Our commonly used mold heating, heat preservation and cooling methods:
l. Overview
Compression molding process is one of the most common and oldest molding methods for plastic materials, and it is the most commonly used process method for studying material properties. It has the characteristics of simple molding device, small equipment investment, simple mold structure, etc. It is still one of the most popular production methods in today's highly developed mechanized and automated production. However, the literature that can be referred to for mold design for compression molding, apart from textbooks, monographs written by several predecessors, and mold design illustrations, few people summarize the key skills in mold design.
The heating, heat preservation, cooling and clamping structure of the mold are an indispensable part of the composite mold design. The structural design directly affects the appearance and internal quality uniformity of the product, and also affects the molding efficiency of the product.
2. Heating, heat preservation and cooling design
2.1 Design requirements for heating pipes
Steel heating is a heating method that must be used in the design of almost all plastic molding molds. It can be designed as one-way wiring, two-way wiring and other forms. The material can be seam pipes, seamless pipes, stainless steel pipes, etc., and the characteristic is heat loss Small size, high thermal efficiency, simple wiring, can be designed as 220V or 380V according to needs, and the wiring is flexible and diverse. However, due to the limitations of its materials and processing technology, it is necessary to pay attention to its oriented characteristics in the mold design.
(L) The heating tube usually has a long cold end at both ends, which cannot play a heating role.
(2) The power design of the heating section should not exceed the limit of 10 watts/cm as much as possible. Such as 30 cm long heating tube, the power should not exceed 300 watts as much as possible. If the design power exceeds this limit, the surface load of the heating tube will be high, and the steel tube will be susceptible to oxidation and corrosion, which will cause a short circuit.
(3) For mold design with a temperature higher than 250°C, it is difficult to use a heating tube. I have used a heating tube to heat up to 420°C, but this molding temperature has a high requirement on the quality of the heating tube, and it is necessary to frequently check the circuit for smoothness and short circuit. Because under this condition, the heating tube, the connecting terminal, the copper wire for the connection, the steel sheet and other media are very easy to oxidize, resulting in a circuit break. Therefore, special treatment is required for the electric transmission medium, try to avoid exposing the conductive wires to the air, and prolong the service life of the wires.
Soldering iron core is usually used as a kind of mold heating tube, which is characterized by high power per unit length (usually a soldering iron core with a diameter of 10mm and a length of 8cm can reach an output power of 150 watts), durable, safe, and not easy to form a breakdown short circuit , Can be buried by drilling blind holes, the disadvantage is that it is difficult to customize the design, and it is easy to be broken and broken during disassembly and replacement.
In the circuit design, insurance measures such as insurance and air switch are indispensable. The operating place must be kept clean and tidy, with good insulation, and be diligent in checking electrical faults during operation to prevent unnecessary dangers.
2.2 Drilling holes for installation of heating pipes
From the perspective of heat transfer, the installation of the heating tube should be as close as possible to the surface of the mold to facilitate the transfer of heat from the heating tube to the mold as soon as possible. In fact, the heating tube does not have a large contact area with the mold. The essence of heat transfer is radiation, and conduction is secondary. Therefore, most of the heating tubes used for mold installation are coated with a coating that enhances infrared radiation. At the same time, a method of limiting the design power (10 watts/cm) is also used to increase the service life of the heating tube.
Therefore, when processing heating tube holes, especially for long heating tube holes, it is not necessary to design a too small fit gap. The effective design method is to fit the heating tube as closely as possible at both ends of the hole, and plugging and plugging can be used. Or design a baffle and other methods. This approach can effectively reduce the heat dissipation area of the heating tube and the loss of radiant heat.
2.3 Burying of heating pipes
The buried heating tube should preferably be filled with the same magnesium oxide powder as the medium in the tube to reduce the heat load on the surface of the heating tube. This method can reduce the surface oxidation of the tube and effectively extend the service life of the tube. If possible, the mounting hole of the heating tube should also be filled with magnesium oxide powder.
