Main runner, runner and gate function are to transport plastic melt from nozzle of injection molding machine to each cavity. It is true that aggregates of gating system can be crushed and reused. However, presence of aggregates means that productivity of injection molding machine is reduced, because material in gating system must also be plasticized in barrel of injection molding machine. For smaller plastic parts, gating system aggregate may account for 50 or more of actual injection volume.

 

Main runner can be regarded as continuation of nozzle channel in mold. In a single-cavity mold, gate that leads directly from main runner to plastic part is called a straight gate.
Productivity of a single-cavity injection mold is usually determined by cooling time of main runner. In addition to providing sufficient cooling for sprue bushing, minimum diameter of inlet on sprue bushing should be as small as possible, cavity should be filled in time.
But there is no universally applicable rule here, because filling of cavity depends on many factors. Main runner should have a draft of 1.5°. Larger demolding slope can make sprue easy to escape from sprue bushing, but when sprue is longer, diameter of sprue will be larger, therefore a longer cooling time is required. Outlet diameter of injection molding machine nozzle should be 0.5mm smaller than minimum hole diameter of sprue bushing, so that no groove will be formed at the top of sprue to prevent aggregate from sprue from coming out.

In a multi-cavity mold, plastic melt must be injected into each cavity through a runner set on mold parting surface. Basic laws that apply to main flow channel also apply to cross section of branch channel. There is an additional factor that must be considered. Cross section of runner is also a function of its length, because it can be assumed that pressure loss in runner is at least proportional to length of runner.
In most cases, pressure loss will be greater because cross section is reduced due to solidification of plastic melt along flow channel wall, the farther away from main flow channel, the greater pressure loss. In addition, main runner and runner system means loss of raw materials and plasticization of injection molding machine. Therefore, runner should be designed as short as possible and cross section should be as small as possible. Length of runner is determined by number of mold cavities and geometric arrangement of each cavity.

Since surface area of circular cross-section runner is the smallest, heat loss relative to cross-sectional area of runner is the least, runner with a circular cross section should be used as much as possible. Because melt in the center of circular cross-section runner finally solidifies, under action of holding pressure, plastic melt can flow for the longest distance along center of circular cross-section runner.

 

Therefore, gate (a section between runner and cavity) should be designed such that melt enters cavity from center of runner with a circular or rectangular section through gate.
At the smallest section of runner, due to flow friction of plastic melt, steel around gate is locally heated. In this way, under effect of holding pressure, before gate solidifies, melt can continue to be injected into cavity for a longer period of time to play a role in feeding.
When there must be movement between smooth surface and runner, runner with circular cross section cannot be used. In this case, a split runner with a semicircular groove can be used. Advantage of this shape is that it only needs to process runner on one side of mold plate. However, when radius of curvature of semicircular groove runner is same as diameter of male cross section runner, semicircular groove runner contains more than 12.5 more raw materials than circular runner.

Gate is channel between runner system and cavity, turning force drop should be as small as possible. Therefore, from runner to cavity, taper section of gate gradually becomes smaller and less profitable. If plastic part is relatively small and there is no special requirement on whether gate position is visible or not, recommended gate, that is, from runner to plastic part, gate is gradually tapered. Therefore, plastic parts can be cleaned and separated from runner. If wall of plastic parts is relatively thick, runners or cutters are used to cut runners, gate type used is better.
However, in this case, the narrowest section should ensure the shortest possible length allowed when using tool to avoid excessive pressure drop.
For runners with a circular cross-section, gate is practical because melt is injected into cavity from center of circular cross-section. For mold where runner is processed on the side of mold plate, gate can also get same effect.
Use of tunnel (submerged) gates is particularly advantageous because it is opened by mold. Plastic parts and runner system are automatically disconnected. Melt is injected into cavity through a short tunnel at the end of runner. Tunnel is properly designed so that gate is almost invisible;
Therefore, secondary processing of removing gate is not required on plastic part, which is possible. It has been found that this type of gate is not only suitable for PE (it is the first resin to use this gate), but also plastic parts of PS, nylon (polyacrylamide (PA)) and other resins can be successfully molded by this method.
However, it must be noted that some situations are discussed below. There should be a relatively large demolding slope or step where gate is set on the surface of plastic part, otherwise scratches are easy to appear on the wall of plastic part behind gate.