Fusible core hollow molding technology of the hott

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At present, hollow blow molding technology can be used to produce all kinds of hollow containers with large inside and small mouth, such as bottles, barrels, cans, etc., but this method is only applicable to thermoplastic. For thermosetting plastics, because they melt at a certain temperature and soon become solid, it is difficult to form them by pneumatic inflation

Figure 1. Hollow containers made of thermosetting plastics

in fact, if thermosetting plastics can be used to make hollow products like thermoplastics, the products have higher strength and hardness, better air tightness and surface gloss, so as to achieve a better use effect. At present, a hollow core molding method for thermosetting plastics has been developed

processing principle and processing technology

1. Processing principle

fusible core hollow molding is carried out by using the different state changes of thermosetting plastics and core materials at the same temperature. Within a certain temperature range, thermosetting plastics first appear in the usual molten flow state, and then undergo chemical reactions to become insoluble and immiscible solids. The metal material as the core melts into a flowable melt at this temperature. According to this principle, a fusible core with the same shape as the interior of the hollow container can be made of low melting point alloy materials, and it is put into the female mold. After the mold is closed, it is injected with thermosetting plastic, and then it is heated and cured. During the curing process of the plastic part, the core is melted and discharged out of the mold, and the product can be taken out after opening the mold

2. Press molding process

the melt core hollow molding method can adopt injection molding, extrusion molding, press molding or reaction injection molding according to the different plasticization methods of raw materials. Here is the pressing process, as shown in Figure 2

Figure 2 Schematic diagram of hollow forming process of pressing fusible core

put the prepared fusible core into the female mold, and preheat the mold after the core is fixed. When the mold temperature meets the requirements, feed and pressurize, raise the temperature and fill the mold. After the molten material fills the cavity, the temperature is raised to the curing temperature of the plastic, so that the plastic part can be uniformly and continuously loaded and hardened during the experimental process, and the fusible core can be melted at the same time. The melted core is poured into the mold through a specially designed channel to make a new fusible core. After the plastic parts in the mold are cooled, the mold can be opened and the plastic parts can be taken out

key points of production control

1. Temperature control

when using fusible core hollow molding technology to form thermosetting plastics, the most critical thing is to control the temperature. In the previous stage of pressurization and temperature rise, the temperature range of material melting flow must be strictly controlled and maintained for a certain time to ensure that the material fills the mold cavity; After the cavity is full, the temperature should be quickly raised to the curing temperature of the material to harden the plastic parts and melt the core rapidly. At this time, too low temperature will prolong the forming cycle, and the hardening degree of the workpiece cannot meet the requirements, and the core cannot be completely melted, making it difficult to discharge or even blocking the flow channel; If the temperature is too high, it is easy to cause the plastic parts to burn, turn yellow, blister and other undesirable phenomena, and even waste products

2. Time control

in addition, we should also grasp the time length of each stage, the most important of which is the curing and core melting time. Ideally, when the plastic part completely reaches the curing hardness, the core just melts completely. In fact, the core melting time is generally longer than the curing time, especially when the core is large. Therefore, the core material with melting point temperature slightly lower than the curing temperature of plastic can be selected. In this way, when the plastic part is basically cured and self-sustaining, the core begins to melt. When the core is completely melted, the plastic part fully meets the curing requirements. In addition, for larger fusible cores, heaters can be buried in them. When the plastic part begins to cure, the core gradually melts from the center to the outer layer to ensure the time balance between the molten core and curing

core design and production

1. Core material

core material must be low melting point metal materials, such as low melting point alloys prepared with Sn, Pb, Bi, etc. The basic principle of selecting core materials is that the melting point of core materials should be the same as or slightly lower than the curing temperature of plastic parts. Generally, the choice of core material will vary according to the plastic part material and core size

2. Core design

the shape of the core should be the same as the internal shape of the plastic part, and its surface must be smooth and its size must be accurate. In the opening part of the core corresponding to the product, appropriate assembly accessories, such as screw holes, pin holes, splines, square heads, which can be connected with the female mold or whose diameter of the mandrel should be 2.5 times the diameter of the reinforcement, should also be designed. In addition, non molten metal inserts can also be designed in this part as connectors. An appropriate amount of space should be left at the runner opening where the core melts to prevent the product from expanding or deforming due to volume expansion after the core melts. If the core is large, it is best to embed electric heating devices in the core to speed up the melting rate of the core

3. Core making

generally, the core is cast by metal against the mold, which is carried out at the same time as the plastic molding process. That is, during the curing process of plastic parts, the molten core is directly poured into the installed core casting mold through the specially designed melt channel. The number of core casting dies depends on the ratio of core forming cycle to plastic forming cycle. It is usually equal to the integer value of the ratio of core forming cycle to plastic forming cycle plus one. In order to ensure the core quality and smooth pouring, the mold should be equipped with a temperature control system and vibration adding device

in a word, the melt core hollow molding method of thermosetting plastics has simple process and easy operation control. It can be produced on the original thermoplastic production equipment

originally published in mm modern plastics magazine (end)

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