known also as rotomolding or rotocasting,
is a process for manufacturing hollow plastic products. The process consists of introducing a known amount of plastic in powder, granular or viscous liquid form into a hollow, shell-like mold. The mold is rotated and/or rocked about two principal axes at relatively low speeds as it is heated, so that the plastic enclosed in the mold adheres to, and forms a monolithic layer against, the mold surface. The mold rotation continues during the cooling phase so that the plastic retains its desired shape as it solidifies. When the plastic is sufficiently rigid, the cooling and mold rotation is stopped to allow the removal of the plastic product from the mold. Rotational molding has particular advantages in terms of relatively low levels of residual stresses and inexpensive molds. Rotational molding also has few competitors for the production of large (> 2 m3
) hollow objects in one piece. The main advanteages of rotational molding are:
Normally, the only controls on the process are the oven temperature, the time in the oven, and the rate of cooling, and variations in any of these parameters will result in incomplete fusing and consolidation. This results in low strength, low stiffness, lack of toughness in the end product and brittleness. Rotational molding does not rely on centrifugal forces to throw the plastic against the mold wall. The speeds of rotation are slow, and the powder undergoes a regular tumbling and mixing action. Effectively the powder lies in the bottom of the mold and different points on the surface of the mold come down into the powder pool.
A meeting of the Association of Rotational molders in USA brought to light some new developments in the process of rotational molding technique that focus on fast moulding cycle that improves output.
One of the new approaches to faster cycles focuses on resin modification. A Schulman has developed a novel resin modification that reportedly brings the melt to an optimum cure in a shorter time. By attaining optimum cure, Schulman's proprietary Better Parts Faster (BPF) compounding technology is said to attain the best physical properties while shortening oven cycle times up to 25% and decreasing cooling times about 10% compared with conventional methods. BPF was developed to help moulders increase throughput without additional labour or equipment. The technology can be used when a moulder is running several parts of different thicknesses on a single arm. BPF assists by curing the thicker parts faster to bring them more in line with thinner sections' cure times. Also, molders running parts with dissimilar colors can rectify throughput imbalances. BPF technology can be compounded into any polyethylene, regardless of MFR or density. There is reportedly no change in physical properties of the resin.
A new oven less moulding concept in which the mould is electrically heated via a conductive coating was unveiled by Innovex Systems, a supplier of custom machinery and automation for material preheating and dispensing, mould clamping, and parts removal. Its' new HeatPaint Rotomolding system heats and cools the part without an oven or cooling chamber. The mold is painted with a new nano particle conductive coating that receives current via slip rings that carry the power to the mould. The coating can be heated to 225 degree C in 30 seconds. The coating reportedly can take continuous use at 350 degree C and short heat cycles at 425-450 degree C. Only the mould gets hot, so a center spider arm can carry electronic controls that send real-time data to the operator. Innovex is developing a fully automated system that includes automatic mould charging with preheated material. PC software will provide zoned temperature control of the mould and data acquisition.
Currently, the rotational molding industry is at an exciting stage of development. The past decade has seen important technical advances, and new types of machines, molds and materials are becoming available. The industry has attracted significant investment from major suppliers. Important new market sectors are opening up as rotational molders are able to deliver high quality parts at competitive prices.