Types of Mould
Mould : A hollow form or cavity into which molten plastic is forced to give the shape of
the required component. The term generally refers to the whole assembly of parts that
make up the section of the molding equipment in which the parts are formed.
various methods of plastic molding:
1. Injection Molding
2. Blow Molding
3. Compression Molding
4. Film Insert Molding
5. Gas Assist Molding
6. Rotational Molding
7. Structural Foam Molding
8. Thermoforming
9. Extrusion
The functions of a mould
• Allow hot plastic into the mould
.Shape the profile of component
.Conduct heat from injected plastic
.Resist temp., Pressure, stress, etc.
.Eject the part smooth
.Guide the sliding half with respect to Fixed half
.Have good polished surfaces
.Rigid enough to prevent deflection
.Suit moulding machine specification
The functions of a mould cont.,
.Have all external edges rounded off
.Produce consistent quality on long run
.Safe in operation
.Display identification of part, material and project
.Easy to assemble ,dismantle, repair & maintain
.Provision for lifting & loading on machine
.Have quick clamping facility
Injection Mould
• What is Injection moulding ?
• The process of Injection moulding is to melt Thermoplastic by heating and under
pressure force it into a closed mould. The hot plastic takes the shape and solidifies
inside the cold mould thereby plastic loses heat and mould gets heated up.
• Now the mould is opened sufficiently to eject the component.
Injection moulding
Injection moulding is a manufacturing technique for making parts from
plastic materials in production. Molten plastic is injected at high
pressure into a mould, which is the inverse of the product's shape. After a
product is designed, usually by an industrial designer or an engineer,
moulds are made by a mould maker (or toolmaker) from metal, usually
either steel or aluminum, and precision- machined to form the features of
the desired part.
Injection moulding
Injection moulding is widely used for manufacturing a
variety of parts, from the smallest component to entire
body panels of cars. Injection moulding is the most
common method of production, with some commonly made
items including spoons and outdoor furniture.
Injection Molding Process
• Injection Molding Process : With injection molding, granular plastic is fed by
gravity from a hopper into a heated barrel. As the granules are slowly pushed
forward by a screw-type plunger, the plastic is forced into a heated chamber
called the barrel where it is melted. As the plunger advances, the melted
plastic is forced through a nozzle that seats against the mould sprue bushing,
allowing it to enter the mould cavity through a gate and runner system. The
mould remains at a set temperature so the plastic can solidify almost as soon
as the mould is filled.
Rotational moulding
Rotational moulding is known as roto-forming. Rotational moulding is a process
in which a pre determined mass of powder, usually thermoplastics, is charged in
a closed mould and caused to simultaneously rotate into two planes while in hot
oven. The temperature would be between 260 and 485°C depending on
material and product. This causes the powder to melt and uniformly deposit on
the mould surface. When the melting face is completed the moulds are moved,
while still rotating, from the oven into a cooling chamber where the moulds are
cooled by air circulation or water spray, internal cooling to speed the cycle by
means of Nitrogen or water fog is currently being investigated as means of
lowering the cost.
After the cooling cycle the moulds are opened, and moulded parts are
removed. Gasoline tanks, toys, plastics balls etc. are produced by this method.
Rotational moulding
Advantages of rotational moulding are :
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Tooling is simple and inexpensive.
Parts produced are strain free.
Parts are usually made with uniform wall thickness.
No weld lines, sprue marks, gate marks / ejection marks on the parts
No scrap, or very little, is produced.
Hollow parts are easily produced with simple tooling.
Usually no secondary operations are required.
Wall thickness change can be easily made without new tooling or tool
modification.
Disadvantages are:
 Not suited generally for large production runs of small parts.
 Not suitable for production of parts with wall thickness less than 0.5.
Rotational moulding
Extrusion:
Extrusion moulding is used to make continuous shapes such as tubes, rods,
sheets, pipes, wires, and films out of thermo plastic materials. The extrusion
machine is similar to an injection moulding machine. The material is fed into the
machine through the hopper. The screw compresses and plasticizes the material
until a liquid melt is obtained. The melt is then pumped through the die, which
shapes the extrusion. On leaving the die, the extrusion is cooled, sized, solidified
and drawn of by some type of take off equipment. The final product is then cut to
length, reeled up or coiled.
Extrusion:
Blow moulding and Thermo forming
• Blow moulding and Thermo forming will discuss later with more details.