forming polymers

polymers often arrive at manufacturing plants as granules and need to be converted into a piece such as a cable or sheet, they can also be calendared into a film. different methods are used depending on the scale of production, what material is being used and whether it is in a workshop or a factory

METHODS

vacuum forming - vacuum forming is used for lots of different products that need to be cupped from a sheet form. the sheet is heated and then is lowered over a mould, then the air is pumped out pulling the sheet around a mould, then the piece is cooled and lifted. vacuum forming requires a very accurate mould that must have rounded edges to encourage stretching, have a draft angle greater than 3° with vent holes to stop air bubbles forming in dips. vacuum forming moulds cannot be deep and must have a smooth finish with no undercuts. vacuum forming often uses HIPS as it is very flexible and thermoforming

thermoforming -used to form thin sheets of polymer that have been heated, using a male and female mould, the polymer is moved onto the mould using a transport chain and then the sheet is heated and lowered as a vacuum helps the sheet fit the mould

injection moulding - injection moulding is used on complex pieces using two part steel moulds. ABS, PVC and acrylic are used in this process. the granules enter a hopper where they are heated and plasticised before a motorised hydraulic system turns an archimedean screw, this pushes the melted polymer at a high pressure through a barrel into a mould. once the peice is finished flashing is trimmed and spruces and excess polymer is removed, this can be recycled and reprocessed. to remove the piece an ejector pin often has to pop the polymer out which can leave a mark

extrusion - extrusion is used to make thin tubes of polymer by using a heated screw to push melted polymer granules through a die and water tank into pull rollers. molten extruded polymer is known as parison. afterwards the extruded polymer is wound up and packaged. extruders don’t have too make cylindrical forms but are use for pipes or wires, they can also be used to make filament for 3D printing, before the extruders are at the right temperature some waste is produced

blow moulding - this is used for hollow shapes that can be made using a two part mould, this is used on HDPE, PET and LDPE, as air is blown into a mould the polymer is pressed into a mould and dries, this is often used for creating bottles. blow moulding requires an extruded parison to be fed into the mould before blowing and the waste flashing is removed and recycled. pieces are also checked for leaks and deformities as part of quality control

rotational moulding - this is used for hollow shapes that are high spec and heavy duty such as kayaks, these products are often expensive. the mould is heated in an oven using a set weight of granules, then the mould is spun around on a large machine for a set time, then the mould is cooled and the shape is produced. the product shrinks after manufacture meaning it is easy to remove. this process is often done in batch production of bins and lockers

compression moulding - a male and female mould are used as a hydraulic press shapes the polymer, this is done for thick pieces on MF and UF, this can also be used on elastomers on shoes and flexible pieces, after the piece has cooled in a mould an ejector pin removes the pice