1.15 Polymer Processes Flashcards
State two features of vacuum forming
Moulds are relatively easy to make but need a draft angle, air holes, and rounded corners
Not cost effective for one-off manufacture
Small-scale batch and mass production
State two features of thermoforming
Like vacuum forming, except an additional mould is pressed on the surface of the polymer sheet at the same time as the vacuum is applied
Good for fine details such as moulding in logos
Slow process, used in industrial production
State two features of calendering
Heated rollers squash and stretch the polymer to make it thinner
Industrial production method, carried out by specialist manufacturers
Used for continuous production
State two features of line bending
Uses an electrically heated element that provides heat along a line
Quite slow and labour intensive used in school or college workshops
Suitable for one-off or limited batch production
State two features of lamination (lay-up)
Requires a former to mould around
Polyester resin used as the polymer
A top gelcoat is applied which may contain pigment
CFRP requires an autoclave for the product to ‘cure’
State two features of injection moulding
Complex 3D shapes can easily be made
Fast industrial production method
High initial tooling cost
Used for large-scale mass or continuous production
State two features of blow moulding
Produces hollow thin-walled components
High initial tooling cost
Used for continuous production
State two features of rotational moulding
Seamless hollow components with a large wall thickness
Mould continuously rotated through heating and cooling
Large-scale batch or mass production
State two features of extrusion
Like injection moulding expect polymer is forced through a die
Industrial production method
Continuous position
State two features of compression moulding
Uses a pre-weighted ‘slug’ of polymer
Polymer removed from moulds
Large-scale batch production
The mould is placed on the bed of the machine, which is called the
‘platen’. The platen is lowered to the bottom of the machine.
2) Polymer sheet is clamped over the mould and a heater is pulled over the
polymer sheet.
3) When the polymer sheet has softened, the platen is raised into the
polymer and the heat is removed.
4)The vacuum pump is switched on, which sucks the polymer onto the mould.
5)Once the polymer has cooled and returned to a solidified state, the
platen is lowered and vacuum switched off.
6) The mould is removed from the moulding. Excess polymer is then
trimmed off.
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LAMINATION LAYER UP PROCESS
1 A mould or former in the shape of the product or component is
prepared. This might be made from timber, manufactured boards such
as plywood or high density foam.
2 The mould or former is coated with a release agent such as wax or PVA,
or is covered with parcel tape.
3 A top layer of gel coat is applied. The gel coat is a coating of polyester
resin often mixed with a pigment to give a colour to the moulding. The
resin might also have additives included to prevent degradation from
UV and hydrolysis. It using CFRP then curing takes place if a specialist
over; know as an autoclave.
4 Fibreglass matting is cut to size and laid over the former. Polyester resin
is brushed onto the matting and a small roller is used to push out any
air bubbles that may form and to smooth out the matting (this process
is called stippling). The matting is available in a range of stock forms
including chopped strand, woven and ‘tissue’, which is a fine grade
matting for covering the surface.
5 Step 3 is repeated until the desired thickness is achieved, and fine tissue
matting is used on the top layers. A vacuum bag may be used to
compress the layers of GRP before it sets.
6 The GRP is allowed to set.
INJECTION MOULDING PROCESS
1 Thermoplastic granules are poured into the hopper.
2 Ascrew thread is rotated by a motor. This pulls the granules through the
chamber and past electric heaters.
3 The heaters melt the polymer.
4 When a sufficient charge of polymer has melted and formed at the end
of the screw, a hydraulic ram forces the screw thread forward. This
injects the polymer into the mould.
5 The mould is water cooled, which enables the molten polymer to harden
quickly.
6 The mould opens and ejector pins push the moulding out.
7 Any excess polymer is trimmed off the moulding. Formers or jigs may
be used to maintain the dimensional accuracy of the moulding while it
cools and hardens completely.
1 Thermoplastic granules are poured into the hopper.
2 Ascrew thread is rotated by a motor. This pulls the granules through the
chamber and past electric heaters.
3 The heaters melt the polymer.
4 When sufficient polymer has melted, the hydraulic ram pushes the Archimedean screw, forcing the polymer through a steel die. The shape
of the die determines the shape of the extrusion.
5 The extrusion may be supported by rollers as it leaves the die and is
cooled by water or air.
6 The extrusion is then cut to the desired length.
COMPRESSION MOULDING PROCESS
1 A ‘slug? of pre-weighed thermoset polymer is inserted into pre-heated moulds.
2 The moulds are closed and hydraulic pressure is applied.
The pressure ensures that the polymer takes the shape of the mould.
3 The moulds remain closed while cross-linking takes place and the thermoset ‘cures’.
4 When the moulding has cured, the machine opens and the product is removed.
5 Excess polymer known as ‘flash’ is removed.
