Widely used by the industrial sector, rotomolded plastic represents 3% of the plastic market, a sector that is booming - it grew by 2.5% in 2017 and is expected to advance 3% in 2018, according to Brazilian Plastic Industry Association (Abiplast).
An example of this expansion is in the North American market figures. In 2018, in North America, sales in rotational molding already reached the $ 2.46 billion mark (Plastic News). There are several reasons for the sector to continue growing, mainly due to the variety and flexibility of the design of the parts and the low waste of materials provided by rotational molding.
What is rotomoulding
Rotomoulding, also known as rotational casting and rotational molding, is a plastic transformation process for the production of small, medium and large hollow parts, with quality and a high level of mechanical resistance.
The method has evolved in recent years and already allows the production of items of high level of complexity with a lower cost in relation to other techniques, such as blowing and injection, since it requires less investment in tools and equipment.
With the rotational molding process, it is possible to manufacture parts from different materials, such as polyethylene, polypropylene, liquid PVC, naylon, etc.
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Basically, the rotational molding process involves four steps:
- Loading: the mold is loaded with a predetermined amount of material.
- Heating: the molds are rotated biaxially inside a heated oven, so that the material is distributed and melted on the internal surface of the mold;
- Cooling: still in rotation, the molds are removed from the oven and placed in a cooling station; the removal of heat from the molds is done by ambient air, forced air or assisted by water spray (fog).
- Demoulding: after the predicted time for the solidification and cooling of the pieces, the rotating movements are stopped and the molds are opened, so that they are removed.
Discover the many advantages of rotational molding over other thermoplastic transformation methods
There is no need to apply pressure on the plasticized material for molding, which occurs basically by the action of the force of gravity; in all other processes, the material is subjected to great shear forces to be formed.
Low cost in the production of molds, as there are no high molding pressures involved.
The frequent absence of microstructural problems is created by material shear and uneven cooling, such as unfavorable molecular orientations, welding lines, and internal residual stresses, making rotomolded products more resistant to impact than blown injected products.
Uniform and controllable distribution of molded wall thickness; significant changes in the thickness of injected parts are impossible without modifications or the manufacture of a new mold, while in rotational molding this is achieved only by varying the amount of material loaded in the mold; blown and thermoformed are usually thinned in the corners due to stretching.
No burrs, characteristics of the blowing and injection processes, respectively, which require reprocessing expenses.
Several parts of the same part can be produced on a single machine, which makes production much more economical.
How more modern machines optimize production
Currently, rotational molding machines offer automatic control in the production process, either in supply or cooling. Besides that, cylindrical ovens enable more homogeneous air flow in all areas of the oven, generating greater thermal efficiency and greater gas and energy savings.
The most modern machines offer a double temperature control system, one inside the oven and the other fixed close to the mold, precisely measuring the temperature, exactly where it must be calculated and adjusted. The information obtained by the sensors is sent directly to the machine's operating system, making the software itself adapt the ideal time for both the oven cycle and the cooling cycle.
Another relevant feature found in high performance machines, is the system of double air lines in the arms, which allow the rotomolder to have much more control over the manufacturing process and better conditions to adjust it efficiently.
The entire manufacturing process is controlled by CLP, from multiple sensors, such as temperature and pressure, installed on the machine. Supervision software is able to automatically select, through the steps, temperature parameters, oven time, cooling time and rotation speeds of the arm / plate.
As we have seen, the rotational molding process makes it possible to manufacture highly complex parts at much lower costs, in an automated and very efficient way.
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