Part Defects


Flash

is excessive, unwanted material located on the edge of the part. This is a result of material passing through the parting line or between mold components.

Flash near the center of the mold or the gate may indicate low melt temperature as the problem. If the temperature of the melt is too high, the melt viscosity will drop, especially if the material degrades. This high-temperature melt with low viscosity may cause too much material to flow into the mold during 1 st stage fill, resulting in flash.

During 1 st stage injection, excessive amounts of material, high injection velocity, or a cavity filling imbalance can lead to flash. Also, excess 2 nd stage pressure or a low clamp tonnage can also lead to flash. Mold faults such as excessive wear or mold damage and machine faults like an inconsistent check ring or excessive platen deflection can also attribute to flash.


Sinks and Voids  

Sink are depressions on the part surface where the material shrinks away from the mold surface. Voids are sections in the center of the part where material shrinks away from itself, leaving a small cavity within the part. To ensure the defect is a void and not a gas bubble, you should mold parts at various injection speeds. If the defect remains stationary, it is most likely a void. Since both sinks and voids are the result of shrinkage, the causes and corrections are often similar.

A low melt temperature will cause larger pressure losses during injection. A high temperature melt causes additional shrinkage during cooling. Both of these conditions can result in sinks or voids.

During 1 st stage injection, insufficient shot size or low injection speed may cause sinks and voids to form. If too little pressure is used during 2 nd stage packing, insufficient material will enter the cavity to compensate for material shrinkage.

If the 2 nd stage time is insufficient, material will flow back through the gate before it seals which will result in sinks on the part in areas near the gate. A mold that is too cold (causing the polymer to freeze quickly) and a hot mold (that increases the amount of shrinkage) will both cause sinks and voids to occur.


Short Shot

A short shot is an incompletely filled mold cavity. This can be a result of many different variables. A low temperature, high viscosity polymer may prevent the mold from filling enough during 1 st stage fill. During 1 st stage, the packing pressure or injection velocity may not be high enough to complete mold filling.

Trapped gas during 1 st stage fill can cause a short shot. Excessive clamp tonnage can compress mold vents and prevent gas from exiting the mold during fill. Damaged or clogged vents can also result in gas entrapment.

If the 2 nd stage pressure is significantly low, there may not be enough pressure to complete mold filling. Also, a significantly low mold temperature may cause an excessive pressure drop to occur during 1 st stage fill – resulting in a short shot.


Jetting

If the mold or polymer is at a low temperature, the high viscosity polymer may not adhere to the mold surface properly, resulting in jetting. High injection speeds will also create excessive shear at the gate area and jetting may occur. Poor gate design typically contributes to the presence of jetting.

Smaller, more restrictive, gates increase the shear rate within the gate and will increase the likelihood of jetting. When the material is gated into relatively large areas, it is harder to create a smooth laminar flow.


Gate Blush

Gate blush appears as rings or ripples in the gate area of the part. This occurs when material slides across the mold surface rather than forming a fountain flow and freezing to the surface. As more material enters the mold cavity, it erodes the material off of the mold surface, causing the blushed appearance.

If a high injection velocity is being used during 1 st stage injection, the polymer may pass through the gate too quickly – creating excessive shear. A high mold temperature may interfere with the development of the solidified layer of plastic against the mold wall and gate blush can occur. Restrictive gates such as pinpoint, submarine, and cashew gates tend to contribute to the presence of gate blush.

Inadequate cooling around the gate area tends to promote flow front slippage resulting in gate blush.


Burning

Burning, or dieseling, appears as a black, gray, or brown discoloration on the surface of the part. This is the result of gasses and volatiles becoming trapped, compressed, and heated in the mold during 1 st stage injection. This gas will burn the front of the polymer flow front. Burning typically occurs near the end of fill, or where the flow ends, such as the bottom of a boss or rib.

Excessively high melt temperatures and back pressures can often cause the material additives to burn off and degrade during recovery.

If the injection speed of the polymer is excessive for the mold, the polymer may displace more air than can be removed through the vents, resulting in burning. High clamp tonnage and long-term tool usage can cause parting line and component wear, thus reducing the effective vent depth and preventing gas from escaping.


