Rotational Molding: Economy and Versatility In A Single Technique

Rotomolding or rotational molding is a process whereby a hollow mold is filled with liquid or powdered resin and rotated in two directions in a hot chamber until the resin covers the surface of the mold and hardens into a piece. It is a cost-effective process that does not require stress on the resin and produces seamless, stress-free, hollow plastic parts with uniform wall thickness and extensive product design options. It offers great versatility in production and is an attractive option compared to similar conversion processes such as injection blown, thermoforming, or injection molding.

Rotational molding process

Rotomolding is a simple four-step process that uses a closed mold with thin walls and high heat transfer capacity.  It requires an inlet to feed the raw material to be rotated and must have an option to open so that the hardened parts can be removed.

In general, a known amount of dry plastic powder is rotatable positioned on two perpendicular axes at the same time. As the mold rotates slowly, the molten material adheres to the hot walls and evenly covers the entire surface.  This principle of operation makes it possible to provide the material with various important configurations for the production of pieces of different sizes and to distribute them in a uniform thickness over the entire surface.

Mold filling: add the required amount of plastic to the mold for processing, usually finely ground thermoplastic powder. However, the thermoplastic dispersion can be made into non-volatile solvents and some thermosetting resins. The entered amount of material corresponds to the desired weight of the last piece. The material consists of an open cold mold that is coated on the inside with a release material. Then close the mold and place it in the oven to heat.

Heating the mold: after the mold has been prepared, it is subjected to heat and simultaneously rotated on two vertical hinges so that the material can slowly adhere to the inner wall and melt into a continuous mass.  Melting is the process of assembling a solid. This does not mean that the material goes into a liquid state (melts) and then intensifies. Therefore, in the rotational molding process, the particles do not actually melt, but instead sin at the contact points until they form a porous three-dimensional network. Eventually, the viscous material fills the remaining pores through the capillaries.

The heat cycle is the longest step in the rotational molding process.  It usually takes between 7 and 15 minutes to heat the resin and dissolve the particles, although cycles of up to 30 minutes are known.  The duration of this step is directly related to the desired thickness of the final work, as the plastic acts as an insulator when applied to the walls of the mold, reducing heat transfer due to its poor thermal conductivity.  To material that has not yet been worked into the walls.

Mold Cooling: When all of the plastic material has assumed the internal shape of the mold, it continues to cool to harden and complete the desired properties.  At this point, the rotation will continue to keep the part uniform.  As a cooling medium, cold water can be used, which is distributed over the mold in droplets (to achieve rapid cooling), airflow with condensed water vapor (with slower cooling) or cold air (with slower cooling)?

Mold emptying: As soon as the piece is done it’s removed from that mold and another production cycle can begin. This last stage looks simple but may need some high-technology method when the created parts are large or intricately designed.

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