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Heating, insulation and cooling of PET bottle blowing machine molds

1 Overview

The compression molding process is one of the most common and oldest molding methods for plastic materials, and is the most commonly used process method to study material properties. It has the characteristics of a single forming device, low equipment investment, and simple structure of the bottle blowing mold. Today, with highly developed mechanization and automated production, it is still one of the most popular production methods. However, in addition to textbooks, monographs written by several seniors, and illustrations of bottle blowing mold design, the literature that can be referenced for bottle blowing mold design for compression molding is that few people have summarized the key skills in bottle blowing mold design.

The heating, insulation, cooling and clamping structure of the bottle blowing mold are an integral part of the design of the composite bottle blowing mold. Structural design directly affects the appearance and internal quality uniformity of the product, and also affects the molding efficiency of the product.

2 Heating, insulation and cooling design

2.1 Design requirements for heating tubes

Steel heating is a heating method that must be used in the design of almost all plastic molding and blow molding molds. It can be designed in various forms such as one-way wiring and two-way wiring. The material can be seamed pipes, seamless pipes, stainless steel pipes, etc. Features It has small heat loss, high thermal efficiency and simple wiring. It can be designed as 220V or 380V according to needs, and the wiring style is flexible and diverse. However, due to the limitations of its materials and processing technology, attention should be paid to its body characteristics in the design of bottle blowing molds.

(1) Heating tubes usually have longer cold ends 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. For example, for a 30cm long heating pipe, the power should not exceed 300 watts as much as possible. If the design power exceeds this limit, the surface load of the heating pipe will be high, and the steel pipe will be easily oxidized and corroded, causing a short circuit.

(3) For the design of bottle blowing molds with temperatures above 250°C, it is difficult to use heating tubes

I once used a heating tube to raise the temperature to 420°C, but this molding temperature requires high quality of the heating tube, and I need to frequently check whether the circuit is smooth and short-circuited. Because under such conditions, heating tubes, terminal blocks, connecting copper wires, steel sheets and other media are very easy to oxidize, resulting in circuit breakage. Therefore, special treatment is required for electrical transmission properties to avoid exposing conductive wires to the air and extend the service life of the wires.

The soldering iron core is usually used as a type of heating tube in bottle blowing molds. It 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 Breakdown short circuits can be buried by drilling blind holes. The disadvantage is that it is difficult to customize the design and is fragile and broken when replaced.

Insurance measures such as fuses and air switches are indispensable in circuit design. The operating area must be kept clean and tidy with good insulation. During operation, be diligent in checking for electrical faults to prevent unnecessary dangers.

2.2 Installation drilling 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 bottle blowing mold, so that the heat of the heating tube can be transferred to the bottle blowing mold as soon as possible. In fact, there is not much contact area between the heating tube and the bottle blowing mold. The essence of heat transfer is radiation, and conduction is secondary. Therefore, most heating tubes used for bottle blowing mold county installation are coated with a coating that enhances infrared radiation. At the same time, the design power (10 watts/cm) is also limited to extend the service life of the heating tube.

Therefore, when processing the heating tube hole, especially when processing the long heating tube hole, there is no need to design too small a matching gap. An effective design method is to match the heating tube as closely as possible at both ends of the hole. Filling and sealing can be used Or design baffles 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 tubes

Buried heating tubes are best filled with magnesium oxide powder that is the same as the medium in the tube to reduce the heat load on the surface of the heating tube. This method can reduce surface oxidation of the tube and effectively extend the service life of the tube. If possible, it is best to fill the installation hole of the heating pipe with magnesium oxide powder.

2.4 Insulation method of bottle blowing mold

Strengthening the insulation measures of the bottle blowing mold can reduce the heat loss of the bottle blowing mold, enable the bottle blowing mold to reach the predetermined production temperature in a shorter time, and reduce energy waste. Every engineering technician has a unique solution to this problem, I will only talk about my experience.

