Tool Design - Basic Gating Methods and Design Consideration...

What is a gate?

A gate is a small opening (or orifice) through which the polymer melt enters the cavity. Gate design for a particular application includes selection of the gate type, dimensions,and location. It is dictated by the part and mold design, the part specifications (e.g.,appearance, tolerance, concentricity), the type of material being molded, the fillers, the type of mold plates, and economic factors (e.g., tooling cost, cycle time, allowable scrap volume). Gate design is of great importance to part quality and productivity.

Single vs. multiple gates

You’ll usually have better success with a single gate, unless the length of the melt flow exceeds practical limits. Multiple gates always create weld and meld lines where the flows from the separate gates meet. Except for long, narrow parts, a single gate into the body of the part (as opposed to an edge gate) will assure more uniform distribution of material, temperatures, and packing, and better orientation effects. While a single gate into the body of the part might incur a higher initial tool cost, lower scrap rates and higher part quality will quickly justify this expense.

Gate dimensions

The cross section of the gate is typically smaller than that of the part runner and the part, so that the part can easily be “de-gated” (separated from the runner) without leaving a visible scar on the part. The gate thickness is usually two-thirds the part thickness. Since the end of packing can be identified as the time when the material in the gate drops below the freeze temperature, the gate thickness controls the packing time. A larger gate will reduce viscous (frictional) heating, permit lower velocities, and allow the application of higher packing pressure for a longer period of time. Choose a larger gate if you’re aiming for appearance, low residual stress, and better dimensional stability.

Figure 1 below illustrates the terms we use to describe gate size.

Gate location

Select a gate location that will ensure rapid and uniform mold filling. Position weld lines and air/gas vents so they have the least effect on the appearance and strength of the part. Since gates are locations of high residual stress, position them away from areas that will experience high external stress during use.

Position the gate away from load-bearing areas. The high melt pressure and high velocity of flowing material at a gate cause the area near a gate to be highly stressed.

Position the gate away from the thin section areas, or regions of sudden thickness change.
This will avoid Hesitation or Sink marks and voids.

 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:

Gate type

Direct (sprue) gate Tab gate Edge (standard) gate Overlap gate Fan gate Disk (diaphragm) gate Ring gate Spoke (spider) gate Film (flash) gate

Direct (sprue) 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.

Dimensions
The starting sprue diameter is controlled by the machine nozzle. Thesprue 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.

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 used extensively for molding PC, acrylic, SAN, and ABS types of materials.
Dimensions
The minimum tab width is 6.4 mm. The minimum tab thickness is 75% of the depth of the cavity.

Edge (standard) gate

An edge gate is located on the parting line of the mold and typically fills the part from the side, top, or bottom.

Dimensions
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.

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 0.4 to 6.4 mm thick and 1.6 to 12.7 mm wide.

Fan gate

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.

Dimensions
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.

Disk (diaphragm) gate

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.

Dimensions
The typical gate thickness is 0.25 to 1.27 mm.

Ring gate

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.

Dimensions
The typical gate thickness is 0.25 to 1.6 mm.

                                                            Spoke (spider) gate

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.

Dimensions
Typical gate size ranges from 0.8 to 4.8 mm thick and 1.6 to 6.4 mm wide.

                                                                  Film (flash) gate

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.

Dimensions
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.

The following gate types are trimmed from the cavity automatically:
– Pin gate
– Submarine (tunnel, chisel) gate
– Hot-runner (hot-probe) gate
– Valve gate

Pin gate

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.

Dimensions
Typical gate sizes are 0.25 to 1.6 mm in diameter.

Benefits
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.

Submarine (tunnel, chisel) gate

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.

Dimensions
The typical size is 0.25 to 2.0 mm in diameter. It is tapered to the spherical side of the runner.

Hot-runner (hot-probe) gate

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.

Valve gate

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.

Essentials of a well-designed gate

A properly designed gate will deliver a rapid, uniform, and preferably uni-directional mold filling pattern with a proper gate freeze-off time.

Gate location

The gate location should be at the thickest area of the part, preferably at a spot where the function and appearance of the part are not impaired. This leads the material to flow from the thickest areas to thinner areas to the thinnest areas, and helps maintain the flow and packing paths. Gate location should be central so that flow lengths are equal to each extremity of the part. Numerical simulation of the molding process is an effective tool that can be used to compare the effects of various gate designs.

Avoiding common problems

Improperly positioned gates often case the following problems; keep them in mind when designing the delivery system.

Gate symmetrically to avoid warpage
Symmetrical parts should be gated symmetrically, to maintain that symmetry. Asymmetric flow paths will allow some areas to be filled, packed, and frozen before other areas are filled. This will result in differential shrinkage and probable warpage of the parts.
Vent properly to prevent air traps
The gate location should allow the air present in the cavity to escape during injection to prevent air traps. Failure to vent the air will result in a short shot, a burn mark on the molding, or high filling and packing pressure near the gates.
Enlarge the gate to avoid jetting
Gate location and size should prevent jetting, the string appearance or spaghetti-size strands of melt in short shots. Jetting can be prevented by enlarging the gate or by locating the gate in such a way that the flow is directed against a cavity wall.
Position weld and meld lines carefully
The gate location should cause weld and meld lines, if any, to form at appropriate positions that are not objectionable to the function or appearance of the part.

Gate length

Gate length should be as short as possible to reduce an excessive pressure drop across the gate. A suitable gate length ranges from 1 to 1.5 mm (0.04 to 0.06 inches).

Gate size

Gates should always be small at the beginning of the design process so they can be enlarged, if necessary. Reducing the gate size is not as easy as enlarging it.

Gate thickness

The gate thickness is normally 50 to 80 percent of the gated wall section thickness. For manually trimmed gates, the gate thickness can occasionally be the same as the gated wall section thickness. For automatically trimmed gates, the gate thickness is typically less than 80 percent of the gated wall section thickness, to avoid part distortion during gate  breaking. Typical diameters at the gate end for pin and submarine gates range from 0.25 to 2.0 mm (0.01 to 0.08 inches).

Freeze-off time

The freeze-off time at the gate is the maximum effective cavity packing time. However, if the gate is too large, freeze off might be in the part, rather than in the gate, or if the gate freezes after the packing pressure is released, flow could reverse from the part, back into the runner system. A well-designed gate freeze-off time will also prevent back flow of the injected material.

Fiber-filled materials

Fiber-filled materials require larger gates to minimize breakage of the fibers when they pass through the gate. Using small gates such as submarine, tunnel, or pin gates can damage the fillers in filled materials. Gates that deliver a uniform filling pattern (such as an edge gate) and thus, a uniform fiber orientation distribution are preferable to point-type gates.