Polypropylene (PP) Injection Molding Process - 11 Common Defects In PP Injection Molding

The injection molding process of plastic raw materials is mainly plasticizing, filling, cooling and shaping to become finished products. It is a process of heating and then cooling, and also the process of plastic changing from particles to different shapes. In this article, we will explain the PP injection molding process from the perspective of different stages and we present 11 common defects in Polypropylene injection molding.

 

Injection Molding Process For Polypropylene (PP)

1. Screw

Most of PP plastic injection molding processing depends on screw to drive the fluidity, so the design of screw has a great influence. The diameter of screw affects the output, the compression ratio affects the pressure value, and also affects the output and finished product effect, including the mixing effect of various materials. The flow of raw materials mainly depends on the heater, but the friction of raw materials will also produce friction heat energy to accelerate the fluidity, so the screw compression ratio is small, the flow is small, and the rotational speed must be increased, resulting in more friction heat than the screw with large compression ratio.

 

2. Melting

The heater of the unit makes the raw material particles melt into liquid flow gradually. It is mainly suitable to adjust the temperature of different raw materials. Increasing the temperature will accelerate the flow of raw materials, which can increase the efficiency, but it does not necessarily guarantee the yield. Therefore, a proper balance must be achieved.

Good results and PP pyrolysis characteristics are that it is best to make the raw materials flow smoothly to the die head during production, so as to avoid the phenomenon of insufficient filling or reflux. Reflux represents that the flow rate of raw materials is faster than the output rate, and the average flow efficiency is increased, which is one of the available methods for processing, but it also causes abnormal MFR distribution, which may lead to instability The defect rate may increase.

 

3. Mold Or Mold Head

Plastic retyping depends on the mold or mold head. The injection molding product is three-dimensional, and the mold is more complex, and the shrinkage rate should be considered. Other products are flat, strip and needle like continuous product heads. 

Most of the plastic machines are designed like syringes. The extrusion force driven by the screw will cause huge pressure at the small outlet and improve the production efficiency. When the mold head is designed as a plane, how to make the raw materials evenly distributed on the whole surface, the design of the hanger mold head is very important. Pay attention to the extrusion opportunity increases the fishgill pump to stabilize the raw material supply.

 

4. Cooling

In addition to pouring raw materials into runner gate, injection mold also has cooling channel cooling material design. Extrusion molding depends on the cooling channel in the roller to achieve the cooling effect, except for the air knife, cooling water directly poured on the blowing bag, and hollow air blowing and other cooling methods.

 

5. Extension

The elongation of finished products can be enhanced by processing, and the molecular weight distribution will also affect the elongation effect in high-speed production. All extruded products, including fibers, have different elongation. Vacuum and air compression molding can also be regarded as another form of extension.

 

6. Contraction

Any raw material has the problem of shrinkage rate, which is caused by the internal stress caused by thermal expansion, cold contraction and crystallization. Generally speaking, thermal expansion and cold shrinkage are easy to overcome, which can be achieved by prolonging cooling time and maintaining pressure continuously. Crystalline raw materials tend to have larger shrinkage difference than non crystalline materials. PP is about 16% of 1000, but ABS is only about 4%. This part needs to be overcome in the mold or by adding additives to reduce shrinkage rate.

 

11 Common Defects In PP Injection Molding 

Check out all the PP injection molding defects, and get to know the causes and solutions & how to fix them.

 

1. Underinjection

(1) Improper control of process conditions. It should be adjusted appropriately.

(2) The injection capacity of injection molding machine is less than the weight of plastic parts. The injection molding machine with larger specifications should be used.

(3) The runner and gate sections are too small. It should be increased appropriately.

(4) Poor mold exhaust, residual air in the mold cavity caused under injection. The exhaust system of the mold should be improved.

(5) The flowability of raw materials is too poor. The resin with better fluidity should be used.

