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Plastics Molding Processes

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How a plastic product can be recycled is determined by many factors, including which molding process was used to make that product.

Plastic products are manufactured from the various plastic polymers and additives using one or more molding processes. Molding further exposes the plastic resins to heat, pressure, and additional chemicals, which further defines the plastic's structure and behavior.

different molding processes
blow molded
injection molded

plastics science: polymers

Different Molding Processes

The type of molding process that is used depends on the desired shape, behavior, and function of the plastic product being made.

Some plastic containers, such as milk jugs and laundry detergent bottles, are made by a manufacturing process called blow-molding. Other plastic containers, such as margarine tubs and sour cream containers, are manufactured using a process called injection-molding.

Products composed of the same resin but manufactured by different molding processes cannot be mixed together in the remanufacture of recycled content products. If they are mixed, the resulting glop is not usable in either manufacturing process.

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Blow Molded

Blow-mold grade plastics flow slowly, and the plastic used must be thick and tacky. Key characteristics of blow-mold grade are stiffness and strength.

Blow-molded containers are literally blown up like a balloon and will resist the pull of gravity. The mold is filled with plastic resin and then air is forced in through a single orifice. Because the resin holds its shape, it will expand out to fill the dimensions and form of the mold.

The finished product is stronger (better crack resistant, less chance of pinhole failure, etc.) than an injection-molded item. Blow-molding is suited for making bottles, due to their narrow-neck shape.

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Injection Molded

Injection-mold grade plastics have a high melt flow index, meaning the liquid plastic moves fast to quickly fill a mold.

The mold for an injection-molded item has many orifices that allow the runny plastic to fill the mold quickly, before it starts to harden.

Injection molding is preferred for wide-mouth containers, also for objects with complicated shapes (such as toys).

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