The mold is mainly composed of pouring system, temperature adjustment system, molding 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 complex and the most changing parts in the mold, requiring the highest processing finish and precision.
The injection mold consists of a moving mold and a fixed mold. The moving 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 the gating system and the cavity, and the movable mold and the fixed mold are separated to take out the plastic product when the mold is opened. In order to reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.
Gating system
The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, cold material cavity, runner and gate.
The gating system, also known as the runner system, is a set of feed channels that guide the plastic melt from the nozzle of the injection machine to the cavity. It usually consists of a main channel, a runner, a gate and a cold cavity. It is directly related to the molding quality and production efficiency of plastic products.
main channel
It is a channel in the mold that connects the nozzle of the injection molding machine to the runner or cavity. The top of the sprue is concave to connect with the nozzle. The inlet diameter of the main channel should be slightly larger than the nozzle diameter (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection. The inlet diameter depends on the size of the product, generally 4-8mm. The diameter of the sprue should expand inward at an angle of 3° to 5° to facilitate the demoulding of the excess in the flow path.
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cold hole
It is a cavity at the end of the sprue to catch the cold material generated between the two injections at the end of the nozzle, so as to prevent the blockage of the runner or gate. Once the cold material is mixed into the cavity, internal stress is easily generated in the manufactured product. The diameter of the cold slug hole is about 8-10mm, and the depth is 6mm. In order to facilitate demoulding, its bottom is often borne by a demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or a sunken groove, so that the main channel can be pulled out smoothly when demoulding.
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Runner
It is the channel connecting the sprue and each cavity in the multi-slot mold. In order to make the molten material fill each cavity at an equal speed, the arrangement of the runners on the mold should be symmetrical and equidistant. The shape and size of the cross-section of the runner have an impact on the flow of the plastic melt, the ease of product demoulding and mold manufacturing.
If the flow of the same amount of material is concerned, the flow channel with a circular cross-section has the least resistance. But because the specific surface of the cylindrical runner is small, it is unfavorable to the cooling of the debris in the runner, and the runner must be opened on the two halves of the mold, which is labor-intensive and difficult to align.
Therefore, trapezoidal or semicircular cross-section runners are often used, and they are opened on half of the mold with ejector pins. The surface of the runner must be polished to reduce flow resistance and provide 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 one can reach 10-12m, and the extra-small one can reach 2-3m. Under the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the debris in the shunt channel and prolong the cooling time.
Fives
gate
It is a channel connecting the main channel (or runner) and the cavity. The cross-sectional area of the channel can be equal to that of the main channel (or branch channel), but it is usually reduced. So 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 function of the gate is:
A. Control the material flow rate:
B. During the injection, the backflow can be prevented due to the early solidification of the melt stored in this part:
C. Make the passing melt subject to strong shear to increase the temperature, thereby reducing the apparent viscosity to improve fluidity:
D. It is convenient to separate the product from the runner system. The design of the shape, size and position of the gate 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-mentioned effects, but also because it is easier to enlarge a small gate, but it is difficult to shrink a large gate. The gate position should generally be selected at the place where the product is the thickest without affecting the appearance.
The design of the gate size should take into account the nature of the plastic melt. Cavity It is the space in which plastic products are formed in the mold. The components used to form the cavity are collectively referred to as molded parts.
Each molded part often has a special name. The forming part that constitutes the shape of the product is called the die (also known as the female die), and the part that constitutes the internal shape of the product (such as holes, grooves, etc.) is called the core or punch (also known as the male die). When designing a molded part, the overall structure of the cavity must first be determined according to the properties of the plastic, the geometric shape of the product, the dimensional tolerance and the use requirements.
The second is to select the parting surface, the position of the gate and the vent hole and the demoulding method according to the determined structure.
Finally, the design of each part is carried out according to the size of the control product and the combination of each part is determined. When the plastic melt enters the cavity, there is a high pressure, so the molded parts should be selected reasonably and checked for strength and stiffness.
In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the surface in contact with plastic should have a roughness Ra>0.32um and be corrosion-resistant. Formed parts are generally heat-treated to increase hardness and made of corrosion-resistant steel.
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Temperature control system
In order to meet the mold temperature requirements of the injection process, a temperature adjustment system is required to adjust the mold temperature. For injection molds for thermoplastics, the cooling system is mainly designed to cool the mold.
The common method of mold cooling is to set up 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 be installed inside and around the mold. Heating element.
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molded parts
Refers to the various parts that make up the shape of the product, including moving molds, fixed molds and cavities, cores, molding rods, and exhaust ports. The molded part consists of a core and a die. The core forms the inner surface of the product, and the die forms the shape of the outer surface of the product. After the mold is closed, the core and the cavity form the cavity of the mold.
According to the process and manufacturing requirements, sometimes the core and die are composed of several pieces, and sometimes they are made into a whole, and inserts are only used in easily damaged and difficult-to-process parts.
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exhaust vent
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 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 out of the mold through the exhaust port at the end of the material flow, otherwise the product will have pores, poor connection, The mold is not fully filled, and even the accumulated air will burn the product due to the high temperature generated by compression.
In general, the vent hole can be set either at the end of the molten material flow in the cavity or on the parting surface of the mold. The latter is to open 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 seeping 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 must not face the operator to prevent the molten material from accidentally spraying out and hurting people.
In addition, it is also possible to use the matching gap between the ejector rod and the ejection hole, the matching gap between the ejector block and the stripping plate and the core, etc. to exhaust.
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structural parts
It refers to various parts that make up the mold structure, including: various parts for guiding, demoulding, core pulling and parting. Such as front and rear splints, front and rear buckle templates, pressure plates, pressure columns, guide columns, stripping plates, stripping rods and return rods, etc.
01
guide parts
In order to ensure that the movable mold and the fixed mold can be accurately centered when the mold is closed, guiding parts must be set in the mold. In the injection mold, four sets of guide posts and guide sleeves are usually used to form the guide parts, and sometimes it is necessary to set mutually matching inner and outer tapered surfaces on the movable mold and the fixed mold to assist in positioning.
02
launch agency
During the mold opening process, a push-out mechanism is needed to push or pull out the plastic product and its condensate in the runner. Push out the fixed plate and the push plate to clamp the push rod. Generally, a reset rod is also fixed in the push rod, and the reset rod resets the push plate when the moving and fixed molds are closed.
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Side core pulling mechanism
Some plastic products with undercuts or side holes must be laterally parted before being pushed out, and the side core can be successfully demoulded after the side core is pulled out. At this time, a side core pulling mechanism needs to be set in the mold.
