Plastic injection molding is an industrial process used to produce a large variety of commonplace items, from household goods like kitchen utensils and plastic tableware to toys and automobile parts. The process involves creating a design that will be molded, then building a mold to fit the final design. The mold is made of a sturdy material like aluminum or steel, and it must be designed to withstand the huge amounts of pressure that will be exerted on it during the injection molding process. It may seem daunting for someone not familiar with the process to think about creating a custom plastic injection mould, but a qualified designer or engineer can help.
There are two main ways that moulds are built: standard machining and CNC machining. CNC machining offers quicker build times and more precision than standard machining, which can lead to lower production costs. The number of cavities that are incorporated into the mould has an impact on overall cost too; more intricate mould details will increase initial manufacturing costs.
The type of plastic that will be used in the finished product also has an effect on the final production cost. Thermoplastic plastics are most commonly used for injection molding. These are polymers that turn molten under high temperatures and then solidify again when cooled. Other types of plastic that can be used for injection molding include thermoset plastics such as epoxy and phenolic. These are typically more expensive to work with, but they can be very durable and resistant to abrasions.
Using the right plastic resin is vital to the final quality of the injection molded part. It must be free of internal moisture to prevent voids, discoloration and structural weakness in the finished product. Moisture is removed during the injection molding process by circulating coolant through passages in the injection molding machine and a temperature control unit (TCU). This system circulates the coolant at a consistent rate to remove heat from the molds, keeping them at the correct, constant temperature for processing.
Once the molten plastic is inside the mould it must sit there for a period of time known as the holding time. This will vary between thermoplastics and the complexity of the mould and can range from milliseconds to a few minutes. Once the holding time is complete the screws in the injection molding machine will pull back, allowing the melted plastic to be released from the mould.
The ejector pins in the mould will then push the injected plastic out of the mould and into an ejection tray. From here the injected plastic will be pushed into a cooling bath, where it will continue to cool until it is hard enough to be removed from the injection molding machine.
This entire process takes a very short amount of time, usually no more than a minute, to produce multiple copies of the same plastic product. Reducing cycle times allows manufacturers to save on production costs by producing more products in less time, and reduce inventory storage and changeover costs. Quick Mold Change Systems help to speed up the resetting and changing of the injection mould, which again lowers production costs.