PROFESSIONAL BLOG
SEPTEMBER 04, 2026 • 7 MIN READING
Large, hollow and complex? Rotational molding may be the answer
How do you produce a large, hollow plastic component — without molding several separate parts that then have to be welded, screwed or glued together?
The answer may be rotational molding.
Arild S. Johnsen
CEO, Plexx
Through our partner SP Gibo, Plexx can offer rotational molding of large, complex plastic components. The method gives you considerable design freedom, and it is worth considering whenever a product needs to be robust and hollow, and is made in small to medium-sized runs.
What is rotational molding?
Rotational molding works differently from conventional injection molding.
Instead of forcing molten plastic into a tool under high pressure, plastic material is charged into a hollow mold that is then heated and rotated. The material distributes itself along the inside of the mold and gradually forms the finished component.
The result is a large, hollow component in one piece, with no joints. The process gives good control over wall thickness, which can be adjusted and optimized to suit the design and the requirements of the finished product.
That opens the door to products and structures that can be difficult or costly to make by conventional injection molding.
01 - Filling
An accurately weighed amount of plastic powder is placed in the open mold, which is then closed.
02 - Heat and rotation
The mold rotates about two axes inside an oven. The powder melts and coats the inside of the mold evenly.
03 - Cooling
The mold is cooled while it continues to rotate, so the material solidifies to an even wall thickness.
04 - Demolding
The mold is opened and the finished part is removed — hollow, in one piece and without joints.
When size becomes a challenge
Rotational molding becomes especially relevant once components get large.
Tanks, vessels, enclosures, covers, ducting, components for machinery and vehicles, and other large hollow structures are typical examples.
SP Gibo has dedicated capacity for large hollow plastic components and complex geometries. Wall thickness can be adjusted and optimized to suit the design and the requirements of the product.
Threaded inserts in brass or stainless steel can also be molded directly into the component. That gives you solid fixing points for mounting other parts, and can reduce the need for machining and assembly afterwards.
This makes the technology relevant in sectors such as manufacturing, agriculture, transport, heavy vehicles and renewable energy.
"A product that today consists of several parts welded together can often be molded as a single hollow component — without a single joint."
Could several components become one?
One of the most interesting opportunities comes up before production even starts.
Think of a product that today consists of several plastic sheets or components that have to be cut, welded, screwed or glued together.
Could the whole structure be made instead as a single hollow, molded component?
In many cases the answer is yes.
That can cut the number of parts, the assembly work and the number of joints — and at the same time make the structure both simpler and more robust.
This is why we like Plexx to be involved while the product is still on the drawing board.
Lower tooling costs can make smaller runs viable
For conventional injection molding, the investment in a large, advanced mold tool can be substantial.
Rotational molding runs at considerably lower pressure, so the tools can be built differently.
That can mean lower tooling costs, which in turn can make the technology attractive when volumes are not high enough to justify the investment some other molding techniques require.
For many Norwegian industrial companies, that is exactly the situation. You may need hundreds or a few thousand components — not necessarily hundreds of thousands.
Color can be built into the product itself
Another advantage is that the component can be produced with the color you want directly in the material.
In some projects that reduces or removes the need for painting or other surface treatment afterwards.
It can both simplify production and give you a surface where the color is not just a layer on the outside of the product.
More than just the molding
A good plastic product rarely starts at the production machine.
Design, material selection, tooling and production method all have to work together.
Through SP Gibo, Plexx can therefore also offer expertise in design and DFM, mold tool development and prototyping, CNC machining and finishing, and assembly and delivery of complete units.
That means a component does not have to be delivered as a raw plastic part. It can be machined, drilled, trimmed, joined to other components and delivered as a finished unit, ready to fit at your site.
Could your product be designed more simply?
That is often the more interesting question.
Not: “Can you make this part?”
But: “Is there a smarter way to design and produce it?”
If you have a large tank, enclosure, vessel, duct, machine part or other hollow structure that today consists of several components, rotational molding may be worth looking into.
Send us a drawing, a 3D file or a description of the product.
Then we can look together at whether rotational molding — or one of our other production methods — could give you a simpler and more cost-effective solution.
Questions and answers
Rotational molding is a method in which plastic material is charged into a hollow mold that is heated and rotated. The material distributes itself along the inside of the mold and gradually forms the finished component — in one piece and without joints.
In injection molding, molten plastic is forced into a mold tool under high pressure. Rotational molding runs at considerably lower pressure, the tools can be built differently, and tooling costs are often lower. That makes the method attractive for small and medium-sized production runs.
Typical examples are tanks, vessels, enclosures, covers, ducting, components for machinery and vehicles, and other large hollow structures. The method is relevant in manufacturing, agriculture, transport, heavy vehicles and renewable energy.
Yes. A structure that today consists of several plastic sheets or components that are cut, welded, screwed or glued together can in many cases be produced as a single hollow, molded component. That reduces the number of parts, the assembly work and the number of joints.
Yes. Threaded inserts in brass or stainless steel can be molded directly into the component. That gives you solid fixing points for mounting, and can reduce the need for machining afterwards.
Not necessarily. The component can be produced with the color you want directly in the material, so the need for painting or other surface treatment can be reduced or removed.
Through SP Gibo, Plexx can offer expertise in design and DFM, mold tool development and prototyping, CNC machining and finishing, and assembly and delivery of complete units.
Do you have a challenge you would like to discuss?
We advise you so that together we can find the optimal solution – from material selection to finished product.