Rotational molding

Rotational molding is a kind of thermoplastic plastic hollow molding method. This method is to add plastic raw materials into mold at first, rotate and heat the mold, plastic raw materials gradually melt adhesive and evenly coated on the entire surface of the cavity under the action of gravity and thermal energy, forming the shape of the required, then cooling and shaped into products.


The main role

There are many kinds of molding processing of plastics and its composite material, in addition to the common methods of extrusion, injection and blow molding process, rotational molding is also a kind of plastic products processing methods. Rotomolding industry developing rapidly abroad, but domestic rotomolding industry development speed is lower than other plastic molding processing industry due to various factors, such as: the slower processing cycle and the limit of the material.

Traditionally, rotomolding is mainly used in thermoplastic materials, and thermosetting material such as crosslinkable polyethylene development is also fast. As no need high injection pressure, high shear rate or precise polymer meter, so the mold and machine cost is low, and the service life is long.

Since the 1930s, after 80 years of development, Roll forming process equipment and process have improved a lot. From small children toys to common automotive plastic parts until the huge engineering plastics, especially large and non-standard shaped hollow plastic products, due to the limitation of other plastic processing technology, now also can only rely on the roll forming process to complete. Roll forming technology has become an important branch of plastic molding processing technology.


The production process

Roll forming basic process is very simple, putting powder or liquid polymer in the molds, then heating and rotating at the same time around the two vertical axis rotation (rotation and revolution), and then cooling. In the initial heating stage, if use the powder material, porous layer is formed in the mold surface first, then gradually fused with cyclic process, and finally form a uniform thickness of homogeneous layer; If use the liquid material, it flows and coated on the surface of mold first, it stopped flowing completely when reach the gel point. Die then turn into cooling work area, cooling by forced ventilation or spray water, and then be placed in the working area, here, the mold is opened, the finished parts are removed, then for the next round of production cycle.


Characteristics

Advantages

Cost

When costs become one of the factors to consider, rotomolding technology has more market advantage than other types of technology. Compared to blow molding technology and injection molding process, rotomolding technology is able to produce different size of products more easily within the scope of effective cost. Its mold is cheaper, because it doesn't have some internal core parts need to be completed. And can be made into another model with small changes.

Due to the parts in production process are under high temperature and spinning process, different from those formed under great pressure, rotational mold is able to withstand the test of pressure , don't need special processing like injection molding process.

The production cost of conversion products has also dropped, because the convert of the light plastic to heavy plastic often requires more raw materials. For rotomolding technology, saves the cost of simple master mold will be its future development trend of high yields.


Design advantage

Compared with other mold technology, rotomolding technology provides more space design. Under the correct design concept, it support a few parts assembled into a complete mold, which approach greatly reduces of high assembly costs.

Rotomolding making program also includes a series of inherent design way of thinking, such as how to mediate wall thickness, how to strengthen the external settings. If still need to add some auxiliary design, also strengthening rib line step can be added into the design.

Rotomolding technology for product injected with infinite imagination of the designers. Designers can choose the best materials in the production process, including various materials approved by the food and drug administration. Additives added in production process can effectively resist the external objective factors such as the descent of the climate, electrostatic interference, etc. In the design process, there are highlights including insert, thread, handles, pour in device, perfect design of surface. Designers can also designed to be more wall type mold, its internal can be hollow or be filled with bubbles.


Main characteristics of rotomolding technology

1. Low cost of mold. At the same size product, cost is about 1/3 to 1/4 of blow molding, plastic mould injection. Suitable for forming large plastic products.

2. The intensity of product edge is good. Rotomolding products edge intensity can be achieved more than 5 mm thick, completely solve the problem of hollow products edge thinner.

3. Rotomolding support all kinds of inserts.

4. The shape of the plastic products can be complex, and the thickness can be more than 5 mm;

5. Rotomolding can be totally enclosed products;

6. Foam plastic can be fill materials and achieve heat preservation;

7. Do not need to adjust the mold, plastic products of wall thickness can be adjusted (more than 2 mm).


Disadvantages

Roll forming also has its disadvantages: cost increase because of the material must be made via grinding crushed. Processing cycle is long, therefore not suitable for mass production. Less plastic varieties are available; Open and close mold belong to the heavy manual labor.


The Main Application

Roll plastic products can be used in vehicles, traffic safety facilities, entertainment, river waterway dredging and construction, water treatment, medicine, food, electronic, chemical industry, aquaculture, textile printing and dyeing, etc.

1. Roll container rotational molding parts.

This kind of plastic parts are widely used in storing and feeding box, storage tank, a variety of industrial chemicals storage and transport container, such as acid, alkali, salt, chemical fertilizers, pesticides storage tank, chemical industry, industrial coating, rare earth for making the wash tank, reaction tank, turnover box, dustbin, septic tanks, living water tank, etc.

2. Transportation rotational molding parts.

Mainly use polyethylene and PVC paste resin, to rotomolding all kinds of automotive parts, such as air conditioning pipe, spiral pipe, back of a chair, armrest, fuel tanks, fender, door frame and gear lever cover, battery case, snow automobile and motorcycle fuel tank, fuel tank, yachts and water tank, boat and ship and dock of buffer shock absorption, etc.

3. Sports equipment, toys, handicrafts rotational molding parts.

Mainly include: all kinds of PVC paste rotomolding parts, such as water polo, floating ball, small swimming pool, yacht and radiator, plastic pallet, bike seat cushion plate, surf board and so on. Toys such as pony, dolls, toys, sand box model, fashion models, arts and crafts, etc.

