Plastics Molding & Manufacturing/Defects
The defects listed here serves as references for purposes of easier problem troubleshooting and does not meant as definite guides
Problem must be troubleshoot with an objective mind. The solution that solved a problem one day may not solve the same problem on another day. Due to the large number of parameters and the variability of these parameters plus the way of parameter interactions, many solutions may exist for a single problem. Likewise, many problems may be fixed by using a single solution.
A study that took place over a 30-year span (1963 to 1993) by Texas Plastic Technologies* analyzed the root causes of the most common injection-molding defects. The defects studied were process-related and did not include those resulting from poor basic product design. The study found that the defects could be traced to problems with one or more of the following four items: the molding machine, the mold, the plastic material,and the molding machine operator.
The first step on troubleshooting is to visualize the way a process should be running. Most troubleshooting is actually performed after a specific job has been running successfully for an extended period of time. There has been an initial setup and debugging process, and the mold has been accepted for production. Then, after running successfully, parts begin to be molded with defects. This is when the troubleshooter is brought into the picture. This is also when simple analysis, common sense and objectivity must be brought into play.
Visualizing what happens to the plastic as it travels from the hopper through the heating cylinder and through the flow path to the cavity image, you can determine what may have changed to cause defects. A heater band could be burned out, or an injection pressure valve spring may leak, or cooling water lines may have become blocked. Any of these problems will cause specific things to happen. A thorough understanding of the molding process will help determine the cause.
Black Specks or Streaks
Excessive residence time in barrel.
Between 20 and 80 % of the barrel capacity should be injected each cycle. If the plastic stays in the barrel longer than normal, it will begin to degrade. This degradation results in carbonized plastic, which appears as small black clusters. These can be carried through the melt stream and show up as spots or streaks in the molded part, visible on the surface of an opaque part and throughout a transparent part.
Solution: Place the mold in a properly sized machine.
Sprue bushing cracked, nicked, or not seating properly.
Any of these conditions will cause plastic to hang up in the crack, nick, or offset seat of the bushing. The material can overheat due to excessive residence time at that location, and this can cause degradation or carbonizing. Eventually the hung-up resin breaks loose and enters the melt stream and flow path.
Solution: Replace cracked or nicked bushings, and use a blueing agent to check that the bushing is centrally seated against the nozzle tip. Also, check that the nozzle tip opening has an equal or smaller diameter than the sprue bushing to ensure a proper seal.
Contaminated raw material.
Dirty regrind, mixed regrind, improperly cleaned hoppers or grinders, open or uncovered material containers, and even poor-quality virgin material from the manufacturer.
Solution: Dealing with only reputable suppliers, using good housekeeping practices, and properly training material handling personnel.
The operator may inadvertently be causing delayed or inconsistent cycles. This will result in either excessive residence time of the material in the heating cylinder, or overcompensating heater bands. Both conditions will result in degraded material, especially with heat-sensitive plastics.
Solution: Place the machine in automatic mode,with the operator serving as monitor to stop the press if an emergency develops. The operator should be trained to be aware of the importance of consistent cycles, whether or not the machine can run automatically.
Back pressure too low.
As the material is heated and augured through the heating cylinder, air becomes trapped within the melt. One of the uses of back pressure is to force this air out before it gets injected into the mold cavity image.
Solution: Back pressure should be set at 50 psi (345 kPa) and increased in increments of 10 psi (69 kPa) until the ideal setting is reached.
Mold temperature too low.
As a material is injected into a mold, it starts to cool immediately and a skin begins to form on the surface of the part. If this skin forms too quickly, any air that is mixed into the material will not be allowed to escape through the surface as intended, causing a blister effect. A mold that is too cool will cause the skin to form too soon.
Solution: Increasing the temperature of the mold will help allow trapped air to escape by delaying the hardening of that skin.
Use of regrind that is too coarse.
This practice increases the amount of air that gets trapped in the melt because the coarse, uneven particles of regrind create pockets of air between them and the
smaller, consistently sized particles of base material.
