The gating system is one of the most important issues in the design of injection molding molds. The gating system is a complete conveying 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 operation.
The function of the gating system is to smoothly fill the plastic melt to the depth of the cavity to obtain plastic products with clear outlines and excellent internal quality. Therefore, during the filling process, it is required to be fast and orderly, with small pressure loss, low heat loss, good exhaust conditions, and easy to separate or cut the condensate of the pouring system from the product.
Ordinary runner system, also known as runner system, or pouring system, is the necessary passage from the nozzle of molten plastic to the mold cavity. The runner system includes the main runner, the runner and the gate.
The main flow channel is the passage between the nozzle of the machine and the entrance of the diverter channel. It is the first place when plastic enters the mold cavity. It can be understood as the flow path of the molten plastic from the nozzle of the injection molding machine to the end of the diverter channel.
Its size and size are closely related to the flow rate of the plastic and the length of the filling time. Too large causes too much cold material to be recycled, the cooling time is increased, and the storage air is increased. It is easy to cause bubbles and loose tissues, and it is easy to cause overcurrent and insufficient cooling.
If the flow path is too small, the heat loss increases, the fluidity decreases, and the injection pressure increases, resulting in molding difficulties. Under normal circumstances, the main flow channel will be manufactured as a separate gate sleeve and set on the mother formwork. But there are some exceptions. Some small molds, because the requirements are not so high, can directly open the main channel on the master formwork, without using a gate sleeve.
Generally common mainstream roads have the following three structures:
(1) The simplest main stream structure with a fixed mold part consisting of an overall structure This structure is often used for simple molds.
(2) The main stream structure of the fixed mold part is composed of two templates. The main stream can also be directly taught on its two templates. In order to avoid the unfavorable dislodging of the plastic in the runner and simplify the processing accuracy, a small step with a minimum of 0.1mm should be provided at the joint of the two parts.
(3) Inlet-type mainstream channel structure of the gate sleeve The mainstream channel structure most commonly used at present is a structure embedded in the form of a gate sleeve. This main flow channel structure is suitable for all injection molds. In order to prevent the gate sleeve from being hit by the nozzle of the injection molding machine, it should be quenched to have a certain hardness.
Also called split runner or secondary runner. It refers to a section of runner between the end of the main flow channel and the gate in a multi-cavity or multi-gate mold for injection or transfer molding. With the design of the injection molding mold, it can be further divided into a first diverter and a second diverter.
In short, the shunt channel is the transition area between the main flow channel and the gate, which can make the flow direction of the molten plastic smoothly transition; for multi-cavity molds, it also has the function of uniformly distributing plastic to each cavity. Shunt channels are necessary in multi-cavity molds, while in single-cavity molds, some 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 decrease of the melt temperature. Of course, we must also consider reducing the volume of the flow channel.
For different plastic materials, the shunts will be different, but there is a design principle that all shunts design should be adhered to: that is, we must try to ensure that the ratio of the surface area of the shunt to its volume is the smallest. That is, when 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 the smallest.
Also called the inlet, it is the narrow opening between the shunt and the mold cavity, and it is also the shortest and thinst part. Its role is to use the shrinking flow surface to accelerate the plastic. The high shear rate can make the plastic flow good (because of the shear thinning characteristics of the plastic); the heating effect of viscous heating also increases the material temperature and reduces the viscosity effect.
After the precision plastic injection molding is completed, the gate is first cured and sealed, which prevents the plastic from flowing back, and prevents the cavity pressure from falling too quickly, causing the molded product to shrink and sag. After molding, it is easy to cut off to separate the runner system and plastic parts.
The gate can be understood as the last "hurdle" of the plastic in the molten state before entering the cavity through the pouring system, which is the feed channel connecting the diverter channel 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 withdrawn, 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 because of this.
4. Cold Material Well
Also known as a cold cavity. The goal is to store the colder plastic wavefront in the initial stage of filling and filling, prevent the cold material from directly entering the mold cavity, affect the filling quality or block the gate.The purpose is to store the cold material head generated during the two injection intervals and the forward cold material of the melt to prevent the cold material from entering the cavity and cause the next formed product to have flow marks. The location of the cold material well is generally set at the end of the main flow channel, but when the length of the diverter channel is relatively long, a cold material well should also be set at its end.