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Compression molding product defect solutions

Join Date: 2022-08-22

In order to define and address the occurrence of defects in compression molded products, a disciplined approach is critical. This will help avoid false positives during troubleshooting. Below is a table of possible causes of various deficiencies prepared by the MDC study. Ultimately hoping to fix the problem correctly and quickly, the following guide will be useful.


This symptom indicates that thermoplastic and polyester are incompatible.

  • 1, due to the starting thickening is not fast, thermoplastic additives from the matrix complex should make the start of the complex one hour to thicken to 500,000 Cps.
  • 2, some thermoplastic additives are not compatible with polyester.
  • 3, additive excess is bound to cobweb serious, the less it is added the better, but the control of the shrinkage of the firm and the problem will arise.


The appearance of foreign substances on the surface of the product.

  • 1, fillers and stearates and other powders are easy to cause uneven thickening and curing after moisture absorption, resulting in bubbles. The compressed air used to clean up the flying edges of the compression molded products contains water vapor and oil splashes is an important source of contamination and must be closely monitored throughout the compression molding process, which can easily cause a variety of defects such as contamination.
  • 2, molded plastic in the air dust and equipment in the oil splash and other contamination, should be purified from the environment at all.
  • 3, the mold itself has impurities or other pollution, but also inevitably lead to products are contaminated.
  • 4, Other ways of contamination include: flying edge, trimming the debris of the previous product, workbench, mixed charging process.

Surface Cracks

Cracks appear on the surface but do not penetrate the substrate of the product.

  • 1, It is easy to occur in the part of root cut (inverted tip), especially when the mold is opened, there will be large stress on the part, if the product is stuck too tightly in the mold, it is easy to cause cracks in the ejector rod part.
  • 2, Unbalanced or abnormal movement of ejector pin will lead to cracking when demolding.
  • 3, Too fast ejecting speed also leads to excessive stress on the molded products.
  • 4, fully cured parts have sufficient strength to resist the stress during demolding, requiring extended curing time and slightly increased mold temperature to ensure adequate curing.
  • 5, The shorter the flow distance in the mold or the lower the material flow rate, the less chance of glass fiber orientation, which also forces the weak parts of the parts to produce a small chance.
  • 6, Molding occurs in the product fusion line part of the material strength is weak, it is not necessarily able to resist the stress of demolding. Therefore, adjust the size and location of the lay-up material, it is possible to change the situation.
  • 7, the application of an external release agent is conducive to mold release, natural and also reduces stress. But the external release agent is only used for new mold, broken chrome-plated surfaces, shear edges and other occasions.
  • 8, reduce the amount of initiator or reduce its activity, to increase the strength of just release parts is effective.
  • 9, the ejector pin part of the flying edge should be reduced to the small, excessive flying edge will cause parts ejected resistance, increase the risk of crack formation, the ejector pin part of a certain gap to allow its air to escape but can not produce an excessive flying edge.
  • 10, generally make the cavity part of the mold than the core part of 10-15 ℃ to eliminate the aforementioned mold locking and root cutting is beneficial.

Fracture Cracks

Cracks penetrate the substrate of the part.

  • 1, When the mold root cut is severe, it is easy to lock the mold and cause cracking when starting or ejecting.
  • 2, the ejector pin imbalance is serious, and also easy to cause difficulties when the mold rupture.
  • 3, The ejecting speed is too fast.
  • 4, glass fiber orientation, material flow front, and fusion line are parts of the weak area, so the pattern of laying material should make the material flow of the short stroke or first in the weak area stuffed with a small piece of material.
  • 5, the material that is not fully cured will inevitably lead to insufficient strength of the product, increase the curing time and improve the mold temperature can improve the thermal strength of the product.
  • 6, the mold is not installed level, as if to make the mold produce root cut, with the inverted tip will lead to rupture.
  • 7, the ejector pin part of the flying edge is too serious, also similar to the root cut, and should be carefully adjusted to the appropriate gap.

The glass fiber orientation of the surface can be seen with the naked eye.

Please note that phase separation and fiber exposure have roughly the same appearance and are easily confused.

  • 1. Too long material flow distance will increase the flow marks and show the glass fiber orientation, try to maintain the random distribution of glass fiber.
  • 2. Too slow closing speed will form flow marks, so increasing the closing speed is good for the randomness of orientation.
  • 3, Lower viscosity increases the chance of glass fiber orientation.
  • 4, Some low shrinkage additives tend to make the glass fiber flooding to the surface layer. With the re-choice of low shrinkage additives and the number of species, it is possible to mask the glass fiber in the surface layer below.
  • 5. The part of rapid change of wall thickness is easy to cause the orientation of glass fiber.
  • 6. Over-exposure of the material mass to air and loss of styrene can easily cause gelation and flow marks.

