The concept of main runner
The main runner is a conical section that connects injection molding machine nozzle to the branch runner, by which the molten plastic run into the mold cavities. And its diameter is closely related to the speed of plastic flow and the time of filling. If the diameter is too large, it will cause the too much cold material, and the cooling time increases,. Too many air can easily cause bubbles ,loose organization and can easily produce vortex and poor cooling.,
In addition, when the diameter is too large, the heat loss of the melt will increase, the fluidity decreases and the injection pressure loss increases, which result in the trouble of molding. If the diameter is too small, it increases the flow resistance of the melt, and it has the same trouble for molding.
The side gate system runner and the point gate system runner are roughly the same, but they have the different sizes.
The runner for thermoplastic plastic generally set in the sprue, and, the sprue is made of a separate insert fasterned on a fixed plate, but for some of the Small mold, it can also be directly machined on the plate. The sprue can be divided into two categories: two plate mould sprue and three plate mould sprue.
Design principles of runner
Design principles of runner are as follows.
(1)The runner should be as short as possible especially for the pin point gate system or poor flow material. The shorter the runner is, the lighter venting burden the mold has. The less runner material will shorten the molding cycle and reduce the loss of the pressure and temperature.
(2) Ease of part release.The design of runner is of conical normally.The taper for 2 plates mould is 2″~ 4” and The taper for 3 plates mould is 5 “~ 1o”. Roughness Ra 1.6 ~ 0.8 um. The taper must be appropriate, for too large would cause speed reduction and produce vortex and air bubble, while the small taper will increase the flow rate, which will result in injection difficulties, and Stripping difficulty.
(3) The runner dimension shall meet the assembly requirements. To guarantee the smooth injection, the sprue and the injection machine nozzle are not permitted for the material overflow which leads to stripping difficulty.The runner small end diameter D2 is 0.5-1mm larger than the nozzle diameter D1 of injection machine.Normally,D2=3.2-4.5mm;The big end diameter of runner should be bigger than the maximum diameter of branch runner by 10%-20%.
(4) the runner should be designed in sprue instead of in plate or insert style to avoid release difficulties for the plastic may run into the joint part.
(5) The runner should be set in the center of the mold to avoid adopting the inclined runner.