Unreasonable structure of plastic part will cause difficulties in mold manufacturing and plastic part forming; mold engineer should propose an improvement plan for plastic part structure, and notify product designer for confirmation.
When receiving customer information, necessary processing shall be carried out on the information.
According to customer data, analysis of plastic part structure mainly includes following aspects: (1) Requirements of injection molding process for structure of plastic parts; (2) Requirements of mold for structure of plastic parts; (3) Requirements of product assembly for structure of plastic parts; 4) Surface requirements.

Process problems such as shrinkage, depression, air drying, air trapping, deformation, scorching of rubber part are related to influence of factors such as local rubber thickness, gate setting, and cooling of rubber part. Processability of structure of rubber parts should be analyzed from following aspects.

Wall thickness of rubber part should be uniform, avoid sudden changes and design of section thickness disparity, otherwise it will cause uneven shrinkage and cause defects on the surface of rubber part.
Wall thickness of rubber part is generally in the range of 1 to 6mm, and the most commonly used wall thickness is 1.8 to 3mm, which depends on type and size of rubber part.
For plastic parts of built 3D model, use Pro/E for cross-sectional analysis, and uneven wall thickness of plastic parts can be found. Steps are: Analysis ® Model Analysis ® Thickness ® [Given maximum and minimum glue thickness, select starting point and end point of analysis, determine parallel section corresponding to analysis]®Compute, as shown in Figure 3.1.1 and Figure 3.1.2.

 

In addition, wall thickness of rubber part is also closely related to melt filling process; process refers to distance from gate to cavity. Under normal process conditions, size of process is directly proportional to thickness of rubber part. The greater thickness of plastic part, the longer maximum flow allowed. Relations or diagrams can be used to check possibility of plastic parts forming.
Wall thickness of rubber part is 2.5mm, under normal forming conditions, and flow of commonly used materials is as follows:
ABS: process 220 mm; PC: process 120 mm; HDPE: process 280 mm; POM: process 180 mm.
Common problems caused by uneven wall thickness:

1. Local thick glue position is shown in Figure 3.1.1, which is prone to surface shrinkage and depression.

2. As shown in Figure 3.1.2, thin rubber position on both sides of rubber part is prone to forming stagnation.

3. Stop position is shown in Figure 3.1.3. Glue thickness adopts a gradual method to eliminate white printing on the surface; in addition, inner corners of rubber parts are rounded to make wall thickness uniform.

 

4. As shown in Figure 3.1.4, if concave position in the middle of rubber part is too deep, actual formed rubber part will produce arch deformation; solution to deformation is to reduce concave depth and make wall thickness as uniform as possible.

 

 

5. As shown in Figure 3.1.5, surface of sharp corners is easy to produce baking. Way to avoid baking is to add rounded corners.

Role of bone position of rubber part is to increase strength, fix bottom shell, support frame, and button guide. Since joint between bone position and shell of rubber part is prone to shrinkage and depression; therefore, thickness of bone position should be less than or equal to 0.5t (t is wall thickness of rubber part), and thickness of bone position is generally in the range of 0.8-1.2mm.

 

When bone depth is more than 15mm, it is easy to have difficulty in running glue and trapped air. Inserts can be made on mold, which is also convenient for mold saving and exhaust.
If bone depth is less than 15mm, draft angle should be 0.5˚ or more; if bone depth is more than 15mm, thickness difference between root and top of bone should not be less than 0.2mm, as shown in Figure 3.1.6.
In order to improve flow conditions of some deep bones, rubberized rice pads are added to bone position; as shown in Figure 3.1.7, horn bones are added with rubberized rice pads, and insert is made to mold.

 

Selection of location of plastic parts gate and gating form will directly affect quality of plastic parts and whether injection process can proceed smoothly. Gate location and form of rubber parts should be analyzed and determined; gates already determined in customer's rubber parts data should also be analyzed and suggestions for improper parts should be made.

 

Principle of setting gate is as follows:

1. Ensure that flow front of rubber material can reach end of cavity at the same time, and make process the shortest, as shown in Figure 3.1.8;

2. Gate should be fed from thicker part first to help maintain pressure and reduce pressure loss;

3. If there are small cores or inserts in cavity, gate should avoid direct impact to prevent deformation;

4. Location of gate should be at a position where plastic part is easy to remove, and it is convenient to trim and does not affect appearance of plastic part, as shown in Figure 3.1.9;

 

5. Facilitate exhaust in the cavity, so that gas in cavity squeezes into vicinity of parting surface;

6. Avoid "track" effect of rubber material flow, causing rubber parts to produce trapped air and weld marks;

7. Avoid phenomenon of air drying and snake pattern at the gate, as shown in Figure 3.1.10, Figure 3.1.11, and Figure 3.1.12;

 

8. Inflow direction of rubber should be such that when it flows into cavity, it can flow evenly along parallel direction of cavity to avoid anisotropy of rubber flow and cause rubber parts to warp, deform and stress crack, as shown in Figure 3.1.13 and Figure 3.1.14.

 

For some rubber parts with complex filling and flow of rubber material, as well as a mold with multiple cavities or multiple finished products, as shown in Figure 3.1.15, location and size of gate can be determined by CAE (Moldflow software) analysis solve.