Polymer processes

These notes follow the terminology and detail in the teacher material, with each process kept to a concise but properly explained sequence.

Vacuum forming

What it is: A heated thermoplastic sheet is shaped over a mould/former by removing the air underneath it.

Step by step

Place the mould/former in the vacuum former and clamp the thermoplastic sheet securely in the frame above it.
Heat the sheet evenly with the radiant heater until the polymer softens and begins to sag uniformly.
Raise the mould into the softened sheet and switch on the vacuum pump so air is removed through the vent holes.
Atmospheric pressure pulls the sheet tightly over the mould; keep the vacuum applied while the polymer cools and sets to the required shape.
Release the vacuum, lower/remove the mould and take out the formed sheet.
Trim away the excess material and check the moulding for webbing, thinning, incomplete forming and dimensional accuracy.

Common uses: blister packaging, food trays, yoghurt pots and protective covers.

Advantages: relatively low-cost moulds; good for batch production; moulds are easy to modify.

Disadvantages: trimming creates waste; deep shapes can become thin; poor mould design can cause webbing or incomplete forming.

Quality control: even heating, tapered sides/draft angles, rounded corners, smooth mould surface and adequate air holes; check full mould contact and wall thickness.

Safety: burns from heaters/hot plastic, moving platen and sharp trimmed edges.

Thermoforming / pressure forming

What it is: A heated thermoplastic sheet is formed between matched positive and negative moulds using vacuum and pressure.

Step by step

Clamp the thermoplastic sheet securely and heat it evenly until it reaches its forming temperature.
Position the softened sheet accurately between the matched positive and negative moulds.
Close the moulds so the sheet is held securely between them.
Apply vacuum and pressure so the plastic is forced accurately into the mould details and deeper features.
Keep the sheet held in the mould while it cools and solidifies.
Open the mould, remove the component, trim it and check the detail, thickness and dimensions.

Common uses: high-quality packaging, food containers and formed panels.

Advantages: deeper forms and sharper detail than simple vacuum forming; logos and textures reproduce clearly.

Disadvantages: matched tooling is more complex/costly; uneven heating can cause distortion or thickness variation.

Quality control: clarity of detail, depth, trimming accuracy, even heating and wall-thickness consistency.

Safety: burns from heated sheet/moulds and crush risk from closing moulds.

Compression moulding

What it is: A measured charge of material is shaped inside a heated mould using heat and pressure.

Step by step

Clean and preheat the mould, then accurately measure the required charge of moulding material.
Place the charge into the heated negative mould cavity.
Close the positive mould using the press so heat and pressure force the material to flow throughout the cavity.
Maintain the pressure while the material cures and takes the mould shape.
Open the mould and remove/eject the component once it has set sufficiently.
Trim any flash and inspect the component for complete filling, voids, surface defects and dimensional accuracy.

Common uses: electrical components and thermoset/rubber mouldings.

Advantages: repeatable process producing strong, dense components.

Disadvantages: curing takes time; excess flash may need trimming; charge mass must be controlled.

Quality control: charge weight, full cavity filling, pressure, cure, voids, flash and dimensions.

Safety: hot moulds, high pressure, crushing and fumes.

Lamination / hand lay-up composites

What it is: Resin and fibre reinforcement are built up in a mould to create a GRP/CFRP composite component.

Step by step

Prepare the negative mould, clean it and apply a release agent so the finished component can be removed.
Apply the pigmented gel coat evenly to create the outer surface and allow it to become tacky.
Lay the fibre reinforcement into the mould and apply resin using the correct resin/catalyst ratio.
Use rollers to spread the resin through the fibres and remove trapped air, adding further layers or roving where extra reinforcement is required.
Leave the laminate to cure fully to the required thickness.
Remove the cured component from the mould, trim it accurately and check for bubbles, dry fibres, voids or delamination.

Common uses: boat hulls, car body panels, kayaks and large composite structures.

Advantages: relatively low setup cost; suitable for large and complex shapes.

Disadvantages: labour intensive and time-consuming; incorrect resin ratio or trapped air creates weak points.

Quality control: resin/catalyst ratio, even resin coverage, air removal, reinforcement, cure and laminate thickness.

Safety: resin fumes and skin/eye irritation; glass/carbon-fibre dust during trimming; use ventilation/extraction and PPE.

Line bending

What it is: A strip heater softens a narrow line in thermoplastic sheet so it can be bent accurately around a jig/former.

Step by step

Cut the thermoplastic sheet to size, mark the bend line and position it directly above the strip heater.
Heat the marked line evenly until the polymer becomes soft; thicker material may need turning to heat both sides.
Remove the softened sheet and bend it around the prepared jig/former to the required angle.
Hold the sheet against the jig while it cools so it sets and does not spring back.
Remove it from the jig and inspect the angle, dimensions and surface for bubbling, burning or cracking.

Common uses: acrylic display stands, guards, brackets and signage.

Advantages: low-cost and accurate for straight bends.

Disadvantages: limited to local straight bends; overheating can damage the polymer.

