Although the structure of the mold may vary depending on the type and function of plastic, the shape and structure of plastic products, and the type of injection machine, the basic structure is the same. The mold is mainly composed of pouring system, temperature regulating system, forming parts and structural parts. Among them, the pouring system and molding parts are the parts that are in direct contact with the plastic, and change with the plastic and the product. They are the most complicated parts in the mold, the changes are the largest, and the processing brightness and precision are required to be the highest.
The injection mold consists of a movable mold and a fixed mold. The movable mold is installed on the movable template of the injection molding machine, and 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 a casting system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic products. In order to reduce the heavy mold planning and production workload, most of the injection molds use standard mold bases.
The pouring system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, the cold material cavity, the runner and the gate.
The pouring system, also known as the runner system, is a set of feeding channels that lead the plastic melt from the injection machine nozzle to the cavity. It is directly related to the molding quality and production efficiency of plastic products.
It is a passage in the mold that connects the injection nozzle of the injection molding machine to the runner or cavity. The top of the dry runner is concave for coupling with the nozzle. The diameter of the inlet of the main runner should be slightly larger than the diameter of the nozzle (0.8mm) to avoid overflow, and to avoid the blockage caused by the connection prohibition.
The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the main runner should be expanded inward at an angle of 3° to 5° to facilitate the release of the runner debris.
It is a cavity at the end of the main runner to capture the cold material generated between the two injections at the end of the nozzle, and then avoid the blockage of the runner or gate.
If the cold material is mixed into the cavity, internal stress will easily occur in the manufactured product. The diameter of the cold material hole is about 8-10mm and the depth is 6mm. In order to facilitate demoulding, its bottom is often carried by a demoulding rod.
The top of the demoulding rod should be designed as a zigzag hook or a sunken groove, so that the dry runner can be smoothly pulled out during demolding.
It is the channel connecting the main runner and each cavity in the multi-slot mold. In order to make the melt fill each cavity at the same speed, the arrangement of the runners on the mold should be symmetrical and equidistant.
The shape and size of the runner section have an impact on the movement of the plastic melt, the demolding of the product and the difficulty of mold making.
If the movement of the same amount of material is used, the flow channel resistance with a circular cross-section is the smallest. However, because the cylindrical runner is smaller than the surface, it is not good for the cooling of the shunt runner, and this runner must be opened on the two halves of the mold, which is labor-intensive and difficult to align.
Therefore, trapezoidal or semi-circular cross-section runners are often used, and are opened on half of the mold with ejector rods. The surface of the runner must be polished to reduce the movement resistance and provide a faster filling speed. 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 does not exceed 8m, the extra-large can reach 10-12m, and the extra-small 2-3m. On the premise of meeting the requirements, the cross-sectional area should be reduced as much as possible to add shunt debris and extend the cooling time.
It is the channel that connects the main runner (or branch runner) and the cavity.
The cross-sectional area of the channel can be equal to the main channel (or branch channel), but it is generally reduced. Therefore, it is the part with the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great influence on the quality of the product.
The effect of the gate is:
A. Control the flow rate:
B. During injection, backflow can be avoided due to the premature solidification of the melt stored in this part:
C. Make the passing melt subject to strong shear to increase the temperature, and then reduce the apparent viscosity to improve the activity:
D. It is convenient to separate the product from the runner system. The design of gate shape, size and orientation depends on the nature of the plastic, the size and structure of the product. Generally, the cross-sectional shape 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 effects, but also because it is easier for a small gate to become larger, while it is difficult to shrink a large gate.
The gate position should generally be selected where the product is thickest without affecting the appearance.
The gate size planning should take into account the properties of the plastic melt. Cavity It is the space in the mold for molding plastic products.
The components used to form the cavity are collectively referred to as molded parts.
Each forming part often has a special title. The molding parts that constitute the shape of the product are called concave molds (also known as female molds), and those that constitute the internal shapes of the products (such as holes, grooves, etc.) are called cores or punches (also known as male molds).
When designing molding parts, the overall structure of the cavity should be confirmed first according to the function of the plastic, the geometric shape of the product, the dimensional tolerance and the application requirements.
The second is to select the parting surface, the position of the gate and the exhaust hole and the demoulding method according to the confirmed structure. Finally, plan each part according to the scale of the control product and confirm the combination method between each part.
The plastic melt has a high pressure when it enters the cavity, so the molding parts should be reasonably selected and checked for strength and stiffness.
In order to ensure the bright and beautiful appearance of plastic products and easy demoulding, the roughness Ra of any surface in contact with plastic is>0.32um, and corrosion resistance. Formed parts are generally heat treated to increase hardness and made of corrosion-resistant steel.
temperature control system
In order to meet the requirements of the injection process for the mold temperature, a temperature adjustment system is required to adjust the temperature of the mold. Regarding the injection mold for thermoplastics, the cooling system is mainly designed to cool the mold. The common method of mold cooling is to open a cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; in addition to using hot water or steam in the cooling water channel, the heating of the mold can also install electricity in and around the mold. Heating element.
Molded parts refer to various parts that constitute the shape of the product, including movable molds, fixed molds and cavities, cores, molding rods, and exhaust ports. The molded part consists of a core and a die. The core constitutes the inner and outer surfaces of the product, and the concave mold constitutes the outer and outer shape of the product.
After the mold is closed, the core and the cavity constitute the cavity of the mold. According to the process and production requirements, sometimes the core and the die are composed of several pieces, and sometimes they are made as a whole, and inserts are only used in the parts that are easily damaged and difficult to process.
It is a groove-shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the melt.
When the melt is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged out of the mold through the exhaust port at the end of the material flow, otherwise the product will have pores and poor connection. , The mold filling is not satisfied, and even the accumulated air will burn the product due to the high temperature caused by the compression.
Under normal circumstances, the vent hole can be set either at the end of the molten material movement in the cavity, or at the parting surface of the mold. The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die.
During injection, there will not be a lot of molten material oozing out of the vent hole, because the molten material will cool and solidify there and block the channel. The opening position of the exhaust port should not face the operator to prevent accidental injection of molten material and hurt people. In addition, the matching gap between the ejector rod and the ejection hole, the matching gap between the ejector block and the stripper plate and the core can also be used to exhaust air.
It refers to various parts that constitute the mold structure, including: guiding, demoulding, core pulling and parting. Such as front and rear splints, front and rear buckle templates, bearing plates, bearing columns, guide columns, stripping plates, stripping rods and return rods, etc.
1. Guide parts
In order to ensure that the movable mold and the fixed mold can be accurately aligned when the mold is closed, it is necessary to provide guide parts in the mold.
In the injection mold, four sets of guide posts and guide sleeves are generally used to form the guide parts, and sometimes it is necessary to set mutually matching inner and outer cone surfaces on the movable mold and the fixed mold to assist positioning.
2. Launch the organization
During the mold opening process, a push-out organization is required to push or pull out the plastic products and their aggregates in the runner. Push out the fixed plate and the push plate to clamp the push rod.
A reset rod is generally fixed in the push rod, and the reset rod resets the push plate when the movable and fixed molds are closed.
3. Side core pulling organization
For some plastic products with side concave or side holes, it is necessary to carry out side parting before being pushed out, and then the side core can be removed smoothly after pulling out the side core.
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