4 Gate type

As important as selecting the optimal gate size and location is the choice of the type of gate.

Gate types can be divided between manually and automatically trimmed gates.

Manually trimmed gates

Manually trimmed gates are those that require an operator to separate parts from runners during a secondary operation.

• The reasons for using manually trimmed gates are:

• The gate is too bulky to be sheared from the part as the tool is opened.

• Some shear-sensitive materials (e.g., PVC) should not be exposed to the high shear rates inherent to the design of automatically trimmed gates.

• Simultaneous flow distribution across a wide front to achieve specific orientation of.

• fibers of molecules often precludes automatic gate trimming.

• Gate types trimmed from the cavity manually include.

• Sprue gate

• Edge gate

• Tab gate

• Overlap gate

• Fan gat

• Film gate

• Diaphragm gate

• External ring

• Spoke or multipoint gate

Automatically trimmed gates

• Automatically trimmed gates incorporate features in the tool to break or shear the gate as the molding tool is opened to eject the part. Automatically trimmed gates should be used to:

• Avoid gate removal as a secondary operation.

• Minimize gate scars.

• Gate types trimmed from the cavity automatically include:

• Pin gate

• Submarine (tunnel) gates

• Hot runner gates

• Valve gates

4.2 Edge gate

The edge or side gate is suitable for medium and thick sections  and  can  be  used  on multicavity two plate tools. The gate is located on the parting line and the part fills from the side, top or bottom.

Dimensions

The typical gate size is 80% to 100% of the part thickness up to 3.5 mm and 1.0 to 12 mm wide. The gate land should be no more than 1.0 mm in length, with 0.5 mm being the optimum.

4.3 Tab gate

A tab gate is typically employed for flat and thin parts, to reduce the shear stress in the cavity.

The high shear stress generated around the gate is confined to the auxiliary tab, which is trimmed off after molding. A tab gate is often used for molding P.

Dimensions

The minimum tab width is 6 mm. The minimum tab thickness is 75% of the depth of the cavity.

4.4 Overlap gate

An overlap gate is similar to an edge gate, except the gate overlaps the wall or surfaces. This type of gate is typically used to eliminate jetting.

Dimensions

The typical gate size is 10% to 80% of the part thickness and 1.0 to 12 mm wide. The gate land should be no more than 1.0 mm in length, with 0.5 mm being the optimum.

4.5  Fan gate

A fan gate is a wide edge gate with variable thickness. This type is often used for thick-sectioned moldings and enables slow injection without freeze-off, which is favored for low stress moldings or where warpage and dimensional stability are main concerns. The gate should taper in both width and thickness, to maintain a constant cross sectional area.

• This will ensure that:

• The melt velocity will be constant.

• The entire width is being used for the flow.

• The pressure is the same across the entire width.

Dimensions

As with other manually trimmed gates, the maximum thickness should be no more than 80% of the part thickness. The gate width varies typically from 6 mm up to 25% of the cavity length.

4.6  Film or flash gate

A film or flash gate consists of a straight runner and a gate land across either the entire length                            or a portion of the cavity. It is used for long flat thin walled parts and provides even filling.                            Shrinkage  will  be  more  uniform  which  is  important  especially  for  fiber  reinforced                            thermoplastics and where warpage must be kept to a minimum.

Dimensions

The gate size is small, typically 0.25mm to 0.5mm thick. The land area (gate length) must                            also be kept small, approximately 0.5 to 1.0 mm long.

4.7  Diaphragm gate

A diaphragm gate is often used for gating cylindrical or round parts that have an open inside                            diameter. It is used for single cavity molds that have a small to medium internal diameter. It is                            used when concentricity is important and the presence of a weld line is not acceptable.

Dimensions

Typical gate thickness is 0.25 to 1.5 mm.

4.8  Internal ring gate.

4.9  External ring gate

This gate is used for cylindrical or round parts in a multicavity mould or when a diaphragm gate is not practical. Material enters the external ring from one side forming a weld line on the opposite side of the runner this weld line is not typically transferred to the part.

Dimensions

Typical gate thickness is 0.25 to 1.5 mm.

4.10  Spoke gate or multipoint gate

This kind of gate is used for cylindrical parts and offers easy de-gating and material savings.

Disadvantages are the possibility of weld lines and the fact that perfect roundness is unlikely.

Dimensions

Typical gate size ranges from 0.8 to 5 mm diameter.

4.11  Pin gates

Pin gates are only feasible with a 3-plate tool because it must be ejected separately from the part in the opposite direction The gate must be weak enough to break off without damaging the part. This type of gate is most suitable for use with thin sections. The design is particularly useful when multiple gates per part are needed to assure symmetric filling or where long flow paths must be reduced to assure packing to all areas of the part.

Dimensions

Gate diameters for unreinforced thermoplastics range from 0.8 up to 6 mm.  Smaller gates may  induce  high  shear  and  thus  thermal  degradation.  Reinforced  thermoplastics  require slightly  larger  gates  >  1  mm  The  maximal  land  length  should  be  1  mm.  Advised  gate dimensions can be found in the table below.

Dimensions of gates (* wall thickness larger than 5 mm should be avoided).

4.12  Submarine (tunnel) gates

A  submarine  gate  is  used  in  two-plate  mold  construction.  An  angled,  tapered  tunnel  is machined from the end of the runner to the cavity, just below the parting line. As the parts and runners are ejected, the gate is sheared at the part. The tunnel can be located either in the moving mould half or in the fixed half.A sub-gate is often located into the side of an ejector pin on the non-visible side of the part when appearance is important. To degate, the tunnel requires a good taper and must be free to bend.

Dimensions

Typical gate sizes 0.8 mm to 1.5 mm, for glass reinforced materials sizes could be larger.

A variation of the tunnel gate design is the curved tunnel gate where the tunnel is machined in the movable mold half. This is not suitable for reinforced materials.

4.13  Curved tunnel gate.

4.14  Hot runner gates

Hot runner gates are also known as sprueless gating. The nozzle of a runnerless mold is extended forward to the part and the material is injected through a pinpoint gate. The face of the nozzle is part of the cavity surface; this can cause appearance problems (matt appearance and rippled surface). The nozzle diameter should therefore be kept as small as possible. Most suitable for thin walled parts with short cycle times, this avoid freezing of the nozzle.

4.15  Valve gates

The valve gate adds a valve rod to the hot runner gate. The valve can be activated to close the gate just before the material near the gate freezes. This allows a larger gate diameter and smoothes over the gate scar. Since the valve rod controls the packing cycle, better control of the packing cycle is maintained with more consistent quality.