Polymer Processes Flashcards
Name 10 polymer processes
- Vacuum forming
- Thermoforming
- Calendering
- Line bending
- Lamination (lay-up)
- Injection moulding
- Blow moulding
- Rotational moulding
- Extrusion
- Compression moulding
Explain Vacuum forming
- First the mould is placed on the platen ( the bed of the machine ) then the platen and mould is lowered to the bottom of the machine
- Polymer sheet (thermoplastic) is clamped over the mould and a heater is placed over the polymer sheet
- The polymer softens and then the platen with the mould is raised into the polymer and then the heat gets removed
-Then the vacuum pump is switched on - this sucks all the air out and this draws the polymer over the mould
- Once the polymer has cooled and returned to its solidified state - the platen is lowered and the vacuum is switched off - the polymer gets removed and the excess polymer gets trimmed off
What polymers are used in vacuum forming ?
Thermoplastic sheets usually HIPS
What production scale is vacuum forming used for ?
Small scale batch production - as its quite a slow process
What sort of products are made with vacuum forming ?
Things such as packaging, trays, panels
What is thermoforming ?
Thermoforming is nearly exactly the same as vacuum forming - only difference is that there is 2 moulds - the one at the bottom is a negative mould and the one pressing it down is called the positive mould
After the heater has softened the polymer sheet, the vacuum is turned on and then the positive mould is pressed down onto the polymer and the vacuum sucks the polymer down onto the negative mould - the 2 moulds then trap the softened in between them - this gives the polymer extra detail
What’s an advantage of thermoforming over vacuum forming ?
In thermoforming, the polymer can have more detail as there is 2 moulds and they trap the softened polymer in between them which can give the polymer more detail
What is Calendering ?
It is a smoothing and rolling process used towards the end of the manufacturing paper and to make thin polymer sheets and film
Pellets of thermoplastic are heated so that they can melt into a dough-like consistency - it is then fed into a series of heated rollers that roll the polymer out ( IT IS EXTRUDED ) - this polymer becomes squashed and stretched - this is in order to make the polymer thinner
Explain line bending
This process is used to produce bends in sheet thermoplastic such as acrylic PMMA
A strip heater provides heat along a straight line - this makes the polymer sheet pliable so it can be bent
What product is line bending usually used for ?
Line bending is usually to make:
Acrylic boxes, shelves or point of sale displays
What products are made with thermoforming ?
Food packaging
Sandwich boxes
Things with more detail than vacuum forming provides
Explain lamination - lay up process
Firstly, the mould/former is prepared and coated with a release agent such as wax or PVA and this helps remove the product out of the mould later
Then a top layer of gel coat is applied - the gel coat is a coating of polyester resin which is often mixed with a pigment to give a colour to the moulding - the resin may also have additives so that to prevent degradation from UV and water
Then another layer of gel coat is applied until the desired thickness is achieved
Then fibreglass matting is cut to size and laid over the mould - before this the matting is brushed with polyester resin and a small roller is used so that any air bubbles get pushed out
Then the matting gets placed on top of the gel layers then the GRP is allowed to set
What are examples of products made with the lay up process ?
Boats, kayaks
What are products that can be made by injection moulding ?
Anything that has a lot of detail to it
Things such as casing on electrical products like TV’s, computers etc
Explain the process of injection moulding
Thermoplastic granules are poured into the hopper and are pushed through the chamber and by a screw thread (called the archemedies screw ) and this screw is rotated by a motor
Then the plastic granules get pushed past the heaters which melt the granules
Then a hydraulic ram forces the thread forward, which injects the polymer into the mould at a high pressure
The mould is folded and opened
Any excess polymer is trimmed off the moulding
What products is blow moulding used to produce ?
Bottles and lots of other hollow products
What polymers are usually used in blow moulding ?
PET, LDPE, HDPE, PP
Explain the process of blow moulding
A polymer is fed into a hopper
An Archemidies screw pushes the polymer through a heated section which melts the polymer
The polymer is extruded as a tube which us called a parison
The mould sides close around the parison and air is forced into the mould at high pressures - forcing the polymer to the sides of the mould - the polymer then takes on the shape of the mould
The polymer is allowed to cool - the mould opens and the finished bottle is ejected
What products is rotational moulding used to produce ?
heavy duty, seamless hollow objects that need a large wall thickness - for e.g. traffic cones and kayaks and water tanks
What polymers are most commonly used in rotational moulding ?
HDPE and PP
Explain the process of rotational moulding ?
First polymer granules or powder are loaded into a mould which us clamped and sealed
The mould is then transferred to an oven where it is heated to melt the polymer - the mould is then rotated slowly around 2 axes and as the polymer is heated - it coats the inside of the mould
Once the polymer has achieved the correct thickness - the mould is cooled - usually with a water/fan
Then when the polymer has solidified - the part will shrink in slightly - allowing it to be removed
What products is extrusion used to produce ?
Coating electrical wire for insulation - solid rods - hollow tubes - STOCK MATERIAL
Explain the process of extrusion
Polymer granules are loaded into a hopper and an Archimedean screw moves the polymer granules past the heater and this melts the polymer
Then when a sufficient amount of polymer is melted - the hydraulic ram pushes the Archimedean screws, forcing the polymer through a steel die - the shape of the die determines the shape of the extrusion
The extrusion may be supported by roller as it leaves the die and then cut to the desired length
What polymers are usually used in compression moulding ?
Thermosetting polymer such as urea formaldehyde ( UF ) and melamine formaldehyde ( MF )
What products are usually made by compression moulding ?
UF electrical fittings, switches and plugs, MF plates, mugs and bowls
Explain Compression moulding
A bit of pre heated plastic charge is paced onto the lower pre heated mould
The moulds are closed and the hydraulic pressure is applied - the pressure ensures that the polymer takes the shape of the mould
The mould remains closed while the thermoset cures
When the moulding cures - the machine opens and the product is removed
Excess polymer known as flash is removed
So there is a mould that is heated and the polymer is heated and placed into the mould then a positive mould - hydraulic press is pressed onto the polymer and this pressure causes the polymer to be drawn over the mould and then the polymer is left to cure and then the mould opens and the polymer is removed ad then the excess plastic known as flash is trimmed off
