The current synthetic materials industry has become the mainstay of the chemical industry, and the plastics industry is the core of this subject. At the same time, the development of the petroleum industry has promoted the production of plastic products. However, with the expansion of plastic production and application, the environmental pollution of waste plastics has become increasingly serious. After nearly 30 years of exploration, China's waste plastics recycling technology has made certain progress, and recycling enterprises have begun to take shape. However, the limitations of the technology are very large, and the recovery method is single. At present, it has only a certain foundation in the method of melt recycling, and the progress in the utilization of thermal decomposition of oil and gas, recovery of chemical raw materials, incineration and recovery of heat energy is slow, and it can be said that it is slow. Just getting started. Moreover, most of the processing enterprises that recycle and recycle waste plastics are small enterprises and township enterprises, and even many are individual enterprises. Due to limited conditions, there are widespread phenomena such as backward technology, poor production equipment, low quality of workers, and low product quality, which directly affect the economic benefits of enterprises. At the same time, in the utilization of high-tech equipment, most of the imported equipments have high energy consumption issues, which directly increase the operating cost and affect the recycling efficiency. Due to the large equipment throughput, some equipments are â€œnot fullâ€ due to the recycling process. "At half-boot or shutdown, the wearing parts can not keep up, making the performance of the whole machine difficult to fully play; the imported equipment is not suitable for the national conditions, because the domestic waste plastic varieties, ingredients, etc. are different from foreign countries, the application effect of the equipment Affected. In this paper, the development and utilization of plastic molding process are discussed in detail.
1 Introduction of Molding Process The plastic material molding process plays an important role in various forming processes. It is mainly used for the molding of shaped products, so the molding pressure used is higher than other processes. Due to the simple equipment required for the press molding process, and the molding of various materials such as fiber, rag, felt, laminate, wound product, and woven fabric, it is common among composite materials manufacturers of all sizes. use. This test is based on the hot press forming of waste garbage bags and waste mulch film, and provides a theoretical reference for the future treatment of various waste plastics by hot press forming.
2 Factors Affecting the Quality of Molded Products The basic process of the plastic molding process is to put a certain amount of the mold material that has been pretreated in a preheated mold, and apply a higher pressure to make the mold material fill the cavity. Under the predetermined temperature conditions, the molding material is gradually solidified in the cavity, and then the product is taken out from the mold, and then necessary auxiliary processing is performed to obtain the final product. The specific influencing factors of the quality of the products are as follows.
2.1 In addition to the strength and stiffness conditions under the working pressure, the mold used for pressing the mold products should mainly consider the ability to apply pressure to the parts and directions of the product more evenly. A certain draft angle can ensure the smooth ejection of the product and the lateral pressurization. The mold design makes the product as a whole as possible, which not only ensures the strength and rigidity of the product, but also reduces the number of auxiliary processing steps and tooling molds. An overflow tank should be opened on the mold to allow the excess material to be discharged smoothly. The molding surface of the stamper shall be at least polished or chrome-plated, with a surface roughness of up to 0.25 Î¼m to ensure smooth demolding. A temperature measuring hole should be opened near the cavity of the mold. The mold itself should be designed with certain accessories as necessary to ensure that it is easy to demould. The pressing mold used in this test is a tensile and bending resistant mold.
2.2 The size and tonnage of the press machine should meet the requirements of the product and the selected materials and mold structure. The temperature and pressure of the press should preferably be in two modes: manual control and automatic control; the work table of the press or the upper and lower heating plates should be kept as horizontal as possible to avoid the resin from flowing to one side, and the heating plate is in the effective working range. Inside, the temperature difference is controlled at about Â±5 Â°C.
3 Pressing Process During the molding process, the resin in the molding material will undergo three stages of viscous flow, gelation and solidification, and the resin molecules themselves will also change from linear molecular chains to insoluble and infusible spatial network structures. The external conditions required to convert a molded material into a qualified product are called molding process parameters of the molding process. In actual production, it is often called the suppression system, including temperature system, pressure system, demoulding and post-processing (as shown in Figure 1).
Pressing process of molded articles of waste plastic materials
3.1 Temperature regime temperature is a measure of the amount of heat contained in the molding compound. The role of thermal energy is to promote the plasticization of the molding material and the curing of the resin. At the initial stage, as the temperature increases, the molding material gradually changes from a soft solid state to a viscous flow state. After the temperature reaches a certain level, the viscosity of the molding material increases, and the resin The curing reaction begins and the final becomes an insoluble and infusible solid. From the viewpoint of molecular motion, as the temperature increases, the energy obtained by the resin molecules increases, the thermal motion of the resin molecules increases, and the initiator starts to function, triggering the curing reaction of the resin molecules. The amount of heat required at each stage of the molding process is different, which requires control of the corresponding temperature specifications, including charging temperature, heating rate, molding temperature, constant temperature, cooling temperature, and post-treatment temperature.
