Polymers.
Forming, redistribution and addition processes · Technical principles
Revision notes
Polymer Processes
POLYMER PROCESSES
Vacuum forming
Vacuum forming is a thermoforming process used to shape heated plastic sheet over a single mould using air pressure.
How the process works
The mould is placed into the former and a sheet of thermoforming plastic (such as HIPS or acrylic) is clamped over it.
The plastic sheet is heated until it becomes soft and flexible.
The mould is raised and air is sucked out, pulling the plastic tightly over the mould surface.
The plastic cools, is removed from the mould, and excess material is trimmed and finished.
Key characteristics
Produces thin-walled products with sloping sides and rounded corners, as the plastic stretches over the mould.
Detail is limited, especially on deep shapes, due to thinning of the plastic.
Mould requirements
No undercuts or overhangs, with draft angles included for easy release.
Air holes are required to remove trapped air, and corners should be rounded to reduce webbing and tearing.
Advantages
Suitable for batch production with relatively low-cost moulds.
Moulds are easy to modify, and products can be made in a wide range of colours.
Disadvantages
Excess waste from trimming.
Poor mould design can cause webbing or uneven thickness.
Not suitable for highly complex shapes.
Typical materials
Acrylic, HIPS, PVC
Thermoforming
Thermoforming is similar to vacuum forming but uses both positive and negative moulds to increase accuracy and detail.
How the process works
A heated plastic sheet is stretched between a positive and negative mould.
The positive mould pushes into the negative mould while air is removed to create a vacuum.
The plastic cools, then is removed and trimmed.
Key characteristics
Produces deeper shapes and sharper detail than vacuum forming.
Logos and textures can be formed more clearly due to pressure from both sides.
Production notes
Suitable for batch and mass production.
Often finished using die cutting for accuracy.
Compression moulding
Compression moulding uses heat and pressure to shape a pre-measured amount of plastic in a closed mould.
How the process works
A warm, pre-measured slug (charge) of plastic is placed into a heated negative mould.
The positive mould clamps down, causing the plastic to spread and fill the cavity.
The plastic cools and the finished component is ejected.
Key characteristics
Produces strong, dense components with consistent thickness.
Accurate measurement of the charge is essential to avoid flash or incomplete moulding.
Production suitability
Used for batch and large-scale production.
Often requires trimming after moulding.
Lamination (lay-up moulding – composites)
Lamination is a manual composite process used to produce large or complex shapes, commonly using GRP or CFRP.
How the process works
A negative mould is prepared and coated with a release agent such as silicone.
A pigmented gel coat is applied to create the surface finish.
Layers of fibre and resin are built up to the required thickness, with each layer rolled to remove air bubbles.
A sealant layer is applied, the product is left to cure, then removed for post-production.
Advantages
Low setup cost and simple equipment.
Can produce very large components and complex shapes.
Pigment can be added directly to the gel coat.
Disadvantages
Time-consuming mould making and curing.
Resin fumes are toxic.
Air bubbles can cause weak points and delamination.
Line bending
Line bending is used to produce accurate straight bends in thermoplastic sheet.
How the process works
The plastic sheet is cut to size and the bend line is marked.
The sheet is placed over a heated wire until the plastic becomes soft along the line.
The sheet is removed and bent around a jig or mould to set the angle.
Advantages
Low cost and minimal equipment.
Suitable for one-off and small batch production.
Disadvantages
Risk of burning or bubbling if overheated.
Thicker sheets heat unevenly and may need rotating.
Safety risk from exposed heating element.
Injection moulding
Injection moulding is a high-pressure process used to mass-produce complex plastic components.
How the process works
Plastic granules are fed into a hopper and moved forward by a rotating screw while heated jackets melt the polymer.
The screw winds back, the mould closes, and a hydraulic ram injects molten plastic into the mould cavity.
The plastic cools, the mould opens, and the sprue is removed.
Key characteristics
Produces complex, highly detailed components with excellent surface finish.
Very accurate and repeatable, with minimal finishing required.
Advantages
Ideal for mass production with low unit cost at high volumes.
Allows features such as clips, ribs, and living hinges to be moulded in.
Disadvantages
Very high tooling and setup costs.
Typical materials
ABS, HDPE, Nylon, PP, PS
Extrusion
Extrusion is a continuous process that produces products with a constant cross-section.
How the process works
Plastic granules are melted and forced through a shaped die.
The plastic exits the die with a fixed cross-section and is cooled before being cut to length.
Key characteristics
Produces long lengths with identical cross-sections throughout.
Length can be varied without changing the die.
Advantages
Low unit cost for mass production.
