Polymer finishes.
The use of finishes · Technical principles
Revision notes
Polymer Finishing
Polymers are generally considered self-finishing materials. This means that once they are removed from the mould or manufacturing process, they require no additional surface treatments to be functional or aesthetically pleasing.
Overmoulding
Overmoulding involves moulding a second polymer (often TPE - Thermoplastic Elastomer) over specific parts of a base product to provide grip, texture, or color contrast.
Method | Process Description | Best For... |
|---|---|---|
Two-Mould Process | The main product is injection moulded in one machine. It is then manually or robotically moved to a second mould where the overmoulding polymer is injected onto it. | Lower volume production or complex parts. |
Twin-Shot Injection | A high-speed, continuous process. The machine uses a rotating mould (180°) with two cavities. The main body is shot, the mould rotates, and the overmould is shot immediately. | High-volume mass production (e.g., toothbrushes, power tool grips). |
Acrylic Spray Paints (A-level Only)
While polymers are self-finishing, acrylic paints are sometimes applied for specific functional or aesthetic reasons.
Properties: Fast-drying, water-soluble when liquid, but water-resistant and UV-resistant once dry.
Protection: Provides a barrier against UV light and weathering, which can prevent polymers from becoming brittle.
Mass Customisation: Used extensively in the automotive industry (e.g., wing mirrors and bumpers).
Efficiency: It is more cost-effective to spray-paint small batches of different colors than to stop an injection moulding machine to change the color of the polymer granules (which causes downtime and waste).
Adding Pigments
Pigments are the primary method for coloring polymers. They are integrated into the material rather than sitting on the surface.
Application: Pigments can be added during the initial manufacture of the polymer stock form or mixed into the granules during the injection moulding/extrusion process.
Smart Pigments:
Thermochromic: Changes color based on temperature (e.g., baby spoons).
Phosphorescent: Glows in the dark (e.g., emergency exit signs).
GRP (Glass Reinforced Plastic): Pigments are added to the gel coat (the outer layer) during the lay-up process to provide a high-quality color finish.
Functional Coding: Using colors to categorize products, such as color-coded kitchen chopping boards to prevent cross-contamination
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