Flow Lines

Flow lines, or recording, appears as rings or ripples perpendicular to the direction of flow as concentric rings. This occurs when material slides across the mold surface rather than forming a fountain flow and freezing to the mold surface. As more material flows through the mold cavity, it erodes the material off of the mold surface causing this rippled appearance. In general, flow lines result from poor adhesion to the mold surface.

If a low temperature polymer is injected into the mold, it may not adhere properly to the mold surface causing flow lines. A low plastic flow may cause the polymer to cool as it fills the mold, contributing to flow lines. If too little material is injected during 1 st stage, the packing pressure may not be high enough to complete mold filling. This causes too much material to be injected during the slower 2 nd stage packing resulting in flow lines. A low 

mold temperature can reduce the adherence of the polymer to the mold surface thus contributing to the creation of flow lines.


Weld and Meld Lines

Weld and meld lines are very similar in appearance because they both result from the joining of two polymer flow fronts. The difference between a weld line and a meld line is how they are formed. Weld lines are created when two flow fronts meet and stop; this is considered a static interaction. Meld lines occur when two flow fronts meet, but continue flowing afterwards; this is considered a dynamic interaction. The dynamic nature of meld lines allows the polymer chains to better interact and entangle. This generally causes meld lines to have better appearance and strength than weld lines.

Low melt temperature polymers reduce the amount of injection and packing pressure present where the two flow fronts meet, thus causing weak weld and meld lines. Excessive melt temperatures and back pressures can burn and degrade material additives. This degradation creates excessive gasses which can interfere with the molecular entanglement at the point of meld or weld line formation.

A low injection velocity can reduce the strength and appearance of meld and weld lines. However, an excessive injection velocity may cause trapped gas to interfere with molecular chain entanglement at the flow front intersection, causing weak weld and meld lines.


If too little material is injected during 1 st stage, the 2 nd stage packing pressure may not be high enough to create a proper weld or meld line. Also, gasses trapped in the mold during injection due to blocked vents can interfere with weld and meld line formation. Low 2 nd stage packing pressure provides insufficient material to compensate for polymer shrinkage and will result in reduced pressure at the weld or meld line location.


A low mold temperature can reduce the temperature of the polymer at the weld line location, thus reducing the amount of polymer chain entanglement.


Excessive clamp tonnage can compress the mold vents causing gas entrapment which can interfere with proper weld and meld line formation. Remember, inadequate venting will always reduce the strength and appearance of weld and meld lines.


Poor Surface Finish

When the appearance of the part surface looks poor and has inconsistent gloss, it is generally the result of non-uniform adherence of the polymer to the mold surface.

Low melt temperatures cause higher pressure losses in the mold cavity during injection – often resulting in poor surface finish at the end of fill. Excessive melt temperatures and back pressures can cause material degradation, which also affects surface finish. If a low injection velocity is used inconsistently, adherence of the melted polymer to the mold surface near the end of fill may occur.

Too little material injected during 1 st stage fill can create a poor part surface finish near the end of fill. Also, low 2 nd stage packing pressure provides insufficient material to compensate for polymer shrinkage and causes poor surface finish due to reduced pressure while forcing the polymer into the mold cavity.


A low mold temperature can reduce the temperature of the polymer as it contacts the mold surface. This can cause material to draw away from the mold surface, reducing the part appearance. Excessive clamp tonnage can compress the mold vents and allow gas to be trapped between the mold and the polymer melt. A poor surface finish may also be a result of a mold surface that has become corroded or damaged if it was not kept clean and safe.


Dimensional Defects


Large Dimensions Overall

When part dimensions are larger than expected, it is typically the result of shrinkage being less than anticipated across the entire part.


If a low temperature polymer is injected into the mold, the polymer may solidify prematurely, resulting in reduced polymer shrinkage. If a low injection velocity is being used during injection, the polymer cools as it fills the mold and may cause an increase in cavity pressure resulting in larger part dimensions.


If too much material is injected during 1 st stage injection, material will be packed into the mold cavity during 1 st stage injection and the part may be larger than expected after packing. High 2 nd stage packing pressure may force excessive material into the mold also resulting in larger part dimensions. If the mold temperature is too low the polymer will cool at a faster rate which reduces the shrinkage and increase the overall part dimensions.