Solution: Use a finer-gage screen in the regrinder. Another solution is to limit the amount of regrind that is used to less than 5 %. Or you can increase the amount of back pressure on the injection screw, assuming the base material is not too heat-sensitive. Another solution, if others fail, is to use only virgin material. In fact, sometimes this can be done to start the run and regrind can be “salted in” as the run progresses.
Early gate opening.
Solution: There is a slight possibility that blisters will form if the operator were to open the gate too soon, thus not allowing the part to cool (solidify) in the mold. This would have to be precisely timed, however, as the part probably would warp, twist, or otherwise deform drastically before blisters would form.
Improper screw design.
A screw with too low a compression ratio for the material being molded will not properly melt and mix the material. This results in weak bonds between the individual molecules in the material and the part exhibits brittleness.
Solution: Use of an injection screw with a higher compression ratio will help solve this problem. Contact the material supplier for the proper screw design for specific materials.
Although it does not occur with any regularity, condensation in the mold cannot be ruled out as a possible source of moisture, which in turn may cause brittleness in molded parts. This condensation will be especially prevalent in molds that are operated under humid conditions. Cooling water in the mold may be the source of such condensation.
Solution: Use insulation panels between the mold and the press, as well as on all the outside surfaces of the mold. Another is to raise the mold temperature slightly to reduce the tendency to form condensation. A small fan blowing around the mold may be of some benefit, but it should not blow directly on the molding surfaces of the mold.
All types of resins need a small amount of moisture in order to be processed properly, but in range of of 1/10 of 1 %. Some materials such as nylon and acrylonitrile-butadiene-styrene (ABS) are hygroscopic by nature and readily absorb moisture from the atmosphere, even after initial drying. Moisture causes brittleness because the water droplets turn to steam when heated in the injection unit and this steam explodes through the melt stream, causing voided areas. These voided areas are not properly bonded and easily break apart when they are subjected to any mechanical forces after molding.
Solution: Some materials (especially hygroscopics) may require conditioning after molding to put back the moisture that was removed during the mold process. Nylons, for example, normally must be conditioned by either annealing in 300? F (149? C) glycerin for 4 hours, or being placed for 4 days in sealed bags filled with water. Without this conditioning, the plastic will be brittle as the result of proper drying procedures used to mold the plastic.
An operator who is controlling the cycle may cause brittleness if the machine is not kept cycling consistently from shot to shot because the material will tend to degrade in the heating cylinder. Degraded material causes weak molecular bonding, which results in brittle parts.
Injection temperature too high.
High injection temperatures can cause the molten material to be too fluid. This may result in the material being so turbulent that air and gases become trapped in the melt stream. The trapped gases show up as voids in the molded part.
Solution: Reducing the injection temperature allows the material to stiffen, permitting the trapped gases to escape from the melt stream.
Caution: Apparent voids may sometimes turn out to be unmelted particles. If that is the case, reducing the temperature will only make the condition worse; increasing the temperature will help melt the particles.
Section thickness too great.
When a plastic part consists of varied wall thicknesses (instead of one steady thickness), the thicker walls will cool (and solidify) last. There will be a pressure loss in those thick areas as they continue to cool after the thinner areas have solidified. The plastic will pull away toward the solid section and cause a void in the thick section. When the void is on the surface of a part, it appears as a sink mark. When it is below the surface, it appears as a bubble.
Solution: Using metal core-outs to thin the thicker wall (Expensive solutions). Or, if possible, change the wall thickness so that the thicker section is no more than 25% thicker than the thin section to minimize the void.
Excessive moisture can get trapped in the resin as the molding process progresses and show up as bubbles in the molded part. The moisture actually turns to steam during the heating process and cannot escape from the material, so it forms a gas pocket that becomes a void.
Solution: Properly dry the material before molding.
