There are two types of plastics. One is called thermosetting resin which does not soften again once it is formed and hardened, and the other is called thermoplastic resin which becomes soft or hard when its temperature rises or falls. Although thermosetting resin has an older history, the majority of the presently used plastics are made of thermoplastic resins.
There are several types of plastic forming method:
(1) In general, material arrangement and coloring are carried out in separate process.
(2) The drying process may be required for some plastics but not required by others.
(3) Materials are supplied to a molding machine from a hopper.
(4) Materials are heated to be plasticized with a heating cylinder. There are various methods for this process. The most popular method is as follows: The screw in-line feeds materials forwards while the screw itself goes rearwards, and when a specified volume of plastic material for plasticity is completed, the screw is moved forwards and a plastic resin is injected and charged into a closed die to mold a shape.
(5) The resin that has filled up the inside of the cavity of die is cooled in the die and becomes solid
(6) The die is opened to take out molded products. Parts other than products (spure,
runner, fin, defective products, etc.) which are produced by this process are regarded as non-conforming products and they can be used again as molding materials.
Although there is no difference from the injection molding until a material is supplied, when the material is heated plastic in the heating cylinder, the position of screw is fixed, in general. Therefore, a resin which is made plastic is discharged continuously from the die. The discharged resin is molded into the basic shape and finally formed with the sizing die and cooled and solidified. The receiving equipment serves an auxiliary function to receive extruded products. The products are cut or wound up according to their characteristics and purpose of use.
There is no difference from the injection molding and extrusion molding until a material is carried into the hopper. The blow molding is a method which clamps a cylindrical material called “parison” with split molds and blows air inside to blow it up and press it onto the inner wall of the die
The parison is a word which is used only for the blow molding and indicates a material in the form of a tube, pipe (bottomless or bottomed) or a pair of sheets before the material is blown up during the blow molding
There are two molding methods for this parison; one is the direct blow method, in which the material is pushed out in the form of a pipe from the extrusion molding machine and the other is the injection blow method, in which an injection molding machine is used to form the material into a bottomed parison like a test tube.
In the case of the injection blow, the method called “extension blow”, in which the material is blown up not only in lateral direction but also in longitudinal direction for improvement in physical properties.