Small Dimensions Overall 

When the part dimensions are smaller than expected, it is typically the result of insufficient material or excessive shrinkage across the entire part.


If high temperature polymer is injected into the mold, the polymer will solidify slowly which may result in more shrinkage. If a high injection velocity is being used during injection, the polymer will cool less as it fills the mold and may cause a drop in cavity pressure which results in smaller overall part dimensions.


If too little material is injected during 1 st stage injection, there may not be sufficient 2 nd stage pressure to properly fill and pack the mold and the dimensions will be small. A low 2 nd stage packing pressure may not be sufficient to compensate for the part shrinkage as the polymer cools. If the mold temperature is high the polymer will cool at a slower rate and thus will increase shrinkage and decrease the overall part dimensions.


Larger Parts at the Gate

When part dimensions are larger than expected at the gate or smaller near the end of fill, it is generally the result of too much pressure loss across the mold cavity.


Low-temperature polymer causes pressure losses in the cavity to be higher than anticipated, resulting in less pressure being present at the end of fill during both 1 st stage filling and 2 nd stage packing.


Low injection velocity causes the pressure drop across the mold cavity to increase, resulting in part dimensions being larger at the gate and smaller at the end of fill. Also, if too little material is injected during 1 st stage injection, there may not be sufficient 2 nd stage pressure to properly fill and pack the end of fill, resulting in smaller dimensions at the end of fill.


Smaller Parts at the Gate

When the part dimensions are either smaller than expected near the gate or larger at the end of fill, it is generally the result of poor gate seal or reduced pressure loss across the mold cavity.


High temperature polymer causes pressure losses in the cavity to be less than anticipated and can result in additional packing pressure reaching the end of fill. Also, if a high plastic flow rate is being used during injection, the pressure drop across the mold cavity will decrease, resulting in higher cavity pressures at the end of fill during both filling and packing. If the 2 nd stage time is insufficient, material will flow back through the gate before it seals, resulting in smaller dimensions on the part in the area nearest the gate.


Warpage

Warpage is the result of inconsistent or unexpected dimensions across the molded part resulting in a deformed part. A warped part does not match the shape and form of the mold cavity.

In longer parts, warpage may appear as a twist or bend. This is most often the result of inconsistent stresses in the part, resulting in uneven shrinkage. When investigating this defect, it is often a good practice to measure the part dimensions since they may be large or small overall, at the gate, or at the end of fill. In many cases, correcting for these conditions will improve or eliminate warpage.

If a high temperature polymer is injected into the mold, excessive shrinkage can occur as the material cools. On the other hand, if a low temperature polymer is injected into the mold, large pressure losses can occur and may prevent adequate packing across the entire part. Both of these conditions can result in warpage.

Low injection velocity causes the pressure losses to increase and often results in larger part dimensions near the gate. This variable shrinkage may cause the part to twist or warp. A low 2 nd stage packing pressure can result in excessive part shrinkage as the polymer cools causing dimensional instability. Also, high 2 nd stage packing pressures can force too much material into the mold, resulting in excessive molded-in stresses. Warpage typically occurs after the part is molded and the internal stresses are relieved. Although some parts may start to relieve this stress immediately, some parts may warp for hours, weeks or even years after being molded.


Insufficient 2 nd stage time causes material to flow back through the gate before it seals, resulting in warpage. A high mold temperature causes the polymer to cool at a slower rate. This increases part shrinkage and also causes warpage. High mold temperatures may prevent the material from becoming cool enough to maintain the desired dimensions at the time of part ejection. Although a low mold temperature tends to provide more dimensional stability at the time of part ejection, it can also cool the material too quickly. The rapid part cooling may result in excessive molded-in stresses which may cause part warpage after the part is molded. Molded-in stress relief most often occurs when the part is exposed to heat, stress or a corrosive chemical.


Unevenly distributed cooling lines often cause thick sections to receive the same or less cooling than thin sections, causing warpage. Sharp thickness transitions create sharp shrinkage transitions which contribute to warpage. Sharp corners create stress concentrations which can cause the part to buckle as the part shrinks and warps.


