Six Necessary Conditions for Precision Injection Molding

Precision injection molding refers to an injection molding method in which the size repetition accuracy of the molded product is very high, so that it is difficult to achieve the requirements using a general injection molding machine and conventional injection molding process.

In injection molding design, in addition to considering general mold design issues, the following issues should also be considered in particular:

1. Appropriate mold size and tolerance

1.1 The relationship between product size accuracy and mold size accuracy Draw a product drawing and consider mold design, mold making and molding process.

First, the mold drawing size can be obtained from the product drawing size. According to the mold drawing size, the mold is made to obtain the actual size of the mold. This mold can be used to obtain a molded product and obtain the actual size of the product. The question is how this actual size is within the required size tolerance of the drawing.

1.2 Appropriate shrinkage As mentioned above, even in the same resin with the same pigment, the shrinkage varies depending on the molding conditions. In precision molding, the shrinkage change should be small, and the expected shrinkage and actual shrinkage should be as close as possible. The shrinkage rate is mainly estimated by sorting out the actual shrinkage rate of similar products in the past. There are also cases where the actual shrinkage rate is obtained by using experimental molds, and then the production mold is designed and made after correction. However, it is almost impossible to estimate the shrinkage rate completely and appropriately, and it is inevitable to correct the mold after trial molding. As a result of the correction, the concave part will increase in size and the convex part will decrease in size. Therefore, the shrinkage rate is set to a small value for the concave part size and a large value for the convex part size. When the outer diameter of the gear increases, it cannot mesh, and when it decreases, only the tooth gap increases, so the shrinkage rate should be set to a small value.

2. Prevent the occurrence of fluctuations in molding shrinkage rate

Precision injection molding must be based on the premise that the mold can be made according to the desired size. However, even if the mold size is constant, the actual size of the product varies due to the actual shrinkage. Therefore, in precision injection molding, shrinkage control is very important. The appropriateness of the mold design determines the shrinkage rate, and it also varies depending on the resin batch. If the pigment is changed, the shrinkage rate will also vary. Due to the different molding machines, the setting of molding conditions, reproducibility, and the actions of each molding cycle fluctuate, which causes fluctuations in the actual shrinkage rate, so it is difficult to control the shrinkage.

2.1 Main factors affecting the shrinkage rate The mold size can be obtained by adding the shrinkage rate to the product size, so when designing the mold, the main factors of the shrinkage rate need to be considered.

The main factors affecting the molding shrinkage rate are: resin pressure, resin temperature, mold temperature, gate cross-sectional area, injection time, cooling time, product wall thickness, reinforcement material content, orientation, and injection speed. These effects vary depending on the changes in items such as resin and molding conditions.

• Resin pressure Resin pressure has a great influence on the shrinkage rate. If the resin pressure is high, the shrinkage rate becomes smaller and the product size is large. Even in the same mold cavity, the resin pressure varies depending on the shape of the product, resulting in differences in shrinkage rate. In the case of a multi-cavity mold, the resin pressure in each cavity is likely to differ, resulting in different shrinkage rates in each cavity.

(2) Mold temperature. Whether it is amorphous resin or crystalline resin, if the mold temperature is high, the shrinkage rate will increase. Precision molding requires the mold temperature to be maintained at a specific temperature. When designing the mold, attention must be paid to the design of the cooling circuit.

(3) Gate cross-sectional area. Generally speaking, when the gate cross-sectional area is changed, the shrinkage rate also changes. The shrinkage rate decreases as the gate size increases, which is related to the fluidity of the resin.

(4) Product wall thickness. The product wall thickness also affects the yield. For amorphous resins, the shrinkage rate increases with the different tendencies of the resin to affect the shrinkage rate of the wall thickness. The thicker the wall, the greater the shrinkage rate. Conversely, the smaller the shrinkage rate. For crystalline resins, it is necessary to avoid particularly large changes in wall thickness. In the case of multi-cavity molds, if the cavity wall thickness is different, the shrinkage rate will also be different.

(5) Reinforcement material content  When glass fiber is used to reinforce the resin, the more glass fiber is added, the smaller the shrinkage rate. The shrinkage rate in the flow direction is smaller than the lateral shrinkage rate. The difference is large depending on the resin. In order to prevent distortion and warping, the gate shape, gate position and number of gates must be considered.

(6) Directionality  Although there are large differences in directionality, all resins have directionality. Crystalline resins have particularly large directionality, which varies depending on the wall thickness and molding conditions.In addition, there is post-molding shrinkage. The main factors affecting post-molding shrinkage are: internal stress relaxation, crystallization, temperature, and humidity.