Efficient continuous process.
Disadvantages
Limited to simple, uniform cross-sections.
Dies are expensive.
Typical materials
Acrylic, Nylon, Polystyrene, ABS, Polycarbonate
Blow moulding
Blow moulding is used to manufacture hollow plastic products such as bottles and containers.
How the process works
A heated hollow tube of plastic (parison) is formed and placed between two halves of a mould.
The mould closes and air is blown in, forcing the plastic against the mould walls.
The plastic cools, and the hollow product is released.
Key characteristics
Produces hollow shapes with thin, even walls and no joins along the body.
Ideal for airtight and watertight containers.
Advantages
Efficient for mass production with minimal finishing.
Consistent wall thickness.
Disadvantages
High tooling and setup costs.
Typical materials
PVC, PET, Nylon, ABS, HDPE, LDPE, PP, PS
Rotational moulding
Rotational moulding produces seamless hollow products using heat and rotation rather than pressure.
How the process works
Polymer powder or granules are placed into a sealed mould.
The mould rotates while being heated, causing the polymer to melt and coat the interior evenly.
The mould continues rotating while cooling, then opens to release the product.
Advantages
Uniform wall thickness with no internal stresses.
Can produce complex hollow shapes, including double-wall products.
Tooling costs are lower than injection or blow moulding.
Disadvantages
Long cycle times.
Limited range of suitable materials.
Calendaring
Calendaring is used to produce plastic sheets and films with accurate thickness.
How the process works
Plastic is heated to a putty-like state or extruded through a screw.
The material passes through a series of heated rollers with progressively smaller gaps.
The sheet may be embossed, then cooled and wound into rolls.
Advantages
Produces smooth, uniform sheets and films.
Suitable for continuous mass production with good thickness control.
Disadvantages
High setup cost and large space requirement.
Not economical for short production runs.
POLYMER PROCESSES – A-LEVEL COMPREHENSIVE TABLE
Process | Step-by-step process (6 full points) | Typical uses | Quality control (process-specific) | Health & safety (A-Level specific) |
|---|---|---|---|---|
Vacuum forming | 1. A thermoplastic sheet (e.g. HIPS or acrylic) is clamped into a frame above a single mould that includes draft angles and has no undercuts. 2. The sheet is heated evenly until it softens and sags uniformly, indicating it has reached forming temperature. 3. The mould is raised into the softened sheet. 4. A vacuum pump removes air through vent holes, and atmospheric pressure pulls the plastic tightly over the mould surface. 5. The plastic is held under vacuum while it cools and sets to the mould shape. 6. The formed component is removed, excess material is trimmed away, and edges are finished. | Blister packaging, food trays, baths, signage, protective covers | Check even heating and uniform sag before forming; inspect for thinning at deep draws, webbing between corners, tearing, and incomplete pull-down; measure wall thickness consistency and check dimensional accuracy after trimming. | Burns from heaters and hot plastic sheets; pinch hazards from the moving platen; sharp edges during trimming; eye protection and heat-resistant gloves required. |
Thermoforming (pressure / matched mould) | 1. A thermoplastic sheet is clamped into a frame and heated evenly to its forming temperature. 2. The softened sheet is positioned between a matched positive and negative mould. 3. The moulds close, trapping the sheet securely between them. 4. Vacuum and pressure are applied, forcing the plastic accurately into all mould details. 5. The plastic cools and solidifies while held in the mould. 6. The mould opens and the component is removed and trimmed, often using die cutting. | High-quality packaging, appliance panels, automotive interior components | Inspect clarity of logos and textures, depth accuracy, and uniform wall thickness; check trimming accuracy and reject parts with distortion or uneven thickness caused by uneven heating. | Burns from heated sheets and moulds; crush risk from closing moulds; sharp edges after die cutting; guarding and PPE essential. |
Compression moulding | 1. The mould is cleaned, coated with release agent, and preheated to the correct temperature. 2. A pre-measured charge of plastic is accurately weighed. 3. The charge is placed into the heated mould cavity. 4. The mould is closed using a press, applying heat and pressure. 5. Pressure is maintained while the plastic flows, cures, and cools. 6. The mould is opened, the component ejected, and excess flash trimmed. | Electrical components, rubber products, heat-resistant housings | Check charge weight to avoid flash or incomplete filling; inspect for voids, short moulds, and surface defects; check thickness and dimensional accuracy; verify full curing for thermosets. | High heat and pressure hazards; crushing risk from press; fumes from heated polymers; guarding, ventilation, and heat-resistant gloves required. |