2.4.1 Insulation measures for heating plates

Heating plates are usually insulated with asbestos boards or asbestos cloths, but asbestos cloths are not easy to place flat and have a certain impact on ensuring the parallelism of the pressure plates. There are many types of asbestos boards, the most common of which is rubber asbestos board. However, this kind of asbestos board is not suitable for use as a sealing and heat insulation material. It has a certain compressibility and at the same time releases a very difficult to use material at high temperatures. The smell will affect the operating environment and the health of the operator. The insulation of the heating plate should be asbestos cardboard. The common specification is 1000×1000, 35mm thick. The board body is relatively regular, has good parallelism, has average compressibility, and has no problem at high temperatures. Odor occurs.

2.4.2 Insulation measures for bottle blowing molds

There are many insulation measures in bottle blowing mold counties. Aluminum hydroxide insulation cotton can be wrapped with asbestos cloth or glass cloth for insulation. There is also a thermal insulation coating on the market that is currently an ideal material for thermal insulation in bottle blowing molds. It is a mixture of medium-long fibers, slurry and an insulation foam material. It has moderate viscosity and is easy to apply. This material is often used as an insulation layer material for chemical and heating pipes. It contains slightly alkaline material (easy to corrode the bottle blow mold). After being used at 150°C, no negative effects such as burning, melting, or odor were found. At the same time, the material is very light and has strong plasticity, making it easy to form a more beautiful bottle blowing mold surface.

2.5 Cooling method of bottle blowing mold

Water cooling is the cooling method used by most bottle blowing mold counties, but it also has its shortcomings: it requires good pipe sealing, the upper and lower water pipes must be smooth, and it wastes a lot of water resources. When the cooling temperature exceeds 100°C, steam explosion is likely to occur. The advantage is that it has a large heat capacity and can achieve rapid cooling.

Air cooling is an ideal cooling method. It is the opposite of water cooling. It does not require strict pipeline sealing and there is no waste of resources. It can cool bottle blowing molds with temperatures higher than 100C. The cooling speed can be determined by the flow of gas. , and the source is simple and convenient, and production workshops of a certain scale can obtain a more convenient air source.

3 Clamping of bottle blowing molds

The clamping structure of the bottle blowing mold is closely related to the heating, insulation and cooling system of the bottle blowing mold. It also provides certain convenience features for the replacement, loading and unloading of the bottle blowing mold. For the sake of convenience, most designers simply punch a few mounting holes on the bottle blow mold for fixation. For example, most molds do not have a separate heating device, but instead install heating plates on the upper and lower platens of the press to simplify the processing of small and medium-sized bottle blowing molds. In the bottle blowing mold structure, only the modules that constitute the main structure of the cavity are left. At this time, the bottle blowing mold can use the fixing method of the injection blow molding mold to fix the bottle blowing mold on the upper and lower templates with pressure plates. Just design the space for fixing the pressure plate on the heating plate blow mold. This design can be used not only for mobile dies, but also for dies with a simple pull-out mechanism. It is only necessary to consider that the position of the push rod does not conflict with the heating pipe in the design of the heating plate. The mold base of one bottle blowing mold can also be used to universally transform multiple bottle blowing molds to simplify the manufacturing cost of bottle blowing molds.

If the supply tool is high, simple heating plate heating can no longer meet the needs of uniform heating. At this time, an auxiliary heating system needs to be installed on the bottle blowing mold, which can be composed of a heating plate, a heating tube and a soldering iron core.

For bottle blowing molds with simple structures and small sizes, using a heating plate for heating will cause excessive heat loss. A simple heating system designed in the bottle blowing mold can meet the requirements. It should be noted that insulation (usually asbestos cardboard) should be added between the bottle blowing mold and the press fixed plate. At the same time, attention should be paid to the neat arrangement of the power cord and the position of the galvanic hole. This kind of equipment ); Due to its small heat capacity, it is especially suitable for small bottle blowing molds that require repeated heating and cooling or rapid heating and cooling.

4 Conclusion

This article is a summary of actual engineering applications, and it is feasible through practice with the various techniques and methods involved in the article.

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