Hello everyone, I hope this article will give you some inspiration. You can save it for reference. When we design runner, there are many things to pay attention to. Let me tell you in detail:

 

Reduce bends and surface roughness: When designing runner system, runner should have as few bends as possible. Its surface roughness should be controlled at Ra0.6 - Ra0.8um, so that plastic can flow smoothly inside.
Consider the number and layout of cavities: When designing pouring system, you must first figure out whether mold is one mold and one cavity, or one mold with multiple cavities. Pouring system should be designed according to layout of cavity, and it is best to be symmetrical with center line of mold so that plastic can enter each cavity evenly.
Avoid single-sided gates (products with large projected areas): If projected area of plastic product is relatively large, when designing pouring system, do not open gate on one side of mold, otherwise product will be unevenly stressed during injection molding, which is prone to problems.
Convenient removal of runners and guaranteed appearance: When designing runners, you must consider that it is convenient to remove runners later, and when correcting gates, you must not leave marks on plastic products. After all, we must ensure that appearance of plastic products is good.
Precautions for multiple pieces in one mold: If there are multiple products in one mold, do not put plastic products with particularly large sizes in same mold.
Main runners avoid impacting cores and inserts: When designing main runners, you must avoid molten plastic directly hitting small-diameter cores and inserts, otherwise they may bend or break.
Select the shortest process: On the premise that plastic can be smoothly molded and exhaust is smooth, you should choose the shortest process, so that plastic filling time can be shortened and production efficiency can be improved.
Ensure smooth filling and exhaust: Runner must be able to smoothly guide molten plastic to fill various parts. During filling process, there must be no plastic vortex or turbulence, and gas in cavity must be discharged smoothly outside mold.
Shorten cooling and molding cycle (during batch production): When mass-producing plastic products, while ensuring product quality, we must find a way to shorten cooling time and molding cycle, so that more products can be produced.
Main runner machining allowance (when high precision requirements are required): There will be shrinkage here in main runner. If plastic product has a high precision requirement in this part, main runner must have some machining allowance or correction allowance.
Gate position selection: Gate position must ensure that when plastic flows into cavity, it is facing spacious and thick-walled area in cavity, so that plastic can flow in easily.
Weld mark treatment: Try to avoid weld marks on product. If it is really unavoidable, then weld marks must appear in unimportant places of product.

Main runner refers to plastic channel connecting injection molding machine nozzle and branch channel. It is the first section of flow channel that molten material passes through when injected into mold. Its shape and size will directly affect flow rate of plastic and injection time.

 

Figure (1) shows form and design parameters of vertical main runner.
d----Diameter of small end of main runner, that is, diameter of main runner at contact point with injection molding machine nozzle.
d=Injection molding machine nozzle aperture+(0.5-1) mm
L-----Length of main runner. It is determined during design according to specific structure of mold.
A---Taper of main runner. A is generally selected in range of 2°-4°. For plastics with high viscosity, A can be 3°-6°.
However, due to limitation of standard taper reamer, standard taper values should be selected as much as possible, or standard gate sleeves should be selected. Generally speaking, size D is about 10-20% larger than D'.

When designing a mold, main runner often deviates from mold center due to influence of product and mold structure, or due to limitation of pouring system and number of cavities. Sometimes this distance is very large, causing many problems when mold is used. First: when ejecting product, since ejector is not in the center of mold, push plate and ejector fixing plate are prone to deviate, causing ejector to break, and product to deform or damage; second: since main runner is not in the center, it will cause single-sided seam to be too large and cause overflow. Although above problems can be solved by using a three-plate mold structure, this will increase mold cost. Therefore, in above situation, design of an inclined main runner can avoid or improve its shortcomings. Figure (2) shows design parameters of inclined main runner. Inclination angle a is mainly related to properties of plastic. For example, inclination angle a of PE, PP, PA, POM, and other plastics can reach a maximum of 30 degrees; inclination angle of HIPS, ABS, PC, and other plastics can reach a maximum of 20 degrees. SAN and PMMA cannot use inclined main runners. Other design parameters of inclined main runner are same as those of vertical main runner.

Diverter is transition part before plastic enters cavity. It can ensure the best molding effect by changing shape, size and direction of cross section, so that plastic can enter cavity smoothly. Commonly used diverter forms and sizes are shown in Table (1).

 

Product related: Geometric shape, wall thickness, size, dimensional stability, internal quality and appearance quality requirements of product will affect diverter design.
Plastic type: Different types of plastic have different fluidity, melting temperature, melting temperature range, curing temperature, and shrinkage rate, which must also be taken into consideration.
Injection molding machine parameters: Pressure, heating temperature and injection speed of injection molding machine also affect diverter design.
Shedding method: How main runner and diverter fall off is also a factor to be considered.
Cavity layout and gate related: How to arrange cavity, where to choose gate position, what form of gate to use, all of these will affect design of runner.

Low pressure and temperature loss: When plastic flows through runner, pressure loss and temperature loss should be as small as possible.
Curing time later than product: Curing time of runner should be later than curing time of product, which is conducive to pressure transmission and pressure maintenance.
Even entry into cavity: It is necessary to ensure that plastic can enter each cavity quickly and evenly. A balanced runner is a good choice. You can see attached figure (3).
Short length and small volume: Length of runner should be as short as possible, and its volume should also be small.
Easy to process and tool selection: Runner design should be easy to process and tool should be easy to select.
Dimension relationship: Each runner section should be 10-20% larger than next runner section