Subsurface gate

The subsurface gate is a circular or oval gate which submerges and ‘feeds’ into the impression below the parting surface of the plastic injection mould, such as automotive mould,pipe fitting mould,commodity mould,home appliance mould. While similar to the round edge gate in that it is of similar or nearly similar shape and feeds into the side of the impression, it has several advantages over the round gate.

1. The from, being in one mould plate, has no matching problems and precise dimensions can be achieved.

2. If the more oval form is used the filling rate if the impression can be conrrolled independently of the gate seal time

3. The gate is sheared from the moulding during its ejection.

The basic design is shown in figure

1. The runner is terminated a distance X from the impression. A secondary runner, usually of conical form, is machined a an angle Φto the impression wall and is stopped short of the impression wall by a distance L ( Note that this corresponds to the gate land length). The gate is then machined at the same angle Φ to join the secondary runner to the impression.

The moulding and feed systems are removed separately from the mould and this means that a separate runner ejection is advantageous, particularly as a certain amount of deformation of the runner is necessary to remove the secondary runner from the mould.

The gate dimensions which must be considered are land length which because of the form, needs to be L (minimum)=1.9mm. Φis the angle between the centre-line of the secondary runner and the impression wall. This angle is normally between 30 degree and 45 degree. The dimensions for the gate cross-section can be estimated from either the equation suggested for the rectangular gate, if an oval gate is adopted, or the equation for the pin gate, if the circular gate is used.

The subsurface gate can be adopted for feeding into the inside surface of a component, providing at least one of the following conditions are met:

1. There is a suitable member which projects below the general parting line surface and which is also located relatively close to the component wall. The subsurface gate feeds directly in to this member.

2. In designs where it is possible to incorporate a small diameter peg relatively close to the component wall, the subsurface gate feeds directly into the peg and thereby into the impression. The peg is subsequently removes as a post moulding operation.

An example of the latter design is illustrated in Figure 2. The component impression (1), with added ‘peg’ impression (2) is shown in the moulding position at (a). The subsurface gate (3) feed under the base of the component wall, into one side of the peg. The reason why the peg (or other projection) must be relatively close to the wall will now be apparent to the reader. This distance control the length of the subsurface gate, and for practical reasons the gate length must be as short as possible.

An ejector pin (4 is provided immediately below the peg, in addition to the normal ejector system, which in this example is also by means of ejector pins (5). An undercut type ‘ puller’ (6) and associated sprue pin (7) is situated relatively close to end of the runner (8)

During the moulding phase the melt flows into the impression via the runner, subsurface gate and peg (gate) respectively. The moulding, and feed system remain in the moving half as as the mould opens. (figure 2(a)).

Ideally, although not an essential feature of the design, the ejection should take place in two stages. During stage one, the moulding and peg is ‘sheared from the subsurface gate in the process. The undercut type puller ensures that neither the runner or gate is dislodged during this first phase.

Finally stage two ejection occurs, which cause the undercut sprue puller to be ejected, and thereby extract the complete feed system from the mould plate as shown at (c)