(1) Loading temperature. The temperature at which the molding material is loaded into the cavity is called the charging temperature, which is determined by the type of molding material and the quality requirements of the product. The volatilization temperature of the solvent in the molding material, the structure of the product, and the production efficiency should also be considered.
(2) Heating rate. The heating rate refers to the rate of temperature rise when the charging temperature reaches the maximum temperature. For the rapid pressing process, the charging temperature is the pressing temperature, and there is no heating rate. For the ordinary molding process, it is necessary to carefully select and control the heating rate, which is especially important when pressing thick-walled products.
(3) Forming temperature and constant temperature. The maximum temperature refers to the peak temperature of the curing reaction of the resin measured by differential thermal analysis, which is actually the molding temperature of the molding material. In the molding process, the molding temperature is important depending on the type of molding material. After the molding temperature is reached, in order to completely cure the resin and eliminate the internal stress of the molded article, it is generally required to be kept at the molding temperature for a certain period of time, which is called a constant temperature process. The duration of the constant temperature process depends mainly on two factors: one is the time required for the molding material to cure; the other is the unstable heat conduction time, that is, the heating element conducts heat to the cavity through the mold, so that the molding material in the cavity is molded. The time required for the temperature to reach the forming temperature is related to the type of molding material, the heating device, the structure and size of the product, and the ambient temperature.
(4) Cooling down. After the thermostating time of the molding process is over, the cooling process is started while maintaining the pressure. There are two types of cooling methods: natural cooling and forced cooling. Usually, in the pre-flow tunnel around the mold, cold water is blown in or cold air is blown by a fan to force the mold and the molded product to cool down.
3.2 Pressure system The pressure system refers to the requirements of the molding pressure, the timing of pressurization and the pressure relief and deflation in the molding process.
(1) Forming pressure. The function of the forming pressure is to overcome the internal friction force of the molding material in the mold, the friction between the molding material and the inner wall of the cavity, so that the molding material fills the cavity and overcomes the steam generated by the volatile material when the molding material is heated. Pressure to obtain a structurally compact product. The amount of molding pressure depends on the type of molding material, the structure and size of the molded article.
(2) Pressurization timing. The pressurization timing refers to the pressurization operation at a certain temperature after a period of time after charging. A reasonable pressurization timing is one of the key parameters to ensure the quality of the product. Premature pressure, the reaction degree of the resin molecules in the molding material is still low, the molecular weight is small, the viscosity is low, the resin component is easily lost, so that local resin aggregation occurs in the product, while the other part is insufficient in resin; Too late, the degree of resin reaction is too high, the molecular weight is too large, resulting in too high viscosity, and the activity of the molding material is too poor to completely cover the cavity. Both of these situations lead to defective products. Only the right pressurization timing can obtain the molded product with the best performance. We believe that the best timing for pressurization should be on the eve of the curing reaction of the resin to release a large amount of gas.
(3) Pressure relief and deflation. The molding material contains a certain amount of volatile components. Some molding materials, such as phenolic molding materials, are also inherently volatile by-products in the curing reaction. These volatile components and by-products, if not effectively discharged during the pressing process, cause bubbles, cracking, and the like of the finished product. This phenomenon will be especially severe for some rapidly reacting molding materials. Therefore, in actual production, gas venting measures are generally required after pressurization, that is, after pressurization, the gas is depressurized and then pressurized, and then repeated several times to achieve the effect of discharging volatile components and by-products.
3.3 Demolding After the end of a pressing cycle, the formed article should be removed from the mold for the next molding operation. The temperature at the time of demolding should be below 60 Â°C. If the temperature is too high, the resulting product is prone to deformation and shrinkage, and the appearance quality is degraded; if the temperature is too low, the molding cycle is prolonged and the cost is increased.
3.4 After the mold-molded product is demoulded, it is often necessary to carry out a certain post-treatment process in order to meet the requirements for use. The first process is to remove the burr of the product, which can be carried out by manual or mechanical grinding to improve the appearance quality of the product. In addition, it is necessary to perform necessary mechanical processing on the product, such as drilling and tapping. Wait. Since composite products are very sensitive to machining, they should be avoided as much as possible. If opening is required, it can be solved by setting the core in the mold. The tapping can be done by setting a threaded core or a thread with a thread. The thread is directly formed on the molded product, otherwise the composite molded product is improperly processed, and the phenomenon of â€œdroppingâ€ is easily generated, resulting in the scrapped product being scrapped.
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