Hollow molding process for thermosetting plastics

The traditional hollow blow molding technology can only be applied to thermoplastics. Now, using the hollow core molding technology, thermosetting plastics can be made into hollow products with excellent performance.

At present, the use of hollow blow molding technology can produce a variety of hollow containers, such as bottles, barrels, cans, etc., but this method is only applicable to thermoplastics. Thermosetting plastics are difficult to be molded by the air pressure inflation method because they quickly become solid after melting at a certain temperature.

In fact, if a thermosetting plastic can be used to form a hollow product such as a thermoplastic, the product has higher strength and hardness, better airtightness and surface gloss, so as to achieve a more ideal use effect. At present, a melt core hollow molding method that can be used for thermosetting plastics has been developed.

Processing principle and processing technology

1, processing principle

Fused-core hollow molding is performed by using a thermosetting plastic and a core material having different state changes at the same temperature. Within a certain temperature range, the thermosetting plastic first melts and then reacts chemically to become insoluble, infusible solids. The metal material as the core melts into a flowable melt at this temperature. According to this principle, a meltable core of the same shape as the hollow container can be made of a low-melting-point alloy material and placed in a female mold. After the mold is closed, the thermosetting plastic is injected, and then the temperature is increased. 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 the mold is opened.

2, press molding process

Fused-core hollow forming method according to the plastic material of raw materials can be different injection molding, extrusion molding, compression molding or reaction injection molding. The press molding process is introduced here. The process is shown in Figure 2.

Fig. 2 Schematic diagram of blow molding core blow molding process

The pre-prepared meltable core is inserted into the female mold and the core is preheated to preheat the mold. When the mold temperature reaches the required level, feed and pressurize it to heat up and fill the mold. After the frit fills the cavity, the temperature is raised to the temperature at which the plastic solidifies to harden the plastic part while melting the meltable core. The melted core is poured into the mold through a dedicated channel, and a new meltable core is re-formed. After the plastic parts in the mold are cooled, the plastic parts can be opened and removed.

Production control points

1, temperature control

When molding thermosetting plastics using melt-core hollow molding technology, the most critical is to control the temperature. In the previous stage of pressure warming, the temperature range of the material melt flow must be strictly controlled and maintained for a certain period of time to ensure that the material fills the cavity; after the cavity is filled, the temperature should be quickly raised to the solidification temperature of the material in order to make the plastic parts Hardens and melts the core quickly. At this point, the temperature is too low will prolong the molding cycle, and the degree of hardening of the parts can not meet the requirements, the core can not be completely melted, making it difficult to discharge or even block the flow channel; the temperature is too easy to make the plastic parts appear burning yellow Bad phenomena such as blistering and even waste products.

2, time control

In addition, we should also grasp the length of time in each phase, the most important of which is the solidification and melting time. The ideal situation is that when the plastic part completely reaches the curing hardness, the core is just completely melted. In fact, the melting time is generally longer than the curing time, especially when the core is large. For this purpose, a core material with a melting temperature slightly lower than the curing temperature of the plastic can be used. In this way, when the plastic part is substantially cured and self-sustaining, the core begins to melt, and when the core is completely melted, the plastic part fully meets the curing requirements. In addition, for larger meltable cores, heaters may be embedded therein. When the plastic part begins to solidify, the core gradually melts from the center to the outer layer to ensure the balance between the melting core and the curing time.

Core Design and Fabrication

1, core material

The core material must be a low melting point metal material, such as a low melting point alloy made of Sn, Pb, Bi, etc. The basic principle for selecting core materials is that the melting point of the core material should be the same as or slightly lower than the curing temperature of the plastic parts. In general, the choice of core material will vary depending on the plastic material and core size.

2, core design

The shape of the core should be the same as the internal shape of the plastic part. Its surface must be clean and the dimensions must be accurate. Corresponding to the opening of the product in the core, it is also necessary to design an appropriate fitting that can be connected or positioned with the female mold, such as screw holes, pin holes, splines, square heads, etc. In addition, non-melt metal inserts can also be designed at this location as a connector. A suitable amount of space shall be left at the flow passage opening where the core melts and flows out to prevent the core from expanding due to volume expansion after the core melts or deforming the product. If the core is large, it is preferable to bury the electric heating device in the core to accelerate the melting rate of the core.

3, core production

Under normal circumstances, the core is cast from a metal mold and is formed at the same time as the molding process. That is, during the curing of the plastic parts, the molten core is directly poured into the filled core casting mold through a dedicated melt flow path. The number of core casting molds depends on the ratio of core molding cycle and plastic molding cycle. It is usually equal to the integer value of the ratio of the core molding cycle to the plastic molding cycle ratio. In order to ensure the quality of the core and the smooth pouring, a temperature control system and a vibration device should be provided on the mold.

In short, the thermosetting plastic melt-core hollow molding method has a simple process and easy operation and control, and can be produced on the original thermoplastic plastic production equipment.