It is necessary to provide a flow-way in an injection mould to connect the nozzle (of the injection machine) to the each impression. This flow-way is termed as feed system. Normally feed system comprises of
1) Sprue.
2) Runner.
3) Gate.
1) Sprue
The sprue is the channel along which the molten plastic first enters the mould. It delivers the melt from the nozzle to the runner system. The sprue is incorporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle.
The sprue has to demold easily and reliably and therefore has to be tapered. The taper is generally 2° in most cases.
2) Runner
The runner is the flow path by which the molten plastic travels from the sprue (i.e. the moulding machine) to the gates (i.e. the cavity). To prevent the runner freezing off prematurely, its surface area should be small so as to minimize heat transfer to the mould. However, the cross sectional area of the runner should be large so that it presents little resistance to the flow of the plastic but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection.
The following factors must be considered while designing a runner system.
Cross section of runner.
Size of the runner.
Runner layout.
The different types of runner profiles widely used in mould are
1) Half round.
2) Fully round.
3) Rectangular.
4) Hexagonal.
5) Trapezoidal.
3) Gate
Gate is the small orifice which connects the runner to the cavity. It has a number of functions. Firstly, it provides a convenient weak link by which the moulding can be broken off from the runner system. In some moulds the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the mould but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back.
Gate locations should ensure following conditions:
· Ensure a balanced flow (rapid and uniform filling) in the cavity so that certain areas of the part are not over packed.
· Ensure mold fills under realistic temperatures and pressures.
· Minimize weld lines as much as possible, or position them in non critical areas.
· Prevent "jetting" by positioning the gate so that the material flow is smooth and uniform.
· Avoid air entrapment.
· Gate into the thickest section and direct material flow.
Gates are classified in two categories based on the method of de-gating.
- Manually Trimmed Gates: Examples are Direct or sprue Gate, Ring Gate, Edge Gate, Fan Gate, etc.
- Automatically Trimmed Gates: Examples are Pinpoint Gate, Submarine or Tunnel Gate, etc.
