15.0 RUNNERLESS MOULD
15.1 Introduction:
Any mould in which a conventional runner system is not incorporated is called a runner less mould. A mould, which incorporates a direct feed from the nozzle, is a basic runner less mould.
This basic arrangement has been developed in order to reduce the sprue length or to eliminate the sprue altogether by using different types of nozzle design. There are four
15.2 Types of nozzle
1 Long reach nozzle
2 Barb nozzle
3 Anti-Chamber nozzle
4 Multi nozzle manifold
15.2.1 Long Reach Nozzle:
The advantage of keeping the length of the sprue as short as possible is to minimize the pressure drop across the gate and also to minimize the blemish left on the moulding when the sprue gate is removed. With the standard nozzle design, the length of the sprue gate is controlled by the depth of the mould plate. The length of the sprue gate can be reduced quite simply by designing a special nozzle variously known as “long reach nozzle or extended nozzle”. This extended nozzle protrudes in to a pocket machined in the mould plate, thus reducing the length of sprue gate. Heating it using a resistance type of band heaters or a thermocouple controls the temperature of nozzle.
(LONG REACH NOZZLE)
15.2.2 Barb Nozzle:
Conventional sprue gate results in a sizable blemish being left at the injection point. But sometimes this is undesirable. To reduce it to a minimum some form of pin gate is necessary. One method of achieving this form is by the use of a special nozzle in conjunction with a reverse-tapered sprue. This nozzle is variously termed as “barb nozzle or Italian nozzle”. The barb nozzle is similar to the standard nozzle except that there is a projection at
the front which incorporate barb
Barb Nozzle
15.2.3 Ante-Chamber Design:
This is also known as the hot well design. A small mass of material is retained in the antechamber. The plastic material adjacent to the mould I partially insulate the central core of plastic material from the cold mould, allowing it to pass through the antechamber in to the impression. As it is not necessary to remove the sprue from the mould fast moulding cycle can be achieved
Ante-chamber nozzle
15.2.4 Multi-nozzle manifold:
M2ulti-nozzle manifold is used for direct feeding of plastic material for multi imp4ression mould. It consists of a manifold block suitably drilled to provide a flow path for the material in to which the required number of nozzles is fitted. The manifold block is fitted to the cylinder of the machine via nozzle adapter and a lock ring. This permits even filling of the impression but separate temperature control of each nozzle is necessary. Each impression will have its own sprue bush. The bare nozzle or antechamber design can be used if pin gating is required. The number of impression is limited to the number of nozzles which can be accommodated with in the aperture in the platen. In view of this and other limitations, this design is not commonly used
Multi-nozzle manifold
15.3 HOT RUNNER MOULD
15.3.1 Introduction:
Hot runner designs are system by which the runner is kept at an elevated temperature with in the mould. This eliminates the necessity of removing the runner system from the mould as a part of the moulding cycle. The mould contains a heated runner manifold block with in its structure. The block, suitably insulated from the rest of the mould, is maintained at a slowly controlled elevated temperature to keep the runner permanently at melt temperature. The plastic material can be directed to the mould extremities with out loss of heat and with out the pressure losses associated with temperature variations.
The runner manifold block, termed as hot runner unit, is mounted adjacent to the cavity plate and accommodated in a grid. The plastic material enters via a centrally pos1tioned sprue bush, passes through the internal drillings and leaves the unit via a secondary nozzle in line with the impression. When the cavity and the sprue gate breaks at its small diameter, the remaining part of the feed system remains heated with in the unit, ready for the next shot.
15.3.2 The advantages of hot runner moulds are:
- Location of the impression is not confirmed to all central region of the mould. Therefore a large number of widely displaced impressions can be fed at the same time.
- The mould is not restricted to a particular machine.
- It allows for pin gating of moulding on multi-impression moulds.
- It allows for multi point pin gating on single impression and multi impression moulds.
- It allows for side or film gating for large moldings
- There is no feed system for the operator to remove from the mould.
- Thin wall –section moldings fill relatively easily because the melt is at higher temperature close to the impression.
- As there is no runner to cool, the cycle time is marginally reduced.
- On manually controlled machines, the mould opening time is reduced
15.3.3 Limitations of the hot runner unit type of mould:
1) The mould setting time is generally greater than for a corresponding two-plate mould
2) An extended period, waiting for the hot-runner unit to heat is required before production can start.
3) The initial debugging of a new mould is usually more extensive than with standard mould
4) The cost is higher than a standard mould
5) The area of the moulding adjacent to the gate may be blemished with surface heat marks
6) With certain material there are some degradation problem
7) Certain material have drooling problem
8) Tendency for certain heating elements to fail and consume more time to replace.
The hot runner unit is insulated from the rest of the mould parts. Two methods are generally used. One method is to fit small blocks of sindanyo insulating material to all sides of the unit by screws. Second method is to provide an air gap of at least 15mm all round the unit except where contact with the rest of the mould is necessary. The hot runner unit is accommodated in a pocket behind the mould plate
15.4 Valve-Gated Hot Runner Unit.
In this design of hot runner unit, valves are incorporated with in the unit to seal off the gates for particular period of moulding cycle. A needle valve is actuated when required by a hydraulic cylinder via a lever arm in the forward position. The front end of the needle valve seals against a complementary tapered orifice with in the secondary nozzle thereby preventing material flow through the gate. In operation, the valves are kept plunger has moved forward compressing the melt. At high pressure with in the injection cylinder and hot runner unit is developed. The hydraulic actuator then suddenly opens the needle valves and plastic melt enters the impression at high velocity. After the impressions are filled the needle valves are closed allowing the screw plunger to be withdrawn. This technique is known as pre-compressed moulding or explosion moulding.
The advantages are:
- No material can be dropped from the gate as it is positively sealed.
- Faster cycles can be achieved as the screw plunger can be withdrawn immediately the gates are sealed. The screw plunger can start to move forward even before the mould is closed.
- The mouldings produced by this technique exhibit less stress, less entrapped air, less apparent flow marks, thus improving the surface finish and physical properties.
- The amount of scrap is reduced since fewer shorts shot or warped mouldings are produced.
- Because of the fast filling rates, a lower melt temperature is possible. Thus there is less risk of degrading temperature sensitive material
15.5 Insulated Runner System:
This system is based on the principle that plastic can be used with in the mould as an insulation medium because of their low thermal conductivity. This design is a modification of the underfeed type, the difference being that larger runners are incorporated and a latch arrangement is included to keep the feed plate and floating cavity plates locked together. The plastic material forms an insulating akin of frozen material adjacent to the runner wall while the central core remains in the molten state.
Larger than normal runners are necessary to allow for this insulating skin and the diameter of the runner should not be less than 12mm. Quick release latches are necessary to permit the runner to be removed from between the feed and floating cavity plates, incase the runner became solid due to inconsistent conditions.
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