Melt flow rate defines the flow of a polypropylene resin.

An extrusion weight of 2160 grams at 446F (230C) is used.

Melt Index: Term that defines the melt flow rate of a polyethylene resin. An extrusion weight

of 2160 grams at 310F (190C) is used.

Mold: A series of machined steel plates containing cavities into which plastic resin is injected

to form a part.

Mold Changer: An automated device for removing one mold from a machine and replacing it

with another mold.

Mold Frame: A series of steel plates which contain mold components, including cavities,

cores, runner system, cooling system, ejection system, etc.

Mold Release Problems: Excess use of mold release may leave parts oily and weaken the

material.

Mold-Temperature-Control Unit: Auxiliary equipment used to control mold temperature.

Some units can both heat and cool the mold. Others, called chillers, only cool the mold.

Moving Platen: The platen of an injection molding machine that is move by a hydraulic ram

or mechanical toggle.

Non-Return Valve: Screw tip that allows for material to flow in one direction and closes to

prevent back flow and inject material into the mold.

Nozzle: The hollow-cored, metal nose screwed into the injection end of a plasticator. The

nozzle matches the depression in the mold. This nozzle allows transfer of the melt from the

plasticator to the runner system and cavities.

Orange Peel: A surface finish on a molded part that is rough and splotchy. Usually cause bymoisture in the mold cavity.


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Over Molding: Filled with one plastic and then a second shot are injected to encapsulate the

first shot.

Packing: The filling of the mold cavity or cavities as full as possible without causing undue

stress on the molds or causing flash to appear on the finished parts. Over- or under-packing

results in less than optimum fill.

Part Picker: An auxiliary unit usually mounted on fixed platen, which reaches into the open

mold to grab parts and remove them prior to next molding cycle. Also called a robot, the

device is used when you do not want to drop parts from mold upon ejection.

Parting Line: On a finished part, this line shows where the two mold halves met when they

were closed.

Peeling: An open blister.

Pin Marks: See Raised Ejector Site.

Pinpoint Gate: A restricted gate of 0.030 in or less in diameter, this gate is common on hot-

runner molds.

Plasticate: To soften by heating and mixing.

Plasticator: The complete melting and injection unit on an injection molding machine.

Purging: The forcing one molding material out of the plasticator with another material prior

to molding a new material. Special purging compounds are used.

Recovery Time: The length of time for the screw to rotate and create a shot.

Re-grind Problems: See Silver/Splay. Use of re-ground material increases susceptibility for

moisture problems as well as polymeric chain length degradation.

Restricted Gate: A very small orifice between runner and cavity in an injection mold. When

the part is ejected, this gate readily breaks free of the runner system. Generally, the part drops

through one chute and the runner system through another leading to a granulator and scrap

reclaim system.

RMS Roughness: A measure of the surface roughness/smoothness of a material. The root

mean square (RMS) average of the "peaks and valleys" of a surface is determined using aProfilometer. The lower the number, the smoother the surface: a reading of one or two would

be a very polished and smooth surface.

Rockwell Hardness: A measure of the surface hardness of a material. A value derived from

the increase in depth of an impression as the load of a steel indenter is increased from a fixed

minimum value to a higher value and then returned to the minimum value. The values are

quoted with a letter prefix corresponding to a scale relating to a given combination of load

and indenter.

Runner: The channel that connects the sprue with the gate for transferring the melt to the

cavities.

Scratch: Mark made via abrasion, not as specified in visual or cosmetic specification criteria.


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Screw Travel: The distance the screw travels forward when filling the mold cavity.

Short Shot: Failure to completely fill the mold or cavities of the mold. Edges may appear

melted.

Shot: The complete amount of melt injected during a molding cycle, including that which fillsthe runner system.

Shot Capacity: Generally based on polystyrene, this is the maximum weight of plastic that

can be displaced or injected by a single injection stroke. Generally expressed as ounces of

polystyrene.

Shrinkage: The dimensional differences between a molded part and the actual mold

dimensions.