This may cause the temperature controllers for the heating barrel to overrun, thus making the material too fluid. As a result, the material may be injected at too high a speed which may cause gases to be trapped. These will then show up as pockets (voids).
Solution: Ensure consistent cycles by running the molding machine in an automatic mode whenever possible. If this is not possible, instruct the operators so they know the results of running inconsistent cycles.
Excessive injection speed or pressure.
If injection pressure is too high, the resin is forced into the mold so fast that any air trapped in the runner system or mold cavities is not allowed time to be pushed out ahead of the resin flow. Then this trapped air becomes compressed and its temperature rises sharply. The hot air ignites the surrounding plastic resin, which burns until the air is consumed, leaving anblemish.
Solution: Reducing the injection speed and pressure will allow enough time for the gas or trapped air to escape through normal venting methods.
Venting systems are placed in molds to exhaust any gases or trapped air that might be present. If the vents are not deep enough or wide enough, or if there are not enough vents, the air is compressed before it is all exhausted and then it ignites and burns the plastic as described under “Machine” above.
Solution: Vents must be a minimum of 0.3 cm wide. The vent land should not be more than 0.3175cm long. Blind areas, such as the bottom of holes, should have vents machined on the side of ejector pins that are placed there. There should be enough vents on the parting line to equal 30% of the distance of the parting line perimeter. Thus 25.4-cm parting line perimeter would have 12 vents, each 0.64 cm wide and 7.62 cm in total.
Excessive regrind use.
The use of regrind may have to be limited,especially with heat-sensitive materials such as polyvinyl chloride (PVC). Regrind material tends to absorb heat in the injection barrel at a slower rate than virgin, because of the irregular surfaces and larger size of the regrind particles. This results in a longer heating cycle which causes the virgin pellets to overheat and degrade. The degradation takes the form of burned particles which are transported through the melt stream into the cavity.
Solution: Limit regrind use to no more than 5 or 10 %. If the volume of shot size is small (less than 20% of barrel volume), it may require no regrind at all. A possibility is to start with all virgin and slowly build up regrind use by salting in regrind at 2% increments until burning occurs. Then drop back 2% and use the resultant ratio for future molding.
Erratic cycles cause the barrel heating system to heat in erratic steps, resulting in hot spots in the barrel. In these areas, the material is overheated and degraded. Again, the degradation takes the form of burned particles which are transported through the melt stream and into the cavity.
Solution: Run in automatic mode. If not, at least instruct all operators on the importance of running consistent cycles,demonstrating the burning effect.
It is common for molders to allow small oil leaks to become big ones before they consider fixing them. Leaking oil has a tendency to find its way into some unbelievable places such as the feed throat of the injection barrel during material changes. Also, when a machine is lubricated, the greasing is usually overdone and grease drips end up on mold surfaces and machine areas, finding their way into the plastic material.
Solution: Eliminate oil leaks and clean up grease drips. By fixing oil leaks, wiping up grease drips, and cleaning up chemical spills, these sources of contamination will be greatly minimized. Good maintenance and housekeeping can prevent material contamination.
Molds with cams, slides, lifters, and other mechanical actions need periodic lubrication. The tendency to overdo this allows the lubricant to find its way to the molding surfaces and enter the molded part. Also, excessive use of mold release causes contamination problems.
Solution:Use the proper lubricant for specific mold components and use only the amount necessary. Optimize the use of mold release if it is needed at all. It seems to be human nature to think that if a little works, a lot will work better, but that is not the case with mold releases.
Regrind has been found to contain any number of contaminants such as residues from food containers, soft drink spills, dust and dirt particles, and other plastic materials. This is usually due to poor housekeeping and / or material handling procedures.
Solution:This type of contamination can be greatly reduced by proper instruction of personnel, highly visible labeling of regrind containers, proper labeling of trash containers to differentiate them from material containers, tight fitting covers for regrind (and any plastic) material containers, proper cleaning of regrind machines, and care applied during material changeovers.
