Design for Rotomoulding: A Practical Guide for Product Designers

Why Rotomoulding?

Rotational moulding (rotomoulding) unlocks the ability to create large, strong, seamless plastic products that other processes simply can’t. Tooling is generally more affordable compared to other plastic moulding processes like injection or blow moulding - meaning your custom product can become a reality with much lower upfront costs.

We can manufacture small parts (our record is 70 grams), up to very large (more than 1000kg in weight and 7m long). At Melro, we specialise in product and tooling design for rotomoulding, with an in-house team of engineers and designers ready to bring your custom plastic product to life.

Alongside the steel moulds that we hand-fabricate at our Pakenham site, we also work with a trusted network of external toolmakers. That means we can match the right tooling solution to your budget and application.

Your vision, backed by our experience, leads to products that meet your needs. Whether this means achieving some specific performance goals or minimise tooling, production, and freight costs, all while ensuring your product comes out exactly as intended.

The earlier we start working together, the smoother the process and the better the outcome.

1. The Rotomoulding Process – in Brief

Plastic resin is placed into a hollow metal mould which is heated from the outside whilst rotating. The resin melts and coats the inside of the mould. Once its finished “cooking”, it’s removed from the heat and continues to rotate whilst cooling.

After cooling, the part is removed: a seamless, stress-free hollow product. The cycle time is slow compared to other moulding processes and depend upon the thickness of the plastic – normally at least 60 minutes to make a part, but could be up to 7 hours.

One way to increase the production rate is to run multiple identical moulds simultaneously. Polyethylene (PE) is the most commonly rotomoulded material worldwide because of its great combination of chemical resistance, wide processing window, mechanical properties, availability and etc. There are a variety of materials available for more demanding applications including XLPE, PP, Nylon, PVDF, ETFE, ECTFE and etc.

• Seamless hollow shapes are possible
• Wall thickness is relatively uniform, but can also be varied within the same part
• Low internal stresses = toughness and impact resistance
• Flexible batch sizes
• The size is limited to what can fit on a rotomoulding machine, and we have a variety of machines which means we can better choose the most suitable machine for your product
• Its common for a single rotomoulded part to replace multiple other parts
• There is no residue left inside a mould after demoulding, so colour changing from shot to shot is not a big problem
• It is possible to mould in metal “inserts” into parts
• Multi-layer moulding is possible, including using PE foam for stiffness or insulation

2. The Realities of Rotomoulding Design

This is where many first-time designers trip up. A few realities to know:

• Parting lines are unavoidable. Every mould has at least one split line depending on how it has been designed, and that faint line will be visible on the final part. If this matters to you, plan to put it in a discreet location, blend it into a feature, or pick a mould texture that disguises it.
• Breather holes are (normally) mandatory. Every part needs at least one. These can be hidden in functional features or trimmed out areas, or covered up post moulding but they cannot be eliminated.
• Corners should be generous. Sharp inside corners can create thin spots, but outside corners form thicker by default. Larger radii are generally preferred.
• Draft angles/shrinkage. A PE rotomoulded part will shrink ~3% smaller off the mould, which means no draft is required on external surfaces. Internal surfaces normally require at least 1–2°. Draft can also make nested stacking and storage possible.
• Tolerances are looser than injection moulding. There are various methods to improve tolerances where precision is critical.

3. Wall Thickness & Structural Design

• Typical thickness range: 3–25mm for a single skin, but skin-foam-skin parts can be above 100mm in thickness.
• Transitions: It is possible to vary the thickness within the same part.
• Reinforcement: Handles, ribs, and bosses can be moulded in to add stiffness without thickening the whole part.

4. Materials That Work

Most projects use polyethylene (PE) – durable, chemical resistant, impact-tough. But there are alternatives. Choosing resin affects tooling, processing, and recycling.

• Crosslinked PE (XLPE): very tough, but not recyclable
• Polypropylene (PP): rigid and high temp, but more brittle compared to PE
• Specialty resins (nylon, PVDF, ETFE, ECTFE): high-performance, higher cost
• Recycled materials: environmentally friendly - we can source resins made with varying percentages of recycled post-consumer or post-industrial waste

5. Moulded-in features

Designing with rotomoulding allows unique opportunities and there is a very wide variety of options possible.

• Threads can be moulded directly into your part, eliminating the need to add fittings post-production
• Permanent UV-stable graphics (logos, certification info, QR codes, etc.) can be moulded in for branding and traceability

6. Tooling & Cost

Steel moulds – Normally hand fabricated from sheet metal which means you are limited to shapes which can be rolled, folded, pressed, welded and etc. Especially suitable for larger parts with less complex geometry. The costs of some moulds can be surprisingly affordable and we have literally fabricated moulds and been moulding parts within a few days for customers in some cases. Generally, a larger mould is more expensive, but it is mostly a function of the fabrication time.

Aluminium moulds – Less restrictions on geometry compared to hand-fabricated moulds, but often at a higher cost. Gives a lot more flexibility to a designer because you have less restrictions. Cost is a function of the surface area of the mould. Regardless of the tooling type, every insert, undercut, or moving part increases tooling price. Investing properly in tooling upfront saves you from poor mouldability, costly rework, or even full replacement later.

7. Product Testing & Standards

Rotomoulded parts can be engineered to meet demanding standards – but this must be designed in from day one. Depending on application, you may need:

• Minimum/maximum wall thickness at critical locations
• Dimensional tolerances for flatness, flange faces, or fit points
• Impact testing with defined criteria (sample location, temp, energy)
• Leak tests (air, water, submersion)

If you don’t specify these up front, we’ll build to reasonable care – but “reasonable” may not be what your end customer expects.

8. Post-Processing & Assembly

Your design doesn’t finish when the mould opens. Plan for:

• Trimming: outlet holes, edge cuts, clean finishes
• Fabrication & Fitout: adding accessories, fittings, or assemblies
• Branding & Graphics: in-mould graphics, decals, or post-paint
• Packaging & Handling: will it stack, ship, and arrive intact?

How Melro Helps Designers Early

We don’t just “take your drawing and mould it.” We partner with you to:

• Audit designs before committing to tooling, including using FEA to optimise designs
• Recommend resin, wall thickness, and structural features
• Advise on inserts, parting lines, and breather placement
• Prototype and run trials to de-risk full production
• Build tooling that balances cost with durability

When you involve us early, you avoid the pain that comes from “designing in isolation.”

Ready to Start Your Project?

Bring us your idea and we’ll help you turn it into a real-world product that works.