The hottest plastic molding technology III

Plastic molding technology (III)

Huang Hanxiong, South China University of technology, Guangzhou molding equipment. Molding equipment includes molding machine molds and auxiliary equipment. 4.1 molding machine molding machine is the main equipment for molding. The function of the molding machine is to exert pressure on the molding material through the mold. If the fixed mold is used, it also plays the role of closing the mold and ejecting the products. Molding machines can be divided into hydraulic and mobile types. At present, the arrangement form of opening and closing molds can also be used. The hydraulic molding machine is composed of frame transmission mechanism and control system. 3 7 oil storage tank 8, feed pump 9 motor; 10 mold closing pump; Mold closing valve 12 oil return pipe 13 oil delivery pipe; 145 charging valve; 15 pressing plug; 16. The frame of pressing oil cylinder is mainly composed of formwork and column. The lower formwork is fixed, and the upper formwork moves downward under the action of hydraulic pressure, pressing the molding material from top to bottom. This is called the upper press molding machine, and on the contrary, it is called the lower press molding machine. The molding machine with large tonnage, large formwork, large stroke and rapid mold closing is usually of the up pressing type, and smcbmc of various sizes and reinforced thermoplastic molding are also often of the up pressing type. Although there are many ways to move the formwork of the die press, the hydraulic chestnut with lower pressure and booster cylinder can also be used for typical 14, 20MPa. The hydraulic pressure in the hydraulic cylinder acts on the piston and is applied to the molding material through the movable template and mold. In addition to the hydraulic cylinder, hydraulic pump and motor, the transmission mechanism of the molding machine also includes various valves and pipelines of the oil storage tank. Hydraulic power devices usually include various check valves, sequence valves, throttle valves, etc., to produce the required speed and pressure on time. If the factory has many molding machines, it is more advantageous to use hydraulic accumulators. For molding machines with clamping force less than 250 Ka, air cylinders and air pressure can be used to replace the hydraulic system. In order to reduce production costs, sometimes the combination of hydraulic pressure and air pressure is used. According to the control mode of molding machine, it can be divided into manual type, semi-automatic type and full-automatic type. All movements of the manual molding machine are completed by the operator except the ejected products. The mold can be fixed on the mold of the molding machine, or the mobile manual mold can be used. In the semi-automatic control molding machine, the operator takes out the products, cleans the mold and adds materials, then presses the electrical contact button to start a new molding cycle. The electrical control system closes the mold press through the through valve. With the help of time relay limit switch and hydraulic valve and other components, it can completely automatically complete the actions of high-speed mold closing, slow mold closing, full mold closing, mold opening, exhaust, staying, mold closing, curing and mold opening in turn. Every molding cycle needs to be restarted by the operator. The semi-automatic control can reduce the error of the operator to a minimum, and the safety interlock device can be included in the system to prevent failure, so that the operator can adopt manual control in case of emergency. All actions of the automatic molding machine are completed automatically. Automatic molding process usually adopts high-frequency preheating. The performance parameters of hydraulic molding machine mainly include pressure parameters, speed parameters and size parameters. Among the pressure parameters, the nominal pressure is the main parameter of the molding capacity of the molding machine. The commonly used ones are 45 and 50. After the analysis of the quality of molten aluminum in each work area and the evaluation of the operation of the electrolytic cell, 1, etc; Speed parameters include piston stroke speed and ejection speed; Size parameters mainly include piston * large stroke workbench size, main piston diameter, etc. 4.2 molding mold place the molding material in the mold cavity or feeding room of the heated molding mold. The mold is closed on the molding machine and pressurized. The molding material in the mold cavity changes into a flowing state under the action of heat and pressure, fills the entire mold cavity, and then forms into products. Molding has the following basic requirements for its mold. Under high temperature conditions, the hardness should not be significantly reduced; It can resist the friction of plastic during molding ④ it can resist the chemical corrosion of plastic and release agent ⑤ the structure should be conducive to the flow of plastic and the removal of products, and can meet the requirements of molding process. The mold cavity surface is highly polished or chrome plated to ensure the smooth surface of molded products. Therefore, when the molding process conditions are strict, the mold is usually made of hardened steel; When the molding process conditions are not too strict, the mold can be made of low carbon steel or brass; The product batch is very small, and the mold can also be made of plastic. Mold plays an important role in molding. First of all, the shape and size of the mold cavity, the roughness of the parting surface and the performance of the demoulding mechanism have an important impact on the performance of the surface. Secondly, in the process of molding, the mold structure has a great impact on the difficulty of operation and production efficiency. Reducing the manual operation in mold opening, mold closing and demoulding can significantly improve the production efficiency. In addition, molds also have a great impact on the cost of products. Generally, the design and manufacturing costs of molding molds are high. When the batch of products is small, the proportion of mold costs in products will be large, so molds with simple and reasonable structure should be used as far as possible to reduce the cost of products. 4.2.1 the structure of the mold is mainly composed of the upper mold and the lower mold. When the upper and lower dies are closed, pressure can be applied to the plastic in the feeding chamber and the die cavity; After the product is cured, the upper and lower molds are opened and the product is ejected with the help of the ejection device. 