Moulding Flashcards
Blow moulding- process
To create hollow unusual shapes from HPDE, LPDE, PET, PP, PS or PVC.
The hollow length of plastic (parison) goes between the two halves of the mould which are clamped at the bottom to seal the shape. Compressed air is blown into the inside which inflated the plastic pushing it to the surfaces of the mould. The mould is water cooled so sets the shapes in place. The waste then gets trimmed off and the bottle gets tested for imperfections. This is done by adding pressurised air to check that the seams are strong. Any waste gets shredded and reused at the start of the process making it a closed loop system (no waste).
Blow moulding- applications, advantages and disadvantages.
Plastic bottles and containers.
Intricate shapes are formed, hollow shapes with thin walls, mass production and low costs.
High initial set up costs.
Injection moulding- process
To create large quantities of identical, complicated, solid shapes from HIPS, ABS, nylon, PP and PS.
Plastic powder or granules are fed from the hopper and into the steel barrel containing a screw. The heaters that line the outside of the barrel heat the plastic and the screw pushes them along the barrel towards the mould. The screw pulls back and then the hydraulic ram forces the screw forward injecting the plastic through a small channel into the mould. The mould is water cools so once the plastic is cooled the process is repeated.
Injection moulding- applications, advantages and disadvantages.
Patio furniture, bottle tops.
The moulds are made from steel so complicated shapes can be made, mass production and a low unit cost for each moulding. High quality finish or texture can be added to the mould.
High initial set up costs.
Vacuum forming- process
To create shaped products from HIPS, acrylic and PVC.
A mould is attached to the support plate which is then lowered. The thermoplastic sheet is clamped in and heated from below. Once it has softened air is blown under to make it into a bubble ready to be shaped around the mould. The support plate is raised and the hot plastic shapes around the mould. Once cooled trim and finish the edges.
Vacuum forming- applications, advantages and disadvantages.
Yogurt pots, blister packs, food packaging and garden pond liners.
Quick process, batch production, easy to make the moulds.
Can’t get straight edges, weak points from stretching the plastic, lots of waste formed as thermoset, only one side formed at a time.
Extrusion- process
To create long products (hollow or solid) that have a particular cross section.
Plastic powder or granules are fed from the hopper and into the steel barrel containing a screw. The heaters that line the outside of the barrel heat the plastic and the screw pushes them along the barrel towards the mould. The softened plastic flows through a die continuously. The die determines the cross section of the extrusion. As it leaves the die it is cooled by passing through cold water.
Extrusion- applications, advantages and disadvantages.
Window frames and hose pipes.
The thickness of the material is determined by the die. Dies are simple and inexpensive to make.
High set up costs.
Rotational moulding- process
Used to make hollow sealed products from a powder of PE.
The mould is filled with plastic powder and then closed. It rotates and heated which makes the plastic melt and stick to the edges of the mould. The mould is cooled whilst rotating which sets the product.
Rotational moulding- applications, advantages.
Footballs and road cones.
Evenly spread layer of plastic, complex hollow shapes, easy to produce large products, no seem.
