Injection Molding Processes

Over 30% of all plastics are converted using the injection molding process, which provides the capability to mass-produced intricate parts in a precise manner. When injection molded, polyethylene provides advantages over other polymers including relatively low cost, ease of recycling, good low temperature impact strength, and easier processing.

Injection molding has been one of the most important fabrication tools for the plastics industry since the reciprocating screw machine was patented in 1956. Today, it's almost impossible to do anything without using injection molded parts. They are used in automotive interior parts, electronic housings, housewares, medical equipment, compact discs, and even doghouses. Injection molding is used to fabricate pallets, toys, crates, and pails, thin-wall food containers, promotional drink cups, lids, and milk bottle caps.

The injection molding process involves melting the plastic in an extruder and using the extruder screw to inject the plastic into a mold, where it is cooled. Speed and consistency are vital keys to running a successful injection molding operation, since profit margins are normally below 10 percent.

Speed
A molder will maximize output by minimizing cycle time which is the amount of time that is taken to melt the plastic, inject it into the mold, cool, and eject a finished part.

Using larger molds that produce more than one part each time the machine performs a cycle can also increase output. These molds are known as multiple cavity molds.

Consistency
Consistency, or elimination of scrap and downtime, is just as important as output in a successful molding operation. The most consistent processing results from careful control of plastic temperature, plastic pressure as it fills the mold, the rate at which the plastic fills the mold, and the cooling conditions. These four primary molding variables are interdependent and can often be used to understand process changes and solve problems. While the variables apply to almost all injection molding processes, the process will be slightly different in each shop, depending on the application, the plastic being used, and the molder's preferences.

Fill rate
In thin wall applications, the material must be injected into the mold as quickly as possible to prevent the plastic from freezing before the part has been completely filled. The newest resin and machine technologies in this area almost always focus on faster, easier fills. In addition to minimizing cycle time through better filling ability, the molder could realize resin cost savings through the ability to fill thinner molds or achieve higher outputs by using larger, higher cavity molds.

Thin wall molding is accomplished using machines that inject the material in less than one second and are big enough to support large, multiple cavity molds. Thin wall lids and containers tend to be small, so molds may be used to fabricate over 100 small lids at a time.