Polymers production methods - DT

line bending

• Used to create simple bends in polymers

• In things such as acrylic and HIPS

• The sheet material is heated along a line using a machine called a strip heater

• Once the material has softened the heater can be removed

• The sheet bent to the desired angle using a former or a bending jig

• It then must be held in place until the polymer has cooled and become hard again

  

press moulding

• used for more complex shapes such as trays and dishes

• used with acrylic and HIPS

• 2 part formers called a yoke (the upper part) and a plug (the bottom part)

• the polymer sheet is heated in an oven to ensure in it heated evenly

• it is then positioned over the plug and pushed down using the yoke

• these are them clamped together until it has cooled

• one the piece had cooled you can remove any excess plastic

Vacuum forming

• plastic is heated evenly until soft

• then air pressure is used to shape it over a mould

• the heating element is similar to the grill on a cooker and is easily moveable to access the machine

• a sheet of suitable polymer (HIPS or acrylic) is fixed across the top of the machine by clamping

• it must be airtight

• below the sheet is the mould

• when the sheet is hot and soft the heater is moved out the way and the mould is raised and the air between the mould and the plastic sheet in evacuated by an air pump

• the air pressure on the outside of the sheet then presses the plastic into close contact with the mould

• vacuum formed products will have the same thickness throughout

injection moulding

• granular plastic is held in a hopper

• it is moved via an Archimedean screw along a heated tube, called the heating chamber

• once the polymer is melted, it is pushed into the mould with a hydraulic ram

• the use of the hydraulic ram ensures just enough material is injected into the mould each time

• the mould is then cooled so that the moulded plastic can be removed

• PE and ABS is used

• used for lego bricks, bottle caps

Blow moulding

• Granular plastic is fed through into a hopper

• then a pre made mould is fitted into the machine

• a tube of softened plastic is extruded into the mould

• hot air, at a high pressure, is blown into the parison, causing it to expand and fill the mould

• the finished product is then cooled and released from the mould

• uses: poly(chloro-ethene), PVC, PET

• makes: toys, drums, plastic bottles

Extrusion moulding

• feeding the plastic pellets into a hopper

• transported into the heating chamber

• heating chamber, also known as the extruder

• the extruder is a machine that consists of a screw that rotates within the heated chamber

• the screw is designed to to malt the plastic pellets and push them towards the die

• the temp and speed of the screw is controlled to ensure the plastic is heated evenly and at the correct temp

• the molten plastic is then forced through the die to create the desired shape

• the die can be customised

• the extrusion process must happen carefully to ensure that the plastic is extruded properly

• this includes controlling the temp and pressure

• the plastic is then cooled

• and once it is cooled it can be trimmed to remove any excess material

• uses: polyethylene, polypropylene or polystyrene

Rotational moulding

• a mould is filled with a precise measure of plastic paste or powder

• the mould is heated and rotated over all three axes

• this means the powder melts and spins out to the sides of the mould in an even layer

• the mould is cooled down with air and water and then opened

• uses: nylon, polyethylene, polypropylene, polycarbonate

• makes: barrels, oil drums, beach balls, septic tanks