How to Design the Main Runner?

Main Runner Concept

The main runner refers to the tapered flow channel from the nozzle of the injection molding machine to the runner. The molten plastic first passes through it when it enters the mold. Its diameter is closely related to the plastic flow rate and the filling time.

When the diameter is too large, there will be too much recycled cold material, the cooling time will increase, and the increase of trapped air will easily cause air bubbles and loose tissue, which will easily cause an eddy current and insufficient cooling. In addition, when the diameter is too large, the heat loss of the melt will increase. If the diameter is too small, the flow resistance of the melt will increase, which is also not conducive to molding.

The shape of the sprue in the side gate gating system and the point gating system is roughly the same, but the size is different.

The main runner of thermoplastics is generally in the ingate sleeve, and the sprue sleeve is made into a separate insert and set on the fixed template, but some molds can also directly open the main runner on the fixed template without using the Ingate sleeve. Ingate sleeves can be divided into two categories: two-plate mold Ingate sleeves and three-plate mold Ingate sleeves.

Main Runner Design Principles

• The shorter the length of the main runner, the better, especially for the sprue of the point gate gating system or the plastic with poor fluidity, the sprue should be as short as possible. The shorter the sprue, the lighter the mold exhaust burden, the less runner material, shorten the molding cycle and reduce the energy (temperature and pressure) loss of the melt.

• To facilitate demolding, the main runner is mostly conical in design. The taper of the main runner of the two-plate mold is 2°~4°, and the taper of the main runner of the three-plate mold is 6°~10°. When the roughness is Ra=1.6 ~ 0.8μm, the taper must be appropriate. If the taper is too large, the speed will decrease, turbulence will occur, and air will be easily mixed in, resulting in pores, if the taper is too small, the flow rate will increase, making injection difficult, and at the same time make it difficult to de-mold the main runner.

• To ensure that there is no overflow between the sprue and the nozzle of the injection molding machine during injection molding, which will affect the molding.

• If the sprue passes through multiple templates at the same time, be sure to pay attention to the taper of the hole on each template and the size of the hole.

• The sprue channel should avoid mosaic structure as far as possible to prevent the plastic from entering the joint and causing difficulty in molding.

Inclined Main Runner Design

Generally, it is required that the position of the sprue should coincide with the center of the mold as much as possible, otherwise there will be the following adverse consequences.

When the sprue deviates from the center of the mold, the clamping force and the expansion forces are not on the same line, causing the mold to be twisted during production. This torque will cause one side of the mold to open and generate a flash, or cause the core to be dislocated deformation will eventually lead to serious consequences such as the deformation of mold guide pillars and even injection molding machine tie rods.

When the main runner deviates from the center of the mold, the hole of the ejector pin also deviates from the center of the mold. When the product is pushed out, the push rod plate will also be subjected to torque. After the torque is transmitted to the push rod, it will cause the push rod to wear and even break.

Therefore, the design should try to avoid the main runner from deviating from the center of the mold, but in the side gate gating system, it is often due to the following reasons, the sprue position must deviate from the center of the mold.

• The size of the product in one mold with multiple cavities is very different.

• Single-cavity, the product is larger, and there is a larger piercing hole in the middle, which can be fed from the inside, but the middle piercing hole deviates from the center of the mold.

• If the deviation of the main runner from the center of the mold is unavoidable, then three measures can be taken to avoid or reduce the impact of adverse consequences on the mold.

• Increase the push rod fixing plate guide column (middle support edge) to withstand the torsion generated by the eccentricity of the push rod.

• When the mold is large, double top pin holes or multiple top pin holes can also be used, so that when the push rod fixing plate is subjected to multi-point thrust, it is easier to push out in a balanced manner.

• The inclined main runner is used to avoid the eccentricity of the top pin hole (K.O. hole). The inclined main runner is related to the type of plastic. It is suitable for plastics with good toughness, such as PVC, inclined sprue sleeves, and top pinhole PE, PP, PA, etc. the maximum inclination angle can reach 30°. For plastics with general or poor toughness, such as PS, PMMA, PC, POM, ABS, and SAN, the maximum inclination angle can reach 20°.