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Plastic Science: What Are The Composition Of Injection Molds
Jun 05, 2018

Core Tip: Although the structure of the mold may vary due to the variety and performance of the plastic, the shape and structure of the plastic product, and the type of injection machine, the basic structure is the same. The mold is mainly composed of a pouring system, a temperature control system, a molded part and a structural part. The gating system and the molded parts are in direct contact with the plastic and change with the plastics and products. They are the most complex and most varied parts in the mold and require the highest finish and accuracy of the part.


    The injection mold consists of two parts: the moving mold and the fixed mold. The moving mold is installed on the moving mold of the injection molding machine, and the fixed mold is installed on the fixed mold of the injection molding machine. During the injection molding, the movable mold and the fixed mold are closed to form a casting system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated to remove the plastic product. In order to reduce the heavy mold design and manufacturing workload, most of the injection molds use standard mold bases.


    Gating system


    The gating system refers to the part of the flow path before the plastic enters the cavity from the nozzle, including the main channel, the cold material hole, the runner and the gate.


    The gating system, also known as the runner system, is a set of feed channels that direct the plastic melt from the nozzle of the injector to the cavity, typically consisting of a main flow channel, a runner, a gate, and a cold pocket. It is directly related to the molding quality and production efficiency of plastic products.


    Mainstream road


    It is a section of the mold that connects the nozzle of the injection molding machine to the runner or cavity. The top of the main flow channel is concave for engagement with the nozzle. The main channel inlet diameter should be slightly larger than the nozzle diameter (0.8mm) to avoid spiking and prevent the two from being blocked due to inaccurate convergence. The diameter of the inlet depends on the size of the product, usually 4-8mm. The diameter of the main flow path should be enlarged inwards at an angle of 3° to 5° to facilitate the stripping of runners.


    Cold material hole


    It is a hole located at the end of the main channel to trap the cold material produced between the two shots at the tip of the nozzle, thereby preventing the clogging of the runner or the gate. If the cold material is mixed into the cavity, the internal stress is likely to occur in the manufactured product. The cold material hole has a diameter of about 8-10mm and a depth of 6mm. In order to facilitate demoulding, the bottom is often borne by the stripper. The top of the stripping bar should be designed as a zigzag hook or a depressed groove so that the strip can be smoothly pulled out during demoulding.


    Split channel


    It is the channel connecting the main channel and each cavity in the multi-slot die. In order to make the molten material fill the cavity at equal speed, the arrangement of the runners on the mold should be symmetrical and equidistantly distributed. The shape and size of the runner section affect the flow of the plastic melt, the release of the product and the ease of mold making.


    If an equal amount of flow is used, the flow path resistance with a circular cross section is minimized. However, due to the small specific surface area of the cylindrical flow passage, it is unfavorable for the cooling of the diversion channel material, and this diverging passage must be opened on the two mold halves, which is labor-consuming and difficult to align.


    Therefore, trapezoidal or semi-circular cross-section runners are often used, and they are opened on half of the mold with a stripper rod. The surface of the runner must be polished to reduce the flow resistance to provide a faster filling rate. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-sectional width of the runner is not more than 8m, extra large 10-12m, extra small 2-3m. To meet the needs of the premise should be reduced cross-sectional area as much as possible to increase the shunt material and extend the cooling time.




    It is the channel connecting the main channel (or runner) and the cavity. The cross-sectional area of the channel can be equal to the main channel (or runner), but it is usually reduced. So it is the smallest part of the entire runner system. The shape and size of the gate have a great influence on the quality of the product.


    The role of the gate is:


    A, control the flow rate:


    B. In the injection, the early flow of the melt in this part can be prevented to prevent backward flow:


    C. The molten material passing through is subjected to strong shearing to increase the temperature, thereby lowering the apparent viscosity to increase the fluidity:


    D, to facilitate the separation of products and runner system. The design of the gate shape, size, and location depends on the nature of the plastic, the size and structure of the article. Generally, the shape of the cross section of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short. This is not only based on the above-mentioned effect, but also because the small gate becomes larger and easier, and the large gate is difficult to reduce. Gate location should generally be selected where the product is thickest without affecting the appearance.


