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Variations

Twin Sheet Forming

It produces parts that are comparable to rotational-molded parts but with greater detail. Hollow, double-walled parts are produced by simultaneously forming two separate sheets in their respective separate molds then fusing the two parts together.

Twin-sheet forming is the process of simultaneously forming two separate sheets in their respective molds then, by welding at high pressure, creating a hollow, double-walled part that is comparable to a roto-molded part, but with better detail and lower cost.

Twin-sheet forming offers many advantages over most operations including roto-molding. For example, bonding by continuous perimeter weld joint saves labor and reduces costs because there are no adhesives or fasters as would be required with other manufacturing methods. It also offers quicker deliver and lower tooling costs than blow molding. And , twin-sheet formed items are stronger and have increased rigidity because of the double-walled structure.

Blow Molding

It is a process similar to other themoforming methods in that it produces parts by using high pressure air, however, like injection molding, melted plastic pellets are used instead of pre-heated plastic sheets. Containers, such as plastic beverage bottles, are a major application. Generally, if uniform thickness and outside shaping are important criteria for a part, it is a good candidate for blow molding.

Parts are produced by trapping a melted tube of plastic (a parison) between two mold halves and then introducing high pressure air to stretch the parison out to fill the mold cavity.

The following sequence produces the parison and the part:

1. Plastic pellets (approximately 1/16" cubes) are fed into the hopper of an extruder and feed by gravity down to the extruder screw.
2. The screw chamber is equipped with a heating unit which melts the plastic as it is pumped by the screw toward the die head of the extruder.
3. In the die head is a "flow pin" around which the molten plastic flows horizontally at first, and then downward. It emerges from the end of the flow pin as a seamless tube. The tube is extruded to the length required for the part. Extrusion is then halted while horizontally moving mold sections clamp it preparatory to air pressure being applied internally.
4. Air can be applied by a tube (blow pin) inside the parison flow pin, or by needles built into the mold which pierce the parison as the mold closes.
5. As pressure is applied, the parison balloons out to fill the mold cavity. Molds are constructed with internal water lines to provide chilled water for cooling the blown part.
6. Depending on the size of the part, the production rate required, and the capacity of the extruder, multiple molds or molds with multiple cavities may be used and more than one parison at a time extruded.