PLASTIC FORMING METHODS

VACUUM FORMING:

• A process where sheets of thermoplastic polymer are heated and formed to shape by the application of a vacuum to 'pull' the material down onto a simple mould.

THERMOFORMING:

• A form of vacuum forming.
• The main differences are that air pressure and an additional female mould assist the forming process, to enable greater detail to be achieved.

- Advantages of thermoforming:

1. It's a low cost process.
2. It's good for smooth shapes with additional detail.

- Disadvantages of thermoforming:

1. Deep moulds result in a thinning of the wall thickness where is has been stretched.
2. It's limited to simple designs.
3. Trimming is usually needed.

--> Process = Sheet material is heated to just above its softening point and then held securely in a frame between the two mould halves. The mould halves close as the same time a vacuum is applied through the lower mould. The upper mould ensures the required amount of detail is achieved.

EXTRUSION:

• Used to produce plastic products that have uniform cross-section. Examples include curtains rails. window frame sections, guttering etc. The process can also be used to insulate wires and cables with a polymer.

- Advantages of extrusion:

1. Generally a low-cost process that requires only simple dies.

- Disadvantages of extrusion:

1. It can only produce continuous cross-sectional shapes.

--> Process =  (STEP 1) Thermoplastic powder is placed in the hopper; this powder then falls onto the rotating Archimedean screw, which in turn pushes the material towards a heated section of the extruder. (STEP 2) The heaters soften the plastic, which is then forced through the die by rotating the screw. (STEP 3) On exiting the die, the plastic is then cooled using a water jet. (STEP 4) Further along the transfer table, the product is cut to the required length.

CALENDARING:

• A process used to manufacture thermoplastic film, sheet and coating materials. Generally PE and PVC are used to create the sheet. For example, LDPE shopping bags.

--> Process = (STEP 1) The rollers are heated to just above the softening point of the thermoplastic. (STEP 2) During the rolling process, the plastic 'dough' is forced through the gap roller. These rollers determine the thickness of the material. (STEP 3) The final roller is the 'chill' roller that cools the material.

LINE BENDING:

• A method of processing sheet materials, in particular thermoplastics; limited to producing simple shapes.