Foaming rotational moulded products, the how and why

Pentas is since their very first beginning foaming their products. Pentas Moulding has its roots in the production of surfboards. These from PE (polyethylene) rotated surfboards, due to the inherent of the rotational molding process, have a hollow shape. To give them the desired firmness these boards were filled with foamed PU (polyurethane) foam.


Pentas does not deny its origin. The specialism to foam rotational moulded products, in close collaboration with various partners, continued to develop after the surfboard period. This resulted in the fact that Pentas has mastered the full foaming process of rotational moulded products. Pentas applies this technique on products that varies in their application, such as for the stabilization of transport containers, strengthening of hospital table tops, insulated totes / valves, floaters and for tanks wherein stored liquids may not freeze..


Why is it necessary to foam certain products?

  • It provides rigidity, if the product is foamed with PU, the hollow product transforms to a monolithic whole causing little to no movement in the product. This is ideal for luggage trays that are rushed at a high speed on a transport system or for table tops where you’d like to have a stable leaf.
  • It is insulating: because of the structure in the foam it has an insulating effect. This feature comes in handy for the use in insulation bins to keep something cold or hot.
  • It is impact-resistant: because the hollow product is fully foamed with PU foam the walls of the hollow product are strengthened from the inside. This improves the impact resistance; useful for roofs of trucks / tractors, as something could fell on the rooftop.


The foam process briefly explained:

Step 1: For each product that has to be foamed will a custom-made mold has to be developed. This mold ensures that the product retains its shape during the foaming process. In this step, the hollow product is laid in the foam mold, after which the foam mold is closed with clips.

Step 2: Now, the full product will be foamed with a mixture of Polyol and ISO. When these two components come into contact with each a reaction will accor that eventually leads to a foam structure.

Step 3: If the response in step two is completed, the foam mold will be opened agian and the product with a foam core can be removed.


Examples of foamed rotational moulded products

Currently Pentas foams about 50 different products. Below a small selection of the portfolio and the different reasons why they are foamed:



Luggage trays

These custom plastic trays are used at airports to transport luggage at a high speed and to briefly store them. For this tray to provide sufficient support and stability the PE hollow mold is filled with PU foam.
A PE exterior and PU (foamed) interior makes the tray a solid product, where each year thousands suitcases are smoothly, in fully automated conveyor systems, transported.



Bed table (medical)

For a supplier of medical furniture Pentas has co-developed a bed table. The bed table is used in medical institutions as a side table. The chassis slides easily under the bed and the tabletop is comfortably reachable for the patient.

The tabletop is foamed to give more strength to the hollow shape and also to make the top absolutely flat, so for instance a cup of coffee remains neat plane on it.



Coolbox with PU insulating layer

A cooling box is desirable for many agriculture machines and harvesters. The rotational molding process is by its great form freedom excellently suitable for the manufacture of such a cooler; often also a design element in the interior.

A cooler of only PE is not cooler. It is the PU foam that is responsible for the insulating effect in the walls.



Another example of a foamed product are the small floaters, they are often used as line of balls in public swimming pools and recreational lakes. By filling the floaters with PU the upward pressure is several times higher than in a solely hollow PE form.

Keeping the line of balls is thus more difficult, which naturally has a safety-enhancing effect.



  • Foaming is a process where all the variables needs to be correct exactly.
  • Type of foam
  • Temperature of the foam
  • Temperature of the product that is getting foamed
  • Venting of the hollow (rotated) form
  • Adhesion between PE and PU
  • Conservation dimensions PE form

With over 40 years of experience in both the rotational moulding and foaming process Pentas Moulding can call itself a true specialist in this field.

You are wondering if a rotated product, whether foamed or not, is suitable for your application? Then we are looking for YOU!