Injection molding has been one of the most important manufacturing tools for the plastics industry since the reciprocating screw machine was patented in 1956. Nowadays it is practically impossible to do anything without injection molded parts. They are used in car interiors, electronic device covers, household items, medical equipment, CDs and even dog houses. Injection molding is used to make pallets, toys, drawers, and buckets, thin-walled food containers, promotional cups for beverages, and milk bottle lids.
In the injection molding process, the plastic is melted in an extruder and the extruder screw is used to inject the plastic into a mold where it is cooled. Speed and consistency are key elements for the injection molding operation to be successful since the profit margins are generally below 10 percent.
A molder will maximize production by minimizing the cycle time, which is the amount of time needed to melt the plastic, inject it into the mold, cool it and extract a finished part.
Using larger molds to produce more than one part each time the machine cycles can also increase production. These molds are known as multiple cavity molds.
Consistency, or elimination of scrap and unproductive time, is as important as production in a successful molding operation. The most consistent processing is the result of careful control of the temperature of the plastic, pressure as it fills the mold, the speed at which the plastic fills the mold and cooling conditions. These four primary molding variables are independent and can often be used to understand changes in the process and solve problems. While the variables apply to virtually all injection molding processes, the process will be slightly different in each business, depending on the application, the plastic used and the preferences of the molder.
In thin-walled applications, the material must be injected into the mold as quickly as possible to prevent the plastic from hardening before the part is completely filled. In general, the most recent resin and machinery technologies in the area are concentrated in faster and simpler fillings. In addition to minimizing cycle time through improved filling capacity, the molder can save on the cost of resins through the ability to fill thinner molds or achieve better production by using larger molds of higher cavities.
Thin wall molding is achieved by using machines that can inject material in less than a second and are large enough to support large molds and multiple cavities. Thin-walled tops and containers tend to be small, so the molds can be used to make more than 100 small lids at a time.