A molding method similar to rotational casting, the difference is that the material used is not liquid, but sinterable dry powder. During the entire production process, the speed of mold rotation, heating and cooling time must all be strictly and accurately controlled. Compared with other mold processes, rotational molding provides us with more design space.

Introduction

Also known as rotational molding, rotational casting molding. A thermoplastic hollow molding method.

That is, first inject powdered or paste materials into the mold, and through the heating of the mold and the rolling rotation in the vertical and horizontal directions, the materials are evenly distributed in the mold cavity and melted by their own gravity and centrifugal force, and then demolded after cooling to obtain a hollow product. Because the rotational speed of rotational molding is not high, the equipment is relatively simple, the product has almost no internal stress, and it is not easy to deform, sink and other shortcomings. Initially, it was mainly used in the production of small products such as toys, balls, bottles and cans made of polyvinyl chloride paste plastics. Recently, it has also been widely used in large products. The resins used have been polyamide, polyethylene, modified polystyrene, etc.

A molding method similar to rotational casting, the difference is that the material used is not liquid, but sinterable dry powder. The process is to load the powder into the mold and rotate it around two mutually perpendicular axes. It is heated and evenly sintered on the inner wall of the mold to form a whole, and then after cooling, a hollow product can be obtained from the mold.

Also called rotational molding or rotational molding. That is, powdered plastic (such as LLDPE) is added to the closed mold, and the mold is heated while rotating. The plastic melts and evenly adheres to the surface of the mold cavity. After the mold is cooled, a plastic hollow product with the same shape as the mold cavity can be obtained, such as a ship, a box, a barrel, a basin, a can, etc. It usually consists of basic steps such as feeding, sealing the mold, heating, cooling, demolding, and mold cleaning. This method has the advantages of small product shrinkage, easy control of product wall thickness, and low mold cost, but low production efficiency.

Production process

Before the rotational molding process, you need to prepare some good quality molds and place them on a molding machine, which also has two devices for heating and cooling. After they are properly placed, first place several molds on the machine. Then, put early regular plastic resin in each mold, and then slowly rotate the mold from two angles vertically and horizontally and put it in the oven. The melted resin will evenly and tightly bond the hot mold and the outer packaging together. Then, put the mold into a refrigeration cycle device and continue to rotate it until the thickness of each part is consistent. When the mold is frozen, take it off the machine.

Design advantages

With the right design concept, we can combine several parts into a complete mold, which greatly reduces the high assembly cost.

The rotational molding process also includes a series of inherent design thinking methods, such as how to reconcile the side wall thickness and how to strengthen the external settings. If additional design is needed, we can also add the reinforcement ribs to the design.

Rotational molding allows designers to create endless imaginations. Designers can choose the best materials during the manufacturing process, including materials approved by the Food and Drug Administration. Additives added during the manufacturing process can effectively resist the invasion of climate, static electricity and other external objective factors. During the design process, insertion ports, threads, handles, inverted devices, and perfect surface design are all highlights. Designers can also design multi-wall molds, which can be hollow or filled with foam.

Cost Advantage

When cost is also a factor we consider, rotational molding has a market advantage over other types of processes. Compared with blow molding and injection molding, rotational molding can easily produce parts of different sizes within an effective cost range. His mold is relatively cheaper because it does not have some internal cores to be completed. And without the internal core, just a small change can make another model.

Since all parts in the production process are finally formed under high temperature and rotation process, unlike those parts formed under heavy pressure, rotational molding molds do not need to be specially processed like injection molding process to withstand the test of heavy pressure.

As for the production cost of conversion products, it has also decreased, because light plastics often need to consume more raw materials to convert into heavy plastics. For rotational molding, a single type of original mold that saves consumption costs will be its future high-yield development trend.

Features

In terms of moldability

1. The wall thickness of the product is uniform, and there is no waste material or weld; it can form products with a large wall thickness range, such as polyethylene rotational molding parts, with a wall thickness range of 1~16mm. However, due to viscosity, thick-walled products are more difficult to mold. The rotational molding process is particularly suitable for molding plastic products of 2~5mm.

2. Usually only hollow products or shell products can be produced, and it is difficult to process solid products. Moreover, the surface condition of the product is highly dependent on the surface of the mold cavity.

3. The dimensional accuracy of rotational molded products is low, and its dimensional accuracy is usually ¡À5%.

In terms of molding technology

1. Suitable for molding large and extra-large parts. Since the rotational molding process only requires the strength of the frame to be sufficient to support the weight of the material, the mold and the frame itself, as well as the closing force to prevent material leakage, even if large and extra-large plastic parts are processed, there is no need to use very heavy equipment and molds. Therefore, theoretically speaking, there is almost no upper limit on the size of products made by the rotational molding process.

2. Suitable for the production of multiple varieties and small batches of plastic products-because the mold used for rotational molding has a simple structure and low price, it is very convenient to change products.

3. Suitable for processing large hollow products with complex shapes, which is unmatched by other molding processes;

4. Plastic products are easy to change color. When the color of the product needs to be changed, you only need to clean the molding mold.

5. The main disadvantages of rotational molding are: high energy consumption, because in each molding cycle, the mold and mold frame need to withstand repeated heating and cooling; long molding cycle, because it mainly relies on the plastic to conduct heat under static conditions, so the rotational molding heating time is long; high labor intensity, poor product dimensional accuracy.

Application

With the development of rotational molding technology, the application range of its products has been continuously expanded, and the application of rotational molding products has been very extensive. Some representative applications can be exemplified as follows.

1. Container-type rotational molding parts, this type of plastic parts are widely used in water tanks, storage tanks for various liquid chemicals (such as acids, alkalis, salts, chemical fertilizers, pesticides, etc.), gasoline containers (gasoline storage tanks and fuel tanks for automobiles and aircraft), battery shells, etc.

2. Rotational molding parts for automobiles, mainly using polyethylene and polyvinyl chloride paste resins, roto-molding various pipe fittings, such as air conditioning elbows, backrests, armrests, etc.

3. Sports equipment and various substitutes. There are mainly various parts made of polyvinyl chloride paste roto-molding, such as water balls, floating balls, bicycle seats, boats, and shock absorbers between ships and docks. Roto-molded cross-linked polyethylene barrels made from Philips' roto-molding grade cross-linked polyethylene tree "Maricxcl-100" can compete with metal barrels, and it has good chemical corrosion resistance and low maintenance costs. Roto-molded pallets have been commercialized in the United States, Japan and other countries in the early 1970s; surfboards, boats, etc. are also roto-molded parts often mentioned in the literature.

4. Toys, models, handicrafts, etc. Since rotational molding molds can be manufactured using precision casting, electroforming and other processes; the surface of the roto-molded part has a good "copy" effect on the fine structure of the mold cavity surface, so the roto-molding method can make the product quite delicate and beautiful, so it is often used to make products with great ornamental value, especially toys, models, handicrafts, etc.

In addition to the above, rotational molding products are widely used in various boxes, shells, large pipes and other parts, such as turnover boxes, garbage bins, machine shells, protective covers, lampshades, bathrooms, toilets, telephone rooms, yachts, etc. Rotational molding products have been widely used in the fields of liquid chemical storage and transportation, chemical enterprises, industrial coatings, washing tanks and reaction tanks in rare earth production, as well as river and sea buoys, domestic water tanks, etc.

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