Gate location is critical to the effectiveness of the fluid flow and packing pressure in the front profile, and it determines the strength and other characteristics of the molded part.
Because gate locations are generally determined by designers and mold makers, this section is written specifically for these people. However, it is also important for molders to be involved from the design stage to prevent possible problems.
Possible adverse effects
For a part made of semi-crystalline engineered polymer, if the gate is in the wrong place, the part will be scrapped, even if all other designs are correct. These will be confirmed by a number of problems that arise in the application of reinforced and unreinforced resins: weld lines and memory air due to fluid flow in the front section will affect the surface finish of the part and, in the case of fiber-reinforced materials, the mechanical properties of the part. Improving process conditions will not do a good job in this.
If the gate is located in a thinner part of the molded part, dents and voids will form in the thicker part of the part. Since the thin-walled material crystallizes faster (see figure), while the thick-walled part requires a longer holding time, the thick-walled part is no longer fused with melt. In addition to optical and mechanical problems, there is also the problem of constant shrinkage in this area, which will cause warping, even for unreinforced materials.
If there are too few gates or the position is incorrect, the flow distance is too long and the injection filling pressure is too high. Flash can occur if the clamping force is not enough or if the polymer being processed is low viscous and crystallizes too slowly. In addition, due to the very limited machining window, it is not possible to adjust the tolerances precisely by adjusting the molding parameters.
Recommendations for setting up suitable gate locations
Place the gate at the thickest part of the wall;
The gate cannot be located near a high-pressure area;
For long parts, where possible, the gate should be set in the longitudinal direction rather than in the transverse or central direction, especially for molding reinforcement materials.
If there are two or more cavities, the parts and gates should be designed to be arranged symmetrically with the vertical runner as the reference;
For axially symmetrical parts such as gears, records, and impellers, in order to obtain excellent flow properties, it is suitable to use a diaphragm gate in the center, or a multi-point gating method with a three-plate mold
For parts with integral hinges, the gate should be set so that the weld is away from the hinge. Be sure to avoid fluid blockage near the hinges.
Cup-shaped parts such as small frame types and capacitor sleeves should be provided with gates at the base to prevent gas trapping.
In tubular parts, the melt should first be filled with an annular tube from one end and then along the length of the tube. This avoids asymmetry in the pre-flow profile.
Insert molding is performed around the hole forming lock, and the molten core and other metal inserts, melted resin, should flow in an annular pattern along the insert to maintain minimal mounting errors of the insert.
A tunnel gate is used to feed the ejector pin and the gate is opened from the inside to avoid apparent defects such as gate marks on the exposed surface.
Gates should be set up to avoid fluid blockage during the filling process as much as possible (e.g. complex parts, multi-cavity molding of different shapes, etc.).
These recommendations don't cover all possible issues in your app. Depending on the level of complexity that arises in a particular molding, some compromises sometimes have to be made. However, during the design phase, if possible, try to think about the issues we are discussing. In this case, simulated filling experiments are extremely useful.
