Thermoplastics vs. Thermosets

Polymers get their strength from a process called polymerization. During polymerization, small molecules called monomers combine to form long polymer chains. Thermosets are polymerized during processing while thermoplastics are polymerized before being processed. During processing, the polymer chains in thermosets fuse together, or cross-link. Once these polymers cross-link, they undergo a chemical change which prevents them from being melted and reprocessed. An egg is an example of a natural polymer which thermosets. Once the egg is heated, it solidifies and cannot be melted again.

Thermoplastics are long polymer chains that are fully polymerized when shipped by the resin manufacturer. Thermoplastics can be re-ground, melted and re-processed while retaining most of their original properties. An example of a natural thermoplastic material is wax. It can be melted and formed. Once cooled, the hardened wax can be melted and formed again. Unlike thermosets, most plastics companies prefer thermoplastic materials because they can be reprocessed and recycled.

Amorphous vs. Semi-Crystalline

Thermoplastic polymers can be categorized into two types; amorphous and semi-crystalline. Amorphous polymers melt gradually when heated. During cooling, amorphous polymer chains solidify slowly in a random orientation. By the end of the cooling phase, they shrink about one half of a percent. Common amorphous polymers include ABS, polystyrene, polycarbonate, and PVC.

Semi-crystalline polymers melt quickly, once heated to their melting temperature. The rapidly melting polymer is easy to process compared to amorphous polymers. As a semi-crystalline material cools, portions of the polymer chains remain in a random state – while portions orient into compact structures called crystalline sites. These crystalline sites increase the strength and rigidity of the polymer. During cooling, semi-crystalline polymers shrink up to three percent – much more than amorphous polymers. Semi-crystalline polymers include nylon, polyester, polyethylene, and polypropylene.

Capillary Rheometry

The capillary rheometer melts the polymer inside a small barrel, and then a plunger forces the polymer melt through a small capillary. The rheometer measures the amount of force required to push the polymer through the capillary. The shear stress on the melt equals the force divided by the surface area of the plunger. The shear rate is a measure of how fast the material is being tested.

The shear rate is determined by the rate of flow through the capillary, and the die geometry. The viscosity of the material is equal to the shear stress divided by the shear rate. In capillary rheometry, the viscosity is usually determined at different temperatures and shear rates.

When the viscosity data is graphed, it provides a good representation of how the material behaves during processing. If capillary rheometry data can be obtained, it is a good method of comparing the flow characteristics of different resins. Always compare capillary rheometer data from similar shear rates and temperatures.

Melt Flow Index

Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. This method uses a standard testing apparatus with a standard capillary to measure the flow of the material. The melt flow indexer tests the polymeric material at a single shear stress and melt temperature. The melt flow index is the measure of how many grams of polymer pass through the capillary over 10 minutes.

A higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. The value obtained through the melt flow index test is a single data point. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials. The melt flow index value is given for each material by virtually all material suppliers.

Spiral Flow Test

The spiral flow test uses a mold with a long spiral flow channel emanating from the center. Notches are etched along the flow path to help identify the length the polymer has flowed within the mold. The mold can be filled using either a constant velocity (constant shear) or constant pressure (constant strain) to determine the polymer behavior.

The behavior of the polymer can be evaluated based on process output data such as flow length, part weight, and pressure at transfer. When using the spiral flow test, it is best to use a mold which has a channel thickness similar to the parts actually being molded.

In-Mold Rheology

In-mold rheology uses a variety of injection velocities combined with machine data to generate a rheology-curve. This curve plots the effective viscosity of a polymer to help determine when shear-thinning occurs. As the shear rate (or flow rate) of the polymer increases, the viscosity decreases. This rheological behavior is unique to polymers and is called ‘shear thinning’.

When graphing this, viscosity is plotted on the vertical, ‘Y axis’ and shear rate is plotted on the horizontal, ‘X axis’.

The apparent shear rate equals 1/(fill time).

The effective viscosity equals (fill time)*(transfer pressure).

Shear thinning will appear as a steep decline in the viscosity of the polymer as the shear rate increases. Once most of the shear thinning occurs the polymer’s viscosity starts to level out. After this point, the viscosity will remain relatively consistent - resulting in a more stable process. For this reason, you should process on the right hand side of the curve.

