Injection Moulding Process:

Injection moulding is the process that moulds plastic through heat and pressure, by injection molten plastic polymer into the desired mould. The process starts with feeding the granular or powdered plastic, most often of the thermoplastic type, into the hopper above the heating cylinder of the machine. The resin falls into and is pushed along the heated tube by reciprocating screw until a sufficient volume of metal plastic is available at the injection nozzle end. The entire screw is then plunged forward to force the plastic into the mould. Each shot may produce one or several parts, depending on the die used. The ram is held under pressure for a few seconds so that moulded part can solidify. It then retracts slightly, and the mould opens. The knockout pins eject the moulded piece. The sprue and runners are trimmed off, usually in a separate trimming die, The Fig. 1.1. shows the different stages of the injection moulding process.

Injection moulding provides the highest production rate of producing plastic parts at a low cost. The time required per shot will vary with the material and the size of the mould, but 300 to 400 shots per hour in fully automatic equipment are not uncommon.

Injection moulding is the most commonly used process for moulding plastics because it gives a good surface finish and can be used for very complex mouldings. Injection moulding, although requiring substantial capital investment, becomes very economically viable in mass production and gives a very low unit production cost. Injection moulded plastics are invariably thermoplastic because thermosetting plastic assumes their final shape through heat and so cannot be moulded with this process. Familiar products manufactured by injection moulding include; computer enclosures, milk crates, CD cases and mobile phones.

Injection Mould

When dealing with injection mould tools, toolmakers and engineers refer to the various mould component parts with a universally adopted terminology. Basic tool types and components can vary enormously both in shape and size, some having similar names, which can easily lead to confusion when encountered by people new to the trade.

Mould Impression

The injection mould _.is an assembly of parts containing within it an ‘impression’ into ti plastic material impression may, therefore, be defined as that part of the mould which imparts shape to the moulding.

The impression is formed by two mould members

(i) The cavity, which is the female portion of the mould, gives the moulding its external form.

(ii) The core, which is the male portion of the mould forms, the internal shape of the moulding.

 

Cavity And Core Plate

The basic mould, in this case, consists of two plates. Into one plate is sunk the cavity which shapes the outside from of the moulding and is therefore known as the cavity plate. Similarly, the core which projects from the core plate forms the inside shape of the moulding. Fig 3.2 shows a simple hexagonal container. When the mould is closed, the two plates come together forming a space between the cavity and core which is the impression.

Sprue bush.

During the injection process, plastic material is delivered to the nozzle of the machine as a melt; it is then transferred to the impression through ‘a passage. In the simplest case, this passage is a tapered hole within a bush as shown in Fig. 3.3. The material in this passage is termed the sprue and the bush is called a sprue bush.

Runner And Gate Systems

The material may be directly injected into the impression through the sprue bush or for mould containing several impression (multi impression mould) it may pass from the sprue bush hole through a runner and gate system Fig. 3.4. before entering the impression.

Register Ring

Fig. 3.4. Feed system for multi- impression mould.

If the material is to pass without hindrance into the mould the nozzle and sprue must be correctly aligned. To ensure that this is so the mould must be central to the machine and this can be achieved by including a register ring. Fig. 3.5 shows the sister ring.

Guide Pillars And Bushes

To mould an even-walled article it is necessary to ensure that the cavity and core are kept in alignment. This is done by incorporating guide pillars on one mould plat which then enter corresponding guide bushes in another mould plate as the mould closes. An example with guide pillars mounted on the core side and corresponding guide bushes in the cavity side is shown in Fig. 3.5. The size of the guide pillar should be such that they maintain alignment irrespective of the applied moulding force; this they are normally able to do. All the constituent parts of the basic mould have now been described and a cross-section drawing of the assembled mould shown in Fig. 3.1.

Fixed Half And Moving Half

It can be seen in Fig. 3.5 that various moulds parts fall naturally into the two sections or halves. Hence, that half attached to the stationary platen of the machine (indicated by the dotted line) is termed the fixed half. The other half of the mould attached to the moving platen of the machine is known as the moving half. Now it has to be decided in which of the two halves the cavity or core is to be situated. Generally, the core is situated in the moving half and the over-riding reason why this is so is as follows.

The moulding, as it cools will shrink on to the core and remain with it as the mould opens. This will occur irrespective of whether the core is a fixed half or moving half. However, this shrinkage on the core means that some form of ejector system is almost certainly necessary. The motivation for this ejector system is easily provided if the core is in the moving half. Moreover, in the case of our single-impression basic mould, where a direct sprue feed to the underside of the moulding is desired the cavity must be in the fixed half and the core in the moving half.

FEEDING SYSTEMS

It is necessary to provide a flow- way in the injection mould to connect the nozzle (of the injection machine) to each impression. This flow way is termed the feed system. Normally the feed system comprises a sprue, runner and gate.

A typical feed system for a four-impression, two plate-type moulds is shown in Fig. 3.6. It is seen that the material passes through the sprue, main runner, branch runners and gate before entering the impression. It is desirable to keep the distance that the material has to travel down to a minimum to reduce pressure and heat losses.

RUNNER

The runner is a channel machined into the mould with the plate to connect the sprue entrance i.e., the gate to the impression. In the basic two-plate mould the runner is positioned on the parting surface while on more complex designs the runner may be positioned below the parting surface.

The wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the moulding there must be no machine marks left which would tend to retain the runner in the mould plate to ensure that these points are met, it is desirable for the mould designer to specify that the runner is polished in line of draw.

Runner Cross-section Shape

The cross-sectional shape of the runner used in a mould is usually one of four forms. shows various cross-sectional tapes of the runner.