An operator can cause contamination through actions such as snacking at the molding machine station. Potato chip salt and soft drink spills are common sources of material contamination. Dust from sweeping can enter the hopper if it is not covered. And, in rare cases,operators have been known to force a break by intentionally throwing trash in the hopper.
Solution:Instruct operators on the importance of maintaining good housekeeping practices and hold supervisors to the same standards.
Cracking / Crazing (Finer cracks)
Stresses can be molded into a product through the molding machine by excessive packing or too fast a filling rate. The plastic is injected and held against the restraining surfaces of the mold cavities. When the part is ejected, the cooling process continues (for up to 30 days) and the highly pressured plastic may begin to relieve. If the skin of the molded part is not yet solid enough, it will split open in the form of cracks.
Solution: Reduce the injection pressure and speed to the lowest numbers that will successfully mold the part. This reduces the tendency to mold in stress.
Insufficient draft or polish.
Draft angles should be an absolute minimum of 1° per side to facilitate easy removal of the part from the mold. Ejector pressure may cause cracked parts if less than that is used. Also,rough cavity surfaces (and other undercuts) cause a drag on the part as it ejects. This may cause cracking if the ejection pressure is increased to push the part over this rough surface.
Solution: Make sure cavity surfaces have a high polish when the mold is built and that they are re-polished as the need arises.
Because moisture turns to steam as it travels through the heating barrel, it creates a condition in which material particles do not properly bond in the area where moisture is present. This improper bonding causes weak areas in the molded part, and these may crack when parts are handled.
Solution: ensure proper drying of the material prior to molding to minimize the tendency to crack due to moisture content.
Erratic cycling will cause hot spots in the heating barrel. The material in these areas may degrade due to overheating. This degraded material will not properly bond with surrounding material particles and will cause weak areas in the molded product that may develop into cracks.
Solution: Inform and show operators that inconsistent cycles will cause defects such as cracking and encourage them to use automatic cycling whenever possible to eliminate the Influence of operator inconsistencies.
Injection speed too slow.
Injection speed determines how fast the material is injected into the mold. If it is too slow, the material tends to cool off and solidify before the mold is filled. Because the material fills the cavity in a tongue-on-tongue fashion, a layer may begin to solidify before the next layer bonds to it. This results in a separation after the part is ejected from the mold and manifests itself as delamination, creating a scrap part.
Solution: Increase injection speed in small increments (2 percent of the total speed) until delamination is eliminated. If flashing or burning occurs, the limits have been reached and another source of the defect should be investigated.
Mold temperature too low.
If the mold temperature is too low, the incoming layers of molten material may cool off too soon and not bond to each other. On ejection, these unbonded layers separate, causing delamination.
Solution: Increase the mold temperature in 5 °C increments until the delamination is eliminated. Then increase by an additional 5 °C F step to compensate for thermal fluctuation in the mold
Foreign materials or additives.
If a pigment is being used to color the resin, it may not be compatible,such as where soap is used in the manufacturing of the pigment. If a color concentrate is used to color the resin, it must be of a material compatible with the base resin. And, if accidental mixing of two incompatible resins has occurred, they will not bond. In all these cases, nonbonding of the materials used will result in delamination of the molded layers of the finished product.
Solution: Check with suppliers of any additives to make sure the proper grade is being used. Also, confirm that all materials are properly identified to ensure that incompatible materials are not being mixed.
Excessive mold release.
If a mold release is required at all, it is necessary to limit its use. Too much mold release will cause a penetration of molded layers by the mold release itself. This will keep the layers from bonding and result in delamination.
Solution: Keep mold release away from presses unless absolutely necessary, and then use only as a quick fix until the cause of the sticking can be rectified. Operators should be made aware of the problems caused by excessive mold release use.
Excessive injection pressure.
Too much injection pressure will partially overcome the clamp pressure of the machine and cause the mold to open slightly during the injection phase. If this happens, a small amount of plastic actually seeps out of the mold. This seepage is called flash. Also, excessive pressure may force plastic into the clearance hole around ejector and core pins. This is also flash.
