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  • What is the cause of poor mold release in silicone injection molding process?

What is the cause of poor mold release in silicone injection molding process?

Usually after silicone injection molding process, the plastic parts need to be separated from the mold cavity. However, sometimes it is difficult to detach the plastic parts from the injection mold, which will cause patterns or scratches on the surface of the plastic parts, and in serious cases, the plastic parts may be deformed or even broken. So what are the reasons that cause the silicone injection molding process not to release smoothly? Here I will take you to understand.

1. Mold failure

Mold failure is one of the main reasons for poor demolding, which is caused by rough surface of the mold cavity, and mold wear scratches or too large gaps at the inserts. If the mold cavity and runner surface defects such as indentations, dents, etc., the plastic parts are easy to adhere to the mold, resulting in release difficulties, so you should try to improve the surface finish of the runner, cavity surface chrome plating.

2. Improper control of process conditions

Process conditions control improper specific nozzle temperature is too low, the cooling time is too short and injection material cut off, will cause the mold is not wave, so in the exclusion of sticky mold and mold release failure, should be appropriate to reduce the injection pressure, shorten the injection time, reduce the barrel and melt material temperature, extend the cooling time, as well as to prevent the melt material cut off.

3. Strong state of decompression

When the plastic part is a box-shaped part without hole on the bottom side and has a flange folded, because the flange folded part is in close contact with the cavity, the decompression state is stronger than the dynamic mold, and the plastic part is easy to stay in the fixed part of the mold. If a groove is made in the core to force the plastic part to stay on the dynamic mold, the product and the core are closely attached to form a decompression state, and when the top bar ejects the plastic part, the phenomenon of top white or whitening will occur. In this case, the plastic parts should be ejected from both sides of the moving mold and fixed mold with air valves, which is especially necessary for deep plastic parts such as cylinders and baskets.   

4. Overfilling

When molding with excessive injection pressure, the molding shrinkage rate is smaller than expected, and it becomes difficult to release the mold. In this case, if the injection pressure is reduced, the injection time is shortened, and the temperature of the melt and mold is lowered, it becomes easy to release the mold. In this case, it is more effective to use a mold release agent that reduces the friction between plastic and mold. For the mold, it is also effective to improve the brightness, eliminate the side wall convexity, honing, and increase the top bar. When forming deeper parts, blowing compressed air between the mold and the part is more helpful to release the mold.

5. Improper use of mold release agent

The purpose of using mold release agent is to reduce the adhesion between the surface of the plastic part and the surface of the mold cavity to prevent them from sticking to each other. As the release effect of the mold release agent that is affected by the chemical effect, also affected by physical conditions, and, molding raw materials and processing conditions are different, the selected release agent species and dosage must be determined according to the specific circumstances, if used improperly, often can not produce a good release effect.

6. Parts stick to the static mold

There are two reasons for this, namely, the nozzle and the cavity on the place where there is stuck, or static mold release resistance is greater than the dynamic mold, so that the parts stick in the static mold. Due to the resistance between the nozzle and the cavity and caused by the static mold on the situation are: nozzle corner radius R than the corresponding radius of the mold R larger, clamping the mold so that the nozzle and the mold is not centered, or nozzle and mold between the leakage of plastic, etc.. Any one of these situations will jam the part and make the part stick to the static mold. To prevent this from happening, the mold should be installed correctly. The reason for high release resistance in static molds is due to low finish or convexity of the side walls. In this case, a Z-type pulling bar should be installed on the side of the moving mold to pull the parts. And in the mold design, it is necessary to consider fully not to happen this phenomenon. The parts in the moving and static mold on both sides with - a certain temperature difference is also effective.

7. Raw materials do not meet the requirements of use

If the raw materials in the packaging and transportation mixed with impurities, or pre-drying and preheating process of different grades of raw materials mixed, as well as the barrel and hopper mixed with foreign objects, can lead to plastic parts sticky mold. In addition, the raw material particle size is not uniform or too large on the sticky mold also has a certain degree of impact, therefore, for the molding of raw materials should do a good job of purification screening work.
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