2.2 Measures that can be taken

(1) Runner, gate balance  As mentioned above, the shrinkage rate changes with the resin pressure. In the case of single-cavity molds with multiple gates and multi-cavity molds, gate balance is required to perform the same mold filling. Resin flow is related to the flow resistance in the runner, so it is best to perform runner balance before taking gate balance. (2) Cavity arrangement In order to make it easier to set the molding conditions, it is necessary to pay attention to the cavity arrangement. Since the molten resin heats up the mold, under the general cavity arrangement, the mold temperature distribution is concentric with the gate as the center. Therefore, when selecting the cavity arrangement of a multi-cavity mold, it is necessary to take both the flow channel balance and the concentric arrangement with the gate as the center.

3. Preventing molding deformation

The reason for molding deformation is that there is internal stress under uneven shrinkage, so it is necessary to prevent uneven shrinkage. In the case of a circular product with a hole in the center of the gear, a gate must be set in the center. However, when the shrinkage rate in the flow direction of the resin is greatly different from that in the vertical direction, there is a disadvantage of producing an ellipse. When a higher roundness precision is required, a 3-point or 6-point gate is required.

However, it is necessary to pay full attention to the balance of each gate. When using a side gate, a 3-point gate will increase the inner diameter of the cylindrical product. In the case where gate marks are not allowed on the outer surface and end surface, using fewer inner multi-point evenly distributed gates can obtain good results.

4. Preventing deformation during demolding

Precision products are generally small, with thin wall thickness, and some have many thin ribs. The design of chess sets must take into account that the products are not deformed, and they can be properly demolded. For resins with low shrinkage.

When the molding pressure is high, it is necessary to pay attention to the fact that the products are easy to remain in the mold cavity. . When molding gears with resins with low shrinkage, the cavity of the gear part is preferably designed on the template on the ejection side.

When using ejector pins, it is necessary to pay attention to the number of ejector pins without deformation and the ejection position. Gears with holes require core pins. In this case, in order to facilitate parallel ejection during ejection, they need to be set on the ejection side template. For angular products, punching templates can be used for ejection, and using such templates for ejection can prevent deformation. Generally, precision products have a small draft angle. In order to reduce the demolding force, mirror processing is required, and the grinding direction must be the demolding direction. A block core that is easy to grind should be set according to the demolding direction.

5. Minimum mold manufacturing error

5.1 Appropriate mold structure according to the desired processing method In order to obtain the desired product size with the desired precision, the corresponding mold size must be available, and the mold needs to be processed with extremely high precision, and is limited by the precision processing machinery.

In order to maintain the mold precision, high wear resistance is required, so quenching is required. The precision of quenching molds processed by grinders and EDM machines can reach within 0.01 mm.

When processing molds with EDM machines, it is necessary to pay attention to the increased wear of the electrode end. When processing the gear cavity in Figure 3 and processing with wire EDM machines, the structure that can be processed through should be designed as much as possible.

From the perspective of preventing grinding deformation and shortening processing time, steel with less quenching deformation should be selected, and the shape with less quenching deformation should be designed. When the shape is complex, quenching cooling is not easy to be uniform, and quenching deformation is easy to occur.

5.2 Total split mold

In order to process the quenched parts to a higher precision, a grinder should be used. Therefore, it is necessary to use inserts to form a split mold.

This type of mold has the following features:

(1) Because appropriate materials can be selected, mold materials with appropriate hardness can be used.

(2) Mold materials with high corrosion resistance and wear resistance can be used.

(3) Heat treatment can be performed separately, so it is easy to set heat treatment conditions.

(4) Mold materials with good mirror finish can be used, and mirror finish operation is also easy, so the mirror finish can be improved.

(5) Because it is easy to grind in the direction of mold removal, it is advantageous to use molds with small mold removal angles.

(6) Due to hardening, the mold accuracy retention time can be extended and the mold life is long.

(7) It is easy to set exhaust at any position, so mold filling is easy.

(8) Grinding is easy.

(9) The accuracy of mold parts can be improved, so the accuracy of products may be improved.

(10) The spare mold cavity and core can be made within a small tolerance, so the parts are highly interchangeable and easy to maintain.

(11) Because grinding is the main process, the processing efficiency is high. (12) There are many parts, and the processing accuracy of each part needs to be greatly improved.

(13) Limited to specific processing methods

(14) Use full quenching

6. Prevent errors in mold accuracy

To ensure the positioning of the sliding parts in each cycle, it is necessary to prevent fluctuations in mold accuracy. In order to maintain the accuracy of the sliding parts, the sliding parts should be quenched and ground. The side core sliding part should have a positioning and withdrawal part.