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The process of precision mold plastic injection molding includes preparation before molding, injection process, and post-processing of products.
Preparation before precision mold plastic injection molding:
In order to ensure the smooth progress of injection molding and the quality of plastic parts, factory preparation work should be carried out before injection molding:
(1) Inspection and pre-treatment of raw materials. Before forming, the raw materials should undergo appearance and process performance inspection, dyeing, and drying. The inspection of raw materials includes the determination of color, particle size, uniformity, flowability, thermal stability, shrinkage, moisture content, and other aspects. In order to make the formed plastic parts more aesthetically pleasing or meet the requirements of usage (parts in optical instruments need to be black to prevent light leakage), it is usually necessary to dye the plastic raw materials. Thermoplastics contain more or less water and volatile substances, and an appropriate amount of water and volatile substances can play a plasticizing role. However, when the content of moisture and volatile substances exceeds a certain amount, it can cause defects in the plastic part, and in severe cases, bubbles can be generated, affecting the quality and accuracy of the plastic part. Therefore, it is necessary to dry it at this time. Drying treatment refers to the use of high temperature to reduce the moisture content in plastics. The methods include oven drying, infrared drying, hot plate drying, high-frequency drying, etc. Thermoplastics usually use the first two drying methods.
(2) Preheating of inserts. Due to the significant difference in shrinkage rates between metal and plastic inserts, significant internal stress is generated around the insert during cooling, resulting in a decrease in strength and cracking of the plastic layer around the insert. Therefore, preheat the metal insert before molding to reduce the temperature difference between the insert and the plastic.
For plastics that are less prone to stress cracking during molding and have smaller inserts, preheating is not necessary. The preheating temperature is limited to not damaging the zinc or chromium coating on the surface of the metal insert, usually between 110-130 ℃. For aluminum alloys or copper inserts without coating on the surface, the preheating temperature can reach 150 ℃.
(3) Cleaning of the material barrel. Before injection molding, when the remaining plastic in the injection machine barrel is different from the plastic to be used or has the same color, it must be cleaned.
(4) Selection of release agent. During injection molding, the demolding of plastic parts mainly relies on reasonable process conditions and correct mold design. However, due to the complexity of the plastic parts themselves or the stable control of process conditions, demolding difficulties may arise, so demolding agents are often used in actual production. Due to the different types and forms of injection materials, the structure of plastic parts, the presence or absence of inserts, and usage requirements, the preparation work for various precision molds and plastic parts before molding is not entirely the same.