In-Mold Rheology Curve

ABS (Acrylonitrile Butadiene Styrene)

Acetal or POM (Polyoxymethylene)

Acrylic or PMMA (Polymethyl Methacrylate)

CPVC (Chlorinated Polyvinylchloride)

HDPE (High Density Polyethylene)

HIPS (High Impact Polystyrene)

Ionomer

LDPE (Low Density Polyethylene)

LLDPE (Linear Low Density Polyethylene)

PA-11 (Nylon-11)

PA-12 (Nylon-12)

PA-4/6 (Nylon-4/6)

PA-6 (Nylon-6)

PA-6/10 (Nylon-6/10)

PA-6/12 (Nylon-6/12)

PA-6/6 (Nylon-6/6)

PAEK (Polyaryletherketone)

PBT (Polybutylene Terephthalate)

PC (Polycarbonate)

PC/ABS (PC/ABS Alloy)

PC/PET (PC/PET Alloy)

PEEK (Polyetheretherketone)

PEI (Polyetherimide)

PES (Polyethersulfone)

PET (Polyethylene Terepthalate)

PETG (Polyethylene Terepthalate Glycol)

PP (Polypropylene)

PPO (Polyphenylene Oxide)

PS (Polystyrene)

PVC (Polyvinylchloride)

SAN (Styrene Acrylonitrile)

TPC-ET (Thermoplastic Copolyester Elastomers)

TPE (Thermoplastic Elastomer)

TPO (Thermoplastic Polyolefin)

PPS (Polyphenylene Sulfide)

PSU (Polysulfone)

PUR (Polyurethane)

1^st Stage Injection

During this stage, the mold is filled using screw velocity control. There should always be enough injection pressure available to ensure the machine can maintain the desired velocity setpoint.

1^st to 2^nd Stage Transfer

Transfer should take place using screw position. The mold should be approximately 95% full at the time of transfer. The resulting part should be a visual short shot.

2^nd Stage Packing Pressure

Pressure must be high enough to finish filling the mold cavity and pack out all sinks and voids. 2^nd Stage Packing Pressure is typically 50-75% of 1^st Stage Pressure.

2^nd Stage Time

Determine the appropriate 2^nd Stage Time for your process by performing a gate seal study. The Gate Seal Study will help determine an adequate 2^nd Stage Packing Time at which the part weight does not increase with an increase in 2^nd Stage Time.

Screw Delay or Decompression Before Recovery

After 2^nd Stage Pack there is a large amount of pressure present in front of the screw. Either screw delay or screw decompression should be used before recovery to prevent damage.

Screw Recovery

During screw recovery, screw recovery should consume 80% of the overall cooling time. If there is a long cooling time, then a significant screw delay can be used to reduce the time the material remains in the barrel.

Screw Decompression After Recovery

When the screw travels forward for injection the pressure holding the check ring in the forward position can interfere with the check ring movement. Screw decompression is necessary to prevent interference. The proper amount of ‘screw suck back’ should be equal to the amount of ‘check ring travel’.

Flash

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 Shots

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.

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.

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.

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.

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.

Troubleshooting Visual Defects

Troubleshooting Dimensional Defects

Troubleshooting Material Defects

Troubleshooting Cycle-Related Defects

Resin-Based Purging Compounds

These purging compounds are comprised of a single base material. The resin-based purging compounds most often used are the Final Production Material, Base Material, or Regrind Material.

Resin-based purging operates by the principle that new material will push most of the old material out of the screw and barrel. In practice, this method tends to be very effective in removing a large amount of the material residing in the screw channels due to material conveyance.

Unfortunately, resin-based purging lacks any additives to help remove contaminants that are stuck to the screw flights and check ring. For this reason, more resin-based purging compound is needed when compared to mechanical or chemical purging compounds.

Mechanical Purging Compounds

These purging compounds are comprised of base carrier resin combined with and a tough additive. Although some molders will make their own proprietary compounds, most mechanical purging compounds are purchased with the resin and additive combined. Non-abrasive compounds are available for virtually all molding applications while abrasive purging compounds can damage many molds and equipment.