Knit Lines/Flow Lines

Excessively fragile parts of the parts all occur in the area where the material flows together, and the reinforced glass fiber in this area is not easily led into laps and bridges, so the fusion marks are the weak areas of the parts' strength.

  • 1. Too long material flow distance and block lay-up method will lead to glass fiber orientation and fusion line, and it is effective to add the material mass directly to the part where fusion marks are likely to occur.
  • 2. Fast mold closing speed is likely to cause glass fiber orientation, too high mold temperature produces gel and affects the material to fuse better, reducing the closing speed, and reducing the mold temperature can make serious fusion marks tend to slow down.
  • 3. Specific mold design, such as too long material flow distance, material mass diversion and cores and other material flow fronts to form holes and lead to fusion marks, if the fusion line occurs in the edge of the part, then set the overflow port is effective.
  • 4. In some cases, it is advantageous to place a specific fiberglass mesh or woven yarn in advance in the area where fusion marks are likely to occur.

dark spots

  • 1, The upper die should be flattened on the stop block of the lower die at the same time to keep the uniform pressure on the material.
  • 2. Insufficient pressure leads to uneven material flow, or the thick and thin cross-section may have poor contact with the mold surface, resulting in localized dark spots.
  • 3, The small pieces of gel may also form the dark spot, the correction method: reduce the time of contact between the material mass and the mold, reduce the mold temperature, reduce the material activity or increase the speed of mold closing.
  • 4, If the mold temperature is too low, there will be independent cold or hot pieces, resulting in uneven curing and shrinkage.
  • 5, If the shrinkage of the material mass is not well controlled, the material will be prematurely peeled off the mold and lose light when curing, the variety and level of low shrinkage additives should be optimized.
  • 6. The material mass is exposed to the air for too long, causing to gel and dark spots.

The product is locally defective and not filled.

  • 1. Check whether the upper mold falls evenly on the stop block and then add some material appropriately to keep the overpressure of about 0.03.
  • 2, The material group contacting the surface of the mold for too long has to gel, it will prevent the material flow to fill the mold smoothly, and lead to the lack of meat.
  • 3, Close the mold too slowly to cause it to gel.
  • 4, Insufficient pressure.
  • 5. Too long distance of material flow, causing to gel in the first.
  • 6, The viscosity of the material mass is too large.
  • 7, The material flow activity is too high or the mold temperature is too high and causes to gel, changing the initiator and resin is effective.
  • 8, Bad exhaust, not escaped air caused by the lack of meat, gaps and scorching.
  • 9, Multi-cavity compression mold in a cavity is overfilled with material resulting in pressure imbalance in another cavity.

Phasing (Thermoplastic Separation)

Low shrinkage additives are separated from the material mass during the material flow to form a sticky surface, thermoplastic resin enrichment, and uneven color.

  • 1. Excessive exposure and contact with the hot mold can easily cause phase separation. Shortening the lay-up time is effective to keep the homogeneity of the material mass.
  • 2. Very short material flow distances often lead to phase separation. Increasing the material flow distance and material flow motion will allow for better mixing of low shrinkage additives in the system, thus reducing the chance of phase separation.
  • 3. Excessive pressure can cause separation from the matrix due to the affinity of the thermoplastic additive to the hot mold core, so reduce the pressure appropriately.
  • 4, Thermoplastic likes to adhere to the surface of the hot mold, melt and separate from the matrix, and reduce the mold temperature appropriately to reduce this phenomenon.
  • 5, The viscosity of the mass is too low, when the flow of thermoplastic additives is easily separated from the matrix, for example, the initial viscosity rise rate is less than 500,000Cps / hour, it is possible that low shrinkage additives are not locked in the matrix, increasing the viscosity of the mass and the initial rate of a viscosity increase.
  • Some thermoplastics tend to phase separate more than others, so adding too much of this thermoplastic can increase phase separation, and new thermoplastic additives are carefully selected. The first is the viscosity increase, followed by the compatibility of homogeneity.
  • 7. Use small amounts of thermoplastic additives in bulk formulations, use ultra-fine particles of filler or increase its feeding volume to target thermoplastic additives and avoid using unconventional mold release agents.


An individual hole or group of small holes with a diameter of 1.0mm or less appearing on the surface.

