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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.
Decrease injection pressure.
Decrease injection speed.
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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
Decrease injection pressure.
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 back pressure.
Decrease melts temperature.
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 injection pressure.
Decrease injection speed.
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
Decrease injection speed.
Increase cold slug area in size or number.
Increase injection pressure.
Increase melts temperature.
Increase mold temperature.
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 back pressure.
Increase cylinder temperature.
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
Decrease injection speed.
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 booster time.
Decrease cylinder temperature.
Decrease holding pressure.
Decrease injection pressure.
Decrease injection speed.
Decrease overall cycle time.
Increase gate size and/or change gate location.
Increase mold temperature.
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 booster time.
Decrease cylinder and nozzle temperature.
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Decrease injection pressure.
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 back pressure.
Increase injection pressure.
Increase injection speed.
Increase melts temperature.
Increase mold temperature.
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.
Decrease melts temperature.
Decrease nozzle temperature.
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 booster time.
Decrease injection pressure.
Decrease injection speed.
Decrease injection-hold.
Decrease mold close time.
Decrease nozzle temperature.
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.
Increase melts temperature.
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
Decrease back pressure.
Decrease nozzle temperature.
Increase booster time.
Increase the melt temperature.
Increase injection pressure.
Increase injection speed.
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 cylinder temperature.
Increase hold-time.
Increase holding pressure.
Increase injection pressure.
Increase injection speed.
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.
Decrease injection speed.
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Decrease melts temperature.
Dry material.
Increase injection pressure.
Increase injection-hold.
Increase mold temperature.
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 pressure.
Increase injection speed.
Increase injection hold.
Increase melts temperature.
Increase mold temperature.
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(