Explanation of terms related to injection molding (4)

1. What are the preparations before injection molding?

 (1) Inspection of raw material appearance and determination of process performance;

 (2) Preheating and drying of materials;

 (3) . preheating of inserts;

 (4) . cleaning of material barrel;

 (5) Selection of release agent.

2. What factors limit the size of plastic parts?

 (1) Limited by the fluidity of plastics. Under certain equipment and process conditions, the plastic with good fluidity can form larger plastic parts; On the contrary, the size of plastic parts that can be formed is smaller.

 (2) The size of plastic parts is also limited by molding equipment. The size of injection molded plastic parts is limited by the injection volume, mold locking force and template size of the injection machine; The size of plastic parts formed by compression and injection molding shall be limited by the maximum pressure of the press and the maximum size of the working table of the press.

3. What national standard does the dimensional tolerance of plastic parts follow?

GB / t14486-93 standard for dimensional tolerances of engineering plastics and molded plastic parts.

4. What is the apparent quality of plastic parts?

The apparent quality of plastic parts refers to the apparent defect state of plastic parts after molding, such as common material shortage, overflow, flash, depression, air hole, weld mark, crazing, warpage and shrinkage, dimensional instability, etc,

5. What national standard does the surface roughness of plastic parts follow?

The surface roughness of plastic parts shall comply with GB / t14234-1993 standard for surface roughness of plastic parts – surface roughness achieved by different processing methods and materials.

6. What are the factors causing dimensional instability of plastic parts?

Factors causing dimensional instability of plastic parts include:

 (1) The electrical and hydraulic systems of the injection machine are unstable;

 (2) . the feeding amount is unstable;

 (3) Uneven plastic particles and unstable shrinkage;

 (4) . the forming conditions (temperature, pressure and time) change and the forming cycle is inconsistent;

 (5) The gate is too small, the size of multi cavity feeding port is inconsistent, and the feeding is unbalanced;

 (6) The precision of the die is poor, the action of the moving parts is unstable, and the positioning is inaccurate.

7. Why do plastic parts have demoulding slope? What does its size depend on?

As the plastic shrinks after cooling, it will be tightly wrapped on the core of punch core pulling, or the plastic part will cling to the cavity of concave model due to adhesion. In order to facilitate demoulding and prevent scratches on the surface of plastic parts during demoulding, the internal and external surfaces of plastic parts must have a reasonable demoulding slope along the demoulding direction. The size of demoulding slope depends on the performance of plastic parts, geometric shape, such as height or depth, wall thickness and cavity surface state, such as roughness, processing grain, etc. The demoulding slope of hard plastic is higher than that of soft plastic; For plastic parts with complex shape or more forming holes, a larger demoulding slope shall be taken; If the plastic is high and the hole is deep, take a smaller demoulding slope; With the increase of wall thickness, the force of inner hole to wrap the core is large, and the demoulding slope should also be larger

8. What requirements should the minimum dimension of plastic part wall thickness meet?

The minimum dimension of wall thickness of plastic parts shall meet the following requirements: sufficient strength and stiffness; When demoulding, it can withstand the pushing force of the pushing mechanism without deformation; It can bear the fastening force during assembly. The minimum wall thickness of plastic parts varies with the variety of plastic and the size of plastic parts.

9. The role of reinforcing ribs on plastic parts?

The main function of the reinforcing rib is to strengthen the strength and stiffness of the plastic parts without increasing the wall thickness and avoid the warpage and deformation of the plastic parts. In addition, the reasonable arrangement of reinforcing ribs can also improve the mold filling fluidity, reduce the internal stress of plastic parts, and avoid defects such as pores, shrinkage cavities and depressions.

10. Why do plastic parts often use arc transition?

In order to avoid stress concentration, improve the strength of plastic parts, improve the flow of melt and facilitate demoulding, transition arc shall be adopted at the connection of each inner and outer surface of plastic parts. In addition, the circular arc also makes the plastic parts beautiful, and the mold cavity will not crack due to stress concentration during quenching or use.

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