  • 1. Too short length of material flow may capture air causing pinholes, properly extend the distance of material flow to facilitate exhaust.
  • 2. The material is too close to the edge of the mold, although it is good to fill the mold but often does not eat enough pressure, properly increase the amount of material, so that the material group is subject to uniform positive pressure.
  • 3, volatile substances (external release agent, press lubricant, moisture) and other staining in the mold or material mass, curing easy to produce gas and become pinholes.
  • 4. Too low viscosity causes material flow too fast, easy to capture air, and too high viscosity material flow is not smooth and will lead to the gel will also capture air, so the viscosity of the material mass should be preferred.
  • 5. Too low pressure causes uneven material flow, which is very easy to produce to gel and capture air.
  • 6, Too high mold temperature, and very slow mold closing will also cause to gel and lead to pinholes.
  • 7, Laying the material into small independent pieces of small pieces can trap a small amount of air to lead to pinholes.
  • 8, mold closure can let the air escape, you can avoid bubbles, pinholes, scorching, etc. Therefore, we should clean the shear edge and ejector pin in time.
  • 9, Sometimes due to the mold is too complex to force the material flow distance is long, resulting in the gel at the end of the material flow to capture air, which can be set up in the mold overflow slot (overflow), can let the air captured by the material mass overflow out of the mold cavity, eliminate gas.
  • 10, If the material is exposed to the air for too long before molding, so that it is too dry and hard, it will not flow easily and the air will be trapped.
  • 11. If the mold temperature of the upper and lower mold is too close (3℃) especially when the mold core temperature is higher than the mold cavity, the shear edge gap becomes smaller or even sealed. The passage of air is closed, generally speaking, as long as it does not stick in the mold cavity, always the mold cavity temperature is higher than the core.


The surface of the product is poorly colored, usually dark, rough and accompanied by pinholes, which have started to cure before the material flow.

  • 1. The material mass stays on the mold surface for too long, causing premature curing that is to gel.
  • 2. Too slow mold closing speed provides the opportunity to gel before the end of the material flow, increasing the closing speed is effective.
  • 3. Too high mold temperature or too active initiator can easily cause gel.
  • 4. The material mass is exposed to air for too long and becomes dry and hard, and the dry material mass stops the material flow, causing pegging and trapping air.

Resin-Rich Area

The fiber content of a certain area on the surface of the product is too low.

  • 1. Excessively long material flow can cause glass fiber orientation and a shortage of glass fiber in the extended material flow.
  • 2. Excessively fast mold closing speed increases the material flow speed, resulting in glass fiber orientation and uneven distribution of glass fibers, while slower mold closing speed enables the base material to drive the orderly flow of glass fibers and reduce resin enrichment.
  • 3. Too low viscosity will not be able to drive the flow of glass fiber together, which will easily cause resin enrichment in the material flow range.

Surface of the ribbed part on the back is sunken, or the back has umbilicals, tabs or thick flesh here

  • 1. This defect is caused when the material mass is laid directly on the ribs or tabs, forcing the glass fiber to squeeze directly into the ribs and tabs. If the flow of the material is extended, it may prompt the glass fiber to bridge over the top of the ribs or tabs and prevent the shrinkage of the material on the ribs and tabs, which will not cause depression, so the laying method and the material flow distance should be optimized.
  • 2, When the mold cavity and the mold core are at approximately the same temperature, the inner and outer surfaces of the product will be cured almost simultaneously, while when increasing the mold temperature of the cavity side (usually the outer surface) will make it cured first and prevent the surface from depression, generally by 10-15℃.
  • 3. Preferably low shrinkage additives to control its shrinkage to small.
  • 4, Increase the chance of bridging the glass fiber in the rib or tab area, and it is advisable to use longer glass fiber.
  • 5. If the ratio of rib width and surface thickness is not appropriate it is easy to cause surface depression, generally the rib is too narrow or the surface is too thin is unfavourable, some literature recommends b/t=0.75 (b is the rib width, t is the surface thickness).
  • 6. Too high pressure will increase the rib appearance, use the secondary pressure method, that is, 10-30sec after forming the original pressure of 25-50% of the pressure reduction pressure to the end of the molding.


Dark strips and blemishes on the surface of the product transfer and remain on the surface of the mold, usually due to abnormal internal release agents and incompatible low shrinkage additives in the formulation at a certain temperature.

  • 1. very low mold temperature, the internal release agent failed to melt (also including low shrinkage additives), causing parts to sticky, resin separation and slagging.
  • 2. The material mass is stopped on the surface of the mold for too long, resulting in the separation of the internal release agent and low shrinkage additives before they arrive.
  • 3, Increase the distance of the material flow to play the role of mixing again, of course, thoroughly cleaning the mold surface is also effective.
  • 4, The residue from the surface of the previous part will be transferred to the surface of the next part to produce the same defects, so carefully clean the mold surface and spray the external release agent.
  • 5, Low shrinkage additives and the base resin is not compatible, will be separated during the molding and cause the mold and parts to form scum at the same time.
  • 6, The excessive amount of low shrinkage additives will even increase this defect.
  • 7, The use of inhibitors to delay the occurrence of gelation, or the choice of low activity initiator, reduce the mold temperature, these are the material flow movement of low shrinkage additives that tend to weaken the separation.

