Question

Explain the rotational molding process. What are the process’s advantages and disadvantages? What type of part geometry is typically produced by this process?

Answer 1

ROTATIONAL MOLDING PROCESS :

  Rotational molding, known also as rotomolding or rotocasting, is a process for manufacturing hollow plastic products. For certain types of liquid vinyls, the term slush molding is also used.

1.heated hollow mold which is filled with a charge.

2.It is slowly rotated with two perpendicular axes.

3.During the heating phase and to avoid sagging or deformation also during the cooling phase.

4. it was a slow process restricted to a small number of plastics.

5. introduced to the rotational molding process in the early 1950s.

6.first applications was to manufacture doll heads.

7. The machinery was made of an E Blue box-oven machine, inspired by a General Motors rear axle, powered by an external electric motor and heated by floor-mounted gas burners.

8.This process of rotational molding led to the creation of other plastic toys.

9.This allowed large hollow containers to be manufactured in low-density polyethylene.

10.Rotational molding machines are made in a wide range of sizes. They normally consist of molds, an oven, a cooling chamber, and mold spindles. The spindles are mounted on a rotating axis, which provides a uniform coating of the plastic inside each mold.

11.all rotation molding systems have a number of parts including molds, oven, cooling chamber and mold spindles.

Order of materials most commonly used by industry:

• Polyethylene
• Polypropylene
• Polyvinyl chloride
• Nylon
• Polycarbonate.

Advantages:

• A hollow part can be made in one piece with no weld lines or joints ! The end product is essentially stress-free ! The molds are relatively inexpensive ! The lead time for the manufacture of a mold is relatively short ! Short production runs can be economically viable ! There is no material wastage in that the full charge of material is normally consumed in making the part ! It is possible to make multilayer products
• The process also has inherent design strengths, such as consistent wall thickness and strong outside corners that are virtually stress free.
• The process also has inherent design strengths, such as consistent wall thickness and strong outside corners that are virtually stress free.
• add prefinished pieces to the mold alone is a large advantage.
• It lies in the molds themselves. Since they require less tooling, they can be manufactured and put into production much more quickly than other molding processes.
• cost advantage with rotational molding is the minimal amount of material wasted in production.

Limitations& Disadvantages:

  The main disadvantages of rotational molding are: The manufacturing times are long ! The choice of molding materials is limited ! The material costs are relatively high due to the need for special additive packages and the fact that the material must be ground to a fine powder ! Some geometrical features (such as ribs) are difficult to mold.

• it is a low pressure process, sometimes designers face hard to reach areas in the mold.
• designers have to keep in mind that it is not possible to make sharp threads that would be possible with injection molding.
• heating and cooling involve transfer of heat first from the hot medium to the polymer material and next from it to the cooling environment.
• The rig for rotational molding usually has a relatively small wall thickness and is manufactured from metals with a high thermal conductivity (aluminum, steel).
• water cooling processes are possible, there is still a significant down time of the mold.
• the mold transfers much more heat than plastic can absorb; therefore, the mold temperature must vary linearly.
• The rotational velocity in rotational molding is rather low (4 to 20 rpm)

Ever since its inception, a characteristic feature of the rotational molding industry has been its abundance of innovative designers and molders taking what is basically a very simple, and some would say crude, process and creating complex, hollow 3-D shapes in one piece. Geometry and shape have to be used particularly effectively because, the dominant polymer, polyethylene, has a very low inherent modulus and thus stiffness. In order to impart stiffness and 6 Rotational Molding Technology rigidity to the end product it is necessary to use many types of special geometrical features, many of which are unique to rotational molding. It is also necessary to encourage the plastic powder to flow into narrow channels in the mold, and this only became possible with the special grades of high quality powders developed for the process and with the additional control over heating that became available in the oven machines.