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Pretreatment of Plastic Injection Molding Raw Materials

Pretreatment of Plastic Injection Molding Raw Materials

Due to the different supply methods (powder or pellets) and properties of plastics, the structure of the parts (with or without inserts) and the use requirements are different. The preparations for various products before molding are also different, which should be determined according to the specific situation. According to the characteristics of various plastics, the raw materials should be pretreated as follows before the plastic injection molding.


1. Inspection of plastic injection molding raw materials


The inspection of raw materials includes three aspects:


(1) Whether the raw materials used are correct and whether they can meet the use requirements of the product (variety, specification, brand, etc.).


(2) Appearance inspection (color, particle shape and uniformity, with or without impurities, etc.).


(3) Physical property inspection (including melt flow rate, fluidity, thermal stability, water content index, shrinkage rate, etc.).


2. Granulation and dyeing of plastic injection molding raw materials


If the raw material is powder, it needs to be granulated before molding. If plastic injection molding products require a certain color, the raw material needs to be dyed. The most common method for coloring injection molding parts is to use the color masterbatch for coloring. That is, mix the raw material particles and the color masterbatch evenly in a certain proportion, and directly add them into the hopper of the injection molding machine.


The method is simple and practical and can make the color uniform. However, the cost is high, and it is only suitable for the molding of the screw injection molding machine. If a plunger injection molding machine is used, it will cause color spots or color patterns on products due to uneven plasticization and mixing. For plastic injection molding with powder as the raw material, granulation and dyeing are generally used. That is, the powder and color masterbatch are extruded and granulated to obtain granules with uniform color.


3. Preheating and drying of plastic injection molding raw materials


Various plastic particles often contain different degrees of moisture, solvents, and other volatile low molecular substances. Their existence often causes defects such as silver streaks, shrinkage holes, and air bubbles in injection molding products. The high temperature will also cause cross-linking and degradation of plastics, which will result in reducing the product performance and appearance quality. Therefore, before the plastic injection molding, most plastics need to be preheated and dried.


Plastics such as polyamide, polycarbonate, polysulfone, polymethyl methacrylate, polyphenylene ether, etc. have a tendency to absorb moisture because their molecular structure contains amido groups, ester groups, nitrile groups, and other hydrophilic groups. Therefore, they should be dried in general. Although the hydrophilic ability of plastics such as polystyrene and ABS is not strong, they generally need to be dried as well. For polyolefin, polystyrene, polyvinyl chloride, polyoxymethylene, and other plastics that are not easy to absorb moisture, if the packaging is tight and the storage and transportation are good, it is generally not necessary to dry.


There are many drying methods, which should be selected according to plastic properties, production batches, and specific drying equipment. Small batches are mostly dried in a hot air circulation oven and infrared heating oven. Whereas, boiling drying and airflow drying are commonly used in large batches. Plastics that are easily oxidized and discolored at high temperatures, such as polyamides, should be dried in a vacuum oven. Drying temperature, time, and material layer thickness are the main factors affecting the drying effect.


The higher the temperature is, the faster the low molecules and water volatilize. However, the softening temperature or melting point of the plastic cannot be exceeded, the material will easily become soft and agglomerated, which will make feeding difficult. The drying temperature is generally around 100°C. A longer drying time is beneficial to improve the drying effect, but it is easy to cause energy waste. And it will also cause decomposition and discoloration of plastics with poor thermal stability. If the material layer is too thick, due to the poor thermal conductivity of the plastic, the drying effect of the surface layer and the center layer will be different under the same drying conditions.


Therefore, the thickness of the material layer should be 20-50mm. It should be noted that the pellets should be properly sealed and stored after drying to prevent the plastic from absorbing moisture again and losing its drying effect. Some plastics that are particularly sensitive to moisture at the plastic injection molding temperature should also consider the use of a sealed or heated hopper during the molding process.

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