Shelf-Life Stability

The additives in some formulations are not conducive to the storage of the material mass and can affect the premature gelation of the material mass.

Carbon black, iron black, cobalt blue and other large doses of pigments will stimulate early gelation, the correct choice of pigments and the use of the appropriate dose becomes an important factor.

  • 1. The use of polymerization inhibitors can effectively extend the storage period. However, we must pay attention to fully mixing with the matrix material, so that it has a small impact on the subsequent curing time.
  • 2, The ambient temperature of the dwellings directly affects the application period, and it is recommended to store in a cold room at 10-15℃.


Physical bonding between the product and the surface of the mold, results in difficulties in release and cracks.

  • 1, Scum, mold contamination or poor mold maintenance can cause the product to stick to the mold. Spraying an external mold release agent can help to supplement the internal release agent.
  • 2. Incomplete curing prevents the material flow from completing, and the shrinkage after curing is easy to hold the very well-fitting core part, so it is beneficial to increase the mold temperature and extend the holding time.
  • 3. The off-center material flow can cause the mold to skew, and once the pressure is released, the mold returns to its original position and mechanical locking of the grip occurs.
  • 4. The direct cause of the mold leads to slightly inverted slightly, easy to happen trivial fixed parts, need to polish or re-plating surface.
  • 5. Too much or too little shrinkage, in some molding conditions is also prone to mechanical locking, to adjust the type and amount of low shrinkage additives.


Abrasion of the darkened area, along the direction of the material flow, especially with the color material surface in the glass fiber-rich area of the scratch.

  • 1. Over-extension of the lay-up method makes the glass fiber bundle easily oriented and leads to surface scratches, try to make the lay-up material retract some.
  • 2. There are already individual gelatinized blocks in the material mass, which appear on the surface like scratches, reduce the mold temperature; make the contact time between the material mass and the mold surface reduce, prevent gel and eliminate the scratches. We should also pay attention to the use of suitable inhibitors and initiators in the formulation of the mass.
  • 3, The viscosity of the pellet is too low, which will cause the orientation of glass fiber in the movement and lead to scratches (texture), increasing the viscosity of the pellet to reduce the orientation and can reduce the texture.
  • 4, Once the low shrinkage additive is separated from the matrix, it will make the material flow disintegrate, and the texture and scratches will appear.
  • 5, not well maintained or mold surface is not chromium-plated, hardness is not enough will be material flow abrasion, causing texture, especially when the light color or white material mass contains high content of crystalline Ti02, its crystal may also lead to the abrasion mold surface.
  • 6. Excessive dryness of the material mass will cause to gel and produce texture.
  • 7, Oil, grease and dirt in the material flow can cause texture on the surface of the product, check carefully whether there is pollution in the material mass and raw material.
  • 8, If the pigment is not fully dispersed, texture like scratches will appear on the surface.

Surface Waves

Visible irregular flow marks.

  • 1. Excessively long and turbulent material flow tends to cause glass fiber orientation, which causes uneven accumulation on the mold and leads to ripples.
  • 2. Unleveled molds tend to have uneven pressure on the material mass when curing, resulting in surface slope, laying the material evenly, and when the upper mold touches the stop block, the pressure added to the material mass should be balanced.
  • 3. Too low mold pressure can actually cause uneven pressure distribution, causing the mass to shrink and leave the mold surface, resulting in ripples.
  • 4, Too low viscosity of the mass causes orientation or turbulent material flow, leading to ripples.
  • 5, Closing the mold too quickly can cause turbulent material flow and lead to ripples.
  • 6. The material mass that has been gelatinized can split the material flow and cause uneven pressure on the material, which is useful to reduce the mold temperature. And the material activity, through the polymerization inhibitor and initiator regulation is also necessary.


Deformation of the product, excessive shrinkage or excessive internal stress.

  • 1. Excessively long material flow can cause glass fiber orientation, resulting in inconsistent glass fiber distribution, which also tends to cause stress concentration.
  • 2. In the edge state of curing products due to the low mechanical strength, size change, to increase the mold temperature and pressure holding time.
  • 3, After curing, the product shrinks or expands too much causing warpage.
  • 4, The use of cold setting fixture can prevent warpage.


After painting, the surface of the part is lightly white.

  • 1, In most of the material systems of A-class surfaces, the surface of the product is prone to whitening, this appearance weakens the decorative nature of the painted, use PS or other non-whitening low-shrinkage additives.
  • 2. Use light color grading coating to reduce whitening.
  • 3, Using whitish low shrinkage additives mixed with PS and then shared.

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