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)
Injection of the plastic into the mold cavity
Molten material cooled (during this process, steps 1-3 are preparing for next cycle)
Return to Step 4 for the next cycle
Calculating an injection molding cycle is as follows:
Cycle = Mo+Mc+I+C
Mc = Time to close the mold (this is the time it takes to actually close the tool)
I = Time to inject material into the mold
C = Cooling Time (Time to solidify molten material)
To = Time to open a mold and eject the part (these can overlap and together make up total open time)
The production rate of injection molding is the Cycle Time multiplied by the number of mold cavities in the tool. Typically for prototype and low volume production a tool will have only a single cavity and little to no automation. Full production molds for extremely high volume applications like caps and closures may have dozens of cavities and full automation with extremely short cycle times and very high productivities. These factors make injection molding extremely cost effective once the initial tooling has been completed and process has been stabilized.