Polymer Processes

Question 1

Name 10 polymer processes

Answer 1

1. Vacuum forming
2. Thermoforming
3. Calendering
4. Line bending
5. Lamination (lay-up)
6. Injection moulding
7. Blow moulding
8. Rotational moulding
9. Extrusion
10. Compression moulding

Question 2

Explain Vacuum forming

Answer 2

• 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

Question 3

What polymers are used in vacuum forming ?

Answer 3

Thermoplastic sheets usually HIPS

Question 4

What production scale is vacuum forming used for ?

Answer 4

Small scale batch production - as its quite a slow process

Question 5

What sort of products are made with vacuum forming ?

Answer 5

Things such as packaging, trays, panels

Question 6

What is thermoforming ?

Answer 6

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

Question 7

What’s an advantage of thermoforming over vacuum forming ?

Answer 7

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

Question 8

What is Calendering ?

Answer 8

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

Question 9

Explain line bending

Answer 9

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

Question 10

What product is line bending usually used for ?

Answer 10

Line bending is usually to make:

Acrylic boxes, shelves or point of sale displays

Question 11

What products are made with thermoforming ?

Answer 11

Food packaging
Sandwich boxes
Things with more detail than vacuum forming provides

Question 12

Explain lamination - lay up process

Answer 12

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

Question 13

What are examples of products made with the lay up process ?

Answer 13

Boats, kayaks

Question 14

What are products that can be made by injection moulding ?

Answer 14

Anything that has a lot of detail to it

Things such as casing on electrical products like TV’s, computers etc

Question 15

Explain the process of injection moulding

Answer 15

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

Question 16

What products is blow moulding used to produce ?

Answer 16

Bottles and lots of other hollow products

Question 17

What polymers are usually used in blow moulding ?

Answer 17

PET, LDPE, HDPE, PP

Question 18

Explain the process of blow moulding

Answer 18

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

Question 19

What products is rotational moulding used to produce ?

Answer 19

heavy duty, seamless hollow objects that need a large wall thickness - for e.g. traffic cones and kayaks and water tanks

Question 20

What polymers are most commonly used in rotational moulding ?

Answer 20

HDPE and PP

Question 21

Explain the process of rotational moulding ?

Answer 21

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

Question 22

What products is extrusion used to produce ?

Answer 22

Coating electrical wire for insulation - solid rods - hollow tubes - STOCK MATERIAL

Question 23

Explain the process of extrusion

Answer 23

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

Question 24

What polymers are usually used in compression moulding ?

Answer 24

Thermosetting polymer such as urea formaldehyde ( UF ) and melamine formaldehyde ( MF )

Question 25

What products are usually made by compression moulding ?

Answer 25

UF electrical fittings, switches and plugs, MF plates, mugs and bowls

Question 26

Explain Compression moulding

Answer 26

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