1 male mold 2 upper mold 3 upper fixing plate 4 female mold; 5. Lower fixing plate; Heating plate; 7 base plate; 8 ejector rod; 9 support rod 10 lower formwork ejector plate; 12 safety pin; 13 jacking rod fixing plate; 14 products; 15 guide post sleeve; 16. The mold for guide pillar molding can be divided into mold cavity, feeding room, guide mechanism, lateral parting, core pulling mechanism, demoulding mechanism and heating and or cooling system. The mold cavity is the part of the directly molded product, and the mold cavity in 4 is composed of male mold 1 and female mold 4. The shape and size of the mold cavity are mainly determined by the products. Because the compression ratio of thermosetting molding material is generally large, the mold cavity before part clamping usually cannot accommodate all the plastic amount of a molding cycle, so a section feeding room is generally set on the mold cavity. The guide mechanism is composed of guide post 15 and guide post sleeve 14. When molding products with side holes and side recesses, the mold must be equipped with various lateral parting and core pulling mechanisms to ensure product demoulding. The demoulding mechanism is composed of ejector rod 8, ejector rod fixing plate 13, ejector rod plate, etc. 4.2.2 classification of molds the shapes and sizes of plastic products are ever-changing. It is very important to design or select appropriate molds according to the molding process performance of plastics and the requirements for product performance. According to the structural characteristics, the molding dies can be divided into overflow type, non overflow type and semi overflow type. Overflow mold overflow mold 53 During molding, the amount of material added each time is not required to be very accurate, but it must be excessive. In order to avoid excessive waste, the excess material should not exceed 5% of the product quality, generally about 2 Because there is no mating surface between the male mold and the female mold, the excess material will overflow from the overflow surface when closing the mold. Because it is very thin, it can be solidified quickly, which helps to avoid the progressive overflow of materials. The excess material attached to the product after demoulding becomes a burr, which must be removed. Phantom overflow mold as non overflow mold, semi overflow mold 1 overflow surface 2 male mold; 3 guide post; 4 female mold; 5 ejector rod 6 product overflow mold has no feeding room and the loading volume is limited, so it is suitable for plastics with small compression ratio. * it is better to use pellets or pre pressed billets for molding, and it is rarely used for plastics with large compression ratio such as strips and sheets. Overflow mold is suitable for molding flat or dish shaped products, especially products without strict requirements on strength and size, such as button decorations, gaskets and other small parts. Overflow mold has the advantages of simple structure, low cost and long service life; The products are easy to demould. When forming flat products, there is no ejector, which can be taken out by hand or blown out with compressed air. The disadvantage of this kind of mold is that the compactness of the product is poor; It is difficult to ensure the uniformity of thickness and strength of mass-produced products due to the possible difference between overflow and each feeding amount; Excessive feeding leads to a certain waste of raw materials. The continuous part of the non overflow mold part has no overflow surface, and the pressure exerted by the molding machine completely acts on the plastic to prevent the plastic from overflowing from the mold cavity. Because there is almost no overflow loss during the molding of this kind of mold, the accurate weighing method must be used for feeding, otherwise the thickness of the product cannot meet the requirements. If the feeding is insufficient, the strength of the product will be reduced, and even become waste. The ejection device must be set for the non overflow mold, otherwise the product is difficult to demould. The non overflow mold is especially suitable for plastics with poor fluidity, high molding pressure or high compressibility, such as cotton cloth, glass cloth or long fiber impregnated molding materials, and is suitable for molding products with complex shapes, thin walls and long processes. The advantage of non overflow mold is that the pressure on the molding material is large, so the product has good compactness and high mechanical properties; Because the mold has no supporting surface, the product with very small wall thickness can be formed, and the burr is perpendicular to the nail surface of the product, which is easy to remove, and the product has no obvious overflow trace. The disadvantage of this kind of mold is that it must be accurately weighed when feeding. Generally, it should not be designed as multiple cavities, because the slightly uneven feeding will cause the pressure of each cavity to be different, resulting in the plastic under pressure of some cavities, which cannot guarantee the product performance; There is wear on the sliding mating surface between the male mold and the female mold. Semi overflow mold is a kind of mold with the structural characteristics of the above two types of mold, which can be divided into two types: supported surface and unsupported surface. The semi overflow mold with supporting surface is not only equipped with a feeding room, but also suitable for plastics with high compressibility because of the feeding room. In this kind of mold, the overflow of plastic is limited, because when the male mold enters the female mold, the overflow can only overflow from the slots opened on the male mold, and the number depends on the need. If no groove is set on the male mold, an outward inclined plane can also be set at the entrance of the female mold, 5. When the height of the product has precision requirements, this kind of mold is preferred, but this kind of mold is not suitable for cloth based and fiber-based plastics, because this kind of plastic is easy to accumulate on the supporting surface, so that the pressure on the plastic in the mold cavity is not high enough. Secondly, the thicker burr formed is difficult to remove, and roll grinding method is used to remove it as much as possible. The small half overflow die 5 without supporting surface is similar to the non overflow die, but the only difference is that the length of the female die is generally 1.5, and the inclined angle is about 23 above 2.5, so a overflow groove is formed between the female die and the male die. During the molding process, when the male mold enters the female mold and does not reach the section, the plastic can overflow from the overflow trough, but it is limited by a certain amount; After the male mold reaches the sixth section, the situation is exactly the same as that of the non overflow mold. Therefore, this kind of mold is suitable for plastics with large compressibility and poor fluidity. Semi overflow mold is widely used in all kinds of thermosetting molding materials, and multiple cavities can be set; Each feeding amount is slightly excessive and does not need to be very accurate. The volumetric method can be used for feeding, which is convenient; The overflow volume is generally only 25% of the product quality, and the overflow thickness can be controlled below 0.15; It can ensure that the products are dense, with high dimensional accuracy and uniform performance. According to the use of molding dies, they can be divided into mobile fixed and semi fixed = types. Mobile mold or manual mold is a kind of mold that is loaded and unloaded outside the machine. It does not have a heating system and is not fixed on the molding machine, but is placed between the upper and lower heating plates of the molding machine. After the product is cured, the whole mold is removed from the molding machine and demoulded on the demoulding frame. In addition to pressurization and heating, mobile molds are manually operated, with high labor intensity and low production efficiency; In the above manual operation