(6) If the barrel temperature is too low, the injection pressure is insufficient or the injection time is too short, the under injection will also be caused. The control quantity of relevant process parameters should be increased accordingly.

 

2. Overflow flash

(1) Insufficient clamping force. The injection molding machine with larger specifications should be used.

(2) The pin hole or guide pin of the mold is seriously worn. Repair shall be done by machining.

(3) Mold temperature or injection pressure is too high. Should be reduced appropriately.

 

3. Surface porosity

(1) When the size of runner and gate of thick wall plastic parts is small, it is easy to produce surface pores. The size of runner and gate should be enlarged properly.

(2) The wall of the plastic part is too thick. The wall thickness should be reduced as much as possible in the design.

(3) If the molding temperature is too high or the injection pressure is too low, the surface of the plastic parts will produce pores. The molding temperature should be reduced and the injection pressure should be increased.

 

4. Flow mark

(1)The melting material and mold temperature are too low. The barrel and mold temperature should be appropriately high.

(2) The injection speed is too slow. The injection speed should be increased appropriately.

(3) The nozzle diameter is too small. The nozzle with larger diameter should be used.

 

5. Silver Streaking 

(1) The content of moisture and volatile matter in raw materials is too high. The raw materials should be pre dried.

(2) The exhaust of mould is poor. The exhaust hole should be added to improve the exhaust performance of the mold.

(3) When the silver streaks always appear in a certain position, it is necessary to check whether there are surface scars on the surface of the corresponding mold cavity. If there is a reflection of the surface scars, the surface scars of the die cavity should be removed by machining.

(4) When different kinds of resins are mixed, there will be silver streaks. Mixed use of different resins should be prevented.

 

6. Weld line

(1) The temperature of melting material and mould is too low. The barrel and mold temperature should be increased.

(2) The gate location is unreasonable. The gate position should be changed.

(3) The volatile content in the raw material is too high or the mold exhaust is poor. The volatile substances in the raw materials should be removed and the exhaust system of the mould should be improved.

(4) The injection speed is too slow. It should be speeded up appropriately.

(5) There is no cold hole in the mold. The cold material hole should be added.

(6) The design of gating system is unreasonable. The filling performance of the gating system should be improved to make the melt flow smoothly in the mold cavity.

 

7. Bubbles

(1) The size of gate and runner is too small. It should be increased appropriately.

(2) The injection pressure is too low. It should be raised appropriately.

(3) The moisture content in the raw material is too high. The raw materials should be pre dried.

(4) The wall thickness of plastic parts changes too much. The shape and structure of plastic parts should be designed reasonably to avoid rapid change of wall thickness.

 

8. Cracking and whitening

(1) The temperature of melting material and mould is too low. The barrel and mold temperature should be increased.

(2) The structure design of the gating system is unreasonable. The structure of runner and gate should be improved to avoid turbulence during filling.

(3) The cooling time is too short. Should be extended appropriately.

(4) Injection speed and pressure are too high. It should be reduced appropriately.

 

9. Bending deformation

(1) Mold temperature is too high or insufficient cooling. The mold temperature should be reduced or the cooling time should be prolonged.

(2) Uneven cooling. The cooling system of the mould should be improved.

(3) Gate selection is unreasonable. The reasonable gate form should be selected according to the specific situation.

(4) The core of the mould is off center. It should be checked and corrected.

 

10. Shrinkage deformation

(1) Insufficient injection pressure. It should be raised appropriately.

(2) Mold temperature is too high. It should be reduced appropriately.

(3) The gate section area is too small. It should be increased appropriately.

(4) The processing temperature is too low. The barrel temperature should be increased properly.

 

11. Vacuum hole

(1) Insufficient pressure holding. The injection time should be prolonged.

(2) Mold temperature is too low, barrel temperature is too high. The mold temperature should be increased properly to reduce the barrel temperature.

(3) The flowability of raw materials is too good. The resin with lower melt index should be used.