    Gate size design should take into account the nature of the plastic melt. Cavity It is the space for molding plastic products in the mold. The components that make up the cavity are collectively referred to as molded parts.


    Each molded part often has a special name. The molded parts that make up the product's shape are called female molds (also known as female molds), and the shapes that make up the product's internal shape (such as holes, slots, etc.) are called cores or punches (also known as male molds). When designing a molded part, the overall structure of the cavity must first be determined based on the properties of the plastic, the geometry of the product, dimensional tolerances, and usage requirements.


    The second is to select the parting surface, the location of the gate and the vent hole, and the demoulding method according to the determined structure. Finally, according to the control product size for the design of each part and determine the combination of parts. Plastic melt has a very high pressure when it enters the cavity, so the molded parts must be checked for reasonable material selection and strength and stiffness.


    In order to ensure that the surface of the plastic product is smooth and beautiful and easy to release, the surface with the plastic has a roughness Ra>0.32um, and it must be corrosion-resistant. Molded parts are generally heat treated to increase hardness and are made of corrosion-resistant steel.


    Temperature regulation system


    In order to meet the mold temperature requirements of the injection process, a temperature control system is required to adjust the temperature of the mold. For injection molds for thermoplastics, the cooling system is primarily designed to cool the mold. The common method for mold cooling is to open a cooling water passage in the mold and use circulating cooling water to take away the heat of the mold. In addition to heating the mold, hot water or steam can be used in the cooling water passage, and electricity can be installed inside and around the mold. Heating element.


    Molded parts


    Molded parts refer to the various parts that make up the shape of the product, including moving molds, fixed molds and cavities, cores, forming rods, and exhaust ports. The molded part consists of a core and a die. The core forms the inner surface of the article and the die forms the outer surface of the article. After mold clamping, the core and cavity form the mold cavity. According to the process and manufacturing requirements, sometimes the core and the die are assembled from several pieces, sometimes made in one piece, and the inserts are only used in parts that are easily damaged and difficult to machine.


    exhaust vent


    It is a trough-shaped air outlet opened in the mold to discharge the gas that the original and the melt brought. When the molten material is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged from the mold through the exhaust port at the end of the stream. Otherwise, the product will have air holes, poor connection, and the like. Dissatisfaction with mold filling, even the accumulation of air due to the compression generated high temperatures and burn the product.


    Under normal circumstances, the vent hole can be located either at the end of the melt flow in the cavity or on the parting surface of the mold. The latter is a shallow groove that is 0.03-0.2mm deep and 1.5-6mm wide at the side of the die. During the injection, the exhaust hole will not leak much molten material, because the molten material will cool down at this place and solidify the channel. The position of the exhaust port must not be facing the operator to prevent accidental ejection of molten material. In addition, the clearance between the ejector rod and the ejector hole, the clearance between the ejector plate and the ejector plate and the core, and the like can be used for exhaust.


    Structural parts


    It refers to the various parts that make up the mold structure, including: various parts for guiding, stripping, core pulling, and parting. Such as before and after the splint, front and rear buckle template, bearing plate, pressure column, guide column, stripping template, stripping rod and return rod and so on.



Guide member


    In order to ensure accurate centering of the movable and fixed molds during mold clamping, guides must be provided in the mold. In the injection mold, four sets of guide columns and guide bushes are usually used to form the guide parts. Sometimes it is necessary to set the inner and outer taper surfaces that are matched with each other on the movable mold and the fixed mold to assist positioning.


    2. Launch agency


    In the mold opening process, an ejector mechanism is required to push or pull the plastic product and its aggregate in the flow path. The fixed plate and the push plate are pushed out to hold the push rod. In the push rod, a reset rod is generally fixed, and the reset rod resets the push plate when the movable and fixed molds are closed.


    3. Side pull core mechanism


    Some plastic products with undercuts or side holes must be laterally typed before they are pushed out. After pulling out the lateral cores, they can be demoulded smoothly. At this time, the side core-pulling mechanism needs to be set in the mold.