Plastic Forming, Cutting & Moulding Processes

Cambridge Design and Technology students have to be familiar with how to cut, shape and form plastic using a range of different methods. This article introduces various plastic forming processes, including vacuum forming, blow moulding, injection moulding, line bending and so on, helping students prepare for examination questions on this topic.

Note: A felt pen should be used to mark out plastic as a pencil doesn’t write on plastic easily (the felt pen can be wiped off afterwards – don’t use a vivid or permanent marker as it will leave marks all over the plastic)!

Vacuum forming

• Thin plastic sheet (i.e. PVC) is heated and sucked down around a ‘former’ (mould)
• See detailed information about vacuum forming
• Only works for thin plastic and relatively simple moulds
• Commonly used for blister packaging

Shrink wrapping

• Wrap product with plastic film (i.e. PVC or PE)
• Apply heat using a heat gun or pass through a heat tunnel to warm the plastic
• Plastic shrinks and contracts, pulling tight around the product’s shape
• Forms tight protective seal – used as protective packaging – often holds multiple products together

Laser cutting and engraving

• File created using CAD software such as Techsoft Design or Adobe Illustrator (different colour used for cutting and engraving)
• Sheet of material placed in bed of laser cutter, depth set correctly
• Extraction fan turned on
• File sent to laser cutter, ensuring to enter speed and other settings as appropriate for that material according to manual
• Material laser cut / engraved and lifted from bed, removing all scraps and debris

Vinyl cutting

Thin sheets of vinyl can be cut using a vinyl cutter. Learn more about vinyl cutting in the article about cutting lightweight materials.

Blow moulding

• Molten tube of plastic lowered into a mould
• Blow air in through tube, so it inflates and presses against the walls of the mould (like blowing up a balloon)
• Cool and harden
• Open mould and remove
• Creates hollow plastic parts in one piece
• Commonly used for bottles and containers

Injection moulding

Note: Injection moulding appears very commonly in AS exam questions.

• Plastic granules fed into machine via a hopper (funnel-shaped container that sits on top of the machine)
• Plastic pellets pushed through a heating chamber by a rotating screw, melting until liquid
• Molten plastic injected into mould cavity using high pressure (the mould has two halves that clamp together forming an internal cavity)
• Cools and harden inside the mould
• Mould opened to release the part
• Widely used
• Fast production, with consistent quality and accurate dimensions (able to rapidly repeat many products all the same)
• High costs to set up (not good for short runs)

Extrusion

• Plastic granules are fed into machine via a hopper (funnel-shaped container that sits on top of the machine)
• Machine is hot and melts plastic
• Molten plastic is pushed through a die with a shaped metal opening using a rotating screw
• Continuous shape emerges – a long, continuous profile matching the shape of the die (like squeezing toothpaste from a tube)
• Plastic cools and hardens as it moves out
• Cut to length as required

A die is a pre-made metal plate or block that shapes material – sometimes by flowing through it (hole), sometimes by cutting it (sharp edge – i.e. a die cutter), and sometimes by pressing/forming it (solid shaping surface).

Compression moulding

3D printing

• Create digital 3D model using appropriate CAD software such as Blender
• Save file as STL file
• Slice into layers using software linked to 3D printer such as Bambu Studio and add ‘supports’ as necessary
• Send file to 3D printer which heats plastic filament until molten and prints model layer by layer
• Remove printed item, clean the build plate and remove any support material

A2 students only: Stereolithography (SLA) is a type of 3D printing that uses a UV laser to selectively cure liquid photopolymer resin layer by layer, producing high-resolution parts with smooth surfaces — ideal for detailed work like jewellery or dental models, however it needs more work afterwards (sanding, washing, curing under UV light). Fused Deposition Modeling (FDM) instead melts a plastic filament and extrudes it through a moving nozzle, building objects layer by layer like a precise hot-glue gun; it’s cheaper, cleaner, and the most common technology in desktop 3D printers, but produces visible layer lines and less fine detail.

Line bending

• A strip heater is used to heat the plastic in a line, so that it can be bent (alternatively a blow torch can be used, moving this up and down the plastic in a line) – this heats the plastic just along the line where it needs to bend
• Use a jig and/or former to help bend the item over the edge of a table or around a former, and hold the item in shape while it cools
• Safety precautions:
  • Tie back long hair and keep loose clothing away from the strip heater
  • Use heatproof gloves
  • Do not touch the heating element or the softened plastic directly, and keep hands away from the hot area when moving or positioning the plastic.
  • Work in a well-ventilated area, to minimise inhaling harmful fumes
  • Do not overheat the plastic (remove it once it becomes flexible, before it bubbles, burns, or gives off smoke)
  • Switch off the strip heater after use and allow it to cool before moving or storing it.
  • Let the plastic cool fully before handling it or removing it from the former.

Hot wire cutting

Expanded polystyrene can be cut using a hot wire cutter, but fumes are toxic (forced air-extraction must be used and never do when wire is red-hot, as melts it too much, decomposes, toxic fumes, and fire risk).

Bandsaw or handsaw

• Can be used to cut plastics such as acrylic sheet or Styrofoam sheets (see the article about shaping wood for more information about these tools)
• When cutting acrylic, this makes a rougher edge than cutting with a laser cutter, which then needs sanding (see below).

Manual cutting (scissors or guillotine)

• Only suitable for one-off creations, or very small batches

Wet and Dry sandpaper

• A special type of sandpaper that can be used either dry or wet.
• Often used wet to stop the sandpaper clogging with plastic dust.

Brasso

• A metal polish (the name comes from “brass”) that you put on a cloth and rub onto metal to clean / polish door handles / instruments / stainless steel items etc (remove oxidation and make them shiny)
• Can be used on cut edges of acrylic to remove fine scratches and make a clear, shiny edge
• Cannot be used on wood as it is oily and stains the surface
• Give the acrylic a professional, transparent appearance
• Typical process: cut the acrylic with the bandsaw → smooth edges with wet and dry sandpaper (progressing through finer grades) → final polish with Brasso to achieve clear, professional-looking edges.

Sample examination questions (AS Design & Technology)