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
Finishes can improve appearance, durability, corrosion resistance and surface performance.Vacuum forming: a heated thermoplastic sheet is drawn over a former and held while it cools.Injection moulding: granules are melted and driven by a screw through a nozzle into a closed mould.Extrusion forces softened material through a shaped die to produce a continuous cross-section.Blow moulding: air expands a hot polymer parison against the inside of a mould.Rotational moulding coats the inside of a heated rotating mould before the hollow product cools.Compression moulding uses heat and pressure to form a measured charge inside a mould.A pattern forms a cavity in sand before molten metal is poured, cooled and removed.Die casting injects molten metal into a reusable metal die under pressure.Investment casting forms a ceramic mould around a sacrificial pattern for detailed components.MIG welding feeds a consumable wire electrode into a gas-shielded weld pool.TIG welding uses a non-consumable tungsten electrode and separate filler where required.Spot welding clamps sheet between electrodes and creates localised heat through electrical resistance.Steam softens lignin so timber can be bent around a former and held until dry.Thin veneers are glued and clamped around a former to create strong curved components.Flexography uses a relief plate and rollers to transfer ink to the substrate.Offset lithography transfers the image from plate to blanket cylinder and then to the substrate.Screen printing pushes ink through open areas of a mesh stencil onto the substrate.A focused laser follows a programmed path while assist gas helps clear the cut.Milling removes material with a rotating multi-point cutter while the work is accurately positioned.In turning, the workpiece rotates while a cutting tool removes material to create cylindrical forms.FEA divides a model into elements so stresses and deformation can be calculated.CFD predicts how fluids move around or through a product.A go/no-go gauge quickly checks whether a feature lies within its permitted tolerance.Vernier callipers measure external, internal and depth dimensions accurately.A micrometer uses a precision screw to measure small dimensions accurately.A tensile test records how a standard specimen responds as an increasing axial load is applied.Hardness tests compare resistance to permanent indentation under controlled conditions.Anthropometric data provides measured human dimensions for designing appropriate product sizes and clearances.Ergonomics applies knowledge of people to improve comfort, safety, usability and performance.Bauhaus design favoured functional forms, modern materials and design suited to industrial production.Art Deco is recognisable through geometric decoration, symmetry, luxury materials and machine-age influence.Memphis challenged restrained modernism with bold colour, pattern, geometry and playful forms.Arts and Crafts valued skilled making, natural inspiration, material honesty and resistance to poor-quality mass production.The life-cycle model helps explain how sales, competition and redesign priorities change over time.Sustainable design decisions should reduce resource use and keep products and materials useful for longer.
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