With the rapid development of electronics, telecommunications, medical treatment, cars and other operations, the demands for high precision and high function of plastic products are accumulated day by day, and precision injection molding requires not only high standard precision and low warping deformation, but also excellent optical function.Injection molding is one of the most important plastic forming methods, and how to travel the injection molding technology water. In order to produce high precision plastic products and invent products with high added value, the planning of mould is an important link.

In the planning of injection moulding, besides general mould planning, special consideration should be given to the following items:

1) In order to obtain the required standard tolerance products, it is necessary to consider the appropriate standard tolerance of the die.

2) Consideration should be given to the unswerving shortening rate of seizure prevention.

3) Consideration should be given to preventing seizure and deformation.

4) Consideration should be given to preventing the occurrence of demoulding deformation.

5) Make the faults of die making smaller.

6) Consideration should be given to prevent the die accuracy from being undetermined.

1. Appropriate Die Standards and Tolerances

1.1 Relation between Standard Accuracy of Products and Standard Accuracy of Dies

Draw product drawings, consider die planning, die manufacturing and forming process.

First of all, the drawing standard of the die can be obtained from the drawing standard of the product. According to the drawing standard of the mould, the mould was made and the practical standard of the mould was obtained. The moulded products can be obtained by using this die, and the product practice standard can be obtained. The question is how this practice standard is within the standard tolerance required on the surface.

1.2 Appropriate Shortening Rate

As mentioned above, even in the same resin with the same pigment, the shortening rate varies with the forming conditions. In precision forming, the change degree of shortening rate should be small, and the estimated shortening rate and the practice shortening should be as close as possible. It mainly uses the practice shortening rate of similar products to estimate the shortening rate, and also uses the test model to find the practice shortening rate, which is then corrected and planned to produce the production model.

However, it is almost impossible to estimate the shortening rate properly, and it is inevitable to correct the die after trial forming. Correction effect, concave will increase the standard, convex will reduce the standard. Therefore, for concave standard, the shortening rate will be set at a small value, and for convex standard, the shortening rate will be set at a large value. When the standard of gear outer diameter becomes larger, it can not mesh, and when the gear outer diameter becomes smaller, the clearance becomes larger, so the shortening rate should be set at a small value.

2. The shortening rate of preventing seizure and moulding is not firm.

Precision injection moulding, there is a need to do according to the required standards for the production of molds as a condition. However, even if the die standards are relative, the product practice standards vary with the practice shortening. Therefore, in precision injection molding, the control of the shortening rate is very important. Whether the die planning is suitable or not to allocate the shortening rate varies according to the batch of resin, and if the paint is changed, the shortening rate varies. Because of the different moulding machines, the setting and reproducibility of moulding conditions and the action of each moulding cycle are not determined, and the actual shortening rate is not determined, so the shortening of the operation is difficult.

2.1 Main factors affecting the shortening rate

Die standard can be obtained by product standard plus shortening rate, so in die planning, the main reason why the shortening rate needs to be considered is home.

The main factors affecting the forming shortening rate are as follows:

(1) resin pressure, (2) resin temperature, (3) mould temperature, (4) gate cross-sectional area, (5) injection time, (6) cooling time, (7) product wall thickness, (8) reinforcement material basin, (9) orientation, (10) injection speed. These effects vary with changes in resin and forming conditions.

(1) Resin pressure

Resin pressure has a great influence on the shortening rate. If the resin pressure is large, the shortening rate will be smaller and the product standard will be large. Even in the same cavity, the resin pressure is different due to the different shape of the product, so the shortening rate is different. In the case of multi-cavity mode, the resin pressure in each cavity varies slightly, and the shortening rate of each cavity varies.

(2) Mould temperature

Whether the right or wrong crystalline resin or crystalline resin, if the die temperature is high, the shortening rate will increase. Precision moulding should adhere to the die temperature at a specific temperature. In die planning, there is a need to pay attention to cooling circuit planning.

(3) Generally speaking, the shortening rate also changes when the gate cross-section area is changed. The shortening rate decreases with the increase of gate standard, which is related to the activity of resin.

(4) Wall thickness of products

The wall thickness also affects the yield. For amorphous resins, the shortening rate decreases with the increase of the wall thickness due to the different tendency of the resin to shorten the wall thickness. On the contrary, the shortening rate decreases with the increase of the wall thickness. As for crystalline resins, it is necessary to prevent the wall thickness from changing very much. In the case of multi-cavity mode, it is assumed that the wall thickness of the cavity varies, and the shortening rate varies.

(5) Reinforcement material content

When glass fiber reinforced resin is used, the more the glass fiber is added, the smaller the shortening rate is, and the shortening rate of the moving direction is smaller than that of the transverse direction. According to the difference of resin, in order to prevent misunderstanding of flying warpage, it is necessary to consider the gate shape, flying direction and the number of gates.

(6) directivity

Although there are great differences in orientation, there are orientations for resins. The orientation of crystalline resins is very strong, which varies with wall thickness and forming conditions.

In addition, there is also shortening after seizure and shaping. The main factors affecting the shortening after moulding are: (1) steep internal stress, (2) crystallization, (3) temperature, (4) humidity.

2.2 Available Approaches

(1) Runner, Gate Balance

As mentioned above, the shortening rate is changed by the resin pressure. The phenomena of multi-point gate and multi-cavity mode in single cavity die are discussed.

Gate balance is necessary for the same filling. Resin activity is related to resistance in the runner.