Material Defects


Splay, Bubbles, and Blisters 

Splay appears as streaking on the part surface in the direction of flow. Bubbles are small pockets of gas within the part, which are similar in appearance to voids. Bubbles are easiest to detect in translucent parts and can occur in both thin and thick sections. Blisters are small bumps on the surface of the part.


All three defects are caused by moisture, air, gases or volatiles present in the resin or mold surface. They appear most common in hygroscopic materials such as Nylon, polycarbonate, and Acetal.


As a result, improper material handling is the most common cause of all three of these defects. If the material is not properly dried, the moisture will escape the polymer during injection and cause these defects. If a dried material is removed from the dryer and is not used immediately, it can re-absorb moisture from the air. Many contaminants such as fluids, grease, and oils can vaporize in the barrel, resulting in splay, bubbles, and blisters.


Excessive melt temperatures and back pressures can cause material degradation. Sprue break often creates an air bubble in front of the nozzle. This may introduce moisture for highly hygroscopic materials. Excessive screw decompression may draw air into the nozzle and barrel. Any of these conditions may cause splay, bubbles, and blisters.


Brittleness, Cracking, and Crazing

Brittleness is a reduction in the impact resistance of the product. Cracking appear as fractures passing through the part. Cracks are typically located at areas of stress concentration such as corners, ribs, and bosses. Although crazing is similar to cracking, these typically appear as miniature striations on the part surface, but do not pass all the way through the part. Crazing can be located anywhere on the part surface.


Although different in appearance, all three defects have similar origins which contribute to premature part failure. When heated in the barrel, water will hydrolyze, breaking up the molecules into hydrogen and oxygen atoms. These atoms can attack the polymer chains and break them up, resulting in weaker molded parts. Many contaminants such as fluids, grease, oils, and other polymers can interfere with physical properties of the molded part.


Excessive melt temperatures and back pressures can cause material degradation. This degradation can break down the polymer chains and reduce the strength of the molded part. If the plastic flow rate is too high, the polymer may encounter too much shear, causing the polymer chains to break. This results in property loss which can contribute to brittleness, cracking, and crazing.

A low mold temperature may cool the part too quickly, resulting in molded-in stresses. A long cooling time may also cause molded in-stresses which may damage the part during mold opening or ejection.

If the breakaway speed is too fast, the part can become damaged it if sticks to the cavity. The breakaway distance is the distance the mold travels at the reduced breakaway speed. If the part is not properly cleared when breakaway ends and the mold opening speed increases, the part will become damaged. Sharp thickness transitions create stress concentrations due to variations in shrinkage and can cause brittleness, cracking, and crazing.


Delamination

Delamination occurs when the polymer separates into layers as it fills and packs the mold. This condition dramatically affects the intermolecular entanglement and attractions between the polymer layers. Delamination can appear as flaking, large bumps on the part surface, or as split layers at the point of part failure.

In most cases, delamination occurs when the polymer is stressed, degraded, or contaminated. Many contaminants such as water, fluids, grease, oils, and other polymers interfere with physical properties of the molded part, resulting in defects such as delamination.

Very low melt temperatures can cause excessive injection pressures to be used during injection and packing, causing the polymer to separate into layers. Excessively high melt temperatures and back pressures can cause material degradation and delamination.


Contamination

Contamination is the presence of material other than the polymer intended to be processed. Contamination often appears as black or colored specs or streaks in the polymer or on the part surface.

A material can be contaminated by a vartious things such as foreign particles such as dust or dirt, cross contamination from other materials, material originating from either the barrel or inferior regrind.

When processing black or dark colored parts, contamination may be very difficult to see.

Poor housekeeping contributes to an increased presence of dust, contaminants, and particulates in the workplace. Practices such as blowing off equipment with air hoses or brushing off particulates with a broom contributes to the presence of airborne contaminants.

When additives, regrind, and colorants are added, it is important to not expose the additive or base resin to airborne particulates.

Open containers and improperly sealed lids will allow airborne particulates such as dust to contaminate the material. An open lid in a dusty environment, a dirty bucket or scoop, or an unclean material mixer will introduce contaminants to the process. Any leaks or dust present in a material delivery system such as a vacuum loader and centralized delivery system will contaminate the material passing through any material delivery system. Since these systems rely on moving air to transport material from one place to another, dirty or improperly installed filters will also contaminate the material.


