Knowledge about rotomolding
2023-11-21 04:08:02
The rotomolding process is an important branch in plastic molding processing. Since its inception in the 1940s, after more than half a century of development, its equipment and processes have become more and more perfect, and it has been widely used in developed countries in Europe, from small children's toys to automobiles. Plastic parts, up to huge engineering plastic products. In particular, large and special-shaped hollow products can only be completed by the rotational molding process due to the limitations of the traditional process.
First, the rotational molding process
Process principle
The plastic is first added to the mold, and then the mold is continuously rotated and heated along the two vertical axes. The plastic in the mold is gradually uniformly coated and melted and adhered to the entire surface of the cavity under the action of gravity and heat. It is prepared for the desired shape and then cooled and shaped.
Second, the control points of rotational molding (influencing factors)
1. Effect of raw resin properties
The fluidity, particle size distribution, apparent density, and particles of the resin powder all have an effect on the molded product. According to the experimental analysis, the higher the peak position of the particle size distribution, the finer the particles, and the better the comprehensive performance of the product. However, considering the comprehensive production cost and efficiency, it is best to set the peak particle size distribution between 60 and 80.
2. Effect of heating temperature and time on rotational molding
The selection of heating temperature and heating time is affected by many factors, among which the most important ones are molding resin, mold and product.
For example, the temperature of the polyethylene rotomolding heating furnace is usually between 250 ° C and 400 ° C. The heating conditions of the specific product during rotomoulding can be determined experimentally. The initial test conditions can be 220 ° C for 10 min.
If the product is poorly plasticized or there are still tiny bubbles remaining in the wall of the product (the gas trapped in the melt is not fully discharged), the heating time should be extended appropriately or the heating temperature should be appropriately increased; if the color of the product is yellow, the material is overheated. A thermal oxidative aging phenomenon occurs, and it is necessary to appropriately shorten the heating time or appropriately lower the heating temperature.
3. Effect of rotation speed and heating rate on rotational molding
Normally, the linear speed of the mold is controlled to 6-35 rpm.
This depends on the shape and structure of the product. If it is a cylindrical product, the line speed can be high; if it is an irregular product such as an edge, a corner, a lap, etc., the line speed should be lowered accordingly. If the required rotational speed cannot be correctly estimated, the rotational speed can be adjusted by the state of the powder in the mold.
4. Effect of cooling on rotational molding
A very important part of the entire rotomolding process during cooling. When all the materials are melted in the cavity, the heat preservation can be started.
At present, the cooling methods in the rotomolding industry generally include water mist cooling and air cooling. The water mist is highly efficient, but the product is prone to shrinkage deformation. The air cooling efficiency is low, but the quality stability is good.
5, the influence of release agent
The silicone resin type release agent, if properly applied, can make the product easy to take out from the mold, thereby improving production efficiency, reducing damage that may occur when the product is released from the mold, and the release of the release agent once can be used repeatedly.
It is known from practice that the surface of the cavity must be cleaned before the silicone resin is applied. The silicone resin should be evenly coated, and the coating should not be too thick. After the coating is applied, the solvent should be slowly heated to evaporate the solvent, and then heated at a high temperature (220 ° C to 250 ° C) for 2 hours or more to crosslink, thereby obtaining A layer of release agent with good mechanical strength.
Third, the main features of rotomolding
advantage:
1. The cost of rotomolding mold is low - the product of the same size and size, the cost of rotomolding mold is about 1/3 to 1/4 of the cost of blow molding and injection molding, suitable for large plastic products;
2, the edge of the rotomolding product is good - rotomolding can achieve a thickness of more than 5mm at the edge of the product, completely solving the problem of thinner edges of hollow products;
3, rotomolding can be placed in various inserts;
4, the shape of the rotomolded product can be very complicated, and the thickness can exceed 5mm;
5, rotomolding can produce fully enclosed products;
6. Rotomolding products can be filled with foaming materials to achieve heat preservation;
7, no need to adjust the mold, the wall thickness of the rotomolded product can be freely adjusted (2mm or more);
8. Suitable for molding large and extra large parts;
9. Rotational molding is easy to change the color of the product - rotomolding molding adds the material directly to the mold, so that all the materials enter, and the product is removed from the mold before adding the materials needed for the next molding. When we use more than one mold to shape the same plastic products, we can also add different colors of materials in different molds, and simultaneously rotate plastic products of different colors;
10, saving raw materials - the wall thickness of the rotomolded products is relatively uniform and the chamfer is slightly thicker, so the material efficiency can be fully utilized, which is conducive to saving raw materials; in addition, in the rotomolding process, there are no runners, gates, etc. After the waste is debugged, almost no material is returned during the production process, so the process has a very high utilization rate of materials.
Disadvantages:
1. Long processing cycle, not suitable for mass production;
2. The energy utilization rate is low, the liquefied oil is heated, and a large amount of heat loss causes energy waste;
3. The selection of molding materials is limited. At present, there are only a few kinds of materials used for rotational molding. 99% of the rotational molding products in China are PE, and 95% of them are rotomolding grade LLDPE suitable for rotational molding. Although various plastics such as nylon, polycarbonate, and ABS are also formed by a rotational molding process, the application surface is very narrow;
4, the material cost is high, need to choose special additives and make fine powder;
5. It is difficult to mold some shapes of products (such as rib-like products);
6. Relative to injection molding and blow molding, each time the product is unloaded and the raw materials are opened, the labor intensity of the workers is large.
Fourth, the application range of rotomolded products
1, container type rotomolded parts
It is widely used in tanks for storage of water tanks and various liquid chemicals (such as acid, alkali, salt, chemical fertilizers, pesticides, etc.), gasoline containers (gasoline storage tanks, fuel tanks for automobiles and airplanes), and housings for batteries.
2. Rotomolded parts for automobiles
Mainly used polyethylene and polyvinyl chloride paste resin, rotomolded into various pipe fittings, such as air conditioning elbows, backrests, handrails and so on.
3, sports equipment and various substitutes
There are mainly various parts of PVC paste rotomolding, such as water polo, floats, bicycle seat cushions, boats and buffer shock absorbers between boats and docks.
4, toys, models, crafts, etc.
Because the rotomolding mold can be manufactured by precision casting, electroforming, etc.; the surface of the rotomolded part has a good effect on the "replication" of the fine structure of the mold cavity surface, so the rotomolding method can make the product quite exquisite and beautiful, so it is commonly used For the production of products with greater ornamental value, especially toys, models, crafts and so on.
5, various cabinets, shells, large pipes and other parts
Such as turnover boxes, garbage bins, machine casings, lampshades, bathrooms, toilets and yachts. Rotomolding products have been widely used in the fields of river and sea buoys, domestic water tanks, etc. in the storage and transportation of liquid chemicals, chemical enterprises, industrial coatings, washing tanks for reaction of rare earths, and reaction tanks.
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