Rotational molding, also known as rotational molding, is a processing method for plastic hollow products. It began in the 1940s, but due to the slower processing cycle and materials used, it is much slower than other plastic processing industries. Originally used mainly for the formation of polyvinyl chloride paste resin. In the 1970s, it developed rapidly and achieved industrialization. Rotational molding is the fastest growing plastic molding method in the plastics processing industry today. In terms of key equipment investment, mold cost, wall thickness uniformity, and low stress concentration of the final product, rotational molding has the advantages of blow molding and injection molding. In addition, rotomolding can also form large and complex shapes, which is difficult to achieve by other molding methods.
At present, there are about 1000 rotomolding enterprises in the world, and 2/3 are distributed in Europe and the United States. Plastics that can be used for rotational molding are PVC paste, polyolefin, nylon, polycarbonate, polyester, EVAC, ABS and PS. The development of powdered polyethylene in the 1960s has given vitality to the development of the rotational molding industry. Until today, 90% of the raw materials for rotational molding are still various grades of polyethylene resin. There are few reports on other rotomoulding materials. Especially for the rotational molding of cast nylon, this article will focus on the technology in this area.
1. Cast nylon features:
It is well known that caprolactam monomer anion polymerized nylon, commonly called cast nylon (MCPA6), has the advantages of general nylon product, high molecular weight, high crystallinity, low polymerization temperature, simple process, high crystallinity and molecular weight. Large and uniform distribution, low density, good mechanical properties, shock absorption and wear resistance, self-lubrication, corrosion resistance, temperature range of use, etc., is an excellent and comprehensive mechanical properties of engineering plastics. It has been widely used in mechanical parts such as bearings, rollers, bushings, gears, turbines, sheaves, fork wheels, retaining rings, baffles, etc., and is used in various fields such as machinery, textile, petrochemical, and defense industries.
2. Casting nylon rotomolding characteristics Rotational molding is a plastic processing method, which not only greatly improves the utilization rate of raw materials, but also has high economical efficiency, and the comprehensive performance of the product can be designed with high added value and no need after Processing, especially for the molding of large-scale seamless composite hollow products, has advantages that other methods can't match, and the application prospect is very broad.
A key factor limiting the development of rotational molding is the powdered raw materials required for forming. The abrasion resistance, tensile strength and flexural strength, and heat distortion temperature of polyethylene materials are insufficient to meet the requirements of more end users. Other plastics, such as polycarbon and nylon, can meet the higher demands of users, but at a higher cost. This is not only the cost of the resin itself, but also the processing of these high-performance resins into powders. Adding auxiliaries and protective atmospheres during processing to protect these heat-sensitive resins that are easily degraded at the high temperature of rotational molding.
The use of a liquid rotomolding system can effectively solve the problem of high-performance powder resin roll molding. The use of a liquid rotomolding system can reduce the rotational molding cycle (shortening the heating cycle, reducing or even eliminating the cooling cycle), and the liquid rotomolding system can obtain a smooth surfaced article due to the low viscosity of the liquid relative to the metal mold. Because of these advantages, liquid rotomolding systems have long been used, such as the polyvinyl chloride paste system that emerged in the 1940s, the caprolactam system that began in the 1960s, and the polyurethane system that appeared in the 1970s.
Cast nylon is usually used in static casting, and it is introduced into the field of rotational molding. It has the advantages of one-time molding from monomer raw materials to parts, and its molding temperature is lower than the melting point of nylon, and the rotational molding energy consumption is small. It has greater economics. In the melt rotomoulding of reactive caprolactam, the rotational molding temperature is about 170 ° C, which is lower than the melting point (210 ° C) of the final product nylon 6, which is a significant difference from the rotational molding of nylon polymer powder. In the case of caprolactam rotomoulding, accompanied by melt polymerization, less heat needs to be replenished from the outside; after rotomolding a good product, the mold can be taken out of the mold without or with a slight cooling. During the entire molding cycle, the rotomolding mold is only changed within a range where the temperature difference is not very disparate, and it is not required to repeatedly undergo high temperature heating and low temperature cooling as in the case of powdery material rotomolding. This is superior to rotomolding of nylon powder.
Casting nylon rotational molding has two outstanding features compared to conventional rotomolding processes compared to conventional rotomoulding materials. The first point is that its molding process is a mixing process of chemical processes and physical processes, while ordinary polyethylene rotational molding is only a physical process. In this process, the nylon monomer undergoes an anionic polymerization reaction, and the monomer is reacted in one step to form a nylon polymer article. The second point is that it is liquid rotomolding. It is different from ordinary powder rotomolding. This brings superiority to the rotomolding process and also puts higher requirements on the molding process.
3. Development status of cast nylon spinning at home and abroad French nylon supplier Rodia has developed two grades of rotomolded products. Tehnyl RTM C207L Natural is a nylon 6 with excellent heat and light resistance. The C207 Black only has good heat resistance. Other grades based on nylon 6 and nylon 66 are still under development. Rhodia predicts that its application in areas such as water tanks, hydraulic oil tanks, automotive oil transmission parts, and sound insulation boards that have high requirements for heat stability is expected to increase.
