Two categories of gates
Gates can have a variety of configurations. They are classified into two categories- manually trimmed and automatically trimmed-based on the method of de-gating.
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.
The following gate types are trimmed from the cavity manually:
|Direct (sprue) gate|
Edge (standard) gate
Disk (diaphragm) gate
Spoke (spider) gate
Film (flash) gate
A direct (or sprue) gate is commonly used for single-cavity molds, where the sprue feeds material directly into the cavity rapidly with minimum pressure drop. The disadvantage of using this type of gate is the gate mark left on the part surface after the runner (or sprue) is trimmed off. Freeze-off is controlled by the part thickness rather than determined the gate thickness. Typically, the part shrinkage near the sprue gate will be low; shrinkage in the sprue gate will be high. This results in high tensile stresses near the gate.
The starting sprue diameter is controlled by the machine nozzle. The sprue diameter here must be about 1.0 mm larger than the nozzle exit diameter. Standard sprue bushings have a taper of 2.4 degrees, opening toward the part. Therefore, the sprue length will control the diameter of the gate where it meets the part; the diameter should be at least 1.5 mm larger than or approximately twice the thickness of the part at that point.
- A smaller taper angle (a minimum of one degree) risks not releasing the sprue from the sprue bushing on ejection.
- A larger taper wastes material and extends cooling time.
- Non-standard sprue tapers will be more expensive, with little gain.
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 used extensively for molding PC, acrylic, SAN, and ABS types of materials.
The minimum tab width is 6.4 mm. The minimum tab thickness is 75% of the depth of the cavity.
An edge gate is located on the parting line of the mold and typically fills the part from the side, top, or bottom.
The typical gate size is 6% to 75% of the part thickness (or 0.4 to 6.4 mm thick) and 1.6 to 12.7 mm wide. The gate land should be no more than 1.0 mm in length, with 0.5 mm being the optimum.
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.
The typical gate size is 0.4 to 6.4 mm thick and 1.6 to 12.7 mm wide.
A fan gate is a wide edge gate with variable thickness. It permits rapid filling of large parts or fragile mold sections through a large entry area. It is used to create a uniform flow front into wide parts, 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:
1. The melt velocity will be constant
2. The entire width is being used for the flow
3. The pressure is the same across the entire width.
As with other manually trimmed gates, the maximum thickness should be no more than 75% of the part thickness. Typical gate sizes are from 0.25 to 1.6 mm thick. The gate width is typically from 6.4 mm to 25% of the cavity length.
A diaphragm gate is often used for gating cylindrical or round parts that have an open inside diameter. It is used when concentricity is an important dimensional requirement and the presence of a weld line is objectionable.
This gate is essentially a flash gate around the inside edge of the part. Since the diaphragm is fed from a concentric sprue (or stub-runner drop), uniform flow to all parts of the gate is asy to maintain.
The typical gate thickness is 0.25 to 1.27 mm.
Like a diaphragm gate, a ring gate is also used for cylindrical or round parts, but it is not always recommended. With a ring gate, the material flows freely around the core before it moves down as a uniform tube-like extrusion to fill the mold.
The typical gate thickness is 0.25 to 1.6 mm.
This kind of gate is also called a four-point gate or cross gate. It is used for tube-shaped parts and offers easy de-gating and material savings. Disadvantages are the possibility of weld lines and the fact that perfect roundness is unlikely.
Typical gate size ranges from 0.8 to 4.8 mm thick and 1.6 to 6.4 mm wide.
A film gate is similar to a ring gate, but it is used for straight edges. It consists of a straight runner and a gate land across either the entire length or width of the cavity or a portion of the cavity. It is used for acrylic parts, and generally for flat designs of large areas where warpage must be kept to a minimum.
The gate size is small, approximately 0.25 to 0.63 mm thick. The land area (gate length) must also be kept small, approximately 0.63 mm long.
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.
– Maintain consistent cycle times for all shots.
– Minimize gate scars.
This type of gate relies on a three-plate mold design, where the runner system is on one mold parting line and the part cavity is in the primary parting line. Reverse taper runners
drop through the middle (third) plate, parallel to the direction of the mold opening. As the mold cavity parting line is opened, the small-diameter pin gate is torn from the
part. A secondary opening of the runner parting line ejects the runners. Alternatively, the runner parting line opens first. An auxiliary, top-half ejector system extracts the runners from the reverse taper drops, tearing the runners from the parts.
Typical gate sizes are 0.25 to 1.6 mm in diameter.
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
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. If a large diameter pin is added to a non-functional area of the part, the submarine gate can be built into the pin,
avoiding the need of a vertical surface for the gate. If the pin is on a surface that is hidden, it does not have to be removed.
Multiple submarine gates into the interior walls of cylindrical parts can replace a diaphragm gate and allow automatic de-gating. The out-of-round characteristics are not as good as those from a diaphragm gate, but are often acceptable.
The typical size is 0.25 to 2.0 mm in diameter. It is tapered to the spherical side of the runner.
A hot-runner gate is generally used to deliver hot material through Heated runners and electrically heated sprues directly into the cavity, producing runnerless moldings. The packing cycle is controlled by the freeze-off of the part near the gate. The very hot aterial at the gate is torn from the part as the cavity is opened.
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 smooths over the gate scar. Since the packing cycle is controlled by the valve rod, better control of the packing cycle is maintained with more consistent quality.