Plastic injection molding is an efficient manufacturing process that allows you to produce large volumes of products in a relatively short amount of time. It’s also one of the most economical forms of plastic manufacturing, offering a low per-piece cost. The injection molding process uses a heated liquid thermoplastic, which is then injected into a pre-designed metal mold to shape the plastic into the desired product. The injection molding process is ideal for a wide variety of materials, including thermoplastics and thermoset polyurethanes.
Injection molding is often a more cost-effective option than other types of manufacturing, such as CNC machining. Additionally, there is little waste when using the injection molding process. The melted scrap plastic that isn’t injected into the mold can be remelted and reinjected into the same plastic mould to create additional parts, further reducing your overall production costs.
A key component of the plastic injection molding process is consistent moisture removal. Moisture can cause voids, discoloration and structural weakness in injection molded parts, which will result in poor performance and appearance. Moisture can also degrade the original characteristics of the hygroscopic plastic used in the injection molding process, such as its strength and color stability.
The process of injection molding starts when operators load plastic granules into the hopper of the injection machine. The granules then enter the barrel through a reciprocating screw, where the heat from the machine’s barrel and screw heaters melt the granules into a molten plastic. The molten plastic is then injected into the mould via a sprue bushing or gate. The sprue bushing directs the molten plastic into the cavity images, or runner channels, which are machined on the faces of the A and B plates. The sprue bushing must be designed to seal tightly against the nozzle of the injection machine’s barrel.
Once the sprue and runners have been filled with molten plastic, the injection machine’s injection speed and dwell pressure are regulated to ensure a smooth, even flow of melted plastic. The injection machine will usually change from injection speed control to dwell pressure control when the runner is 95-98% full of molten plastic, so that the molten plastic has enough time to fill out the remaining areas of the mould.
After the molten plastic has filled out all of the cavities in the mould, the injection machine will shut down, unclamp and open the mould to remove the injected part from the custom plastic injection mold. Once the molded plastic is ejected from the plastic injection mold, the machine will then close the mould, readying it for the next shot.
While a custom plastic injection mold can be expensive to design, build and test, it will last for many years in the factory and save you time and money on future projects. Plastic injection molding can also be more complex than other manufacturing processes, such as CNC machining, allowing you to include features in your product that would be impossible or extremely costly to create with other methods.