Research on Application of injection mold based on

2022-07-28
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Research on the application of injection mold based on LOM technology

Abstract: This paper introduces the process methods and technical characteristics of various injection molds based on LOM technology, such as silicone mold, epoxy resin mold, metal spray mold, etc., and studies the design steps, manufacturing technology, reactive low-pressure injection process and some technical keys of paper-based reactive low-pressure injection mold, The results show that the injection molded parts made by this rapid tooling technology are of good quality and suitable for the requirements of small and medium batch production in the research and development of new products

key words: rapid prototyping, rapid tooling, mold design, injection molding mold, paper-based mold

introduction

rapid prototyping (RP) is a high-tech manufacturing technology developed in the late 1980s. According to its material and process characteristics, it is mainly divided into LOM (Laminated Object Manufacturing), SLA (stereolithography) process SLS (selective laser sintering) process and FDM (fused deposition modeling) process have been widely used in automobile, motorcycle, home appliance, aviation, medical and other industries

however, there are still some limitations in the application of various rapid prototyping technologies at present, which is mainly reflected in the fact that there are still great differences in material and performance between the directly made rapid prototyping parts and the actual parts, and they are often not used to measure various data related to strength and performance. In the production of product covers, although the paper prototypes made by LOM technology have high hardness, they are prone to delamination and cracking. The photosensitive resin prototypes made by SLA process are brittle. The prototypes made by SLS process directly sintered plastic powder have good performance, but the current cost is high. The FDM technology is not satisfactory in the production cost and support of prototypes. In addition, the requirements for a certain number of trial parts are often difficult to meet. The application of rapid prototyping technology in mold has led to the emergence of rapid tooling (RT), which has rapidly developed from an application direction of rapid prototyping technology to a relatively independent emerging technology

1. Rapid tooling process based on LOM technology

rapid tooling technology is generally divided into direct method and indirect method according to whether RP prototype transition is required in the manufacturing process. For example, rapid tooling processes such as silica gel mold, epoxy resin mold and metal spray mold reproduced from RP prototype belong to indirect rapid tooling technology, while direct rapid tooling technologies mainly include SLS process 3DP (three dimensional printing, also known as three-dimensional printing forming technology) process, lens (laser engineering net shaping, also known as laser engineering net shaping technology) process and other direct rapid metal mold technologies, and various wood molds, lost foam molds and plastic injection paper-based molds based on LOM technology also belong to direct rapid mold technology

the rapid prototyping parts made by LOM technology have similar strength to hardwood, can withstand high temperature of about 200 ℃, and have good mechanical strength and stability. After appropriate surface treatment, such as spraying varnish, polymer materials or metals, they can be used as the master mold of various indirect rapid molding processes, or directly make paper-based molds for injection molding, Various rapid molding processes based on LOM technology have been widely used in the production of plastic panels

2. Indirect rapid molding process based on LOM technology

in the application of rapid molds for injection molding, its basic principle is to first use the paper rapid prototype made by LOM technology to make silica gel mold, epoxy resin mold, metal spray mold and other rapid molds through different mold reproduction technologies, and then carry out thermal injection or reactive injection according to the mold conditions to obtain plastic parts for production or trial production. Due to the mature reproduction technology, simple process and various methods, it can integrate the needs of downstream users to select the appropriate process according to the specific requirements of product quality and quantity. It is one of the commonly used rapid tooling technologies at present

2.1 silica gel mold

silica gel mold is the most common rapid tooling, also known as soft tooling. It is one of the rapid tooling manufacturing methods with the widest application range and the most mature technology. It is generally used for trial production molds or transition molds for making other molds. Silicone rubber materials used in silicone mold are mainly divided into room temperature vulcanized silicone rubber (RTV silicone rubber for short) and high temperature vulcanized silicone rubber (HCV silicone rubber for short). The former is generally cured at room temperature and can withstand a temperature of about 300 ℃; The vulcanization temperature of the latter is generally about 170 ℃, which has better performance, and the working temperature can reach more than 500 ℃

the manufacturing process of silica gel mold is generally as follows: take the paper-based prototype as the master mold, preset the parting surface on the master mold, make the parting backing, and spray the release agent; Prepare silica gel in proportion for pouring; Pour the other half after complete curing; Cutting the silicone mold along the predetermined parting surface; Take out the prototype and complete the silicone mold making. In the absence of vacuum equipment, attention should be paid to selecting appropriate silicone rubber materials and taking reasonable exhaust measures

otherwise, the parameters of silicon detection are not satisfactory. The rubber mold has the characteristics of short manufacturing cycle, low cost, simple process, good material elasticity, easy demoulding and good copying performance. The reactive low-pressure injection molding process is easy to operate and does not need vacuum equipment, but the exhaust conditions are poor, which is easy to produce entrained bubbles and filling defects. The silica gel mold also has some shortcomings, such as low precision, short service life, and can not be used in the hot injection molding process. When used as an injection mold, the service life is generally 10 ~ 50 pieces. In addition, the characteristics of easy demoulding sometimes cover up the design defects of no demoulding angle or reverse demoulding, and affect the inspection effect of product design quality