2.4 Mold insulation method
Strengthening the heat preservation measures of the mold can reduce the heat loss of the mold, make the mold reach the predetermined production temperature in a short time, and reduce energy waste. Every engineer and technician has a unique set of solutions to this problem, I will only talk about my experience.
2.4. l Heat preservation measures of heating plate
Asbestos board or asbestos cloth is usually used for heat preservation of heating plate, but asbestos cloth is not easy to lay flat, and it also has a certain influence on the guarantee of parallelism of the pressure plate. There are many types of asbestos board, the most common is rubber asbestos board, but this kind of asbestos board is not the right material for sealing and heat insulation. It has a certain compressibility and it will release a very difficult The smell affects the operating environment and the health of the operator.
Asbestos cardboard should be used for the heat preservation of the heating plate. The common specification is 1000×1000, 3-5mm thick, the plate body is relatively regular, the parallelism is good, the compressibility is relatively average, and there is no peculiar smell under high temperature.
2.4.2 Thermal insulation measures of the mold
There are many thermal insulation measures for the mold, and aluminum hydroxide thermal insulation cotton can be wrapped with asbestos cloth or glass cloth for thermal insulation. There is also an insulation coating on the market that is currently an ideal material for mold insulation. It is a mixture of medium and long fibers, slurry and a kind of insulation foam material. It has moderate viscosity and is easy to apply. This material is often used as an insulation material for chemical and heating pipelines, and it is slightly alkaline (easy to corrode molds). After being used at 150°C, no negative effects such as scorching, melting, odor, etc. were found. At the same time, the material is very light, and the plasticity is strong, and it is easy to form a more beautiful mold surface.
2.5 Mold cooling method
Water cooling is the cooling method adopted by most molds, but it also has its shortcomings; it requires that the pipelines have good sealing performance, and the upper and lower water pipelines must be unobstructed, which wastes water resources. When the cooling temperature exceeds 100°C, steam explosion is likely to occur. The advantage is that the heat capacity is large, and the temperature can be quickly cooled.
Air cooling is an ideal cooling method. It is the opposite of water cooling. It does not require tight pipe sealing, and there is no waste of resources. It can cool molds with a temperature higher than 100°C. The cooling rate can be determined by the flow of gas. And the source is simple and convenient, and a certain-scale production workshop can obtain a relatively convenient gas source.
3. Clamping of the mold
The clamping structure of the mold is closely related to the heating, heat preservation and cooling system of the mold, and at the same time, it provides certain convenient features for the replacement, loading and unloading of the mold. Most designers simply drill a few mounting holes on the mold for the convenience of the drawing. For example, most of the molds do not design heating devices separately, but install heating plates on the upper and lower pressure plates of the press to simplify the processing of small and medium-sized molds. Only the modules that constitute the main structure of the cavity are left in the mold structure. At this time, the mold can be fixed by an injection mold—fix the mold on the upper and lower templates with a pressure plate. Design the space for fixing the pressing plate on the heating plate mold. This design can be used not only for movable stampers, but also for stampers with simple ejection mechanisms. It is only necessary to consider that the position of the ejector rod does not conflict with the heating tube in the design of the heating plate. It is also possible to use the mold base of one mold to perform universal transformations on multiple molds to simplify mold manufacturing costs.
If the mold is taller, the heating of the heating plate alone cannot meet the needs of uniform heating. At this time, an auxiliary heating system needs to be installed on the mold, which can be composed of a heating plate, a heating tube and a soldering iron core.
For a mold with a simple structure and a small size, heating with a heating plate will cause a greater heat loss. A simple heating system designed in the mold can meet the requirements. It should be noted that heat insulation (usually asbestos cardboard) should be added between the mold and the fixed plate of the press for heat preservation, and attention should be paid to the neat arrangement of the power cord and the position of the galvanic hole. This design); due to its small heat capacity, it is especially suitable for small molds that require repeated heating and cooling or rapid heating and cooling.
4. Conclusion
This article is a summary of practical engineering applications, and many techniques and methods involved in the practical article are feasible.
