Injection mold is a part that endows plastic with shape and size during molding. Although the structure of the mold may be ever-changing due to the variety and performance of plastics, 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 gating system, forming parts and structural parts. Among them, the gating system and molding parts are the parts in direct contact with plastics, and change with plastics and products, which are the most complex, most varied and require the highest processing smoothness and precision.
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, etc. Molded parts refer to all kinds of parts that constitute the shape of products, including moving mold, fixed mold and cavity, core, molding rod and exhaust port, etc. Typical mold structure is shown in the figure.
Mainstream avenue
It is a passage in the mold that connects the nozzle of the injection machine to the runner or cavity. The top of the main runner is concave to engage with the nozzle. The inlet diameter of the main runner should be slightly larger than the nozzle diameter (0. 8 mm) to avoid spillage and prevent the two from blocking due to improper connection. The diameter of the inlet depends on the size of the product and is generally 4-8 mm. The diameter of the main runner should be expanded inward at an angle of 3 to 5, so as to facilitate the demoulding of the runners.
cold slug well
It is a cavity at the end of the main runner, which is used to collect the cold material produced between 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, the internal stress will easily occur in the manufactured product. The cold material cavity has a diameter of about 8-lOmm and a depth of 6mm. In order to facilitate demoulding, its bottom is often borne by demoulding rod. The top of the demoulding rod should be designed into a zigzag hook shape or a sunken groove, so that the main runner can be pulled out smoothly during demoulding.
subchannel
It is the channel connecting the main runner and each cavity in the multi-groove die. In order to fill each cavity with molten material at equal speed, the distribution of runners on the mold should be symmetrical and equidistant. The shape and size of the cross section of the runner have an influence on the flow of plastic melt, the demoulding of products and the difficulty of mold manufacturing.
If the flow rate is equal, the resistance of the flow channel with circular cross-section is the smallest. However, the specific surface of the cylindrical runner is small, which is unfavorable to the cooling of the shunt. Moreover, the shunt must be set on the two mold halves, which is labor-consuming and easy to align.
Therefore, trapezoidal or semi-circular cross-section shunt channels are often used, which are set on half of the mold with demoulding rod. The runner surface must be polished to reduce the flow resistance and provide faster mold filling speed. The size of the runner depends on the plastic variety, the size and thickness of the product. For most thermoplastics, the cross-sectional width of the shunt is no more than 8m, which can be as large as 10-12m and as small as 2-3m. On the premise of meeting the needs, the cross-sectional area should be reduced as far as possible, so as not to increase the shunt and prolong the cooling time.
Gate
It is a channel connecting the main runner (or shunt) with 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. Therefore, it is the smallest cross-sectional area in the whole flow channel system. The shape and size of the gate have great influence on the product quality.
The functions of the gate are as follows: a, to control the flow speed; b, to prevent backflow due to the premature solidification of the molten material in this part during injection; c, to make the passing molten material undergo strong shear and raise the temperature, thereby reducing the apparent viscosity and improving the fluidity; d, to facilitate the separation of products from the runner system. The design of gate shape, size and position depends on the properties of plastics, the size and structure of products.
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, which is not only based on the above-mentioned effects, but also because it is easy for the small gate to get bigger, but difficult for the large gate to shrink. Generally, the gate position should be selected at the thickest place without affecting the appearance of the product. The design of gate size should consider the properties of plastic melt.
Cavity
It is the space for molding plastic products in the mold. Components used to form cavities are collectively referred to as molding parts. Each molded part often has a special name. The molded parts that make up the shape of products are called female dies (also called female die), and those that make up the internal shapes of products (such as holes and grooves) are called cores or male dies (also called male dies).
When designing molded parts, the overall structure of the cavity should be determined according to the properties of plastics, geometric shapes of products, dimensional tolerances and application requirements. Secondly, according to the determined structure, the position of parting surface, gate and vent and demoulding method are selected. Finally, design each part according to the size of the controlled product and determine the combination mode between each part. The plastic melt has high pressure when it enters the cavity, so it is necessary to select reasonable materials and check the strength and stiffness of the molded parts.
In order to ensure that the surface of plastic products is bright, clean, beautiful and easy to demould, the roughness Ra > 0.32 um of the surface in contact with plastic should be corrosion resistant. Generally, the hardness of molded parts is improved by heat treatment, and corrosion-resistant steel is selected.
exhaust port
It is a groove-shaped gas outlet opened in the mold, which is used to discharge the original gas and the gas brought by the melt. When the melt is injected into the mold cavity, the air originally stored in the mold 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 welding, unsatisfactory mold filling, and even the accumulated air will burn the product due to high temperature caused by compression.
Under normal circumstances, the vent hole can be located at the end of melt flow in the mold cavity, or on 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. During injection, there will not be much melt seeping out of the vent hole, because the melt will cool and solidify there and block the passage. The opening position of the exhaust port must not face the operator, so as to prevent accidental spraying of molten material from hurting people.
In addition, the fit clearance between ejector rod and ejector hole, and the fit clearance between ejector block and stripper plate and core can also be used to exhaust air.
structural parts
It refers to all kinds of parts constituting the mold structure, including guiding, demoulding, core pulling and parting parts. Such as front and rear clamping plates, front and rear buckle templates, bearing plates, pressure columns, guide columns, stripping plates, stripping rods and return rods, etc.
Heating or cooling device
This is a device for solidifying and shaping molten materials in the mold. For thermoplastics, it is generally the channel of cooling medium in the positive and negative molds, and the cooling purpose is achieved by the circulating flow of cooling medium. The introduced cooling medium varies with the type of plastic and product structure, including cold water, hot water, hot oil and steam. The key is efficient uniform cooling. Uneven cooling will directly affect the quality and size of products. The arrangement of cooling channels and the selection of cooling medium should be considered according to the thermal properties (including crystallization) of the melt, the shape of the product and the mold structure.
Introduction and composition of injection mold
Plastic products are usually produced in batches or in large quantities, so it is required that the mold should be of high efficiency and high quality when used, with little or no processing after molding, so the mold design must consider:
1. Determine the parting surface and gate position according to the service performance and molding performance of plastic parts.
2. Considering the manufacturability in mold manufacturing engineering, the design scheme is determined according to the equipment status and technical strength, so as to ensure that the mold is easy to process from the whole to the parts, and easy to ensure the dimensional accuracy.
3. Considering injection productivity, increase injection times per unit time and shorten molding cycle.
4. Show the size, hole, column, convex and concave structures with precision requirements in the mold, that is, the plastic parts will not be processed or processed less after molding.
5. The die structure is simple and applicable, stable and reliable, with short cycle and low cost, which is convenient for assembly, maintenance and replacement of wearing parts.
6. Selection and treatment of die materials.
7. Standardized production of dies: try to select standard parts such as standard die frame, common ejector pin, guide parts, sprue bushing and positioning ring.
Basic composition of injection mold
1. gating system: the channel through which molten materials flow into the cavity from the injection machine nozzle, including main runner, branch runner, gate, cold well, drawing rod, etc.
2. Molded parts: parts of molded plastic parts, such as cores, cavities and other auxiliary parts.
3. Temperature control system: used to adjust the temperature of the mold.
4. Ejection system of plastic parts: including side parting mechanism, secondary ejection mechanism, first reset mechanism and several sequential distance parting mechanisms, etc.
5. Installation part: the part that reliably installs the mold body on the injection machine.
6. Connecting system: A connecting system that integrates all structural members into a whole.
7. Guide system: to ensure the movement accuracy of various structural members, such as guide posts and guide chutes.
