I add more details, actually a gate also balances the resin filling activities amount cavities, via adjusting the gate size, the gate location, size and quantity are decided with input from the plastic part shape, appearance, resin selection, molding machine capability, molding cycle time, secondary processing requirement, economic factors, and so on.

a gate will elevate the temperature of the cooling resin by friction as it flows through the narrow gate, and reduce flow marks and weld marks, facilitate cutting off the connection between the runner and the plastic part, easing secondary processing.

actually the function and purpose of a gate is to control the amount and flow direction of the melt resin, seal the resin inside the cavity until the parties sufficiently solidified enough for injection, and prevent it from flowing back.

when we design a gate for a mold, in general, it is easier to full up the cavity with a gate of a larger sectional area. But it will take a longer time to cool and seal the gate, and after injection molding, it is more difficult to cut off. A smaller gate sectional area leads to the opposite result. Subject to the constrains of the plastic part material, shape, and appearance, a good gate design is the one that leads to least residual stress and deformation.

hey! Every one, actually the most important part in a feeding system is gate, because gate is the end of the runner, it is the entrance to the cavity. The location, quantity, and size of the gate have great impact on the part’s appearance, molding efficiency and dimensional accuracy. Therefore, gate design needs to consider the plastic part’s shape, material flow direction, weld mark creation, and gate processing.

a special runner is single cavity runner, typical single cavity mold designs use a central feed approach, if the plastic part’s projected area on the parting line is continuous. That is, it does not have a runner. The slug flows directly to the cavity from the gate through the spure.

unbalanced runner designs have the advantage of shortening runner’s total length and thus enjoy an extensive application in the industry.

the purpose is to increase the resistance to the material flow at the near-by cavities, for practical reason, a smaller gate size is used, to allow enlargement after mold test, when necessary, until all cavities have the same material intake at the same rate. Unbalanced runner design is not appropriate for plastic parts of high technical requirements.

what is an unbalanced runner? Unlike a balanced runner design, an unbalanced runner design is that the runner lengths from the spure to each cavity may be different. And the gate has different sizes, the length of the gate near the spure should be greater than the one away from the spure, or the sectional area near the spure’s gate should be smaller than the one away from the spure.

what is a balanced runner? The balanced runner is that all runners have identical length, shape, and sectional dimensions, from the spure to each cavity. The design is to ensure each cavity has a balanced material flow and is filled out at the same time. for example, radiate runners, linear runners, Y-shape runners, X-shape runners, H-shape runners.

various runner designs can be found in a multi-cavity mold, runner layout is designed for the mold cavities and thus depends on the cavity layout, the basic types of layouts for runners and cavities are balanced and unbalanced.

during mold opening, the well can pull the slug off the spure bush. The diameter of a cold-slug well should be larger than the larger diameter of the spure, while it’s length should be about the same as the larger diameter of the spure, to promote the material flow.

what is the function of cold-slug? It’s purpose is to store cold slug created intermittently by the nozzle due to heat loss and low temperature. Cold slugs could clog or slow down the material flow in runner and cavity and cause cold scars or cold marks on plastic part.

please don’t forget cold-slug, it well usually locates at the end of the spure. That is, at the end of the runner and on the core-retainer plate, on directly opposite side of the spure.

but how to design a spure bush, I have some principles in the following:

when a sprue allows the melt plastic material to flow into the cavity, that is called direct gate, in that case, the plastic part will be left with a trace by the sprue. To keep the sprue bush in place, while being under pressure, a step design is used to fix it on a locating ring. The hole inside the spure bush is usually conic in shape. The smaller end of the hole is a curve, the larger taper the hole has, the easier the spure to pop out. Spure bush and nozzle may come in contact on a planer surface, it facilitates a right location and tight fit.

we have discussed too much about the feed system of a injection mold. Today I want to say more details about feed system assembly, sprue is mentioned at first, where is the sprue of a feed system? Sprue is located at the entrance of a mold, it allows the melt plastic material to flow into the runner or cavity. A sprue bush is inserted on the cavity plate or runner plate.

so when we select a feeding system, what is the primary principle? I think it is to maintain the temperature and pressure as it flows out of the nozzle and fill out the cavity easily. Commonly, it prefers a “ short and thick” spure, runner, and gate. Other considerations are also given to the requirement of machine’s injection volume, plastication ability, part’s appearance, machining, dimension precision, physical properties, cycle period, and material waste.

in that case, in practice, how to make a decision on gate and runner design is very important. Because every mold has it’s making schedule. So the decision is usually based on the shape and the size of the plastic part and it’s material properties. Ideally, the runner and the gate should be an integral effort in the plastic part design and mold design process.

Although the design of runner and gate has a great impact on the part’s molding, and quality. It is difficult to calculate and design an optimal feeding system with a simple formula, for it is difficult to model the acceleration of the flow inside the cavity, going through the gate at an elevated temperature due to friction.