4. All kinds of large or non-standard rotational molding parts.

Rack, machine shell, shield, chimney, agricultural sprayer, furniture, canoeing, camping car canopy, playground equipment, planter, bathroom, toilet, telephone, advertising display card, chair, highway isolation pier, traffic cone, buoy, bull barrels, and the construction barrier and so on.


From products, rotational molding products cover from ordinary people use goods to key industrial development products, its function also change from a single container performance to structure performance. Along with the development of the roll-plastic equipment and mold, structural complexity of plastic products have been developed to the point of beyond imagination. Products, unable to complete with injection molding, blow molding, extrusion process, are able to be done through rotomolding technology, geometry size of products support from 0.5L to 50sqm. If there is no limitation of transportation, its product size can even bigger. From the classification of the product, can be roughly divided into cylindrical tank (0.25sqm ~ 50sqm), frustum of a cone shape exposure barrel (20L ~ 3000L), square exposure barrel (30L ~ 2000L), with the need of function, can also be molding is silent type of all kinds of containers, to meet the needs of different industries. Some shortcomings includes: high unit cost, low production efficiency, need special equipment to form the mass production, and high transport costs due to light weight but large volume.

Compared with foreign countries, Chinese rotomolding industry has some problems. Rotomolding industry in China develop quickly, but the overall size is small, the technology is low, there are three major bottlenecks in development.


The principle

Rotational molding is depend on the mold high speed rotating centrifugal force, making the raw material is uniformly distributed on the inner wall of the mold. It is atmospheric pressure molding, low residual stress, avoiding products defects such as warp and simplifying mold greatly. It significantly reduce manufacturing costs, becoming the most competitive method in production of medium, large and super large fully enclosed and semi-enclosed hollow seamless container.

The biggest advantage of rotational molding is the simple mold and low cost. It is usually used in the production of large-scale hollow products, but need larger production equipment and the corresponding design and reasonable mold. Rotational mold must ensure uniform heat of the mold parts in the process of spinning, this technical difficulties is the greatest gap between domestic equipment and foreign equipment. Especially in the production of some large plastic products, uneven heating often leads to local contraction and impact strength decrease, making low finished product quality. In the mold design, rotational molding requires low mold pressure, but in order to ensure the good liquidity, raw materials mold design needs to be reasonable and concise, the domestic plastic mold design level is often difficult to satisfy. For these reasons, more than 80% of Chinese enterprises import the production equipment and mold from abroad, which increased the production cost in China and weaken the advantages of low cost in equipment, has affected the Chinese rotational molding technology application development.


Compared with other methods of molding, rotational molding is more suitable for forming large jointless plastic products. It uses powder particles material, the very competitive molding method in powder plastic molding process. But at the same time, it require highly in particle size of plastic powder and surface roughness, also requires the material has excellent mobility and thermal stability. 90% plastic products apply special PE rotational molding raw material, and domestic plastic special material manufacturer is seldom, mainly rely on imports. Resin consumption in global rotational molding is 8% ~ 9% of total resin production, while China data has less than 1%, it shows that Chinese rotomolding dedicated enormous room for growth. Improving rotomolding special material production and increasing product variety, is suspends in front of the petrochemical enterprises serious topic, also is a very good opportunities.


The development trend

Rotational molding is very suitable for the production of small batch, many varieties, colorful large-scale hollow plastic products, such as: sports goods, highway guardrail, art and design for the park glowing spheres, boats and mobile terminal, vacuum cleaner, and various other valuable goods. It belongs to the processing technology of high added value, in today's plastic products under a trend of small amount of diversification, its development prospect is more positive.


However, rotomolding industry professionals in China is very few, compared with research professionals in injection molding and extrusion process, technology research personnel is hard to find. Rotomolding talents are growing by themselves in all enterprises in China, the number is not much, and plastic basic theory research in the blank, which is not good for the long-term development of the industry. In addition, the lack of talent, also make industry overall level is difficult to improve, has become China's weakness in rotomolding development..


Rotational molding technology research has made great achievements, but compared with injection molding, extrusion, blow molding, there is still many shortcomings. Development trend of rotational molding are mainly concentrated in the following aspects: 1) computer simulation of heat conduction in the process of heating and cooling, establish the corresponding mathematical model, using automation technology for precision control of various process parameters; 2) the solution to warping and contraction of the products, to eliminate air bubbles, shorten the processing cycle. More effective process improvement is to choose the appropriate time to mold pressurized (69 kPa), can shorten the processing cycle by 25%, bubble greatly reduced at the same time, improve the toughness of the products. In addition, to flow air into mold, reduce the temperature gradient between inside and outside wall of mold, and at the same time of strengthening cooling to reduce product crystallization structure caused by temperature difference caused by the differences of buckling deformation; 3) develop new technology, new materials, new equipment.


Foreign companies has made progress in R&D of resin and special additives. New developed resin fluidity and toughness is good, and resistance to environmental stress cracking can reach more than 1000h. Plastic foam molding, rotomolding crosslinking molding, plastic multi-layer composite forming technology has been developed.


Rotational molding is the most economic and effective method in the production of large-scale hollow plastic products. To increase the popularization and application in domestic, we must absorb foreign advanced technology, develop plastic raw materials, improve the automation level of roll-plastic equipment, shorten the period of mold design and manufacturing, making rotational molding, injection molding, extrusion and other processing technology go ahead in hand.


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