Common Problems and Solutions of Injection Mold Trial (1)

  • Jun 09,2022

Shrinkage marks


--> Characteristics of injection molded part defects

Usually associated with surface marks, and is formed by the shrinkage of plastic from the mold surface.


--> Possible cause of the problem

a) The melting temperature is either too high or too low.

b) Insufficient plastic in the cavity.

c) The side that touch the plastic overheat during the cooling phase.

d) The runner is unreasonable and the gate section is too small.

e) Whether the mold temperature is compatible with the characteristics of the plastic.

f) The product structure is unreasonable (too high, too thick, obviously different thickness).

g) The cooling effect is not good, and the product continues to shrink after demolding.


--> Solution

a) Adjust the temperature of the injection cylinder.

b) Adjust screw speed for correct screw surface speed.

c) Increase injection volume.

d) Make sure to use the correct gasket, increase screw forward time, increase injection pressure, increase injection speed.

e) Check that the shut-off valve is installed correctly, as abnormal operation will cause pressure loss.

f) Lower mold surface temperature.

g) Correct the flow channel to avoid excessive pressure loss; according to actual needs, appropriately expand the cross-sectional size.

h) The mold temperature is properly controlled according to the characteristics of the plastic used and the product structure.

i) Improve product structure when possible.

j) Try to keep the product cool enough.



The product has low dimensional accuracy


--> Characteristics of injection molded part defects

Variation in weight and dimensions during injection molding exceeds the production capacity of the mold, injection machine, and plastic combination.


--> Possible cause of the problem

a) The plastic input into the injection tank is uneven.

b) The temperature or fluctuation range of the injection cylinder is too large.

c) The capacity of injection molding machine is too small.

d) Unstable injection pressure.

e) Screw reset is unstable.

f) Variation of operating time, inconsistent solution viscosity.

g) Injection speed (flow control) is unstable.

h) Plastic are not suitable for the mold.

i) Consider the influence of mold temperature, injection pressure, speed, time and holding pressure on the product.


--> Solution

a) Check if there are enough cooling water to ensure the correct temperature.

b) Check if thermocouples is with poor quality or loosed.

c) Check that if the thermocouple which used with the temperature controller together is the correct type.

d) Check the injection volume and plasticizing capacity of the injection molding machine, then compare with the actual injection volume and injection volume per hour.

e) Check that there is a stable melted material for each operation.

f) Check if the backflow prevention valve leaks and replace if necessary.

g) Check if incorrect feed settings.

h) Ensure that the screw is stable in the return position of each operation, that is, no more than 0.4mm change.

i) Check the inconsistencies in operating hours.

j) Use back pressure.

k) Check whether the hydraulic system operates normally and whether the oil temperature is too high or too low (25-60℃).

l) Select the plastic which is suitable for the mold (mainly consider the shrinkage rate and mechanical strength).

m) Readjust the entire production process.


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