Thermoplastic injection molding – working principle

Functional principle of Thermoplastic injection molding

Plasticizing by screw

Piston injection molding machines were used for the process up to and including 1956. Today, however, so-called screw-piston injection molding machines are common in plastic molding ( Thermoplastic injection molding ). The functional principle of the injection molding process is that plastic granules are usually drawn from a funnel into the screw flights. There they are cut up and sheared. This creates friction heat, which, in conjunction with the heat supplied by a heated cylinder, ensures relatively homogeneous melting of the plastics. These melted plastics then collect in front of the tip of the retreating snail.

Inject into the cavity

The so-called injection phase comes next. The screw’s back is compressed throughout this process, either manually or hydraulically. At pressures between 500 and 2000 bar, the plastic melt is forced out. It passes through the nozzle, which is pressed onto the injection molding tool, the non-return valve, and the sprue into the shaping cavity of the injection molding tool. Alternatively, it may pass through a hot runner system, which is especially typical of contemporary series tools. It’s tempered here. Reduced pressure then keeps applying pressure to the melt until the connection—also known as the sprue—has frozen or solidified.

Demoulding and shot preparation

The forceful injection type enables you to partially offset and make up for the volume loss that happens during cooling. This implies that the final product’s intended surface quality and dimensional correctness may be ensured. After that, the screw starts to twist, preparing the injection molding ( Thermoplastic injection molding ) compound for the next molded item. The thermoplastic molding has time to cool inside the mold. The liquid plastic center, or soul, solidifies. The completed molded part is then automatically expelled when the tool opens.


Usually, upon ejection, the sprue is also immediately removed. It is also feasible to use injection molding without a sprue; however, this requires that the tool’s sprue be made appropriately. These days, the ejected molded pieces are typically sent straight for additional processing or stored in an organized fashion employing handling devices.

Thermoplastic injection molded parts

These days, injection-molded goods come in weights ranging from a few milligrams to almost fifty kilograms. Additives and fillers can be used to optimize the thermoplastics used in injection molding, both for the injection molding process and for the intended application afterwards. Glass beads or glass fibers are examples of very hard inorganic materials that are occasionally used as fillers.

Plastics used for injection molding and possible end products

Vehicle construction uses thermoplastic injection molded ( Thermoplastic injection molding ) parts composed of polyolefins, like polypropylene. This produces things like bumpers and dashboards. For transparent items like taillights or headlights, transparent plastics like Plexiglas (PMMA) and polycarbonate are utilized. These materials can be colored if needed. Injection molding is the main application for polystyrene (PS) and its copolymers, such as ABS (acrylonitrile-butadiene-styrene), in toys and home appliances. In mechanical and electrical engineering, technical polymers such as polyamides (PA) and polyoxymethylene (POM) are utilized. Usually utilized in much lesser quantities, these are mostly found in expensive parts and goods.