Unveiling the Composition of Plastic Mold Structure: Clear and Easy-to-Understand Guide

The injection molding of molds is a method used for mass-producing certain complex-shaped components. The specific principle involves pushing heated and melted plastic materials into the mold cavity of the plastic mold under high pressure through the screw of the injection molding machine. After cooling and solidification, plastic molded products are obtained.

Plastic molds consist of two parts: the movable mold and the fixed mold. The movable mold is installed on the moving template of the injection molding machine, while the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form the gating system and the cavity. When opening the mold, the movable mold and the fixed mold separate to facilitate the removal of plastic products.

Part A: Plastic mold structure may vary greatly due to different types and properties of plastics, shapes and structures of plastic products, and types of injection machines. However, the basic structure remains consistent.

1.      Plastic mold structure can be divided by function, mainly composed of: gating system, temperature control system, molding component system, venting system, guiding system, ejector system, etc. Among them, the gating system and molding components are in direct contact with the plastic, varying with the plastic and product, making them the most complex and variable parts of the mold, requiring the highest processing cleanliness and precision.

2.     Gating system: refers to the part of the plastic flow path from the nozzle to the cavity, including the main runner, sub-runner, cold slug well, gate, etc.

3.     Molding component system: refers to the various components that form the shape of the product, including the movable mold, fixed mold, cavity (concave mold), core (convex mold), molding rod, etc. The core forms the inner surface of the product, while the cavity (concave mold) forms the outer surface shape of the product. After molding, the core and cavity form the mold cavity. According to process and manufacturing requirements, sometimes the core and cavity are composed of several blocks, sometimes they are made as a whole, with inserts used only in vulnerable or difficult-to-process areas.

4.     Temperature control system: to meet the temperature requirements of the injection process, a temperature control system is needed to adjust the mold temperature. For injection molds used with thermoplastic plastics, the main focus is on designing cooling systems to cool the mold (or heat the mold). The common method of mold cooling is to open cooling water channels inside the mold, using circulating cooling water to take away the heat from the mold; mold heating can also be achieved by using hot water or hot oil through cooling water channels, or by installing electric heating elements inside and around the mold.

5.     Venting system: designed to exhaust the air and gases generated during the injection molding process from the cavity to the outside of the mold. Poor venting can result in surface defects such as gas marks (silver streaks) or burning. The venting system of a plastic mold usually consists of a slot-shaped vent opening inside the mold to exhaust the original cavity air and the gas brought in by the melt.

During the injection of the melt into the cavity, the air originally trapped in the cavity and the gas brought in by the melt must be exhausted out of the mold through the vent opening at the end of the flow, otherwise, the product may contain air bubbles, poor weld lines, incomplete filling, or even burn due to accumulated air being compressed and heated. Generally, the vent hole can be located at the end of the melt flow in the cavity or on the parting surface of the mold.

The latter is a shallow groove opened on one side of the concave mold, with a depth of 0.03-0.2mm and a width of 1.5-6mm. During injection, the vent hole will not have much molten material leaking out, as the material will solidify and block the passage at that point. The location of the vent opening should not face the operator to prevent accidental splashing of molten material. In addition, venting can also be achieved by using the clearance between the ejector rod and ejector hole, the mating clearance between the ejector block and the stripper plate, and the mating clearance between the core and the cavity.

5.     Guiding system: designed to ensure accurate alignment of the movable and fixed molds during mold closing, guiding components must be installed in the mold. In injection molds, four sets of guide pillars and guide bushes are usually used to form the guiding components, and sometimes inner and outer tapered surfaces that match each other are also set on the movable and fixed molds to assist in positioning.

6.     Ejector system: generally includes ejector pins, front and rear ejector plates, ejector pin guide rods, ejector pin return springs, ejector plate locking screws, etc. After the product is cooled and formed in the mold, when the mold is opened for demolding, the plastic product and any flash in the runner are pushed or pulled out of the mold cavity and runner positions by the ejection mechanism - ejector pins driven by the injection machine's ejector rod, so as to prepare for the next injection molding cycle.

Part B: Plastic molds are generally composed of several parts according to their structure, including mold frame, mold core, auxiliary components, auxiliary systems, auxiliary settings, and dead corner processing mechanisms.

1.     Mold frame: Generally, it is not necessary to design it ourselves, as it can be ordered directly from standard mold frame manufacturers, saving a significant amount of time required for mold design. Therefore, it is referred to as the standard mold frame for plastic molds. It forms the basic framework of the plastic mold.

2.     Mold core: The mold core is the core part of the plastic mold and the most important component inside the mold. The forming part of the plastic product is located inside the mold core, and most of the processing time is spent on the mold core. However, for relatively simple molds, there may be no mold core, and the product is formed directly on the mold plate. This was the case for most early plastic molds, which were relatively primitive.

3.     Auxiliary components: Common auxiliary components of plastic molds include locating rings, sprue bushings, ejector pins, knockout pins, support pillars, ejector plate guide pins and bushings, and runner vents. Some of these are standard parts that can be ordered together with the mold frame, while others need to be designed separately.

4.     Auxiliary systems: The auxiliary systems of plastic molds include gating systems, ejector systems, cooling systems, and venting systems. Sometimes, due to the high temperature requirements of the plastic materials used, some molds may also have a heating system.

5.     Auxiliary settings: Auxiliary settings for plastic molds include lifting ring holes, knockout (KO) holes, and so on.

6.     Dead corner processing mechanisms: When plastic products have dead corners, the mold may have one or more structures to handle them, such as sliders, inclines, hydraulic cylinders, etc. Most domestic literature refers to these mechanisms for handling dead corners as "core-pulling mechanisms."

In fact, plastic molds are not complicated. Regardless of the variations in plastic products, the structure of the mold for forming these plastic products boils down to the aforementioned aspects. The differences between molds mainly lie in their sizes. The positions or methods of various auxiliary components, auxiliary settings, and auxiliary systems may vary. The methods, structures, and sizes of dead corner processing may also differ. Of course, designing molds to be simple to process, easy to assemble, long-lasting, reasonably priced, and capable of producing good-quality molded products requires significant design experience. Good experience can help address problems encountered during design and processing and provide better handling of design changes.

Part C: Structure composition of injection machines: A general-purpose injection machine mainly consists of injection unit, clamping unit, hydraulic drive system, and electrical control system.

The injection unit primarily functions to uniformly melt the plastic and inject a certain amount of molten material into the mold cavity with sufficient pressure and speed. The injection unit consists of plasticizing components (screw, barrel, and nozzle), hopper, transmission device, metering device, injection and moving cylinders, etc.

The clamping unit's role is to open and close the mold, ensuring reliable closure of the molding mold during injection and facilitating product demolding. The clamping unit consists mainly of front and rear fixed plates, moving plates, tie bars connecting the front and rear plates, clamping cylinders, linkage mechanisms, mold adjustment devices, and product ejector devices.

The hydraulic system and electrical control system ensure that the injection machine operates accurately and effectively according to the predetermined requirements (pressure, speed, temperature, time) and sequence of actions during the process. The hydraulic system of the injection machine mainly consists of various hydraulic components, circuits, and other auxiliary equipment, while the electrical control system mainly comprises various electrical components and instruments. The hydraulic and electrical systems are organized together to provide power and control for the injection machine.