The Plastic Injection Molding Process

Update: 01-11-2019

The plastic injection molding process is a manufacturing method for producing plastic parts. Before the process begins, an experienced mold maker must construct a mold or tool in order to produce a part. The construction of the mold will include two halves (core and cavity) and contain all the geometry and features that make up the part specifications.


Once the mold is constructed, it is then loaded into an injection molding machine where the process is performed. A hopper holds thermoplastic pellets, which are fed into the barrel of the machine. Friction between the rotating screw and barrel melts the pellets. Hydraulic or electric toggles close the mold halves and press them together with hundreds of tons of pressure. The rotating screw then advances a preset distance injecting a prescribed volume of the molten plastic into the mold cavity under high pressure (many thousands of PSI). Once the material enters the mold, it begins to cool and solidify to conform to the shape of the mold. After the material cools sufficiently, the mold is opened automatically and the part is ejected from the core by a number of ejector pins.


The injection molding cycle is as follows:


Material enters the barrel
Material melts and mixes
Volume of material (Shot sizes in barrel is created)
Mold closes
Injection of the plastic into the mold cavity
Molten material cooled (during this process, steps 1-3 are preparing for next cycle)
Mold opens
Part ejects
Return to Step 4 for the next cycle


There are two types of injection molding machines: electric and hydraulic. Previously, hydraulic machines were predominately used in the industry. Today electric presses are fast becoming the molding machine of choice. The reason for this is because electric presses use far less energy and are nearly 100% repeatable. While the cost of an all electric machine is typically 30% higher than a hydraulic presses, higher demand is closing the gap on cost. It is estimated that in the future, hydraulic machines will be a thing of the past, as more injection molding companies are making the switch to stay competitive.


Injection Molding machines are rated by tonnage, ranging from less than 5 tons to 6000 tons. The tonnage is the reference of clamping force that the machine can exert to keep the core and cavity of the mold closed during the molding process. The clamp force required is determined by the projected area of the part being molded.

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