The hottest rotational molding process and equipme

Rotational molding process and equipment and their research progress

1 rotational molding process and its advantages

rotational molding is a process method for the molding of hollow plastic products, also known as rotational molding, rotational molding or rotational molding. The process of rotational molding is to add plastic powder into the mold, then heat the mold and make it rotate continuously along two mutually perpendicular axes. Under the action of gravity and heat, the resin in the mold is gradually and evenly coated, melted and adhered to the inner surface of the mold, so as to form the required shape, and then cool the mold and demould to get the product

rotational molding originated in Britain in the 1940s, and developed greatly in Europe in the 1950s, and then spread to the United States, Japan and other places. With the maturity of polyethylene powder technology, it has increasingly become a very competitive molding method in powder plastic molding process. In recent years, the rotational molding industry has continued to develop at an annual growth rate of 10 ~ 15%, and has become the best molding and processing method for the production of medium, large or super large fully closed and semi closed hollow seamless containers with a small batch of plastic industry as the key emerging industry. At present, the global consumption of resin for rotary molding has accounted for 8% - 9% of the total output of resin

compared with the traditional molding methods of hollow plastic products (such as injection molding, blow molding, thermal molding, etc.), the most remarkable feature of rotational molding process is that the resin heating, molding and cooling processes are carried out in the non pressure mold. Such characteristics make it show a series of advantages

the mold is simple and low-cost, which is suitable for forming large-scale plastic products

it is suitable for the production of multi variety and small batch plastic products

suitable for molding products with complex shapes and composite products

save raw materials

the wall thickness of the product is uniform without joints

the product has no residual force and is not easy to deform

it is easy to change the color. When you want to change the color of the product, you only need to clean the forming mold

2 rotational molding machine

2.1 several typical rotational molding machines

swing rotational molding machines: they have the advantages of economically producing products with large volume and simple shape, and the disadvantage is that the degree of automation of the system is not high, resulting in low efficiency and heavy operation

shuttle roll molding machine: with two shuttle machines, it can economically produce large storage tanks, containers and small products. It is an entry-level rotational molding equipment with simple operation and low maintenance cost

clamshell roll molding machine: low initial cost, small floor area, less manual labor and good product quality. The double shaft rotation of the machine is provided by the main and auxiliary shaft variable speed gear motors during the rolling process. The rotating arm can be straight or curved, and can handle small or large molds

vertical roll molding machine: three arm type and six arm type. Its rotating arm operates in the same plane, with separate heating, cooling or loading/unloading stations. The maximum mold swing is usually limited to 900 ~ 1200mm, which limits the size of products. It is mainly used to make doll parts, toys, balls and auto parts

fixed rotating arm rotational molding machine: the equipment can range from a small 1000mm swing to a large 3800mm swing. It has three stations: heating, cooling or loading/unloading. The equipment is efficient and easy to maintain, and occupies a dominant position in the field of rotational molding. Usually heating is the key factor in the whole cycle, so it becomes the control factor of equipment operation

independent rotating arm type rotational molding machine: it is a type of equipment commonly used in the rotational molding industry. It has an air circulation heating chamber and an air/water cooling chamber. It provides five stations, including an automatic frame to control each rotating arm to rotate around the central table, and each rotating arm and frame are independent, providing more automated and perfect processing means, which fully reflects the flexibility of the independent rotating arm rotational molding machine

clam type rotational molding machine: This machine is the exclusive product of Canadian FSP company. The hot box of the machine can also be used as a cold box. The structure is divided into upper and lower parts. The upper part can be opened and closed. The shape and opening method are like clams. The rotation diameter of the die ranges from 1.2 to 3.6m

2.2 development status of rotary molding equipment at home and abroad

from the perspective of heating methods of rotational molding, direct fire heating and hot box heating are commonly used at present. Direct fire heating is generally used in the production of large-scale products, while foreign advanced rotary molding manufacturers mostly use hot chamber heating three arm four station and four arm five station double shaft rotating independent rotating arm equipment

developed countries not only continuously improve the configuration of rotational molding equipment, but also vigorously improve the automatic control level of equipment. For example, the rotalog rotary molding control system developed in the UK includes two parts: temperature measurement and control display. The temperature measuring part is sealed and installed in an insulating box on the mold frame. The temperature in the mold measured by the thermocouple is sent to the monitoring computer outside the heating room in real time through radio signals, and the computer displays the temperature/time diagram in the mold. Compared with the temperature/time diagram of the heating chamber, this diagram can visually display the heating and heat transfer of the mold and materials in the rotational molding equipment. The operator can use rotalog control system to reduce and optimize heating and cooling cycles, improve and ensure product quality, reduce production costs, and accurately evaluate molds and new materials

foreign rotational molding plants generally have many different rotational molding equipment. For example, spincast company in the United States has 7 rolls with a maximum rotation diameter of 4m, diamond company has 9 rolls with a maximum rotation diameter of 4.3m, and amerikart company has 10 rolls with a maximum rotation diameter of 3.3m

compared with the advanced level of foreign countries, the manufacturing and auxiliary technology of rotational molding equipment in China is still relatively backward, and there is no professional rotational molding equipment manufacturer at present. In terms of the use of domestic rotational molding equipment, there are the following characteristics