Solution: Reducing the injection pressure minimizes flashing conditions. If the mold design allows, begin the molding process with very low pressure and slowly increase from shot to shot until the cavities are filling properly. This should be done in 345-kPa increments when the mold is almost filled.
Inadequate mold supports.
Components called support pillars are used in the construction of a mold to supply extra support behind the cavity retainer plates on the ejector half of the mold. They are used to keep the mold from collapsing during the injection phase of the molding cycle. If there are too few pillars, or they are not properly designed or located, the mold will tend to deflect when the injection pressure is applied. The mold will open slightly and flash will occur.
Solution: It is best to use a few large-diameter pillars instead of many with small diameters because the smaller-diameter pillars tend to press into the support plates.
Improper flow rate.
Resin manufacturers supply materials in a variety of flow rates. Thin-walled parts may require easy-flow materials,while thick-walled parts may be able to use stiffer materials. The stiffer materials are usually stronger. If a fast-flowing material is used, it may creep into small crevices where thick materials could not. Flash could be the result. But even with thicker materials, if the flow rate changes to slightly thicker yet, more pressure may be required to inject the material and this could blow the mold open, also causing flash.
Solution: Use a material that has the stiffest flow possible without causing nonfill or flashing the mold by consulting with a materials supplier.
Erratic cycling can cause the material in the barrel to overheat slightly. This will cause the material to flow easier and it may begin to flash in areas that were not flashing before.
Solution: Instruct the operators on the importance of proper and consistent cycles. Use the machine’s automatic mode if at all possible to minimize the operator’s influence.
Inadequate injection pressure.
Flow lines may be the result of improperly bonded material. If injection pressure is too low, the tongues of material that enter the cavity are not packed together to form smooth layers against the molding surface. The material actually starts to wrinkle as one layer tries to crawl over the already cooling layer outside of it.
Solution: Increase the injection pressure to force the layers together quickly while they are still hot enough to bond tightly.
Mold temperature too low.
Generally, a hot mold will allow the molten plastic to flow farther before cooling off and solidifying. This results in a very dense part that minmizes the formation of flow lines.
Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the cavity properly. Start by using the material supplier’s recommended temperature and increase in increments of 5 °C until the flow lines disappear. Allow the mold to stabilize for 10 shots between each adjustment.
Improper flow rate.
A material that is too stiff (low melt index) may not flow fast enough to pack the mold before it solidifies and the flow front may not be able to squeeze out the flow lines that form.
Solution: Use a material that has the fastest flow possible without causing flashing conditions. Ask the material supplier.
Erratic cycles cause hot and cold spots to form in the heating barrel. The material in the cold spot areas may not get to the proper temperature for molding before injection. If this happens, the material will have a slower flow rate. The slow flow rate may cause the cavity to be underpacked, which can result in flow lines.
Solution: Instruct the operators on the importance of maintaining consistent cycles. Show them samples of the variety of defects caused by inconsistent cycles.
Nonfill (Short Shots)
Insufficient material feed.
The most common cause of nonfill is not enough material prepared in advance for injection into the mold.
Solution: Increase the amount of material being fed to the mold by adjusting the return stroke of the injection screw so that more material is transferred from the hopper system with each rotation of the screw. Adjust this setting until there is between a 0.3- and 0.6-cm cushion at the front of the injection cylinder.
Venting is used to remove trapped air from the closed mold so molten material will be able to flow into every section of the mold. If the air is not removed, it acts as a barrier to the incoming plastic and will not allow it to fill all sections of the mold. The result is nonfill. The mold should be vented even before the first shot is made.
Solution: Vent the runner first, and then create enough vents on the parting line to equal 30 percent of the length of the perimeter surrounding the cavity image. An additional approach is to use a vacuum system in the mold to help pull the trapped air out before injecting material.
Improper flow rate.
