Defective molding may be derived from improper setting of the injection parameters or material conditions.
Corrective action or improved procedure should be in practice by finding out the cause of the defective molding.
Typical phenomena of the defective molding, possible causes and actions for improvement shall be explained as follows:
5-1 Short filling (short shot)
A short shot is a condition in which resin cannot be fully filled into the mold. This will cause poor appearance and poor function of the product.
Possible causes of the short shot are as follows:
• Resin fluidity is poor.
• Amount of resin measured by the machine is too less.
• Product wall thickness is too small.
• Air pocket due to flow pattern of the resin.
• Air ventilation of the mold is poor.
• Resin temperature is too low.
• Temperature of the cavity surface is too low.
• Injection speed is too slow.
• Injection pressure is too less.
5-2 Flashes or burrs
Flashes are thin filmy resin adhered to edges of the product. Flashes may cause difficulty in assembly of the molded parts and may be hurting when handled.
Flashes can be resulted from following conditions:
• Resin viscosity is too low.
• Clamping force of the mold is not enough.
• Dimensional problem of the cavity.
• Clearance on cavity parts is too much.
• Rigidity of the mold plate is not enough.
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• Resin temperature is too high.
• Cavity surface temperature is too high.
• Injection speed is too fast.
• Injection pressure is too high.
5-3 Sink mark
Sink mark is a concavity on the product surface. It causes poor appearance and poor function of the molded product.
Sink marks can be resulted from following conditions:
• Contraction factor of the resin is too high.
• Wall thickness of the product is too thick.
• The gate is too small.
• Runner is too small.
• The gate location is too far.
• Cavity temperature is partially too high.
• Cooling capacity of the mold is too less.
• Holding pressure is too low.
• Time for holding pressure is too short.
• Cooling time is too short.
5-4 Void
Void is a bubble in the product. There can be classified to two causes.
One is caused by water or air mixed into the material and another is caused by vacuum cavity created when material is contracted. Void results in weak strength of the product or poor appearance if the products need to be transparent.
Void can be resulted from following reasons.
(In the case of mixed air)
• Preheating of the pellets is not enough.
• Rotation speed of the screw is to fast.
• Staying time of the resin is too long.
• Resin temperature is too high.
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• Injection speed is too fast.
• Air ventilation in the mold is not enough.
(In the case of vacuum void)
• Holding pressure is too low.
• Time for holding pressure is too short.
• Gate is too small.
• Runner is too small.
• Amount of cushion is too less.
• The wall of the product is too thick.
5-5 Flow mark
Flow mark is a wavelike mark of the resin flow left on the surface of the product. Appearance and surface quality should be affected.
Flow marks can be caused by following reasons:
• Resin flow is poor.
• Resin temperature is too low.
• Gate is too small.
• Injection pressure is too low.
• Holding pressure is too low.
• Surface temperature of the cavity is too low.
• Injection speed is too low.
5-6 Weld line
Weld line is a line mark on the surface of the product appeared around merging area of the resin flow. Appearance and strength of the product should be affected.
Weld line can be caused by following reasons:
• Resin flow is poor.
• Resin includes additives such as fiber.
• Surface temperature of the cavity is too low.
• Staying time of the resin is too long.
• Location of the gate is not proper.
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• Air ventilation is not enough.
• Injection pressure is too low.
• Injection speed is too slow.
• Holding pressure is too low.
• Time for holding pressure is too short.
• Preheating of the pellets is not enough.
5-7 Burn
Burn is a phenomenon in that resin burns when air is heated upon compression around final filling area and air pocket area. Burn should affect appearance and quality of the product.
Burn can be caused by the following reasons:
• Air pocket can be created.
• Air ventilation is not enough.
• A certain gas has plugged the vent line.
• Injection speed is too fast.
• Resin temperature is too high.
• Gate is too small.
5-8 Jetting
Jetting is a snaking mark appeared on the product surface, this tends to appear when the gate is too small.
Jetting can be caused by the following reasons:
• Gate is too small.
• Gate is located around the thick wall of the product.
• Injection speed is too fast.
• Injection temperature is too low.
