POLYMER PROCESSING

Processing is the source of much of the variability in properties of synthetic polymers. Different processing conditions determine the difference between a milk jug and, to a great extent, high strength PE fibers (ultra oriented). Processing is one component of a series of steps that lead to a plastic product or component from raw feed stocks. Although each commodity polymer has a somewhat different sequence of industrial stages involved in bringing it to market, a simple and fairly exemplary case is a polyethylene bottle of shampoo.
The various grades polymers are partly composed of blends of different branch content, molecular weight and density from different synthetic reaction conditions. For example, a film blowing grade of polyethylene might contain a blend of linear low density polyethylene, branch content metallocene polyethylene and low density polyethylene.
Polymers offer certain unique problems in processing. They are viscoelastic fluids which display high viscosity and broad transition temperatures. Machinery is large and involves a significant energy input. Polymers degrade (chain breakup) resulting in a loss of properties with exposure to high temperatures and stress for an extended period of time.
Some basic processing operations and machinery are extrusion, screw extruder, fiber drawing, injection molding, blow molding, dip coating, calendering, mixing/dispersion, melting, modification, and introduction of additives.

Mechanical + Thermal
Thermoplastic – solidified by cooling and reheated by melting
Thermosets – retain their structure when reheated after polymerization (usually crosslinked)
Elastomers – rubbers, deform readily with applied force
Thermoplastic elastomers
other
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Chapter 1. Primer/introduction

Major Processes
Extrusion
Injection Molding
Blow Molding
Thermoforming
Rotomolding

Formulation
Additives are used to modify properties and/or lower costs
Additives: heat stabilizer, light stabilizer, lubricant, colorant, flame retardant, foaming agent, plasticizer
Reinforcement: particulate minerals, glass spheres, activated carbon, fibers
Blends, alloys, laminates
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Chapter 1. Primer/introduction

Extrusion

Blown-Film Extrusion

Injection Molding

Co-injection Molding
1. Inject Skin
2.a. Inject Core
2.b. Continue injecting core until cavity is filled
3. Inject Skin again to purge sprue.

Injecting hollow parts with complex interior geometry

Injection – Compression Molding
Very similar to squeeze casting

Extrusion Blow Molding

Injection Blow Molding

Thermoforming

Calendaring
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Chapter 1. Primer/introduction
Thin and thick section calendaring is used to make wide sheets (8-12 ft).