Quality control: bend-line position, even heating, jig angle and absence of bubbles/burn marks.

Safety: burns from the heating element and hot polymer.

Injection moulding

What it is: Molten thermoplastic is injected under high pressure into a closed metal mould.

Step by step

Feed thermoplastic granules through the hopper into the heated barrel, where the rotating Archimedes screw moves them forward and the material melts.
As molten polymer collects in front of the screw, the screw winds back so a measured amount of material is prepared for the moulding cycle.
Close and clamp the mould halves, then move the screw forward like a ram to force the molten polymer through the nozzle and into the mould cavity.
Maintain pressure while the cavity fills, then allow the polymer to cool and solidify inside the mould.
Open the mould and use ejector pins to release the finished component.
Remove the sprue/flash where required and check for short shots, flash, sink marks, warping and incorrect dimensions.

Common uses: bottle caps, casings, toys, clips and detailed plastic components.

Advantages: very fast and repeatable for mass production; excellent detail and surface finish.

Disadvantages: high mould/tooling cost; incorrect machine settings can repeatedly produce defective parts.

Quality control: melt temperature, injection pressure, complete filling, shrinkage, flash, short shots and dimensions.

Safety: high temperature and pressure, hot nozzle/molten polymer and crushing at the mould.

Extrusion

What it is: Molten thermoplastic is forced continuously through a shaped die to produce a constant cross-section.

Step by step

Feed plastic granules through the hopper into the heated barrel.
A motor turns the Archimedes screw, moving the granules forward while heat and friction melt the polymer.
The screw forces the molten polymer continuously through a shaped die which forms the required cross-section.
The hot extrusion passes through a cooling stage using air or water so it solidifies without excessive distortion.
The finished extrusion is pulled through continuously and cut to the required length or wound onto reels.

Common uses: solid rods, hose pipes, pipes, window profiles and other continuous sections.

Advantages: efficient continuous production and low unit cost at high volumes.

Disadvantages: limited to constant cross-sections; poor cooling can cause distortion.

Quality control: barrel/die temperature, material flow, cross-section dimensions and cooling.

Safety: hot die/molten polymer, moving machinery and cutting hazards.

Blow moulding

What it is: A heated hollow tube called a parison is inflated inside a two-part mould to produce a hollow plastic product.

Step by step

Form the heated thermoplastic parison and position it between the open halves of the mould.
Close and clamp the mould around the parison so it is sealed in position.
Force compressed air into the parison so it expands and presses against the inside surfaces of the mould.
Keep the product in the mould while the polymer cools and solidifies to the required hollow shape.
Open the mould and remove the component.
Trim excess material and inspect the walls, neck/seams and product for leaks.

Common uses: shampoo and detergent bottles and other hollow containers.

Advantages: efficient mass production of seamless hollow forms.

Disadvantages: wall thickness can vary and excess material may require trimming.

Quality control: wall/base thickness, blow pressure, sealing, neck accuracy and leak testing.

Safety: hot polymer/moulds, compressed air and moving mould parts.

Rotational moulding

What it is: Polymer powder melts and coats the inside of a heated mould while the mould rotates on two axes.

Step by step

Measure the required quantity of polymer powder/granules, place it inside the mould and seal the mould.
Heat the mould while it rotates slowly on two axes so the polymer begins to melt.
Continue rotating as the molten polymer repeatedly coats the inside surfaces and builds the hollow wall.
Move the mould to the cooling stage but continue rotation so the material does not pool or sag while solidifying.
Once cooled, stop the rotation, open the mould and remove the seamless hollow component.
Trim and inspect wall-thickness uniformity, incomplete fusion, porosity and dimensional accuracy.

Common uses: water tanks, wheelie bins, kayaks, traffic products and playground equipment.

Advantages: large seamless hollow forms and relatively lower tooling cost than injection moulding.

Disadvantages: long heating/cooling cycle and limited suitable materials.

Quality control: powder quantity, heating, biaxial rotation, cooling and wall thickness.

Safety: hot moulds/ovens, rotating machinery and manual-handling risks.

Calendaring

What it is: Softened polymer is passed through a series of rollers to produce continuous sheet or film of controlled thickness.

Step by step

Heat the polymer until it becomes soft and workable, then feed it into the calendaring rollers.
Pass the material through a series of heated rollers with progressively smaller gaps so its thickness is reduced evenly.
Where required, pass the sheet through an embossing roller to add a texture or pattern.
Pass the formed sheet through chill rollers so it cools while maintaining the required thickness and surface.
Trim the sheet edges and wind the finished material onto rolls or cut it into sheets.

Common uses: PVC sheets, vinyl flooring and plastic films.

Advantages: smooth, uniform sheet with good thickness control; efficient continuous production.

Disadvantages: high setup cost and large equipment; not economical for short runs.

Quality control: sheet thickness, surface/embossing quality, roller temperature, gap and alignment.

Safety: crushing/entanglement at rollers, burns, fumes and need for guards/emergency stops.