Forming and processing polymers

Forming Polymers

• Polymers can be formed in various ways depending on different factors

Factors

• Whether the product is made in a school workshop or industrial facility
• What product or part is being formed or molded
• The applicable scale of production
• What polymer material is used
• Which forming methods are commonly used in a school workshop

Processing Polymers

• Polymers in powder or granule form get processed to produce usable stock for manufacturing

Vacuum Forming

• Vacuum forming is used both in school workshops and industry. Products include plastic yoghurt pots and bath tubs
• High impact polystyrene (HIPS) is the most common plastic for vacuum forming in schools
• Polyester, ABS, and acrylic are used in the industry

Vacuum Forming Process - Three Main Stages

• Sheet of thermoplastic is heated and pressed onto a former (mould) by atmospheric pressure
• vacuum reduces pressure below the softened thermoplastic
• The plastic takes the mould's shape, cools, and sets before the mould is removed

Mould Making - Features for a Good Product

• Corners and edges should be rounded with a small radius
• Positive draft angle greater than 3° is needed for easy removal
• Vent holes are needed where there are dips in the profile
• The profile should not be too deep
• It should have a smooth finish
• Avoid undercuts

Line Bending

• Strip heaters are used for line-bending to create a permanent fold in thermoplastic such as acrylic
• This is often used to create small products in school workshops
• It is ideal for creating display stands, menu holders, and photo frames

Line Bending Process

• Marking out should be done carefully before placing the sheet on the strip heater
• A line is marked using a chinagraph pencil or non-permanent marker
• The strip heater is heated
• The marked line on the workpiece is placed across the heating strip
• The time a line needs heating depends on the polymer sheet's type and thickness
• The polymer sheet is allowed to heat through
• The line is tested for flexibility and bent to the required angle when pliable
• Once set, the workpiece can be cooled in a water tray

Laminating or Layup

• Thermosetting polymer resins can be combined with fibres to produce products such as speed boats, rowing boats, canoes, caravan panels, and even bumper cars

Layup Process

• Fibre strands or matting, such as glass fibre, carbon fibre, or Kevlar, create composites
• The mould may be made from timber, board or metal
• A release agent is applied to the mould
• Polyester resin gel coat is applied and cured
• Matting is laid into the mould and resin is mixed with catalyst and brushed onto the matting
• In the industry, chopped strands may be sprayed onto the mould with resin. The product is then rolled to remove air bubbles and allowed to set

Thermoforming

• Like vacuum forming, thermoforming forms a heated, softened polymer sheet
• Unlike vacuum forming, thermoforming uses both male and female moulds
• The male mould is lowered onto the heated sheet, pressing it into the female mould
• A vacuum assists in pulling the polymer into the mould

Injection Moulding

• Injection moulding is used in industrial production to produce complex shapes
• Moulds are generally steel and made in two parts
• Polymers suitable for injection moulding: ABS, acrylic, and PVC

Injection Moulding Process

• Thermoplastic granules are fed into a hopper
• The hopper feeds an Archimedes screw that drags the granules past a heater where they become plasticized
• When soft, pliable plastic reaches the screw's end, a hydraulic piston forces it into a mould

Ejection and Finishing

• Water cools the plastic quickly
• The mould is separated, and ejector pins release the moulding that is repeated
• Any excess polymer flashing is trimmed and recycled

Blow Moulding

• Blow moulding uses a two-part mould and compressed air to make hollow shapes
• Polymers like HDPE, PET, PP, and LDPE are used to produce hollow plastic products like garden water butts, bins, and plastic toys
• It is also used to create packaging, such as bottles and containers

Blow Moulding Process

• Polymer granules are heated and extruded to create a plastic tube known as a parison
• The parison is fed into the mould and pinched at the bottom as the mould closes tightly
• It is filled with heated compressed air, which inflates the parison to fill the mould
• The plastic cools and takes the shape of the mould

Blow Moulding Finishing

• The cooled moulding is released and finished
• Any excess or flashing where the mould closed on the parison is trimmed automatically
• The waste material is recycled and reused
• Products are tested to ensure they are airtight and watertight. Rejects are sent for recycling

Rotational Moulding

• Similar to blow moulding, rotational moulding creates hollow plastic forms
• HDPE and PP polymers are commonly used
• The process produces a seamless product
• It is used for water tanks, communal recycling bins, kayaks, and plastic slides

Rotational Moulding Process

• Polymer granules or powder are weighed, based on the desired wall thickness, and put into the mould
• The mould is placed on a rig in a large oven and heated
• As it heats, it rotates slowly in two axes allowing the polymer to coat the inside of the mould evenly
• Once the desired thickness is reached, the mould is cooled
• There is a slight shrinkage as the polymer cools

Compression Moulding

• Compression moulding forms thermosetting polymers using a hydraulic press
• Polymers like urea formaldehyde and melamine formaldehyde create simple profiles like electrical fittings
• Elastomers are also formed using rubber shoe soles and silicone cake moulds for example

Compression Process

• Two separate male and female moulds are used, which create a void when pressed together.
• The moulds are heated, and polymer is placed between them
• Pressure is exerted, heating the charge and forcing it into the void
• It is then left to cool before the part is removed using an ejector pin
• The amount of polymer used is slightly greater than the amount needed to fill the void