Sprue or Direct Gates

Type of gate
Sprue or Direct Gates are used with single cavity moulds, generally for thick walled
parts. In this pressure control is not necessary and the gate can be enlarged to ensure ease
of filling and packing to avoid sinks.
Gates are also described by their location in the mould, for example edge gated, tab gated
or submarine gated.
1; tab gating : used where it is desirable to eliminate gate defects from the part. The
gate is located on a tab which is attached to the part and which is removed in a
finishing operation.
2; submarine (tunnel) gating : this is particularly suitable for automatic moulding
because the part automatically delegates when it is ejected from the mould.
3; edge gating : occurs at the side of the part. It is usually used with parts which are
manually finished.

Round Pin Point Gates

Types of Gates
Round Pin Point Gates are widely used for automatic moulding because of the ease of
demoulding. The gate should be sharp edged at the runner and flared at the part, to
encourage separation at the runner gate interface, rather than affecting on the part surface.
Most moulds used for polyethylene use this type of gate.

Location of Gates

The location of the gate is extremely important to the satisfactory operation of the mould.
Gates must be located so that air contained in the cavity can be vented through easily
placed vents, usually at the parting line.
Weld lines should be avoided by locating the gate so that the flow proceeds uniformly out
from the gate in the direction of the greatest dimension, and by choosing the correct gate
design.
Gates should not be located at any location which is flexed or subject to mechanical
stress since the gate area is normally a weak spot in the part, due to high residual stresses.
Gate or land length has a major effect on pressure drop through the gate. Gate lengths are
usually fairly short, to minimize pressure drop.

functions of a gate.

Restricted gates are used in multi-cavity moulds where they perform the
following major functions :
1; provide a point between the part and the runner with low mechanical strength,
allowing easy separation.

2; provide a point in the runner system with a large surface-to-volume ratio which
will freeze rapidly when flow stops, sealing the cavity.

3; create a back pressure in the runner system to promote equal filling of all cavities
simultaneously.

Gate design of a mould

A gate is the connecting section between the runner system and the part. Gates can be
relatively large, providing full flow to the part or can be of smaller cross section than the
runner, restricting and accelerating the flow. Large gates are used to mould large thick
parts, or to avoid surface blemishes which might be caused by flow patterns from small
gates. Restricted gates are circular in cross section and are about 1.5 mm

A three plate mould

A three plate mould permits move gating of multiple moulds and makes it easy to
readily choose the moulded part from the runner system. In this system, when
the mould first opens, the cavity plate and the moveable or force plate open
together releasing the runner system and breaking the gate. As the mould
continues to open, separation occurs between the cavity plate and the force plate,
releasing the part.

The hot manifold

The hot manifold mould is another form of the hot runner system where in the
plastic in the runners is kept molten by heat from electrical heaters inserted into
the runners, sprues and gates.