(1) the roll molding machine used is mainly heated by direct fire, and there is a small amount of hot chamber heating three arm four position independent rotary arm roll molding machine, and there are few other roll molding machines

(2) the effective data collection equipment for the independent rotating arm of hot box heating is almost all imported from abroad, mainly from the United States, Italy, Germany and Japan, and almost no domestic products. However, there are few imported direct fire heating swing machines, and most of the direct fire heating swing machines used are imitations of domestic manufacturers

(3) the number of rotational molding equipment in domestic rotational molding plants is small, and the scale of rotational molding plants is small

3 rotational molding mold

3.1 overview

mold is an indispensable and important equipment in rotational molding

the rotational molding mold is simple and non pressure molding mold, so the wall is thin and light. The rotational mold is composed of upper and lower mold halves and fixed with plywood. In order to exhaust the gas formed in the mold cavity and prevent the deformation of the product, an exhaust hole is set on the mold

due to the ideological bias of rotational molding, domestic manufacturers dare not invest more in industry in research and development. There are three common methods for mold production: steel plate welding, aluminum alloy casting and electroplating molding. Rotational molding does not require high strength of mold material, but it must have good thermal conductivity and be able to withstand the stress alternation in the process of frequent heating and cooling. Obviously, low carbon steel, aluminum, stainless steel, etc. are ideal molding materials. When choosing the mold making method, the following three aspects should be considered: the complexity of the structure, the number of molds and the surface quality requirements of products. Among these three requirements, the shape and size of the products dominate. We can choose different production methods according to different situations. At the same time, cost is another important factor in manufacturing molds. Mold production cost includes mold materials, mold thickness, position of parting surface, number of molds, surface treatment, etc. Rational design of the structure of the rotational molding die and correct selection of the die materials are the primary guarantee for obtaining high-quality products with smooth surface, no bubbles, uniform wall thickness and no stress

the wall thickness of the mold shell is designed according to the design specifications of atmospheric pressure vessels, that is, from the stiffness calculation, combined with the product thickness, heating method and molding method. Generally, steel plates are used for larger molds, with a thickness of 2-4mm, and the inner wall of the mold should be polished. In addition, the thickness of the mold depends on the rotation direction and heating method. The wall of the mold used for the conduction heating of hot liquid is thicker, generally 7.9 ~ 12.7mm, to ensure uniform heat transfer and no hot spots; The wall of the mold heated by hot air circulation is thinner, generally 5 ~ 7mm

3.2 cast aluminum mold

the castings produced by traditional sand casting have low precision and long molding cycle, and the surface quality of the castings can not meet the requirements of rotational molding mold. After casting, they need to be polished manually. Therefore, the production of aluminum mold for rotational molding must adopt precision casting method, and introduce rapid molding technology to shorten the manufacturing cycle of the mold. Only in this way, the advantages of less investment and quick effect of rotational molding can be fully displayed. There are many methods of modern precision casting. It is very economical to use gypsum mold or ceramic mold for single castings with large size and complex shape, such as rotational mold. If combined with the hot mold differential pressure pouring method, the fluidity of the cast aluminum alloy is greatly improved, and the wall thickness of the rotational molding mold is further reduced, which is very beneficial to speed up the heat conduction in the rotational molding and shorten the molding cycle

3.3 rapid molding technology

rapid molding technology is a method of processing various metal molds directly or indirectly by using the prototype made by rapid prototyping Molding Technology RPM (rapid prototyping molding). It is a high-tech integrating the latest achievements in CAD/CAM, chemical industry, materials and precision machinery. At present, the relatively mature rapid modeling methods include laser modeling SLA, sheet stacking LOM, fuse deposition FDM and selective laser sintering SLS. In the manufacturing of rotational mold, it is very advantageous to use LOM Prototype with surface coating treatment instead of wood mold to directly manufacture gypsum mold, ceramic mold (large parts) or convert the prototype into gypsum mold and ceramic mold (small parts) through silicone rubber mold transition, and then cast metal mold with gypsum mold and ceramic mold to realize precision casting of aluminum mold

4 quality control of rotational molding products

4.1 common quality problems

simple and practical is a major advantage of rotational molding, but it also brings some congenital defects to this technology. For example, bubbles are easy to appear inside the products, while holes are easy to appear on the surface; Products are prone to bending, shrinkage, discoloration, etc. These phenomena not only affect the appearance of products, but also seriously damage the mechanical properties of products. Therefore, it is of great significance to study the formation and disappearance of bubbles, the formation and disappearance of bending and shrinkage phenomena, and the impact strength of products to improve the quality of molded products

4.2 bubbles and pores

bubbles are formed because air is entrained between powder particles when they melt and bond together. In industrial production, the method of increasing heating temperature is often used to reduce or eliminate bubbles in products. Because when the heating temperature is increased and the bubbles in the product are prolonged or eliminated, the viscosity of the material decreases and the bubbles diffuse easily. At the same time, because there is more time to make the bubble diffuse more thoroughly. Although this method is effective in eliminating bubbles in products, it also has side effects. For example, increasing the melting temperature will prolong the molding cycle and reduce the production efficiency; Under high temperature, the material is oxidized, which reduces the mechanical properties of the products, especially the impact strength

4.3 impact strength

there are many factors that affect the impact strength of products, such as heating temperature, heating time, cooling speed, particle size