The Plastic Injection Process

The injection molding process is the most common way to produce a wide range of different plastic parts. It involves a high-speed, fast-curing process that uses a variety of thermoplastics and thermosets. The plastic resins are injected into a mold that is closed, then allowed to cool and solidify. The resulting part is then removed from the mold and can be further processed into a finished product. The injection molding process is extremely efficient, producing a large number of identical plastic products in a short time period. It is also very economical compared to alternative manufacturing processes such as blow molding or extrusion.

The plastic injection process involves several stages including clamping, injection, cooling, ejection, and trimming. A hydraulically powered clamping motor actuates the mold’s clamping bars that secure the two halves of the clamshell-style mold together. The clamping pressure, referred to as tonnage, must be sufficient to keep the mold securely closed while the molten plastic is injected and subsequently cooled. Clamping forces that are too weak can lead to leaks of the molten plastic, development of flashes, and inconsistent parts. Forces that are too strong can cause damage to the mold and equipment. Fractured hydraulic cylinders, cracked platen and mold plates, and crushed ejector pins are some of the possible defects caused by improperly set clamping pressure.

Once the two mold halves are secured, the molten plastic is injected into the cavity through the sprue and runner channels of the injection unit. A gate, which resides at the end of each runner channel, directs the flow of the melted plastic into the cavities. The sprue, runners and gates must be carefully designed to minimize entrapped gasses that can cause premature part rejection during the cooling and solidification stages.

After the molten plastic is injected, it must be left to cool and solidify into the desired shape of the final product. During the cooling stage, some shrinkage of the plastic may occur. The cycle time of the injection molding process varies and is influenced by the size of the part, the maximum wall thickness, and several thermodynamic properties of the plastic material.

A reciprocating screw in the injection machine pushes and rotates a barrel containing thermoplastic pellets, melting them by friction with the shaft. The melted plastic is injected into the mold at thousands of pounds of pressure through the injection nozzle. The injection process requires a precise injection pressure and controlled temperature to ensure consistent, high quality production. The injection pressure must be sufficient to fill the cavity completely while compensating for thermal shrinkage and other factors. The cured thermoset materials used in injection molding require special treatment after the injection process to prevent cracking, dripping, and warping. A trimming process is usually required to cut off the excess plastic materials from each molded part before they are separated. This step can be performed manually or through automated trimming systems. Some manufacturers utilize a combination of both manual and automatic machining to ensure quality control.plastic injection

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