The hopper is mounted directly atop the feed throat and can be a difficult component to clean. If your company uses a sliding hopper or a sliding feed throat shut-off, there are often contaminants and dust present in the sliding mechanism which can get into the barrel.


Excessively high melt temperatures and back pressures can cause material degradation. This degradation can affect the appearance of the molded part as well as stick to the screw. Degraded material can stick to the screw flights, nozzle or hot runner system for days, months, or even years before breaking off and contaminating a different batch of material in the future.


Poor Color Distribution 

Poor color distribution is inconsistent coloration of the molded part. This is typically the result of poor material mixing, resulting in an uneven distribution of colorants throughout the material.

Improperly mixed colorants will not distribute evenly when melted inside the barrel. As the screw rotates, the polymer chains should have some backflow within the screw flights to ensure the additives are properly mixed. Inadequate back pressure can minimize the backflow and poor color distribution may occur.


Cycling Problems

Part Sticking and Ejector Pin Marks In some cases, the part may stick to the core or cavity side of the mold. This complicates either mold opening or part ejection. When the resistance to part ejection is too high, the ejector pins can apply too much force to the part surface, resulting in ejector pin marks. Although ‘ejector pin marks’ is a common industry term, any form of ejection such as blades, sleeves, and lifters can cause similar defects. In both part sticking and ejector pin marks, difficulty in removing the part from the core or cavity is most often the cause.

If a low temperature polymer is injected into the mold, it may solidify prematurely, resulting in reduced polymer shrinkage. This most often causes the part to stick to the cavity side of the mold.

If a high temperature polymer is injected into the mold, the polymer may solidify slowly. This can result in increased polymer shrinkage, causing the part to stick to the core.

If too much material is injected during 1 st stage injection, material will be packed into the mold cavity during 1 st stage. This typically causes the part to stick in the mold during ejection. Low 2 nd stage packing pressure can result in excessive part shrinkage, which can cause the part to stick to the core. High 2 nd stage packing pressures can force too much material into the mold causing the part to stick to either the core or cavity.

A high mold temperature will cause the polymer to cool at a slower rate, thus increasing part shrinkage and causing part sticking. A low mold temperature may cool the part too quickly, causing molded-in stresses. These stresses often contribute to the presence of ejector pin marks.

A long cooling time may also cause molded in-stresses which may cause the part to become damaged during mold opening or part ejection.

If the two mold halves separate too quickly, the part can stick to the cavity and become damaged. If the part is not properly cleared when the slower mold breakaway speed ends, the part can stick and become damaged. If the part is ejected too fast, it may stick to the core. Parts without draft angles are more likely to stick to the mold. These parts also tend to create vacuum forces which can hold the part to the core or cavity. Many toolmakers polish cores and cavities perpendicular to the direction of mold opening and part removal. This polishing technique may create undercuts which can interfere with the part removal process.


Occasional Part Hang-Up

Occasional part hang-up is when part sticking occurs occasionally, but not consistently. In multi-cavity molds, a large filling imbalance can cause some filled mold cavities to begin packing while other cavities are short during 1 st stage. This situation can cause variable short shots, especially in hot runner molds where cavity filling imbalances are in excess of 6%. The filling imbalances may cause occasional part hang-up.

During mold filling, the gasses present in the mold must escape the mold as the material fills the mold cavity. If gas is trapped in some cavities while other cavities are not blocked, it can cause a significant cavity filling imbalance and occasional part hang-up.

Inadequate cooling will cause the mold to heat up with time. This heating up of the mold can increase the amount of shrinkage that occurs during part cooling causing part sticking. When the part sticks, the process will typically stop as the part is removed. Once this happens, the mold has a short time to cool, often causing the mold to run well until the mold heats up again and hang-ups randomly occur.


Nozzle Freeze-Off

Nozzle freeze-off is when the material in the nozzle cools too much – resulting in a blockage of flow. In most cases, nozzle freeze-off is the result of an improperly heated or insulated nozzle.

