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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.


Why Use Injection Molding:
The principal advantage of injection molding is the ability to scale

production en masse. Once the initial costs have been paid the price

per unit during injection molded manufacturing is extremely low. The

price also tends to drop drastically as more parts are produced.

Other advantages include the following:


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:


The sprue
The runners
The gate locations
Any overflow material that leaks out of the part cavity itself (a

condition called “flash”).

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.


Lastly, the gate is the part of the channel after the runner that

leads directly into the part cavity. After an injection mold cycle

(typically only seconds long) the entirety of the molten plastic will

cool leaving solid plastic in the sprue, runners, gates, part

cavities themselves, as well as a little bit of overflow potentially

on the edges of the parts (if the seal isn’t 100% right).

Thermoset material, such as an epoxy resin that cures once exposed to

air, is a material that cures and would burn after curing if one

attempt is made to melt it. Thermoplastic material by contrast, is a

plastic material that can be melted, cool and solidify, and then be

melted again without burning.


With thermoplastic materials the material can be recycled are used

again. Sometimes this happens right on the factory floor. They grind

up the sprues/runners and any reject parts. Then they add that

material back into the raw material that goes into the injection

molding press. This material is referred to as "re-grind".


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.