Fault Analysis and Troubleshooting Methods for Common Issues in PP Injection Molding

Polypropylene (PP), due to its excellent chemical resistance, mechanical properties, and molding flowability, is widely used in injection molded products such as automotive parts, home appliance housings, daily necessities, and medical devices. However, during the injection molding process, various molding defects often occur due to material properties or improper control of process parameters. Here we analyze common issues in PP injection molding and provides corresponding troubleshooting methods.

Common Problems	Cause Analysis	Troubleshooting Methods
Short shot	Improper control of process parameters	Adjust as appropriate.
	The injection capacity of the injection molding machine is less than the weight of the plastic part.	Use a larger injection molding machine.
	The cross-section of the runner and gate is too small.	Increase as appropriate.
	The flow distance of the melt in the mold cavity is too long or there are thin-walled sections	Set a cold slug well.
	Poor mold venting; residual air in the mold cavity causes short shot	Improve the mold venting system.
	The flowability of the raw material is too poor.	Use a resin with better flowability.
	Low barrel temperature, insufficient injection pressure, or too short packing time can also cause short shots.	The relevant process parameters should be appropriately increased.
Flash or burrs	Insufficient clamping force.	A larger injection molding machine should be used.
	The mold's pin holes or guide pins are severely worn.	Repair should be carried out by machining.
	There are foreign objects or impurities on the mold's parting surface.	They should be removed.
	The mold temperature or injection pressure is too high.	It should be appropriately reduced.
Surface pores	Surface pores are likely to occur when the runner and gate sizes of the mold for thick-walled plastic parts are too small.	The runner and gate sizes should be appropriately increased.
	The wall thickness of the plastic part is too great.	Wall thickness should be minimized as much as possible during the design stage.
	Excessively high molding temperature or low injection pressure can both cause surface pores on the plastic part.	The molding temperature should be appropriately reduced and the injection pressure increased.
Flow marks	The melt and mold temperatures are too low.	The barrel and mold temperatures should be appropriately increased.
	The injection speed is too slow.	The injection speed should be appropriately increased.
	The nozzle orifice is too small.	Replace with a nozzle of larger diameter.
	No cold slug well is provided in the mold.	Add a cold slug well.
Silver streaks	The moisture and volatile content in the raw material is too high.	Pre-dry the raw material.
	Poor mold venting.	Add vent holes to improve the mold's venting performance.
	Poor contact between the nozzle and the mold.	Adjust the position and geometry of both components.
	Silver streaks always appear in specific locations.	Check whether there are surface scratches on the corresponding mold cavity. If surface replication occurs, use machining methods to remove the scratches from the mold cavity surface.
	Mixing different types of resin can cause silver streaks.	Prevent mixing of different types of resin.
Weld lines	Melt and mold temperatures are too low.	Increase barrel and mold temperatures.
	Improper gate location.	Adjust the gate location.
	Excessive volatile content in the raw material or poor mold venting.	Remove volatile substances from the raw material and improve the mold venting system.
	The injection speed is too slow.	Increase the speed appropriately.
	No cold slug well is provided in the mold.	Add a cold slug well.
	Foreign matter or impurities on the mold cavity surface.	Clean the mold cavity.
	Improper gating system design.	Improve the gating system to ensure smooth melt flow in the mold cavity.
Black streaks and burn marks	The injection molding machine specification is too large.	Replace with an injection molding machine of smaller specification.
	The resin has poor flowability.	Use an appropriate amount of external lubricant.
	The injection pressure is too high.	Reduce appropriately.
	Poor mold venting.	Improve the mold's venting system by adding vent holes or using an insert structure, and appropriately reduce the clamping force.
	Improper gate location.	Change the gate position to ensure uniform melt flow within the mold cavity.
Bubbles	The gate and runner dimensions are too small.	Increase as appropriate
	The injection pressure is too low.	Increase appropriately.
	The moisture content in the raw material is too high.	Perform pre-drying treatment on the raw material.
	The wall thickness variation of the plastic part is too great.	Design the shape and structure of the plastic part reasonably to avoid abrupt changes in wall thickness.
Cracking and whitening	Melt and mold temperatures are too low.	Increase barrel and mold temperatures.
	The mold gating system is poorly designed.	Improve the runner and gate structure of the mold to prevent turbulence of the melt during filling.
	Cooling time is too short.	Extend the cooling time appropriately.
	The design of the ejection device for demolding is inadequate.	Use a pneumatic demolding device.
	Injection speed and pressure are too high.	Reduce appropriately.
Warping deformation	Mold temperature is too high or cooling is insufficient.	Lower the mold temperature or extend the cooling time as appropriate. For long and thin plastic parts, use a fixture to hold them during cooling.
	Uneven cooling.	Improve the mold's cooling system to ensure uniform cooling of the plastic part.
	Improper gate selection.	Select an appropriate gate type based on the specific situation. Generally, a multi-point gate can be used.
	Mold eccentricity.	Inspect and correct.
Poor demolding	Injection speed and pressure are too high.	Reduce appropriately.
	The surface finish of the mold cavity is too rough.	Enhance the surface finish through polishing and electroplating.
	Improper control of mold temperature and cooling conditions.	When the plastic part sticks to the mold core, increase the mold temperature and shorten the cooling time. If the plastic part sticks to the cavity surface, lower the mold temperature and extend the cooling time.
	The ejection area of the demolding mechanism is too small.	Increase the ejection area.
Shrinkage deformation	Insufficient packing pressure.	Extend the packing injection time appropriately.
	Insufficient injection pressure.	Increase appropriately.
	The mold temperature is too high.	Reduce appropriately.
	The gate cross-sectional area is too small.	Increase as appropriate
	The processing temperature is too low.	Increase the barrel temperature appropriately.
Vacuum hole	Insufficient packing pressure.	Extend the packing injection time appropriately.
	The mold temperature is too low and the barrel temperature is too high.	Increase the mold temperature appropriately and lower the barrel temperature.
	Insufficient injection pressure.	Increase appropriately.
	The flowability of the raw material is too high.	Use resin with a lower melt index.