MOULDS AND INJECTION MOULDING MACHINE

 

MOULDS AND INJECTION MOULDING MACHINE

 

MOULDING

Moulding of plastic material comprises of forming an article to the desired shape by the application of heat and pressure in a hollow form or cavity.

 

The following major methods of transforming plastic materials into finished form are as follows.

 

THERMOSET PLASTIC MOULDING METHODS

·     Compression Moulding

·     Transfer Moulding

·     Injection Moulding.

 

THERMO PLASTIC MOULDING METHODS

·     Injection moulding

·     Blow moulding

·     Extrusion moulding

·     Roto moulding

·     Thermo forming

·     Calendaring

 

 

INJECTION MOULDING

It is one of the most widely used method for processing plastics (usually thermoplastic). In this phenomenon the raw material is plasticised outside the mould in a separate cylinder where it is heated. The plasticised material is then injected into the cavity and is allowed to cool by means of cooling water or oil circulating in the mould. After the material is cooled and solidified the mould is opened and component is ejected.

 

ADVANTAGES OF INJECTION MOULDING.

1.     Parts can be produced at high rate even with high volume of moulding.

2.     Different colours with matt finish can be obtained.

3.     Scrap is minimized as runners, gates and sprues are reground.

4.     Close tolerances can be achieved even for small parts.

5.     Moulding with reinforcement and inserts (metallic and non metallic) can be moulded with ease.

6.     Thermo set elastomers can also be processed in this moulds occasionally.

 

DISADVANTAGES

1.     Comparatively costly process.

2.     Skilled workers are necessary for manufacturing and operating these moulds.

3.     Assembly is not easy as there are more number of mating parts.

 

FOUR CAVITY- INJECTION MOULD

Component is having offset feeding system so the mould can be constructed Four cavity center feeding feeding directly from the sprue runner & gate. There is a edge gate provided at the side core  itself.

 

The basic mould(two plate mould) consists of two parts namely a fixed half and moving half. When these two halves are opened moulding can be extracted such as assembly is sometime reffered as single day light mould. When mould is opened here is only one space or as it is normally termed as daylight.

This type of mould consists of two main units namely.

1)    Fixed Half.

2)    Moving Half.

 

INJECTION MOULDING MACHINES

Injection moulding machine essentially consists of the units namely

Ø     The mould-clamping unit.

Ø     The injection unit.

Ø     Controlling unit.

Types of injection moulding machines

Mainly there are three types of machines.

1)    Hand injection-moulding machine.

2)    Piston type injection-moulding machine.

3)    Reciprocating screw type injection moulding machine.

 

HAND INJECTION MOULDING MACHINE

The hand injection moulding machines, which are available in ranges of capacities upto 2 ounce (28.4 grams). They are generally being used without thermo regulators.  The temperature controls provided along with the machine under the category thermostat, which are only indirect controls.

 

PISTON TYPE INJECTION MOULDING MACHINE

In this type of machines a cold piston moves inside a hot cylinder, which contains plastic materials. In order to facilitate dispersion of heat and get a homogeneous melt the cylinder is provided with a torpedo.  Actually when the piston moves forward the molten material diverges at one end of the torpedo.  Before getting shot through the nozzle.

 

THE PRINCIPAL VARIABLES THAT MUST BE CONTROLLED ARE

1.     The amount of plastic material introduced into the cylinder.

2.     The pressure applied to the piston.

3.     Piston speed.

4.     Temperature of the heating cylinder (nozzle temperature may also be controlled)

5.     Temperature of the mould.

6.     Piston forward time.

7.     Mould closing time.

8.     Mould clamping force.

9.     Mould opening time.

 

SCREW TYPE INJECTION MOULDING MACHINE

The screw type injection-moulding machine consists of a screw rod rotating In a hot barrel capable of elevating continuous melt of plastic.  By operating screw intermittently, one can get an intermittent flow of plastic through the nozzle. Because of the shearing action and material flowing in different patterns there is a homogeneous in the melt.

 

In screw type machine the mechanical energy from the screw is converted into heat, which is distributed uniformly throughout plastic as the screw rotates and mixes it.  Heating time and thermal degradation is reduced, and mixing of pigment in the plastic is more homogeneous.

 

INFLUENCE OF MATERIAL PROPERTIES ON THE MOULDING

As the plastic cools in the mould, it contracts, so the moulding smaller than the mould in which it was formed.  When the mould is being designed from a dimensioned drawing of the part, a shrinkage allowance must be added to the dimensions of the mould.

 

Shrinkage allowances are obtained from the plastics resin producer, usually given as a range of shrinkage values, the exact value depending on the shape of the moulding conditions.