Use of a material with too low a melt index may result in the material beginning to solidify before the entire cavity has been filled.
Solution: Increasing the flow rate by 2 or 3 points may be enough to ensure the material flowing long enough to completely fill the cavity before it solidifies.
Erratic cycling may cause cold spots in the heating cylinder, and the material in these areas will flow at a slower rate than the surrounding material. When it enters the cavity, the slower material will solidify sooner than the rest and cause a non-fill condition.
Solution: Instruct the operators on the importance of maintaining consistent cycles. Demonstrate defects caused by inconsistent cycles. Again, utilize the machine’s automatic cycling if possible in order to minimize the operator’s influence.
Barrel temperature too high.
If the barrel temperature is too high, the resin absorbs an excessive amount of heat. The heat causes excessive expansion of the resin molecules and increases the amount of voided area between these molecules. After injection, and on cooling, the skin of the molded product solidifies first and the remaining resin closes up the molecular voids as it cools, pulling the already solidified skin with it. The greater the amount of void volume, the greater the degree of pulling and the greater the total shrinkage. This results in defects similar to that shown.
Solution: Decrease the barrel temperature to allow the resin to stay molten without creating excessive void areas. The shrinkage will return to normal. Published shrinkage data will provide the normal shrinkage for a specific material, but shrinkage rates may vary, depending on direction of flow. Material suppliers will provide range data for all directions of flow.
Mold temperature too high.
Generally a hot mold causes the material to stay molten longer, which may result in the required skin not properly forming before ejection of the product. When this occurs, the still cooling material continues to shrink because there is no normal restraining skin to hold it from shrinking too much, and the product will shrink beyond the normal dimensions.
Solution: Decrease the mold temperature until the material maintains the proper flow and fills the cavity without shorting. This should be done in 5 °C increments, and once the desired level is reached, a single increase of 5 °C should be added to compensate for fluctuation in the temperature control units.
Improper flow rate.
A material that is too stiff may not get fully packed into the cavity. If packing does not occur, the density of the part is too low and the part will be allowed to shrink beyond normal expectations.
Solution: Raise the melt index by 2 or 3 points; the increase may be enough to allow full packing of the cavity and minimize excessive shrinkage.
Early gate opening.
If the operator is speeding up the cycle by opening the gate too soon, the cooling plastic may not have had a chance to solidify enough to form a proper skin on the molded product. If this skin is not solid, the remaining plastic that cools pulls the skin with it and,with nothing restraining it, continues to shrink beyond expected rates.
Solution: Instruct the operator that cycles that are too fast may cause defects in the molded product that are not even visible.
Insufficient injection pressure or time.
Injection pressure must be high enough to inject material into the mold and force the material to fill every part of the mold until the mold is packed solidly. When properly achieved, this packing ensures that all the resin molecules are held as closely as possible to each other. In such a case, the molecules will not be able to travel very far upon cooling, and sink marks, such as that shown, will be mini-mized. If the pressure (or time the pressure is applied) is too low, there will be excessive voids between the molecules, especially in areas where two walls meet. When the resin cools, these voids will collapse, bringing cooled material into them and causing excessive shrinkage and resultant sink marks.
Solution: In initial molding trials, first estimate injection pressure and time, then adjust until a molded part is formed but is just short of complete filling. Increase the pressure and time in 10-percent increments until a flash-free, completely filled part is obtained. This packs out the mold and helps minimize sink marks.
Excessive rib thickness.
Ribs are normally designed into a part to add strength in a given area. If the rib thickness is the same as the adjoining wall thickness, an excessively thick area is created at the junction of the rib and the wall. This thicker area takes longer to cool, and as it does, it pulls in the already cooled and solidified area around it, resulting in a sink mark.
Solution: The rib wall should be designed to be no more than 60 percent of the adjoining part wall. Thus, if the part wall is nominally 0.25 cm, the thickness of the rib should not exceed 0.15 cm. This keeps the junction area relatively thin so it will cool at the same rate as the surrounding areas and minimize (or even eliminate) sink marks.