Lamination (hand lay-up – composites) | 1. A negative mould is cleaned and coated with a release agent. 2. A pigmented gel coat is applied and left to become tacky. 3. Fibre reinforcement is laid into the mould. 4. Resin is applied and worked into the fibres using rollers to remove air bubbles. 5. Layers are built up to the required thickness and left to cure. 6. The component is removed from the mould and trimmed and finished. | Boat hulls, car body panels, storage tanks, large composite structures | Check resin-to-catalyst ratio; inspect for air bubbles, dry fibres, delamination, and uneven thickness; visual inspection and tap testing for voids; surface finish checks on gel coat. | Toxic resin fumes; skin and eye irritation; carbon/glass fibre dust during trimming; PPE including gloves, goggles, respirators, and good ventilation required. |
Line bending | 1. The thermoplastic sheet is cut to size and the bend line is marked accurately. 2. The sheet is positioned over a strip heater aligned with the bend line. 3. Heat is applied until the plastic softens along the marked line. 4. The sheet is removed and bent around a jig or former. 5. The bend is held until the plastic cools and sets. 6. The finished part is inspected and any surface marks are removed. | Acrylic display stands, guards, brackets, signage | Check bend angle accuracy, straightness of the bend, surface clarity, and absence of bubbles or burn marks; ensure bend occurs exactly on the marked line. | Burns from heating element and hot plastic; fire risk if overheated; exposed hot wire hazard; supervision and PPE required. |
Injection moulding | 1. Plastic granules are fed into a hopper and melted by heated barrels as a screw rotates. 2. The screw retracts to measure a precise shot of molten plastic. 3. The mould closes and clamps shut under high force. 4. Molten plastic is injected into the mould cavity at high pressure. 5. The plastic cools and solidifies inside the mould. 6. The mould opens, ejector pins release the component, and minimal finishing is required. | Bottle caps, phone cases, toys, clips, medical components | Monitor melt temperature, injection pressure, and cycle time; inspect for flash, short shots, sink marks, warping, and surface defects; check part weight and dimensions for repeatability. | Extremely high temperatures and pressures; crushing hazards; hydraulic systems; strict guarding, interlocks, and trained operators required. |
Extrusion | 1. Plastic granules are fed into a hopper and heated in a barrel. 2. A rotating screw melts and pushes the plastic forward. 3. The molten plastic is forced through a shaped die. 4. The extrusion exits with a constant cross-section. 5. The product is cooled using air or water. 6. The extrusion is cut to length or wound onto reels. | Pipes, tubing, window frames, plastic profiles | Continuous measurement of cross-section dimensions; inspection for surface defects; control cooling rate to prevent warping or distortion. | Burns from hot polymer and die; entanglement with moving machinery; cutting hazards; guards and emergency stops required. |
Blow moulding | 1. A hollow tube of heated plastic (parison) is formed. 2. The parison is placed between two halves of a mould. 3. The mould closes, sealing the plastic. 4. Compressed air is blown in, expanding the plastic to the mould walls. 5. The plastic cools and solidifies. 6. The mould opens and the hollow product is trimmed and finished. | Bottles, containers, fuel tanks | Check wall thickness and base thickness; inspect neck and thread accuracy; carry out leak testing for airtight and watertight integrity; inspect for weak seams. | Hot plastic and moulds; high-pressure air hazards; moving mould parts; machine guarding essential. |
Rotational moulding | 1. A measured amount of polymer powder is placed into a sealed mould. 2. The mould is heated in an oven while rotating on two axes. 3. The polymer melts and coats the inside of the mould evenly. 4. Rotation continues during cooling to prevent sagging. 5. The polymer solidifies into a seamless hollow product. 6. The mould is opened and the product removed and trimmed. | Water tanks, wheelie bins, kayaks, playground equipment | Check wall thickness uniformity; inspect for incomplete fusion or porosity; check dimensional accuracy after cooling and trimming. | Hot moulds and ovens; long cycle times; manual handling risks; heat-resistant gloves and lifting aids required. |
Calendaring | 1. Plastic is heated until soft and workable. 2. The material is fed into a series of heated rollers. 3. The rollers progressively reduce the thickness. 4. The sheet may be embossed with texture or pattern. 5. The sheet is cooled using chill rollers. 6. The finished sheet is trimmed and wound into rolls. | PVC sheets, vinyl flooring, plastic films | Measure thickness across the sheet; inspect surface smoothness and embossing quality; monitor roller temperature and alignment to prevent defects. | Crush and entanglement hazards at rollers; burns from heated rollers; fumes; strict guarding and emergency stop systems required. |
Comparison questions (e.g. vacuum forming vs injection moulding)
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