Plastic injection is one of the most common manufacturing methods to produce parts in polymeric material. It is one of the fastest and most cost-effective processes for producing components that require precise dimensional tolerances, which is why it is widely used in various industries. The primary materials for plastic injection are thermoplastics, which include polyethylene (PE), polypropylene (PP) and polycarbonate (PC). The final thermoplastic selection is dependent upon the requirements of the manufacturer, including the desired properties and design of the component.
The plastic injection molding process involves the injecting of melted plastic into a mold to form a product with a predetermined fixed form. The product is then cooled, solidified and released from the mold. The resulting product will usually have a parting line, sprue, gate marks and ejector pin marks. These features are unavoidable due to the nature of the process and are typically caused by misalignments, wear, gaseous vents or clearances between adjacent molded parts contacting the injected plastic.
To produce a plastic injection part, the design of a mold is first developed. The mold is made of two primary components, the injection mould and the ejector mould. The injection mould consists of the A plate and B plate, which are machined to contain a cavity that corresponds to the shape of the finished product. The injection mould also includes a sprue bushing that seals against the nozzle of the injection machine to allow molten plastic to flow through and fill the cavity. The sprue bushing is then guided to the injection mould cavity image via channels that are machined into the A and B plates, referred to as runners.
After the sprue is filled with plastic and the injection mould closed, the screw draws back, releasing pressure and allowing the plastic to cool in the mold. The cooling time can be anywhere from milliseconds to minutes, and is critical for ensuring that the plastic sets correctly in the injection mould before being ejected out of it.
The heat transfer during the injection process is a critical factor in the formation of the molded plastic. However, after the injection process is complete and the molded plastic has cooled, it is necessary to remove the remaining heat from the plastic to prevent warping and decomposition of the plastic.
Moisture in the molded plastic can cause voids, discoloration or structural weakness in the injection molded part, and these defects will result in poor performance and appearance and/or scrap. Consistent moisture removal is especially important for the processing of hygroscopic resins, such as PET, polycarbonate, and ABS.
Jetting is a snake-like stream of molten plastic that can occur when the injection speed is too high, the tool is improperly designed with gates or sprues, or there are excessive walls in the injection mould. Other causes of jetting are shear heat burning the molten plastic before injection, poor runner and gate designs, a small sprue size, excessive injection speeds, and inadequate clamping force.