Advantages, disadvantages, and selection methods of gate position and structural form in MIM injection mold design

 

Date：[2024/3/8]
 

A gate, also known as a feed gate, is a channel that connects the splitter and the melt in the mold cavity. The appropriate choice of gate is directly related to whether the injection molded product can be intact and of high quality. Gate design includes determining the shape and size of the gate section and selecting the gate position. The determination of the shape and size of the gate section is mentioned in many textbooks and will not be repeated here. Based on the characteristics of different injection molded products, the author compares the differences of various types of gates and discusses the selection methods and principles of gate positions and structural forms.
The position of the gate plays a decisive role in the shape of the molten flow front and the effect of the holding pressure, thus also determining the strength and other properties of the injection molded product. For factors that affect the determination of gate position, including the shape, size, wall thickness, dimensional accuracy, appearance quality, and mechanical properties of the product. In addition, the difficulty of processing, demolding, and removing the gate should also be considered. The correct gate position can avoid foreseeable problems.
1 The type and location of the gate
In injection mold design, there are 11 commonly used gate forms based on the structure and characteristics of the gate.
1. Vertical gate: refers to the main gate gate, which is a non restrictive gate
(1) Advantages: Plastic melt directly enters the mold cavity from the large end of the main channel, thus having characteristics such as low flow resistance, short process, and long supply time. This type of gate has a good melt flow state, and the melt flows from the center of the bottom surface of the mold to the parting surface, which is conducive to exhaust; This gate form minimizes the projection area of injection molded products and pouring systems on the parting surface, compact mold structure, and uniform force distribution on the injection molding machine.
(2) Disadvantages: There is a large residual stress at the feeding point, which can easily cause warping and deformation of injection molded products. At the same time, the gate is large, making it difficult to remove gate marks and affecting aesthetics. Therefore, this type of gate is mostly used for injection molding of large and medium-sized long processes, deep cavities, cylindrical or shell shaped injection molded products, especially suitable for high viscosity plastics such as polycarbonate and polysulfone. Additionally, this form of gate is only suitable for single cavity molds.
When designing such gates, in order to reduce the gate area at the contact point with the injection molded product and prevent defects such as shrinkage and deformation, on the one hand, it is advisable to choose a smaller taper of the main runner cone angle (2-4 °), and on the other hand, the thickness of the fixed template and fixed mold seat should be minimized as much as possible.
2. Side gate: In foreign countries, side gates are referred to as standard gates. The side gate is generally set on the parting surface, and the plastic melt fills the mold cavity from the inside or outside. Its cross-sectional shape is mostly rectangular (flat groove), and changing the gate width and thickness can adjust the shear rate of the melt and the freezing time of the gate. This type of gate can be selected for its position based on the shape characteristics of injection molded products, making it easy to process and repair, making it widely used.
(1) Advantages: The gate section is small, which can reduce the consumption of molten material in the pouring system. It is easy to remove the gate and the traces are not obvious. Suitable for injection molded products of various shapes, but not suitable for slender barrel shaped injection molded products.
(2) Disadvantages: Injection molded products and gates cannot separate on their own, resulting in welding marks, significant pressure loss during injection molding, and unfavorable exhaust for deep cavity injection molded products.
3. Fan shaped gate: A fan shaped gate is usually set on the parting surface, feeding from the outer side of the mold cavity. The gate gradually widens along the feeding direction, and the thickness gradually decreases. The plastic melt entering the mold cavity from the gate has a relatively straight wavefront, which can reduce warping deformation and is suitable for forming wide plate shaped plastic products.
4. Thin gate: also known as flat gate. The distribution channel of the gate is parallel to the side of the mold cavity, and its length is usually greater than the width of the plastic product.
(1) Advantages: Plastic melt enters the mold cavity uniformly at a lower speed through a thin gate, and its material flow is parallel, which can avoid warping deformation. It is commonly used to shape flat large-area thin-walled plastic products.
(2) Disadvantage: It is difficult to remove the gate, which increases the production cost of plastic products.
5. Ear protection gate: Ear protection gates are mainly used for high transparency flat plastic products and plastic products with minimal deformation requirements.
(1) Advantages: The ear guard gate is equipped with an ear groove on the side of the mold cavity, and the molten metal generates frictional heat on the side of the ear groove through the gate, thereby improving fluidity. After adjusting the direction and speed, it evenly and smoothly enters the mold cavity at the ear guard, avoiding spray.
(2) Disadvantages: It is difficult to remove the gate and there are large traces of the gate.