Various Methods of Hollow Blow Molding in Plastic Products Production
As mentioned in the last article, blow molding method can be divided into two categories, extrusion blow molding and injection blow molding.
1) Extrusion blow molding
This method is very simple, high yield and wide range of applications. About three quarters of blow molding products are made by extrusion blow molding.
The control of mold wall thickness is very important for improving product quality and reducing production cost during the blow molding process. Through the impact resistance test, it is found that the products with uniform wall thickness not only use less material, but also have higher impact resistance. Modern wall thickness control system has been equipped and utilized servo oil pressure system and computer to form multi-stage control, thus the factory greatly improves the products’ quality, also increase output and shorten the cooling time of finished products.
2) Injection blow molding
Products made by injection blow molding is accurate in size. And they are no need for secondary processing, with stable weight, uniform wall thickness, no scraps and flying edges. Because the injection billet has the bottom, there is no abutted seam at the bottom of these products.
Although injection blow molding can produce the goods with great appearance and high strength, it can not be used for producing large sized products (generally below 4L) or the products with handles, due to the large investment of equipment and mold. It is mostly used for the mass production of small plastic containers according to its high productivity. Besides, the suitable plastics for this method are polystyrene (PS), polypropylene copolymer (PP) and polyethylene (PE).
The main factors that will affect injection blow molding process are temperature, pressure and time. To ensure the product quality, the engineers need control the temperature of four parts, including injection machine, mold billet, mandril in mold billet and blow mold. And adequate injection pressure, holding pressure and blowing pressure can lead to the decreased defects of products.