The pouring system is one of the most important issues in the design of injection molds. The pouring system is a complete delivery channel that guides the plastic melt from the injection molding machine nozzle to the mold cavity. It has the functions of mass transfer, pressure holding and heat transfer. It has a decisive influence on the quality of plastic parts. Whether its design is reasonable affects the overall structure of the mold and the difficulty of its process operation.
The function of the pouring system is to fill the plastic melt smoothly into the depth of the mold cavity to obtain a plastic product with a clear outline and good internal quality. Therefore, it is required to be fast and orderly during the mold filling process, with small pressure loss, less heat loss, good exhaust conditions, and easy to separate or cut the casting system condensate from the product.
The common runner system, also known as the runner system or pouring system, is the necessary passage for molten plastic from the injection nozzle of the injection machine to the mold cavity. The flow channel system includes a main flow channel, a split flow channel, and a gate.
The main channel is a passage connecting the nozzle of the machine to the entrance of the diverter channel, and is the first place where plastic passes through when entering the mold cavity. It can be understood as the flow path of molten plastic from the nozzle of the injection molding machine to the end of the flow path.
Its size and size are closely related to the plastic flow rate and the length of the filling time. If it is too large, too much cold material is recovered, the cooling time is increased, and the enclosed air is increased. It is easy to cause air bubbles and loose tissue, and it is easy to cause over-flow and insufficient cooling;
If the flow path is too small, the heat loss increases, the fluidity decreases, and the injection pressure increases, resulting in difficulty in molding. Under normal circumstances, the main channel will be manufactured as a separate gate sleeve, which is mounted on the mother template. However, there are some exceptions. For some small molds, because the requirements are not so high, you can open the main channel directly on the master template without using a sprue sleeve.
The common mainstream roads have the following three structures:
(1) The simplest mainstream structure of the overall structure of the fixed mold part This structure is often used for simple molds.
(2) The main channel structure composed of two templates for the fixed mold part The main channel can also be directly trained on its two templates. In order to avoid the wrong shape and the unfavorable release of plastic in the flow channel and simplify the processing accuracy, a minimum step of 0.1mm should be provided at the junction of the two parts.
(3) Inlet-type mainstream channel structure with gate sleeve The most commonly used mainstream channel structure is a structure in which the gate sleeve is inserted into the formwork. This mainstream channel structure is suitable for all injection molds. In order to prevent the gate sleeve from being injured by the injection molding machine nozzle, it should be quenched to have a certain hardness.
Also called split runner or secondary runner. It refers to a section of flow channel connecting the end of the main channel and the gate in the multi-cavity or multi-gate mold of injection or transfer molding. With the design of the mold, it can be divided into the first flow path and the second flow path.
Simply speaking, the runner is a transition area between the main channel and the gate, which can make the flow direction of the molten plastic be smoothly changed; for the multi-cavity mold, it also has the function of evenly distributing the plastic to each cavity. The shunt channel is indispensable in multi-cavity molds, and in single-cavity molds, some shunt channels can be omitted. Therefore, when we design the flow channel, we should fully consider how to minimize the pressure loss in the flow channel and avoid the melt temperature reduction. Of course, we must also consider reducing the volume of the flow channel.
For different plastic materials, the flow channel will be different, but there is a design principle that all the flow channel design should be followed: that is, we must try to ensure that the ratio of the surface area of the flow channel to its volume is the smallest. That is, in the case where the length of the shunt channel is constant, the ratio of the surface area or side area of the shunt channel to its cross-sectional area is required to be minimum.
Also called the feed inlet, it is the narrow opening between the runner and the mold cavity, and it is also the shortest and thinnest part. Its function is to use the tightening of the flow surface to accelerate the plastic. The high shear rate can make the plastic flow well (because of the shear thinning characteristic of the plastic); the temperature increase effect of viscous heating also increases the material temperature and reduces the viscosity. effect.
After the molding is completed, the gate is first cured and sealed, which prevents the plastic from flowing back, and prevents the mold cavity pressure from falling too fast, causing the molded product to shrink and sag. After molding, it is easy to cut to separate the flow channel system and plastic parts.
The gate can be understood as the last "sill" of the plastic in the molten state before it enters the cavity through the pouring system, which is the feed channel connecting the runner and the cavity. There are two main functions: first, it controls the flow of plastic melt into the cavity; second, when the injection pressure is removed, the cavity must be blocked, but at the same time, we must ensure that the cavity has not been completely cooled and solidified The plastic will not flow backwards.
4. Cold feed well
Also called cold pocket. The goal is to store the cold plastic wavefront at the initial stage of filling and filling, to prevent the cold material from directly entering the mold cavity, affecting the filling quality or blocking the gate. The purpose is to store the cold head produced by the two injection molding intervals and the forward flow of the melt to prevent the melt from entering the cavity and causing the next molded product to have flow marks. The location of the cold feed well is generally set at the end of the main channel, but when the length of the diverter channel is relatively long, the cold feed well should also be installed at the end.