Cause analysis and solutions for sink marks on molded plastic parts in injection molding industry

The sink marks on the injection molded parts are shrinkage marks caused by the cooling and hardening of the plastic, mainly found in the thick-walled positions, ribs, casings, the back of the nut inserts, etc. What is the main reason for the sink marks? How should we solve this problem?

 

1. In terms of equipment

 

(1) Insufficient supply.

 

The screw or plunger is seriously worn, and the molten material leaks during injection and pressure holding, which reduces the filling pressure and material volume, resulting in insufficient molten material.

 

(2) The nozzle hole is too large or too small.

 

If it is too small, it is easy to block the feeding channel, and if it is too large, the shooting force will be small and it will be difficult to fill the mold.

 

 

 2. In terms of mold

 

(1) The gate is too small or the runner is too narrow or shallow, the runner efficiency is low, the resistance is large, and the melt is cooled prematurely

 

 

The gate should not be too large, otherwise the shear rate will be lost, the viscosity of the material will be high, and the product will not be full. The gate should be set up in the thick-walled part of the product. The necessary cold slug wells with sufficient capacity are set up in the runner to remove the cold slug from entering the cavity and make the filling continue.

 

 

(2) For multi-gate molds, it is necessary to adjust the filling speed of each gate, and it is best to set up gates symmetrically.

 

 

(3) The key position of the mold should be effectively set with cooling water channels to ensure that the cooling of the mold is with a good effect on eliminating or reducing shrinkage.

 

 

(4) The entire mold should be free of sharps and have reliable mold clamping and sealing, and can withstand high-pressure, high-speed, low-viscosity melt filling.

 

 

3. In terms of molding process

 

(1) Increase the injection pressure, holding pressure, and prolong injection time.

 

 

(2) Increasing the injection speed can more easily fill the part and eliminate most of the shrinkage.

 

 

(3) For thin-walled plastic parts, the mold temperature should be increased to ensure smooth material flow; for thick-walled plastic parts, the mold temperature should be reduced to accelerate the curing and molding on the surface of the parts.

 

(4) Extending the cooling staying time of the plastic part in the mold, maintaining a uniform production cycle, increasing the back pressure, and retaining a certain buffer at the front of the screw are all conducive to reducing shrinkage.

 

(5) Low-precision products should be ejected out from the mold as soon as possible to cool them slowly in air or hot water, which can make the shrinkage and dents smooth without affecting to use.

 

 

4. In terms of raw materials

 

The raw material is too soft and easy to be with dents. The effective method is to add a nucleating agent to the plastic to accelerate the crystallization.

 

5. In terms of product design

 

The design of the product should make the wall thickness uniform, and try to avoid the change of the wall thickness. For plastics with large shrinkage such as polypropylene, when the thickness change exceeds 50%, it is best to take the thickened parts to ribs instead .