Principles of each unit of metal injection molding die
Principles of Each Mold Unit
Figure 5.3 shows a schematic diagram of the basic structure of a metal injection molding tool. It consists of a set of steel plates with a clamping plate at each end. The front mold plate is fixed to the side facing the injection unit, sometimes called the fixed platen. The molding tool has a central hole surrounded by a locating ring. When the feedstock is injected into the mold through the sprue bushing, the nozzle of the injection unit is directly facing the central hole. Behind the front mold plate, two cavity plates contain one or more cavities. The front cavity plate is fixed to the front mold plate, and the rear cavity plate is connected to the rear mold plate. The fixed end of the mold is usually called the A-side, and the movable end is called the B-side.
The temperature of the mold cavity is controlled by an oil temperature controller or a water temperature controller, and the oil or water can circulate through the cooling channels within the cavity plates. The design of the cooling channels and the optimal temperature of the mold largely depend on the type of feedstock and require certain engineering experience, making it difficult to give general guidance. The moldtemperature should not be too low, otherwise the rapid cooling of the feedstock will lead to incomplete filling; nor should the mold temperature be too high, otherwise it will increase the cooling time of the feedstock. German company BASF Catamold usually uses oil to heat the mold because the company's feedstock requires very high temperatures, while water is typically used to heat the mold for wax/polymer and water-soluble feedstock systems.
The cavity plates are closed during the injection molding process, and the part is ejected when the cavity plates separate after solidification. The design of the mold cavity should ensure that the adhesion of the part to the rear cavity plate is greater than its adhesion to the front cavity plate, so that when the cavity plates separate, the part will stick to the rear cavity plate and can be ejected by ejector pins; otherwise, the part will not fall off the mold. Some molds have spring-loaded ejector pins on the A-side to ensure the part remains on the B-side when the mold opens.
When the front cavity plate is fixed to the front mold plate, the rear cavity plate is held in place by a support plate, which is connected to the rear mold plate via a spacer block. When the mold is closed, the accurate positioning of the two cavity plates is ensured by guide pins at the 4 corners of the mold. The holes in the spacer block leave space for moving components such as ejector pins and puller bolts, whose ends are connected to the ejector plate. The ejector plate can move in the limited area between the ejector plate retainer and the rear mold plate. The appropriate number of ejector pins can be determined based on the size and quantity of the injected parts.
Mold designers determine the number and distribution of ejector pins by analyzing the friction between the mold and the part. This also requires considering the shrinkage of the part during cooling, which can cause the external contours of the part to be easily pushed out, but the internal contours may adhere to the mold due to shrinkage towards the core.
Setting a draft angle makes the demolding process easier. The draft angle is the angle of a surface parallel to the mold opening direction, and the draft angle is typically 1°.The basic structural components that make up the injection mold have been introduced above. Most of these components are standardized and can be purchased from specialized suppliers. Metal injection molding parts manufacturers rely on these standard component suppliers to save production costs and maintain high quality standards for the molds. Generally, only the mold cavity plate must be customized.
For parts with more complex shapes, the mold cavity may include cores, retractable slides, threads, etc. Adding a plate between the two cavity plates allows for more flexible adjustment of the gate position, which is known as a three-plate mold.
