Molding methods for thermoplastic resins – extrusion, blow molding, vacuum, and compressed air molding

2024-09-06

Plastic products can be found everywhere and have become an indispensable thing in our lives. Plastics include thermoplastic resins that soften when heated and thermoset resins that harden when heated. Injection molding is the most commonly used molding method for thermoplastic resins, but there are many other molding methods for thermoplastic resins. In this article, we will focus on thermoplastic resin molding methods other than injection molding.

The basis of thermoplastic resin molding

The basic molding process of thermoplastic resin is mainly "heating and melting", "molding" and "cooling and solidifying". This process is common to all molding methods. The difference is what method is used in the "molding" stage. For example, in the injection molding process, resin in a molten state is injected into a mold. You should be able to understand the molding methods described below if you focus on "what method is used to mold them?"

Extrusion

Extrusion is a method of extruding resin that has been heated and melted into shape like vermicelli. No matter which side is cut, it is possible to create a product with the same shape. In extrusion, the part of the mouth through which the resin is extruded (i.e., the slit part of the extruder) is called the mold. Since the cross-sectional shape of the extruded product depends on the shape of the mold, it is possible to manufacture various products by changing the shape of the mold. When making rod-shaped products such as tubes, round or square molds can be used; When making flat products such as sheets or films, slit-shaped molds can be used. Extrusion of more than two resins by multiple extrusion machines allows multiple layers of product to be formed through a single mold, or resin is squeezed around a pre-prepared core material to cover the core material.

Blow molding

Blow molding is a method of blowing air into a heated and softened resin to make the resin expand and mold. In the process of blowing glass, molten glass is wound around one end of a metal tube and then blown from the other end to expand it into shape. The same principle applies to blow molding. There are several types of blow molding, and the most representative method is "extrusion blow molding". In the extrusion blow molding process, the heated molten resin is extruded into a cylindrical shape (parison), which is then placed in a blow molding mold, and when the parison becomes soft, compressed air is blown into the inside of the parison to make it expand and come into close contact with the mold. Another method of blow molding is "injection blow molding", in which a test tube-shaped parison is made by injection molding, and then the parison is placed in the mold and air is blown in. Injection blow molding is generally used to make plastic bottles, etc.

Vacuum forming, compressed air forming

Vacuum forming and compressed air molding are formed by softening the extruded resin sheet by heating and then using air pressure to make it come into close contact with the mold. In vacuum forming, the resin sheet and the mold are in a state of depressurization, and the resin and the mold are in close contact. On the other hand, in the case of compressed air molding, molding is carried out by pressurizing the resin sheet with compressed air so that it is in close contact with the mold. Because the compressed air molding method exerts a higher pressure on the resin sheet than vacuum forming, it is possible to create products with more complex and sharp shapes. Vacuum forming and compressed air forming are low-cost and versatile, such as egg crates and food trays, automotive interiors and bumpers, medical device housings, etc.