Excessive regrind use.
Regrind material is usually in the form of much larger pellets than virgin material because of the nature of the regrinding process. These larger, inconsistent particles do not nest well with the virgin particles and gaps are formed that trap air during the melting process. This trapped air impedes the ability of the molten plastic to be packed into the mold. Where trapped air bubbles exist, sink marks may form.
Solution: Limit the use of regrind to 10 to 15 percent in these cases. If more regrind than that is generated, try to use it on another product, or package it up and sell it to a broker.
Early gate opening.
If an operator opens the gate too soon, the cycle is shortened and the molten material may not have solidified enough to restrain still-cooling material from shrinking too much. This may cause sink marks, especially at wall junctions and around bosses.
Solution: Instruct the operator on the importance of maintaining consistent cycles. Demonstrate defects caused by early gate openings. If possible, use the automatic cycle function of the molding machine to minimize operator influence.
Splay (Silver Streak)
Barrel temperature too high.
If the barrel temperature is too high, the resin will decompose and begin to char or carbonize. The charred particles will float to the resin surface during injection. The result is a spray of charred particles, on the surface of the molded part, which fans out in a direction emanating from the gate location
Solution: Decrease the barrel temperature to allow the plastic to stay molten without degrading and charring. The particles will bond together as designed and splay will be eliminated.
Gates too small.
Gates that are too small will cause restrictive friction to the flow of the molten plastic, and this can cause degradation of the material at that spot in the mold. The degraded, decomposed material enters the cavity and may be forced to the surface in the form of a typical splay pattern.
Solution: Examine the gates to make sure there are no burrs. Enlarge the gates so the depth is 50 percent of the wall thickness the gate is entering. The width can be increased until it is as much as 10 times the depth without affecting cycle times.
If the material was not properly dried, the excessive moisture will turn to steam as it travels through the heating barrel. This steam becomes trapped and is carried into the mold cavity, where it is usually forced to the surface and spread across the molding surface of the cavity. It appears as streaks of silvered char, which is splay.
Solution: Make sure that all materials are properly dried. Even materials that are not hygroscopic (such as nylon) must have surface moisture removed before molding. Drying conditions are critical, and material suppliers have documented conditions for specific materials and grades.
Erratic cycling will cause hot spots in the heating barrel. Material will become degraded in these areas and may char. These charred particles enter the melt stream and eventually the cavity where they are fanned out across the molding surface, appearing as splay.
Solution: Instruct the operator on the importance of maintaining consistent cycles.Demonstrate by showing defective parts created by inconsistent cycles.
Inadequate injection pressure or time.
If too little injection pressure or time is used, the plastic material will tend to cool down and solidify before the mold is packed out. Then the individual molecules of the plastic are not packed together, leaving them space to moveinto as the part is cooled. They relax during the cooling period and are allowed to move about. While the outer skin of the product may be solid, the internal sections are still cooling and the movement of molecules here determines the degree of warpage.
Solution: Increase injection pressure or time to contain the cooling molecules in a rigid position (packed) until they are solid enough to prevent movement.
Mold temperature too low.
Generally, a hot mold will cause the material to stay molten longer than a cold mold and allow the molecules to be packed tightly together. This results in a very dense part that minimizes the tendency for warpage.
Solution: Increase the temperature to the point at which the material has the proper flow and packs the mold. Make adjustments in increments of 5.6 °C and allow 10 full shots between changes to allow the machine to stabilize.
Improper flow rate.
It is always best to use the stiffest flow rate possible in order to achieve the greatest property values. However, a material that is too stiff may not flow fast enough to pack the mold before it cools and solidifies. Stresses may be set up as the material stretches in an effort to fill the mold. This stretching results in warpage when the part is ejected from the mold and the stresses are slightly relieved.
Solution: Use a material that has the fastest flow rate possible without causing flash.