The gate is the link between the part and the runner system.
It is normally a restricted area that facilitates separation of the runner from the part. The size, shape and placement of the gate can significantly affect the ability to successfully mold a product. The key feature of the gate is to allow for easy, potentially automatic, separation of the part from the runner system, while allowing for filling and packing of the part. The gate should be relatively small so that it is able to be easily removed from the part. However, too small of a gate can restrict packing of the part, cause over-shearing of the material, jetting, and other gate-related defects. Gates should be located at the thickest area of the part, so that the part does not have defects, such as hesitation.
Types of Gates
Edge gates are the most basic type of gate. They are normally rectangular and attach to the part along its perimeter at the parting line of the mold. They are used when automatic degating is impractical. A narrower gate allows for easier degating but also increases flow rate through the gate and increases the possibility of jetting and other flaws.
Fan gates are similar to edge gates in that they are attached to the part at the parting line and require manual degating. Unlike the edge gate, the fan gate expands out from the runner in the shape of a fan with its widest end opening to the cavity. The fan region can be relatively thick and feeds a thin gate land. A properly designed fan gate will have broad, uniform melt flow front. This can improve melt orientation, reduce the chance of jetting, reduce the stress in the gate region of the part, and reduce the shear rates through the gate. By implementing a fan gate, it allows for multiple restrictive gates to be replace by one fan gate, thus eliminating the formation of weld lines. The major disadvantage of using a fan gate is the width causes problems with degating.
The film, or flash gate, attempts to capture the advantages of the fan gate, while using less space and material. The disadvantage of using a film gate is the flow distribution across the gate and the flow rate through the gate is less predictable than in the fan gate. The melt has the tendency to hesitate at the thin gate land position closest to where it is fed by the runner.
Diaphragm gates are used for cylindrical-shaped parts that are normally open on either end. They are used in three plate cold runner, hot runner, or single-cavity sprue gated molds. Diaphragm gates are generally used when concentricity is an important dimensional requirement. The advantage of this type of gate includes the elimination of weld lines, minimal potential for core deflection, and development of an ideal flow pattern for predictable shrinkage and minimal unwanted warpage.
A tunnel gate is typically conical in shape with the smallest end of the cone attached to the part. The gate is cut in the mold so that it tunnels from the runner to the cavity below the parting line of the mold. During ejection, the gate is sheared from the part. The gate opening must be kept to a minimum to assure that it breaks away from the part during ejection without damaging the part or leaving an excessive gate vestige.
Cashew gates are a variation of tunnel gate, except that they can provide gating in regions that cannot be reached by the standard tunnel gate. The primary limitation is that the curved shape requires that the material in the gate goes through considerable distortion during ejection.
Jump gates can be used to eliminate gate vestige from the outside of a part. The jump gate jumps past the outer wall into an internal feature of the part. It is used when automatic degating is required in a two-plate cold runner mold- but there is a cosmetic or physical concern with the gate being attached to the exterior of the part.
A tab gate can sometimes be used to reduce jetting or other gate related problems by slowing down the velocity of the melt as it exits the gate. The melt approaching the gate is split between the gate and the overflow. This reduces the flow front velocity as the melt initially emerges from the gate. This method may cause hesitation at the gate, and determining whether the gate will work is often left to trial and error.