Injection Moulding VS ROtoMoulding

How do the processes compare?

Differences in materials used

Firstly, let’s look at the materials involved in each technique. Rotomoulding largely uses polyethylene (PE) – a light, versatile resin which is recyclable and can have several different crystalline structures, including linear low-density (LLDPE) and high-density (HDPE).

Polyethylene is the most widely used thermoplastic in the world, accounting for around 30 per cent of all plastics, owing to its low cost, versatility, easy processability, excellent electrical insulation properties, toughness, flexibility and non-toxicity, to name a few properties.

Rotational moulding can also utilise materials such as polypropylene (PP), PVC, and nylon, but this is less common. 

Injection moulding, on the other hand, utilises a broader range of thermoplastics, including: 

• Acrylonitrile Butadiene Styrene (ABS) – an opaque thermoplastic known for its tough and impact resistant properties. It’s commonly used in electronic housings, auto parts, pipe fittings and, importantly, Lego toys!

• Polycarbonate (PC) – a transparent thermoplastic which is lightweight and durable. It’s often used as an alternative to glass, in roofing or safety eyewear.

• Polyamide – a synthetic polymer made of petroleum-based plastics, which is wear-resistant and low cost.

• Polystyrene – a lightweight, versatile synthetic polymer commonly used in packaging and construction due to its insulation and protective properties.

Injection moulding can process both thermoplastic and thermosetting polymers. Whilst thermoplastic can be remelted and remoulded once set due to its low melting point, and offers better dimensional stability, thermosetting plastic cannot be reformed once hardened, making it typically stronger.

Tooling costs

Injection moulds are typically made of aluminium or stainless steel, both of which are expensive and are not easily altered once created. This can become an issue if a customer requires a design change such as adding extra material to a part. To do so, you are required to decrease the size of the tool cavity by adding aluminium or metal to it. This is extremely challenging with this type of material and in many cases may mean scrapping the tool entirely and starting over.

In contrast, due to the lack of pressure needed during this process, rotational moulds tend to be comprised of steel or aluminium. This makes them significantly less expensive and easier to change.

Production times and costs

Looking at rotomoulding first, due to the nature of its heating and cooling cycles, the process was historically restricted to only one or two cycles completed each hour. This limits production volumes and can result in extended production timelines. However, with the introduction of electrically heated tooling and full integrated robotic production cells, this can now be scaled up to bring the process production times and energy usage down. That said, the time needed to reach the production stage is typically shorter than in injection moulding, which can be several months depending on the project.

The tooling and material costs involved in rotational moulding are lower, making it a cost-effective method for small to medium runs. This means that it can produce low to medium batch numbers, which may better suit the customer’s needs and result in less waste.

The production times in injection moulding are much faster, producing large quantities of product quickly. This means that although injection moulding has higher initial tooling costs, when produced at high volumes it offers lower per-part costs than rotomoulding. This also makes it less suitable for projects that require smaller runs. 

Complexity and design flexibility

Rotational moulding has high design flexibility, allowing for the creation of complex shapes, sizes and features that are impossible to achieve with other manufacturing methods. Dimensional stability also supports the production of large, hollow parts, making it ideal for storage vessels, for example. Additionally, the process’s flexibility facilitates the easy incorporation of features like inserts, threads, and double walls.

The nature of the rotomoulding process also means that particularly distinctive shapes and other complex geometries can be made in a single, seamless piece, rather than built from several individually manufactured components. As a result, the finished object doesn’t have as many stress points, which reduces the risk of fractures.

Injection moulding, on the other hand, is a high precision method far better suited to repeatability, allowing it to produce high volumes of identical parts, which is a huge advantage in terms of delivering consistency on a large scale. Injection moulders can achieve designs accurate to within +/- .001 inches – results which are consistent across multiple production runs.