Sink/Shrink: A depression or valley on a component surface that would not normally have a

depression.

Single-Cavity Mold: A mold having only one cavity and producing only one finished part

per cycle.

Sink Mark: A shallow depression or dimple on the surface of a finished part created by

shrinkage or low fill of the cavity.

Slip Agent: Additive used to provide lubrication during and immediately following

Splay Marks: Marks or droplet type imperfections on the surface of the finished parts that

Sprue Gate: A passageway through which melt flows from the nozzle to the mold cavity.

Sprue Lock: The portion of resin retained in the cold-slug well by an undercut. This lock is

used to pull the sprue out of the bushing as the mold opens. The sprue lock itself is pushed out

of the mold by an ejector pin.

Sprue: The feed opening provided in injection molding between the nozzle and cavity or

runner system.

Stack Molds: Two or more molds of a similar type that are positioned one behind the other to

allow for additional parts to be manufactured during a cycle.

Stress Cracking: There are three types of stress cracking:

1. Thermal stress cracking is caused by prolonged exposure of the part to elevated

temperatures or sunlight.

2. Physical stress cracking occurs between crystalline and amorphous portions of the part

when the part is under an internally or externally induced strain.

3. Chemical stress cracking occurs when a liquid or gas permeates the parties surface. All of

these types of stress cracking have the same end result: the splitting or fracturing of the

molding.


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Structural Foam Molding: Process for making parts that have solid outer skin and foamed

core. An inert gas foaming agent (e.g., nitrogen is injected and mix with the plastic material

under high pressure inside the extruder barrel. The mold is filled with a short shot and the gas

expands the material forming a solid skin with a cellular structure inside. This process is often

used for large structural plastic parts with high strength and low weight.

Submarine Gate: A gate where the opening from the runner into the mold cavity is located

below the parting line. Also called a tunnel gate.

Suck-back: When the pressure on the sprue is not held long enough for the melt to cool

before the screw returns. Some of the melt in the cavities or runner system may expand back

into the nozzle and cause sinks marks on the finished part.

Tab Gate: A small removable tab about the same thickness as the molded item, but usually

perpendicular to the part for easy removal.

Tonnage: The measure by which injection molding machines are typically categorized,

representing the clamping force of the injection molding machine.

Vent: A shallow channel or opening cut in the cavity to allow air or gases to escape as the

melt fills the cavity.

Vented Barrel: Special plasticator unit with a vent port over the compression section of the

screw to permit escape of gases prior to injecting melt into mold. Often used when molding

moisture-sensitive resins.

Vertical Flash Ring: The clearance between the force plug and the vertical wall of the cavity

in a positive or semi-positive mold. Also the ring of excess melt which escapes from the

cavity into this clearance space.

Voids: Air pockets in the part which have opened or were not filed with material, leaving an

opening or hole.

Warpage: Dimensional distortion in a molded object. Caused by internal stresses via un-even

material flow, cooling, and compression.

Weld Line: Where melted material flows together during molding to form a visible line or

lines on a finished part that may cause weakening or breaking of the component.

Wisps: Similar to stringing but smaller in size. These also may occur as slight flashing when

the mold is over packed or forced open slightly. Mold-parting-line wear or misalignment can

also cause wisps.

H. Appendix 1: Quick trouble shoots reference


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Defect

BackPressure

BarrelTemps

BridgingofMaterial

ClampPressure

CleanMold

CleanScrew

CleanVents

ConsistentCycleTime

CoolingTime

CushionSize&Consistency

DamagedTooling

DecompressionSettings

EjectorBars(EqualLength

)

EjectoSpeed

EndOfArmTooling

FeedThroatCooling

FillSpeed

FillTime

HoldPressure

HoldTime

HotRunner(Blocked)

HotRunner(Temps/SetPoi

nts)

HotRunner(ZonesMislable

d/NotControllin

MaterialContaminated

MaterialDrying(Time,temp

,&DewPoint)

MaterialDryness(

microsoft word - molding notes rev 03

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