2.2 aluminum filled epoxy resin mold

the process of aluminum filled epoxy resin mold is similar to that of silica gel mold, that is, firstly, the paper-based prototype is used as the master mold, the parting surface is preset on the master mold, the release agent is sprayed, and the two-component thermosetting epoxy resin filled with finely ground aluminum powder is poured. Note that the mixing and curing processes should be carried out in vacuum, and the other half of the pouring is completed according to the above process after curing, After finishing, the aluminum filled epoxy resin mold is made. It is mainly used as a hot injection mold, with a service life of 500 ~ 2000 pieces. However, epoxy resin molds generally have the disadvantage of poor thermal conductivity, resulting in longer production cycle and lower production efficiency compared with steel molds

2.3 metal spraying mold

metal spraying mold mainly includes metal cold spraying mold and plasma spraying mold. The manufacturing process of the metal cold spraying die is as follows: firstly, spray the atomized molten metal on the surface of the RP prototype with a spray gun, form a metal shell with a thickness of about 2mm after the liquid metal solidifies, and then make the die after adding backing, embedding the cooling pipe, removing the prototype, etc. The sprayed surface has good reproducibility and high dimensional accuracy. However, compared with the traditional metal mold, it can not use a large clamping force. In addition, the poor thermal conductivity also leads to the extension of the injection molding cycle, and the service life is generally thousands of pieces

the basic principle of the manufacturing process of plasma spraying die is that the powder melted by high-temperature plasma arc generated by ionized gas is sprayed to the surface of the workpiece to be sprayed with high-speed flame flow and forms a shell. After treatment, the die is made. It can be sprayed with high melting point metal and non-metallic materials. It has the characteristics of high surface hardness, small deformation, stable process and long service life

3. Direct rapid molding process based on LOM technology

the application of LOM technology in direct rapid injection mold is mainly reactive injection mold. According to different injection conditions, it can be divided into vacuum injection molding and low-pressure injection molding. The following is the paper-based reactive low-pressure injection mold process based on LOM technology (hereinafter referred to as paper-based injection mold)

3.1 design of paper-based injection mold

the design principle and steps of paper-based injection mold are similar to those of steel mold design, but considering the particularity of paper-based parts and the difference of injection molding methods, the design of paper-based injection mold has different characteristics

1) size compensation

paper-based injection mold belongs to direct molding, and there is no cumulative error caused by indirect molding processes such as silica gel mold in the reproduction process. Therefore, the factors affecting the dimensional accuracy of final injection molded parts mainly include the loss of conversion accuracy when converting 3D CAD models to STL files, paper-based mold manufacturing error, paper-based mold assembly error, polyurethane injection material shrinkage, etc, Here, the dimension to be compensated is the shrinkage of polyurethane material, that is, the dimension compensation of 3D CAD model in 3D software, which must be considered for larger parts

2) determination of demoulding direction and parting surface design

generally speaking, the demoulding direction has been considered when designing the 3D CAD model. What needs to be done here is to comprehensively consider all links of mold design and manufacturing, and further confirm the rationality of the original design demoulding direction. If there are problems, the 3D CAD model can be modified to redesign the demoulding direction or model structure

the basic principle of parting surface design is similar to that of steel mold, but at the same time, the particularity of paper base parts and the specific characteristics of reactive injection molding process should be considered. The main reason is that the design of parting surface should comply with the principle of maximum projection surface, ensure the dimensional accuracy and surface quality of injection molded parts, simplify the mold structure as much as possible, facilitate the manufacturing and installation of molds, and meet the process conditions of reactive low-pressure injection molding, Consider the location of the weak strength area and the forming direction of the paper base

the determination of demoulding direction and parting surface design are closely related to mold structure, mold manufacturing, injection molding and other links, and are one of the important links for the success of mold design

3) demoulding slope inspection and design

as the surface quality of paper-based mold is worse than that of steel mold, and there is no elasticity of silicone rubber mold, it is in the same position with alloying and heat treatment (see metal strengthening). In addition to the general demoulding slope inspection, in order to facilitate demoulding, the demoulding slope should be as large as possible on the premise of meeting the dimensional tolerance requirements of the workpiece or not affecting the structure and function of the workpiece, Or focus on the parts that are not easy to demould in the mold structure

4) design of core pulling, sliding block and ejector rod

if the part structure needs, the core pulling, sliding block, ejector rod and other structures should also be designed like the steel mold. Whether the design is reasonable is very important for the smooth demoulding of injection molded parts, and it is also one of the technical keys in the design of paper-based injection mold

5) design of runner, gate and exhaust port

for the runner and gate of paper-based mold, it is not necessary to design a complete gating system such as main runner, shunt runner, gate and cold material cavity like the steel mold, but a simple gate design with the mold surface directly to the injection molded parts. When selecting the gate position, it is mainly considered that it should be at the lowest or relatively low point to facilitate the filling of the mold cavity and natural exhaust of polyurethane material, Try to set it at a position that does not affect the appearance of the injection molded parts, and try to set it at a position where there is little cold material left and it is easy to take out. Generally, the exhaust port shall be designed at the relative high point and exhaust dead corner to ensure that the polyurethane material fills the mold cavity

6) mold locking and positioning design

mold locking and positioning design mainly considers that the reactive low-pressure injection molding process is mainly manual operation, and the hole position should be designed at the appropriate position, so that the mold locking and positioning can be fastened with bolts during injection molding

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