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What is Injection Molding:
Injection Molding is a manufacturing process for producing parts in large volume. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession.
Injection Molding produces low scrap rates relative to traditional manufacturing processes like CNC rubber machining which cut away substantial percentages of an original plastic block or sheet. This however can be a negative relative to additive manufacturing processes like 3D printing that have even lower scrap rates.
Note: waste plastic from injection molding manufacturing typically comes consistently from four areas:
A sprue is simply the channel that guides molten plastic from the nozzle of the injection molding machine to the entry point for the entire rubber injection machine tool. It is a separate part from the mold tool itself.
A runner is a system of channels that meet up with the sprue, typically within or as part of the mold tool, that guides the molten plastic into the part cavities within the mold tool. There are two principal categories of runners (hot and cold) which you can read about here.
Typically, quality control departments will limit the amount of regrind that is allowed to be placed back into the press. (Some performance properties of the plastic can degrade as it is molded over and over). Or, if they have a lot of it, a factory can sell this re-grind to some other factory who can use it. Typically regrind material is used for low-quality parts that don't need high performance properties.
Injection Molding is very repeatable. That is, the second part you produce is going to be practically identical to the first one etc. This is a wonderful characteristic when trying to produce brand consistency and part reliability in high volume production.