Mechanical purging compounds operate by the principle that the additive scrubs the screw and barrel while the base resin pushes the material down the barrel. This compound works better than resin-based purging compounds because the scrubbing action of the additive helps to remove stuck material from the screw and barrel.

Chemical Purging Compounds

These purging compounds are comprised of base carrier resin combined with and chemical additive. Some providers sell grades of compound which are pre-mixed and ready to use. Other compound providers sell popular solid and liquid additives which can be added to any of your existing resins. When mixing solid or liquid purging additives, always follow the recommended procedures as well as wear the necessary recommended personal protective equipment. Non-corrosive compounds are available for virtually all molding applications while corrosive purging compounds can damage many molds and equipment.

Most chemical purging compounds operate by the principle that the chemical soaks in the barrel while it expands and breaks down the stuck material. After the manufacturer-recommended period of soak time, the base resin pushes the material down the barrel. Some chemical purging compounds are designed to be left in the barrel for extended periods of time, some must be purged after the designated soak time. Some fast-acting chemicals do not require a soak time, others require a barrel temperature increase, so following the recommended compound-specific purging procedure is very important.

The Four Phases of Purging

Preparation : Involves getting all of your needed materials, tools, equipment, and documentation together before purging takes place. Proper preparation is the most important step in ensuring an efficient and effective purge.

Initial Purge : Involves thoroughly cleaning out the hopper, and then clearing the barrel and screw with a purging compound. You should provide specific procedures to ensure every technician performs this step properly. If this step is performed incorrectly, a large amount of time and purging compound can be wasted.

Final Purge : Includes thoroughly cleaning the hopper again and then filling the barrel and screw with final production material. This is also the step where gate drops are to be removed and cleaned when necessary.

Production : This entails configuring the machine settings for production and then begin molding acceptable parts.

Large Shot Purging

This method typically uses 80-100% of the maximum shot size for fewer purge cycles. Large shot purging is popular because it is relatively fast, but it only cycles the check ring a few times.

Small Shot Purging

This method uses a shot size of approximately 10-20% of the machine’s capacity and includes making many small shots of purge. This cycles the check ring many times, which should help clean the check ring more effectively, but may take a little more time than the large shot purging method.

Continuous Purging

This method uses a high back pressure to hold the screw forward and forces the machine behave more like an extruder. This process is typically the fastest method, but does not provide any check ring cycling. This method is typically best for purging hot runner systems and machines without a check ring.

Closed Mold Purging

Specific purging compounds allow parts to be molded while purging takes place. Purge compound is typically molded into parts using settings similar to the documented production settings. This method reduces the safety risk associated with handling, transporting, and storing purge and can be helpful for processes where part quality is strongly affected by production stoppages.

Dry Purging

Dry purging involves pushing out as much material out of the barrel as possible before adding the next material to be processed. This approach often works well for heat-stable materials with very good conveyance properties, but is not recommended for unstable materials since some material will always remain stuck to the screw as the barrel empties.

Wet Purging

Wet purging is performed by leaving the barrel full of the previous material before adding the next material. This method is usually recommended since the screw flights remain full of material. This maintains positive material conveyance in the screw and tends to be useful for materials which have a tendency to stick to the screw.

Celsius to Fahrenheit (°C to °F)

Millimeters to Inches (mm to in)

Milliliters to Fluid Ounces (ml to fl oz)

Grams To Ounces (g to oz)

bar to psi

Machine Pressure Graph Method

Many injection molding machines provide a hydraulic pressure vs. plastic pressure graph. This graph allows the molder to take a hydraulic measurement from the machine’s pressure gauge, locate it on the graph on the horizontal axis, follow the value to the graph line and then over to the left to determine its corresponding plastic pressure on the vertical Y axis.

To calculate the intensification ratio, use the graph to locate any hydraulic pressure and its corresponding plastic pressure and then divide the plastic pressure value by the hydraulic pressure.

If there are multiple lines on your pressure graph, it is because your molding machine was offered with multiple screw diameters. To use the correct line, determine which diameter screw your machine has and locate its corresponding pressure curve.

