Causes and Countermeasures of Polypropylene Injection Molding Faults

Fault name, cause and countermeasure
Undernote
(1) Improper control of process conditions. It should be adjusted appropriately.
(2) The injection volume of the injection machine is insufficient. The injection machine with larger specifications should be replaced.
(3) The structure size of the gating system is too small. The gate and runner sections shall be appropriately enlarged.
(4) The flow distance of molten metal in the mold cavity is too long or there are thin-walled parts. Cold pockets shall be provided.
(5) The mold exhaust is poor. Mold exhaust should be improved.
(6) The flow performance of raw materials is too poor. The raw materials with good fluidity should be replaced.
(7) The barrel temperature is too low, the injection pressure is insufficient, or the injection time for feeding is too short. Relevant process parameters should be improved.
molding flash
(1) The molding temperature is too high or the injection pressure is too high. It should be properly lowered.
(2) Insufficient clamping force. The injection machine with larger specifications should be replaced.
(3) There are foreign matters on the parting surface of the mold. It should be cleared.
(4) The pin hole or guide pin of the mold is seriously worn. The mould shall be repaired by machining.
Bubble
(1) The injection pressure is too low. It should be properly improved.
(2) The size of gate and runner is too small. It should be enlarged appropriately.
(3) The wall thickness of the product changes too much. The shape and structure of the product should be reasonably designed to avoid sharp changes in wall thickness.
(4) High moisture content in raw materials. The raw materials shall be dried.
Vacuum hole
(1) Insufficient pressure holding. The injection time of replenishment should be appropriately extended.
(2) The injection pressure is too low. It should be properly improved.
(3) The mold temperature is too low and the barrel temperature is too high. The temperature of the barrel should be properly reduced and the mold temperature should be increased.
(4) The fluidity of raw materials is too good. The resin with low melt flow rate should be replaced.
Surface porosity
(1) When the mold runner and gate of thick-wall products are small, it is easy to produce surface pores. The runner and gate size should be enlarged.
(2) The product wall is too thick. The wall thickness shall be minimized during design.
(3) If the molding temperature is too high or the injection pressure is too low, it will lead to pores on the surface of the product. The molding temperature should be properly reduced and the injection pressure should be increased.
Weld mark
(1) The temperature of molten material and mold is too low. The temperature of the barrel and mold should be properly raised.
(2) The injection speed is too slow. It should be accelerated appropriately.
(3) The gate position setting is unreasonable. The gate position should be changed.
(4) The content of volatile substances in raw materials is too high or the mold exhaust is poor. Volatiles in raw materials should be removed and the mold exhaust system should be improved.
(5) There is no cold material hole in the mold. Cold material hole shall be added.
(6) There are foreign matters on the surface of the mold cavity. It should be cleaned.
(7) The design of gating system is unreasonable. The filling performance of the gating system should be improved to make the molten material flow smoothly in the mold cavity.
Burn and black lines
(1) The specification of the injection machine is too large. The injection machine with appropriate specifications should be replaced.
(2) The fluidity of the resin is poor. An appropriate amount of external lubricant should be used.
(3) The injection pressure is too high. It should be properly lowered.
(4) The gate position design is unreasonable. The gate position should be changed to make the molten material flow evenly in the mold cavity.
(5) The mold exhaust is poor. Mold exhaust should be increased.
Silver stripe mark
(1) Poor contact between mold and nozzle. The position and size of both should be adjusted.
(2) The mold exhaust is poor. Mold exhaust should be increased.
(3) Mixed use of different kinds of raw materials will produce silver stripes. The mixing of different resins shall be prevented.
(4) The content of water and volatile matter in raw materials is too high. The raw materials shall be dried.
(5) When the silver strip mark always appears at a certain position, check whether there is surface scar on the corresponding mold cavity surface. If there is a reflection of surface scars, the surface scars of the mold cavity shall be removed by machining.
craze crack
(1) Injection pressure and injection speed are too high. It should be properly lowered.
(2) The cooling time is too short. It should be extended appropriately.
(3) The temperature of molten material and mold is too low. It should be properly improved.
(4) The setting of ejection device is unreasonable. The ejecting sectional area and ejecting point shall be increased as much as possible, and pneumatic demoulding device is preferred.
(5) The structure design of the pouring system of the mold is unreasonable. The size of gate and runner should be improved to avoid turbulence during mold filling.
Bending deformation
(1) The mold temperature is too high or the cooling is insufficient. The mold temperature shall be properly reduced or the cooling time shall be extended. For slender products, the method of cooling after clamping can be adopted.
(2) Uneven cooling. The mold cooling system should be improved to ensure uniform cooling of products.
(3) The mold is eccentric. It shall be checked and corrected.
(4) The gate selection is unreasonable. The reasonable gate form shall be selected according to the specific situation. Generally, multi-point gate can be used.
Excessive contraction
(1) Insufficient injection pressure. It should be properly improved.
(2) The mold temperature is too high. It should be properly lowered.
(3) The processing temperature is too low. The barrel temperature should be properly raised.
(4) The sectional area of the gate is too small. It should be enlarged appropriately.
(5) Insufficient pressure holding. The injection time of replenishment should be appropriately extended.
Die Adhesion
(1) The surface finish of the mold is too poor. Its finish shall be improved by grinding or electroplating.
(2) Injection pressure or injection speed is too high. It should be properly lowered.
(3) Improper control of mold temperature and cooling conditions. When the product sticks to the core mold, the mold temperature shall be increased and the cooling time shall be shortened. When the product sticks to the mold on the surface of the mold cavity, the mold temperature should be appropriately reduced to extend the cooling time.
(4) The setting of ejection device is unreasonable. The ejection sectional area and ejection point shall be increased as much as possible.