• Surface temperature of the cavity is too low.
5-9 Silver streak
Silver streak is a fine silvery line on the surface of the product.
Silver streak can be caused by the following reasons:
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• Preheating of the pellets in not enough.
• Air ventilation is not enough.
• Gate is too small.
• Resin temperature is too high.
• Rotation speed of the screw is too fast.
• Injection speed is too fast.
5-10 Poor luster
Poor luster is a phenomenon in that luster on the product surface is not enough or inconsistent. It affects appearance of the product.
Poor luster can be caused by the following reasons:
• Resin flow is poor.
• Too much variation in the wall thickness of the product.
• Surface finish of the cavity is too rough.
• Preheating of the pellets is not enough.
• Injection speed is too slow.
• Injection pressure is too low.
• Holding pressure is too low.
• Time for holding pressure is too short.
• Surface temperature of the cavity is too low.
5-11 Inclusion of foreign matters
Foreign matters such as dusts, dirt, bugs, etc. may be included in the material. Appearance and material strength may be affected.
Causes may be as follows:
• Preheating management is poor.
• Foreign matters tend to be in at the crashing process.
• Cover of the hopper dryer is left open.
• There may be a fluorescent light above the machine.
• 5 S is not in practice in the factory.
• Management in keeping material bags is not proper.
• Oil and separation agent are spread too much on the mold.
• Air ventilation tube of the mold is plugged.
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• Cleaning of the mold is not sufficient.
• Color change is not proper.
• Staying time of the resin in the cylinder is too long.
• Foreign matters are on the screw.
5-12 Poor releasing of the mold
Poor releasing of the mold is a phenomenon in that the product sticks to the cavity. There are three patterns for this problem. One is that the product sticks to the cavity when the mold is opened. The second is that the product sticks to the core when product is to be ejected. The third is that the product sticks to the slide core. When poor releasing happens, naturally continuous production is interrupted and product surface may be damaged.
Poor releasing can be caused by the following reasons:
• Releasing resistance of the resin is too big.
• Draft angle is too small.
• Cavity surface is too rough.
• Direction of the cavity surface finishing is perpendicular to draft direction.
• Cavity is machined in the shape of under cutting.
• Measured resin is too much.
• Amount of cushion is too much.
• Injection pressure is too high.
• Holding pressure is too high.
• Time for holding pressure is too long.
• Cooling time is too short.
• Cooling capacity of the mold is not enough.
5-13 Bending and deformation
Bending and deformation is a phenomenon of the bent and deformed product.
Bending and deformation can be cause by the following reasons:
• Contraction rate of the resin is too big.
• Wall thickness of the product varies too much.
• Gate location is not proper.
• Filling pattern is not even.
• Surface temperature of the mold varies from one place to another.
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• Holding pressure is too low.
• Time for holding pressure is too short.
• Surface temperature of the cavity is too high.
• Cooling time is not enough.
• Layout of the ejector pins is imbalanced.
• Draft angle is too small.
5-14 Dimension defect
Dimension defect is a phenomenon is that the product dimensions fall outside of specified tolerances.
Dimension defects can be caused by the following reasons:
• There is a fluctuation in the quality of the material by the lot.
• Machining dimensions of the cavity may be in error.
• Contraction rate of the material is out of scope.
• Flow pattern of the resin is not stable.
• Molding condition is not stable.
• Surface temperature of the mold is not stable.
• In the case of multi-molding, balance of the runner and gate is not appropriate.
• Holding pressure is released before gate sealing.
<References>
1) “Reading of Plastic Molding”, written by Y. Sakurauchi, published by (株) 工業調査会 2) “Easy Injection Molding”, written by K. Takano, published by (株) 工業調査会
3) Molding Standard Parts Face 1999-2001, published by (株)ミスミ
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Table 5.1 Variety of Defective Mold and Causes
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Fig. 5.1 Molding Cycle Diagram
Fig. 5.2 How to Experimentally Establish Appropriate Holding Pressure and Time for Holding Pressure
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Fig. 5.3 Relationship between Injection Pressure and Pressure in the Cavity