Recently, BASF Engineering Plastics also introduced nylon resin for rotational molding for the first time. The target market for Capron BR30HS Nylon 6 Resins is in automotive liquid reservoirs; for oil liquids, diesel or coolant reservoirs suitable for small production, another potential use is for automotive air supply pipes, trucks and construction equipment roofs.
The more mature research on cast nylon rotational molding is the nylon block copolymer Nyrim from DSM. The product is formed by copolymerization of a caprolactam monomer and a polymerizable polyol. The polymerization is divided into two steps, the first stage being the reaction of a polyether polyol with a diacylated caprolactam (diacylated caprolactam used as an initiator for the anionic polymerization of the caprolactam monomer) to form an acylated caprolactam-terminated polyamide ester. The reaction is completed in a few seconds in the presence of a base catalyst. This constitutes the prepolymer component provided by DSM. In the next stage, the caprolactam anionic polymerization is activated by the amidated caprolactam-terminated prepolymer, and the carbonyl group on the prepolymer backbone acts as an activation point, causing the activated caprolactam monomer to grow at these points, eventually forming A ( BA) n structure of nylon block copolymer. The soft polyether segments of the copolymer impart high impact and good bending properties to the final article, while the stiff nylon segments impart high heat distortion temperature, rigidity and mechanical strength to the final article. The user can adjust the ratio of the soft segment to the hard segment according to the specific needs, so that the final product can achieve the desired performance between the rigid nylon and the soft elastomer.
Long-term research on nylon liquid rotomolding has been carried out at Queen's University of Belfast, Northern Ireland. The initial reaction temperature, liquid viscosity, heating time, biaxial speed, cold take-up speed and other process factors have been investigated for the properties of the final rotomoulded product. The impact of appearance.
Domestically, the rotational molding of cast nylon is mainly for large-sized parts. The FRP Research and Design Institute of the National Building Materials Bureau is an early and well-researched unit in China. In the early 1970s, a large-scale rotary forming machine was successfully developed. It has developed hollow cast nylon products such as air filter for nylon Benz car, hollow shoe plant, forest fire extinguisher and liquefied petroleum gas tank lining, hollow floating body and helicopter fuel tank. Jiao Bin et al. conducted a detailed systematic study on the casting of nylon rotomolded thin-wall seamless composite hollow products.
Shanghai Jieshijie New Material Co., Ltd. is also a pioneer in domestic casting nylon rotomolding technology. The company concentrates on research and development to carry out long-term research on this technology. In the raw material process, we give full play to the strength and advantages of the company's research and development, increase the investment in basic theory and process formulation research, set up a special project department to carry out research, apply for a number of patents, and develop for rotomolding nylon. Laid a solid foundation. Our company has developed a nylon fuel tank rotomolding product with a focus on the fuel tank. Traditionally, automobile and motorcycle fuel tanks are made of metal materials. Due to the development of the locomotive industry, locomotive designers first think of using high-performance polymer materials instead of metals to reduce vehicle weight and improve fuel efficiency.
Compared with traditional metal materials, plastic fuel tanks have the following characteristics:
(1) Light weight, about 40% to 50% lighter than the same volume of metal fuel tank;
(2) The degree of freedom in design is large, and it can be designed into any shape, so that the space can be fully utilized to increase the oil load;
(3) The development cycle of the forming mold is short, about 1/3 of the development cycle of the metal fuel tank forming mold;
(4) The processing and molding cycle is short, and the product qualification rate is very high, which is especially suitable for large-scale industrial production;
(5) Low thermal conductivity and no explosion;
(6) It has excellent low temperature impact resistance;
(7) Good mechanical performance, especially for external motorcycle fuel tanks, which need better mechanical strength;
(8) Good barrier performance for gasoline and diesel;
(9) It has certain temperature and heat resistance, and can be used for a long time under certain temperature conditions;
(10) Good fuel corrosion resistance.
Among them, (1) to (6) are also advantages of the polyethylene rotomolding tank, and (7) to (10) are only available for casting nylon materials, and polyethylene materials are not provided.
As a general-purpose plastic, polyethylene material does not have superior mechanical and thermodynamic properties as a nylon material for engineering plastics; in addition, as a non-polar material, polyethylene material does not have superior corrosion resistance as a nylon material of a polar material. And resistance to fuel penetration. In addition, nylon is a material that is easy to spray, and has obvious advantages in the paint decoration of the fuel tank. After two years of research and development, our company's cast nylon fuel tank has a complete set of casting technology and technology for casting nylon rotomolded fuel tank. The product has passed the series inspection of authoritative departments at one time and is actively expanding. produce.
4. Conclusion With the development of society, in order to meet the needs of structural design and space saving, hollow products are mostly irregular structural shapes, and conventional molding methods such as blow molding and rotational molding are difficult to form. Casting nylon rotomolding technology not only gives full play to the advantages of materials, but also the molding process is simple, especially the material flowability is good, and it has high economy. It is especially suitable for manufacturing high-performance and high added value of complex structures. Hollow products are bound to become a new profit growth point in the plastics processing and molding industry, play a huge role and have broad application prospects.
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