Generally speaking, rubber molded products must be molded through the corresponding mold for molding processing, a rubber product after high temperature, high pressure vulcanization, from the mold cavity or mold core is commonly known as mold release. Poor demolding is one of the important reasons for the quality defects of rubber products and the impact on production efficiency. It can cause defects such as distortion and tearing of parts, and some even damage the mold, bringing trouble to normal production. Studying the unfavorable factors affecting the demolding of rubber products is of great significance to ensure the quality of products, prevent defects, prevent scrap, and improve production efficiency.
1. Factors affecting the demolding of rubber products
Poor demolding of rubber products mainly means that when the product is ejected, it cannot fall off smoothly. This is caused by many influencing factors, these factors are complex in relation to each other, and the degree of influence and expression are different, mainly including rubber product design, mold design and manufacturing, production process, operation method, mold maintenance, etc.
1.1 The influence of rubber product design on mold release
The design of rubber products directly affects the release performance of products, so the design of products should meet the requirements of easy demolding of products. The main factor affecting the demolding in the design of the product is the demolding slope, in order to open the mold and take out the product, the inner and outer surfaces of the vertical parting surface should be provided with sufficient demolding slope. Although some products have the slope of demolding, the value is too small, and some products only have the slope of the outer surface, ignoring the slope of the inner surface and the internal ribs and perseverance; Some products have no slope at all, which brings difficulties to the product demolding. After the product is baked, the centripetal shrinkage occurs due to the cooling of the product, which produces a large holding force on the core or pin, which hinders the demolding. If the demolding slope is increased, this resistance can be significantly reduced, and defects such as tearing of the product due to lack of slope can also be avoided. The demolding slope is related to the shape and thickness of the product, usually determined empirically, and the slope of the general product is between 1 ° ~ 3 °.
1.2 The influence of mold design and manufacturing on mold release
1.2.1 Influence of mold design on mold release
Rubber mold is one of the main equipment for the production of rubber products, mold pressing principle can be divided into injection mold, die casting mold, pressing mold design is according to the shape, characteristics and use requirements of the product, according to the same rubber products to design several molds of different structures. The mold structure is directly related to the quality of products, production efficiency, mold processing difficulty and service life. Therefore, mold structure design research is quite important. In order to ensure that rubber products have correct geometry and certain dimensional accuracy, mold structure design should follow the following principles:
(1) Master and understand the hardness, shrinkage and use requirements of the materials used in rubber products.
(2) Ensure the shape and contour of the product.
(3) The mold structure should be simple and reasonable, the positioning should be reliable, the installation and disassembly should be convenient, and it should be easy to operate.
(4) The number of mold cavities is appropriate, which is convenient for machining and mold use, and should take into account production efficiency.
(5) The mold should have sufficient strength and rigidity, and strive to be small in shape, light in weight, easy to process, and in line with the production process.
(6) The mold cavity should be convenient for loading and taking out the product, and when vulcanizing, the rubber material should have sufficient pressure.
(7) The mold should have a certain accuracy, finish and reasonable parting surface, which is easy to trim.
(8) The mold should have a broken rubber groove to facilitate cleaning.
(9) The mold design should conform to serialization and standardization, and strive for good versatility.
From the requirements of mold design, it can be seen that the factors affecting the demolding of products include mold stiffness, demolding resistance, ejection mechanism, etc.
1.2.1.1 Mold stiffness
Rubber molds generally use combined molds, so there are interference fits or gap fits in the molds. In the production process, under the action of clamping force or injection pressure, mold parts are prone to elastic deformation, and when the mold is opened, these deformations will cause friction between steel and steel surfaces. If the elastic rebound is large, it will also cause an extrusion force between the rubber and the mold surface, forcing the center of the frame to become an arc, and the rubber material is extruded from the curved frame seam. As a result, the mold opening resistance increases, resulting in difficulty in demolding, causing the product to tear, and even the mold to be scrapped (usually the product has a steel skeleton material). Therefore, mold design should ensure that the molded parts have sufficient rigidity.
1.2.1.2 Demolding resistance
When the product is demolded, it is necessary to overcome the mold opening resistance and ejection resistance. Most of the product quality defects caused by poor mold release are related to this. The ejection resistance mainly comes from the holding force of the product to the core, including the force caused by shrinkage, extrusion, bonding and the friction between the rubber and steel surfaces. These forces either add up or are combined in different ways to affect the release of the product.
1.2.1.3 Ejection mechanism
The ejection mechanism directly affects the ejection effect, the demoulding ejector rod is generally installed in the center position of the mold, and the cross-sectional area of the demoulding ejector rod should not be too small, in order to prevent excessive force per unit area, it is easy to make thin products top break or products with steel skeleton deformation. When producing products with large area and heavy weight, in order to prevent excessive friction caused by the ejection action jamming or the bending of the ejector rod, the mold pad should be equipped with a demolding guide block of about ¢100mm, or more set up the mold ejector rod assembly, but it needs to be balanced.