Troubleshooting Common Molding Defects Investigate the nozzle heat before investigating the thermocouple placement and poor nozzle heating. In many cases, contact of the nozzle with the sprue bushing can result in a large amount of heat transfer. This situation is only found in cold runner molds since hot runner molds use a heated sprue bushing. This heat transfer is very often reduced through the use of a small piece of insulating material between the nozzle and the sprue bushing.

Specialized insulators made of paper and plastic are available to be placed in front of the nozzle. Some molders use common items, such as business cards, to insulate the nozzle from the mold. You can also install a nozzle with a smaller orifice diameter to minimize the amount of freeze-off that occurs.

If the thermocouple is placed too close to the heater band, it may not accurately represent the temperature of the nozzle. This prematurely heats up the thermocouple which signals the temperature controller to turn off before the nozzle if fully heated. The closer the thermocouple is placed to the melt, the more accurately the temperature of the nozzle is measured and controlled.

Inadequate nozzle heating is typically the result of an inadequately sized heater band. Larger sized nozzles will generally require a larger heater band to maintain the desired temperatures. Larger heater bands may also be required if you are processing a high temperature material such as polysulfone or PEEK.

This will often be identified by the nozzle temperature controller. If the temperature controller remains on more than 50% of the time it is working too hard to maintain the desired temperature. Poor nozzle heating can also be the result of a faulty heater band or controller. If the temperature controller is on nearly 100% of the time, or it cannot reach the desired set point, it may be faulty. Your maintenance department should be capable of determining the condition of your heater bands and temperature controller.


Drool and Stringing

Drool and stringing is defined when material flows out of the nozzle, either onto the machine or into the sprue bushing between cycles. This can also occur in the hot runner system – drooling material into the mold from the gate tip. Stringing is a strand of material pulled out of the nozzle or hot runner tip as the mold opens. This is a result of poor separation between the sprue from the nozzle, or the part from the hot runner gate. Both drool and stringing are the result of excessive pressure at the nozzle or hot runner tip, a low viscosity melt due to a high melt temperature, high nozzle temperature, or material degradation.

Verify the plastic temperature variables are still correct. If necessary, make adjustments to decrease material temperature such as reducing the: screw speed, back pressure, or barrel temperature. Degraded material in the drool or stringing is a good indication of excessive screw speed, back pressure, or barrel temperature.

You can increase the amount of decompression to reduce the pressure at the nozzle. When increasing the decompression, you should ensure you do not retract the screw enough to cause gas to be present in the melt. This may result in splay, bubbles, or blisters on or in the part. If the nozzle thermocouple is placed too close to the barrel, it may not accurately represent the nozzle temperature. When the thermocouple is placed too close to the barrel it may be measuring the temperature of the barrel and not the nozzle. This can be tested by measuring the temperature of the nozzle using a temperature probe. Whenever checking the nozzle temperature, always ensure that the barrel has been retracted and the material has been purged, and the screw retracted. This ensures there is no pressure present at the nozzle to prevent injury.

If the hot runner tip thermocouple is too close to the mold base, it also may not be accurately reading the hot runner tip temperature. Although this is generally a mold design issue, you can test the performance of the tip by measuring the temperature of the tip using a temperature probe. Again, you should ensure that the nozzle is retracted, the material purged, and screw retracted to ensure there is no pressure present at the hot runner.


Material may flow easily out of a nozzle or tip if the wrong orifice diameter or design is being used. Smaller diameter nozzles can be used to help counteract nozzle drool. Nozzles with a longer orifice length will also reduce the likelihood of nozzle drool. Many injection molders will use a reverse-taper nozzle to help prevent nozzle drool. Reverse-taper nozzles have a diameter which tapers outward to the nozzle tip and helps pull material from the nozzle as the mold opens.


Smaller diameter hot runner tips can often help promote better gate seal to reduce the likelihood of stringing. Some injection molders will use valve gating systems which open and close the tip either mechanically or thermally to prevent stringing or drool. Mechanical valve gates use a pneumatically or hydraulically activated valve gate to physically block material from flowing through the gate. Thermal valve gates use a heating element which heats up quickly to allow material flow, and cools quickly to help prevent material flow.