Tumbling clamshell oven puts new spin on rotomolding.

Fast and efficient heating as well as improved control of product wall thickness are claimed for an unusual rotomolding machine that uses infrared heating elements in an oven that rotates 360 [degrees]. The PLC-controlled machine comes from Datome in Ollioules (Toulouse), France. Engineer Paul Meuret says the machine is intended for technical parts or special large parts like rafts. The oven is a closed container that closely envelops the mold or molds, which rotate inside it while the oven itself turns also. This unique arrangement places the heaters in close proximity to the mold and permits zoned heating - a capability not usually associated with rotomolding. These features allow you to put the correct amount of heat precisely where you want it, Meuret says.

Why should a rotomolder consider using electric heating when gas is often a less expensive energy source? Meuret replies that its electric infrared elements yield 40-50% thermal efficiency versus approximately 7% thermal efficiency for gas heating. Moreover, the quick on/off ability of the tubular quartz heating elements reportedly adds to both energy savings and precise heating control.

As an example of tight temperature control, Meuret says foamed parts can be made without the assistance of a drop box. This is accomplished by using a mixture of neat resin and resin blended with foaming agent. The two components are selected to have different melting characteristics. Molding is performed with a two-stage temperature profile that causes the neat resin pellets to melt first and "skin" the mold, followed by activation of the foaming agent.

Shoremaster Plastics Inc. in Fergus Falls, Minn., is the first U.S. rotomolder to use a Datome machine, which has been running since last November. Shoremaster is negotiating or a license to manufacture, sell, and service the machines here. The precise heating control is one major reason the company paid a premium over conventional rotomolding systems.

TAKE THIS OVEN FOR A SPIN

The molds are designed to rest in a special carousel frame that rotates on one axis inside the oven, which turns on an axis at 90o to the first. The rotisserie-type carousel typically has up to six mold-carrying arms. Each arm rotates independently of the oven's rotation. This arrangement ensures that no mold is shadowed by another, so all are heated uniformly. To provide additional heat for making especially deep parts, rod or panel heaters can be attached directly to the mold surface and powered through electrical connections on the mold. Up to four 48-in.-diam. molds can be placed in the machine at one time.

The oven is six-sided, and measures 7.8 ft diam. x 10.5 ft long, which Meuret says provides more capacity than a spherical oven with an 11-ft-diam. molding platform. The oven uses 150 30-in.long quartz-tube heaters with reflectors. The heaters are grouped into independently controlled zones of six elements. Meuret says the elements have a max. temperature of approximately 3636 F and heat up fast, helping to trim overall forming times by around 2 min.

Datome transfers mold carousels in and out of the oven using an overhead crane running on twin tracks. Cooling, demolding, and heating operations are set up in a linear pattern under the crane.

Datome recently came up with an alternative mold-handling set-up. In this four-station mold-transfer system, each mold carousel is held by one of four pairs of electronically operated arms that are arranged in a pinwheel and rotate around a common center. At the first station, the arms tilt into the oven and pluck out the mold carousel or insert a new one. The arms index to the cooling, demolding, and charging stations.