1.2.2 The influence of mold manufacturing on mold release
During the mold manufacturing process, the cavity, core surface roughness, and the gap of the mating surface of the insert structure should be strictly checked, otherwise it will affect the mold release of the product. The gap between the insert and the mating surface is too large, the rubber has fluid properties when heated, and the rubber is easy to squeeze during the mold filling process, forming a thick flash, which seriously hinders the demolding and product appearance. In addition, for deep-cavity thin-walled products, when ejected, a vacuum is formed on the surface of the part and the core, resulting in difficulty in demolding. Therefore, when the mold is manufactured, the core should have appropriate air intake holes or the core surface is relatively rough (without affecting the quality of the product), which is conducive to mold release.
1.3 The influence of production process parameters on mold release
The production process parameters are related to the quality defects of the products, among which the injection pressure, holding pressure pressure, vulcanization temperature, glue content, vulcanization time, etc. have a great influence on the demolding. When the injection pressure is too high, it will cause elastic deformation of the mold parts and cause extrusion force. If the holding time is too long, the pressure in the mold cavity increases, which will cause the shear force and molecular orientation stress to increase. At the same time, the holding pressure injection force is too high and the time is too long, which will also cause the process filling, produce large internal stress, and also cause the deformation of the mold parts or the flash between the mating surfaces, making the demolding more difficult. Vulcanization temperature, rubber content rate, vulcanization time are related to the shrinkage rate of rubber, vulcanized rubber is easy to produce return to the original phenomenon at high temperature, large shrinkage rate of rubber after vulcanization, vice versa, small shrinkage. The higher the rubber content rate, the greater the shrinkage, the lower the rubber content, the smaller the shrinkage, the longer the vulcanization time, the greater the degree of cross-linking, the small shrinkage rate, the short vulcanization time, the small degree of crosslinking time, the shrinkage rate is large, and the shrinkage rate is large in terms of process, the shrinkage rate is large in under-vulcanization or per-vulcanization, and the minimum shrinkage rate is large only at the positive vulcanization point, and the holding force of the mold with complex core and insert structure is also large, which is not conducive to the demolding of the product.
1.4 Influence of operation method on mold release
A pair of well-structured molds, such as using the same rubber material and using the same process flow, due to the different operator proficiency and methods, the release effect obtained is also different. Therefore, it is necessary to be familiar with and master the demolding method of the product in order to obtain a good demolding effect. Here are a few reasonable release methods:
(1) Manual demolding is one of the methods for rubber products to take out products from the mold cavity and mold core, which is suitable for small miscellaneous parts and rubber products with high hardness.
(2) Mechanical demolding is mostly used for injection molding molds, die-casting molds and other large rubber products.
(3) Air demolding is the use of compressed air or vacuum suction cup to take out products, its biggest feature is: simplify the mold structure, shorten the opening and closing time, reduce labor intensity, and improve production efficiency.
(4) Deformation demolding is mainly the use of elasticity and good elongation of rubber, using forced compression or elongation deformation, so as to obtain the demolding effect.
(5) Mold core demolding is to knock out or press out the center mold core when demolding, and then open the mold to demold, which is conducive to ensuring the quality of the product and the demolding effect.
(6) Insert demolding is suitable for more complex or products with inserts.
1.5 The influence of mold maintenance on mold release
A pair of molds with reasonable structure, in order to extend its service life and ensure product quality, in addition to careful use, you should also pay attention to mold maintenance. After the mold is used, the following items must be checked:
(1) Whether the mold is deformed, especially the mold cavity or mold frame. After the mold is deformed, it is not conducive to the demolding of rubber products with steel skeleton or high hardness.
(2) Whether there is loosening and hair pulling in the mating part. There will be gaps and marks when the mold is loose and pulled, and after the rubber material is squeezed, it is difficult to release the mold, and even tear the product.
(3) Whether the positioning is reliable. The mold cavity is generally made of multiple templates put together, and incorrect positioning will cause gaps or deformation of the mold.
(4) Whether the surface of the mold cavity is smooth and how is the fouling situation. After the mold cavity is rusted or fouled, a large friction will be generated during the demolding process, which is not conducive to the demolding of the product.
(5) Whether the movable matching core and ejector accessories in the mold are complete.
If there are the above problems, the mold should be repaired, cleaned, rustproof, etc.
2 Precautions
The above analyzes the influence of various factors on the demolding of products, and puts forward corresponding countermeasures for each influencing factor. The fundamental measures to avoid poor mold release can be boiled down to:
(1) The product structure is easy to demold, and there should be sufficient demolding slope.
(2) The mold structure is reasonable, and the mold stiffness is improved as much as possible.
(3) Reasonably determine the production process parameters.
(4) Improve the operation level of workers.
(5) Pay attention to mold maintenance.
(6) Reduce friction and use appropriate mold release agent.
(7) Choose a reasonable and appropriate viscosity formula.
