Polymer Processes/Manufacturing And Finishes

Question 1

Types of Polymer Processes to know:

Answer 1

• Vacuum Forming
• Thermoforming
• Line Bending
• Laminating (Layup)
• Injection Moulding
• Blow Moulding
• Rotational Moulding
• Extrusion
• Compression Moulding

Question 2

How can you tell if something is Injection Moulded

Answer 2

• Shut Lines
• Sprue Mark
• Complex Shape
• Lots of detailed features

Question 3

Process of Injection Moulding

Answer 3

1. Plastic granules are fed into the hopper.
2. The screw in the chamber below the hopper sends the granules forward.
3. Heated jackets around the screw melt the polymer.
4. The screw winds back and the hydraulic ram comes forward into position.
5. The mould is closed and sealed as the ram forces the melted plastic into the mould.
6. The plastic is allowed to cool and the halves of the mould release.
7. The “sprew” is machined off.

Question 4

How can you tell if something has been Vacuum Formed

Answer 4

• Simple, tray shape
• Sloped Sides
• Thin Walls
• Rounded Edges

Question 5

Process of Vacuum Forming

Answer 5

1. The mould is placed into the former.
2. A sheet of thermo-softening plastic, such as HIPS is clamped over the mould.
3. The plastic is slowly heated to become soften.
4. The mould is raised and the air is sucked, drawing the plastic over the mould to take on it’s shape.
5. The plastic is allowed to cool and then removed from the mould.
6. The edges must then be trimmed and finished.

Question 6

What is Thermoforming

Answer 6

Thermoforming is a process of heating a thermoplastic sheet to its softening point. The sheet is stretched across a single-sided mould and then manipulated. Then, it cools into the desired shape.

(In male moulds, the plastic is thickest at the top where the plastic first meets the mould, and thinnest at the flange.)

Question 7

How does Thermoforming Work?

Answer 7

It is very similar to vacuum forming, except there is additional mould that is pressed onto the surface of the polymer sheet at the same time as the vacuum is applied, sucking the polymer down into the mould below. The two moulds trap the softened polymer in between them, giving extra detail to the moulding.

Question 8

What is Line Bending

Answer 8

Line or strip bending is a process used to form straight, small curvature bends in thermoplastic sheet material such as Acrylic. Line or strip bending is a process used to form straight, small curvature bends in thermoplastic sheet material such as Acrylic and ABS. Line bending is carried out in both school and industrial situations.

Question 9

How to tell if something has been Blow Moulded

Answer 9

• Shut Line
• Sprue Mark
• Closed Neck/Hollow Shape

Question 10

Process for Blow Moulding

Answer 10

1. Mould is opened into it’s two halves.
2. A hollow tube of polymer known as the parison is heated.
3. The parison is lower into the mould.
4. Mould is clamped shut, leaving only a small hole for the air hose.
5. Air is forced into the mould at high pressure. The plastic is forced to the mould sides and cools.

Question 11

How to tell if something has been Extruded

Answer 11

• Long regular shape
• Can be tube/bat
• Shapes are linear

Question 12

How does Extrusion Work?

Answer 12

This process can be compared to squeezing toothpaste from a tube. It is a continuous process used to produce both solid and hollow products that have a constant cross-section. E.g. window frames, hose pipe, curtain track, garden trellis.

(It is like Injection Moulding, but the plastic goes through a steel die, rather than into a mould)

Question 13

Process of Extrusion

Answer 13

1. A motor turns a thread which feeds granules of plastic through a heater.
2. The granules melt into a liquid which is forced through a die, forming a long ‘tube like’ shape.
3. The extrusion is then cooled and forms a solid shape.
4. The shape of the die determines the shape of the tube.

Question 14

What is Compression Mouding

Answer 14

Compression moulding is the oldest commercial plastics moulding process and is mainly used to make products from thermosetting materials. A combination of heat and pressure is used to change the material’s form and chemical structure.

Question 15

What is Calendaring

Answer 15

Calendaring is a smoothing and rolling process used towards the end of manufacturing paper. It is also used in making thin polymer sheet and film as stock material for further processing into products later in the product cycle.

Question 16

How does Calendaring Work?

Answer 16

Calendaring works by heating pellets of thermoplastic so that it melts into a dough-like consistency. It is then extruded between a series of heated rollers so that it becomes squashed and stretched to make it thinner. It is then finished on cooling rollers, before being shopped into standard stock form sizes.

(Calendaring is purely an industrial product method, carried out by specialist manufacturers - used in continuous production)

Question 17

What is Rotational Moulding (+ examples)

Answer 17

Rotational moulding is a process used mainly to manufacture hollow shaped products such as footballs, traffic cones and storage tanks up to 3m^3 capacity.

Setup costs are usually quite high, so this process is normally used for large-scale batch or mass production.

Question 18

What is the Process of Rotational Moulding

Answer 18

1. Polymer powder or granules are loaded into a mould, which is clamped and sealed.
2. The mould is transferred to an oven where it is heated to 260 - 370°C, depending upon the polymer used. The mould is rotated slowly (fewer than 20 rotations per minute) around 2 axes, and as the polymer is heated, it coats the inside of the mould.
3. Once the polymer has achieved the correct thickness, the mould is cooled. Usually a fan and/or water is used to cool the polymer.
4. When the polymer has solidified, the part will shrink slightly, allowing it to be removed.

Question 19

What is Compression Moulding (+ examples)

Answer 19

Compression moulding is the oldest commercial plastics moulding process and is mainly used to make products from thermosetting materials. A combination of heat and pressure is used to change the material’s form and chemical structure.

Examples of products made with this can be switches and electrical plugs, or polymer plates, mugs, and bowls for example.

Question 20

What is the Process of Compression Moulding

Answer 20

1. A ‘slug’ or pre-weighted thermoset polymer is inserted into pre-heated moulds.
2. The moulds are closed a 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.

Question 21

What is Overmoulding and why is it used

Answer 21

Some items that have been injection moulded may be over moulded to add a more rubbery material to add more grip for example. An example of where this is used are toothbrushes, or drills (essentially most things with rubber grips on them). Overmoulding is used to improve appearance and functionality of the product. It is done with a special injection moulding machine with two end bits, and a specific type of mould.

The bond between the materials are extremely strong. The process is largely automated

Question 22

Downsides of Overmoulding

Answer 22

Designing the specific equipment takes time and is expensive. Makes the material very difficult to separate, and therefore more difficult to recycle.

Question 23

What is Insert Moulding

Answer 23

Insert moulding is where a part (like a tapped hole or a nut) is inserted into the mould before injection moulding, before the thermoplastic is injected around it. Workers may need to insert the item by hand which may slow down production and cost more.

Question 24

What is Lamination (Lay-Up) (+ examples of products that include it)

Answer 24

The lay-up lamination process is used to make 3D products from fibre-based composites like GRP or CFRP.

Examples of products that are made with the lay-up process include:
• Boats and yachts
• Kayaks
• Trains

Question 25

Process of Lamination (Lay-Up)

Answer 25

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 of PVA, or is covered with parcel tape.
3. a tip 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 resign might also have additives included to prevent degradation from UV and hydrolysis. If using CFRP then curing takes place in a specialist oven, known as an autoclave.
4. Fibreglass matting is cut to size and laid over the former. Polyester resin is brushed onto the matting an 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 used in 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.