Moulding Flashcards
(12 cards)
Injection moulding
Products
- packaging
- toys
- cating for electrical products
Pros
- good for mass production
- low unit cost for high volume
- precision molding
Cons
- high set up costs
- expensive molds to design and make
Injection molding process
- Feeding
Plastic granules are poured into a hopper (a funnel-shaped container). - Heating
The granules are moved through a heated barrel by a rotating screw. As they move, they melt into a liquid. - Injection
The molten plastic is forced into a metal mould using high pressure. - Cooling
The plastic cools and hardens inside the mould, taking its exact shape. - Ejection
Once solid, the mould opens and ejector pins push the plastic part out. - Trimming / Finishing
Any extra plastic (called flash) is trimmed off, and the product is ready for use.
Blow molding (pre form)
Products
- plastic bottles
Pros
- hollow shapes
- good for mass production
- thin walls to reduce weight and cost
Cons
- High set up costs
- Expensive molds to design and make
Blow Moulding (pre form)process
- Feeding
Plastic granules are fed into a hopper and sent into a heated barrel where they melt. - Forming the Parison
The melted plastic is shaped into a hollow tube called a parison. - Mould Closing
The parison is placed between two halves of a mould, which then close around it. - Blowing
Air is blown into the parison, forcing it to expand and take the shape of the mould (like blowing up a balloon). - Cooling
The plastic cools and hardens while inside the mould. - Ejection
The mould opens and the finished hollow product (like a bottle) is removed. - Trimming
Excess plastic is trimmed off, and the product is ready for use.
Blow Moulding (parison)
Products
- plastic bottles
Pros
- hollow shapes
- thin walls to reduce weight and cost
- good for mass production
Cons
- high set up costs
- expensive molds to design and make.
Blow moulding (parison) process
- Feeding
Plastic granules are poured into a hopper and fed into a heated barrel. - Melting
Inside the barrel, the plastic is heated and turned into a molten (liquid) form. - Forming the Parison
The molten plastic is extruded (pushed) through a die to form a parison — a hollow plastic tube. - Mould Closing
The parison is placed between two halves of a mould, which then close tightly around it. - Blowing
Air is blown into the parison, making it expand and fill the inside shape of the mould (like inflating a balloon). - Cooling
The plastic is left to cool and harden in the mould. - Ejection
Once solid, the mould opens and the finished product (e.g. a bottle) is removed. - Trimming
Any extra plastic around the edges is trimmed off to clean up the product.
Vaccum forming
Products
- yoghurt pots
- Blister packs
- Packaging
Pros
- good for batch production
- Inexpensive
- relatively easy to make moulds
Cons
- Accurate mold design needed to prevent webbing
- Large amounts of waste material produced
Vaccum forming process
A thermoplastic sheet is clamped over a mould (also called a former).
The plastic is heated until it becomes soft and pliable.
The air is removed from beneath the plastic using a vacuum.
Atmospheric pressure pushes the softened plastic tightly onto the mould.
The plastic is allowed to cool and harden in the shape of the mould.
The formed plastic is removed and excess material may be trimmed.
Extrusion
Products
- drain pipes , tubes
Pros
- continuous
- high production volumes
- low cost per unit
Cons
- Limited complexity of parts
- Uniform - cross sectional
shape only
Extrusion process
Plastic granules or pellets are fed into a hopper.
The granules are heated in a barrel until they soften.
A rotating screw pushes the softened plastic through a die (a shaped opening).
The plastic takes the shape of the die as it exits.
The continuous shape is cooled using a water bath or air.
The extruded plastic is cut to the desired length.
Rotational molding
Products
- buckets
- footballs
- traffic comes
- bins
Pros
- no seams
- uniform wall thickness
Cons
- low volume production
- labour intensive
Rotational molding process
A measured amount of thermoplastic powder is placed into a hollow mould.
The mould is clamped shut and heated in an oven.
While heating, the mould is rotated on two axes (bi-axial rotation).
The plastic melts and coats the inside walls of the mould evenly.
The mould continues to rotate while it is cooled to maintain an even layer.
Once cooled, the mould is opened and the hollow product is removed.
