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Defects and Solutions of Multi-material Injection Molding

by:Vowin Rapid Prototyping     2023-02-16
The primary mold of multi-color and multi-material injection molding is exactly the same as that of ordinary injection molding PP, ABS, PC, PA, etc. The injection molding of the secondary mold of Shanghai Mulan Injection Processing Factory generally uses TPE material, so it is necessary to master the processing characteristics of TPE. Adhesive strength, sealing quality, etc.

1. Processing equipment

TPE can be molded with a standard thermoplastic injection molding machine equipped with a 3-zone screw. The screw compression ratio is at least 2:1, and the length-to-diameter ratio is at least 20:1. When adding dark TPE color masterbatch, if it is necessary to increase the plasticization rate and change the dispersion effect of the color masterbatch, a barrier screw is required. Open nozzles are generally used during processing, and the use of self-locking nozzles and check valves at the end of the screw will facilitate injection molding.

2. Melting temperature

When processing TPE, adjust the barrel temperature so that the melting temperature is between 180~20°C. Starting from the feeding section of the feeder, each section gradually increases by 10-20°C until the melting section reaches the desired value. The temperature of the self-locking nozzle should be equal to or lower than the temperature of the last section of the barrel by about 10°C. If the melt is overheated, there will be a peculiar smell, which means that the raw material is decomposed by heat.

Generally, for parts coated with TPE, the amount of TPE is small. If the blank is large, resulting in a large secondary mold, a larger injection molding machine must be installed, so that the residence time of TPE in the barrel will be very long, so the set barrel temperature should be lower than the recommended temperature of TPE. The temperature should be determined according to the specific conditions during injection molding. Every time the material is recycled, the barrel temperature should be reduced by 5~10°C.

3. Mold temperature: The mold temperature is generally between 25~40°C. If it is a thin-walled product, it is recommended that the mold temperature be between 40~60°C.

4. Injection molding pressure and speed: In order to obtain a longer injection process, high injection pressure and speed are required when injecting TPE alone, especially when thin-walled products are formed by injection molding. When TPE is injected twice, try to use a lower injection pressure. Pressure and speed to prevent deformation of the primary blank in the mold and control TPE flash.

5. Back pressure and screw speed: It is generally recommended that the back pressure is 20~50bar, and the screw rotation speed should be between 25~75r/min.

The impact of higher back pressure is as follows: ①It can change the dispersibility of color masterbatch and other additives; ②Improve the uniformity of plasticization; ③The plasticization time is increased at a certain screw speed; ④The uniformity is also positive when the screw speed is reduced. influences.

The impact of lower back pressure is as follows: ① resulting in insufficient shear energy and uneven plasticization; ② components with higher melting points will not be completely plasticized.

6. Holding pressure and time: After filling the mold, in order to avoid dents and ensure 100% filling of the mold, less holding pressure can be used. Keep the holding time as short as possible.

The impact of too high holding pressure is as follows: ①The mold is too full; ②It is not easy to demould; ③There will be stratification at the gate; ④The inlet is prone to deformation.

The possible effects of low holding pressure are as follows: ①There are dents in thick-walled parts; ②The shrinkage rate increases.

7. Adhesive effect of secondary molding

In order to ensure a good bonding effect between the primary part and the secondary TPE, Shanghai Mulan Injection Molding Factory should pay attention to the following operations:

①Inject TPE at a higher speed to make the melt disperse quickly. ②The melt temperature of TPE should be appropriately high to ensure that the joint surface of the semi-finished product can be melted. ③Mold temperature should be between 40~60℃. ④Because the holding pressure will cause the displacement of the mutual joint surface, it is usually not necessary to hold the pressure. ⑤ Do not use any release aids, such as release agents, etc. ⑥ Use a mold with TPE surface etching. ⑦ Semi-finished products processed after intermediate storage should usually be preheated before encapsulating or covering TPE. The preheating process does not take a long time, and only the surface of the product needs to be heated. 100℃).

8. Control of one-time forming blank

The surface of primary parts must be free from oil and dust. Controlling the stability of injection molding product quality, including product size and product deformation, is generally achieved through stable injection molding process conditions and product weight control. Whether the quality of the first piece is stable or not will directly lead to the product quality and scrap rate during the second lagging.

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