Making Chunky Things Out of Polymers


List of processing stuff on macrog/mpm/composit/fiber/process

Two ways that polymers can be used to make chunky things (like toys and shoe bottoms) are:

Using a Mold

First, you need a polymer that will work! It can be:

Molding polymers to make objects

Compression Molding - for thermosets
Injection Blow Molding
Extrusion Blow Molding

Compression Molding

Compression molding goes back to 1909, when Leo Bakeland used it to make radio cabinets. Now, compression molding is used to make pot handles, cheap dinnerware, kitchen utensils, electrical receptacles, and even cafeteria trays!

The kind of polymer used is a thermoset, meaning that it forms crosslinks when it gets hot. Those crosslinks are like glue that hold the polymer chains together. So, with heat, crosslinks form, and the polymer gets harder. (That's called "curing" the polymer.)

Now, the folks who make things like this do something that's very smart. They form some of the crosslinks, so the polymer isn't too hard yet. Then, they make pellets or powder out of the polymer.

That puts the polymer in a form that's easy to work with.

Those pellets (or powder) are then put into the mold. This kind of mold has two halves (or platens) that are made out of steel and get very hot. Not only is the mold heated (to about 350 degrees F), but it's put under a lot of pressure (about 1,500 psi).

Under all that pressure and heat, the plastic first starts to get soft. The soft plastic fills the mold. Still feeling pressure and heat, the plastic is cured -- lots of crosslinks form -- and it gets hard. The two halves of the mold are pulled apart, and out pops a product!

These products tend to be kind of heavy and dense. And hey, when you think about how they're made, it's no wonder that these products are used for things that need to stand up to heat, like pot handles.

Injection Blow Molding

Injection blow molding is used to form hollow plastic parts.

Injection blow molding consists of three steps:

  1. injecting plastics into a mold to form a plastic tube;

  2. blowing the tube into a cavity mold to form a hollow part; and

  3. ejecting the part.

At the first station a split half mold closes on a steel rod called a mandrel. The mandrel forms the center of the tube.

The mandrel is held in the mold much like a stick in an ice-cream bar. The mold forms the outside of the tube.

The injection molder forces plastic into the mold and around the mandrel. Once the plastic tube is formed it is called a parison.

The first stage is completed when the mold opens and the mandrel is transferred to the second station.

At the second station a second mold closes on the mandrel and parison. This cavity of the second mold is the shape of the final product. The mandrel, encased in the soft plastic parison, is held at the center of the closed mold. Air is injected through the mandrel, forming a hollow plastic part by blowing the soft plastic parison to the shape of the mold. After the hollow part has cooled, the mold opens and rotates to the third station.

At the third station the finished part is finally ejected off the mandrel by a stripper bar.

Advantages of injection blow molding over extrusion blow mold.

  1. In injection blow molding the part is formed in an enclosed mold, so there is no scrap to be trimmed from the container.

  2. The three stations of the process allow for the formation of a thick-threaded neck with a thin container wall. This feature allowed for the use of injection blow molding of pressure bottles for coke, and safety enclosures for pill bottles.

  3. A third advantage is the ability to mold multiple containers. Many injection blow molding machine use multiple mandrels, forming up to 12 hollow parts at one time.

  4. Allowances can be made for thicker bottoms and mid-sections by increasing the cavity around the mandrel. At the second stage, the parison is stretched by extending the steel mandrel, forcing the plastic to elongate. This elongation stretches and aligns the plastic molecules along the vertical directions of the container. This increases the overall strength of the container.

Extrusion Blow Molding

Extrusion is one of two forms of blow molding used for hollow enclosed products.

An extruder, or ram, is used to force melted plastic through a tube forming die.

The extruded tube, called a preform or parison, descends vertically between a split cavity mold, made of either aluminum or high temperature epoxy.

The cavities shape the final part.

The mold closes on the parison. The tube is trapped inside the mold.

The top and bottom of the tube is sealed as the mold closes.

At some point on the mold, the parison is punctured with a small air tube.

Air pressure is injected into the soft parison causing it to expand against the sides of the mold.

The mold cools the plastic and forms the hollow shape. The mold opens and blown hollow objects is ejected by an upper ejector plate. The pinched ends can be removed by machining.

Typical products made by extrusion blow molding are 55 gallons drums, bottles, trash containers, double walled tool and instruments cases, arm rests, portable hot drink containers, and other hollow objects.

Blow molded containers have irregular wall thicknesses induced by the stretching of the plastic parison during the blowing process.

To overcome the irregular wall thickness the parison is extruded with irregular thickness.

The opening in the die that forms the parison is changed to form different wall thickness.

To control the thickness a computer with a parison control program is connected to the extruder die.

The program automatically opens and closes the die to increase the amount of material to be formed in the parison around threaded areas and the weaker bottoms sections.

The pinch-off mark left at the bottom of hollow containers identifies extrusion blow molded from injection blow molded product.

Thanks to Christopher L. Lester for writing the college-level version of this page.

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