Processes Used to Form Plastic Materials Chapter 15

Plastic stock comes in many forms to accommodate different processes sheets, pellets, powder, granules, rods Produce near-net-shape parts Shrinkage must be considered with plastic Little to no scrap Thermoplastic scrap can be reused

Compression Molding Cold compression molding Used with thermoplastics and thermosets Usually thermosets Done hot and cold Cold compression molding The material is placed between the mold halves Mold is closed Part remains in mold and is transferred to an oven for curing Hot press compression molding Plastic mixture of resin, reinforcement and fillers placed in mold Heated between 225°F and 325°F Closed with pressure from 100 psi to 2000 psi Curing happens in the mold, takes from seconds to 5 min Molds are constructed with heating and cooling vents

Injection Molding High volume production, most widely used Used with thermoplastics and thermosets Primarily thermoplastics Measured amount of liquid plastic forced into a heated mold Called a shot Molds are often cooled by water running through lines in the mold Parts cool in seconds Molding machines are rated by the amount of plastic (in ounces) that they can inject in one shot

Rotational Molding or Rotomolding Makes hollow parts Small to very large parts Plastic is placed in molds Molds are heated, then rotated until cool enough that the plastic has hardened No need for inner core Less expensive tooling

Blow Molding Thin and medium walled hollow products Can be a fast process Starts with a pre formed parison Hollow tube of heat-softened plastic Parison is positioned between two mold halves One end is clamped shut, air tube is placed in other end Air blown into parison, pushing it out against the mold

Injection Blow Molding Hybrid process Combines injection molding and blow molding Two step process Parison is formed by injection molding Parison encircles the mandrel Parison is moved to blowing station Placed in another mold, formed to final shape

Thermoforming Heating plastic to it’s softening point and forcing it over the contours of a one piece mold Many different forms of thermoforming

Vacuum Forming Plastic sheet is heated Vacuum is applied under the sheet drawing it down against a mold