Machine Specifications Method

The specifications for your molding machine can be located either in your machine manual, from other machine-related documentation, or from the manufacturer. The important information you need to obtain is the maximum hydraulic pressure and maximum injection pressure.

To calculate the intensification ratio, divide the injection pressure by the hydraulic pressure.

Hydraulic Cylinder Method

If you know the actual internal dimensions of your hydraulic cylinder you can calculate the exact intensification ratio of your molding machine.

To calculate the intensification ratio, multiply the number of hydraulic cylinders times the internal diameter of the hydraulic cylinder squared and then divide that result by the diameter of the screw squared.

Material Consumption

Before determining the dryer residence time, as well as the minimum and maximum dryer size, you must first determine how much material is being used. For the calculations used in this guide, we will refer to material consumption as a measure of material usage per hour. This is typically represented as kilograms or pounds per hour.

To determine the material consumption, you must know the weight of material consumed during each cycle as well as the time used to produce the product. For injection molding, the weight of the entire shot including any parts, sprue, runners, or gates produced is measured. Likewise, the cycle time is provided by the molding machine.

Use the following formula to calculate the material consumption:

Dryer Residence Time

The residence time is the duration the material remains in the material dryer. Residence time is typically provided in hours. To calculate this, you will need to know the dryer capacity (in kilograms or pounds) as well as the material consumption (in kilograms or pounds per hour).

The following formula can be used to calculate the dryer residence time:

Dryer Capacity

The dryer capacity calculation will provide the minimum amount of material the dryer must hold to keep the material dry. Dryer size is typically provided in kilograms or pounds. To calculate this, you will need to know the recommended drying time in hours as well as the material consumption in kilograms or pounds per hour.

Use the following formula to calculate the dryer size:

Cooling Time

The following is the basic way to calculate the part cooling time:

Total Amount of Heat to be Removed

Use the following formula to calculate the total amount of heat to be removed:

Required Cooling Power

The following formula will help to determine the overall amount of cooling power necessary:

Cooling Power Per Line

Use the following formula to determine the amount of cooling power necessary per cooling line:

Required Volumetric Flow Rate

The following formula will help to determine the necessary volumetric flow rate of coolant:

Printed Materials

Books, manuals, and industry publications are inexpensive, easy to obtain, and can be used repeatedly. But asking employees to read and retain a large body of written information is unrealistic. Most people require interaction in order to learn effectively. Such materials are essential for reference but do not provide measurable results.

Video Training

Training with videos or DVDs is visually interesting and is somewhat inexpensive, but still lacks interaction and progress monitoring. Furthermore, video is a linear presentation that cannot be customized. Video presentations can be lengthy, and studies have shown that retention drops after more than three minutes of continuous video.

Seminars

There are several questions to ask about classroom training, guest lecturers and seminars. Will the workers be able to understand and keep up with the instructor? How much will they retain if they “zone out” during the session? How many times will they have to repeat the session to apply it on the job? What’s the ideal number of employees to train at one time? The information provided may be useful but it is nearly impossible to track the training’s effectiveness.

Online Training

This form of training is continually gaining popularity throughout every industry. This is most effective when it combines 3D animation, digital video from actual production environments, on-screen titling, and professional narration. This formula is designed to captivate the end user and ensure a higher level of retention. Each training course should contain many questions to test the participant’s knowledge while the program is delivered. This type of training should be available 24/7 to allow employees to learn at their own pace. Such training programs should be up-to-date and use ‘industry best’ practices. Furthermore, courses should be easily customized to your specific needs. Online training provides measurable results and is most effective when used in conjunction with plant-specific exercises. This ensures that the knowledge gained during the interactive training can be demonstrated as learned skills out on the production floor.

On-the-Job Training

Hands-on training is often perceived as the best way to train any employee, but it is very costly. To be successful, such training must be conducted by an experienced supervisor or manager with a knack for instruction. However, the right person for the job is often too busy to train new employees. On-the-job training can teach an employee about the visible workings of machines, but not the inner workings. These are the primary reasons we suggest a structured or “blended” approach.