Rotomoulding can be used to create thick-walled plastic products in a variety of sizes and materials. It is a versatile process that allows for complex shapes without seams or joints. While more expensive than blowmoulding for high volumes, rotomoulding is well-suited for low volume or customized parts due to relatively low setup costs. Proper design, material selection, and manufacturing techniques are keys to optimizing the strength and lifespan of rotomoulded products.

1. Frequently Asked Questions About Rotational Moulding
At Francis Ward, we are often asked a variety of questions relating to rotomoulding, our
processes, products and capabilities, so we have put together a list of the most frequently
asked for your information.
However if you cannot find the answer you require please get in touch.
What thicknesses of plastic can be moulded using rotomoulding?
It is possible to mould very thick wall sections using the rotational moulding process.
Large capacity tanks have been moulded with wall thicknesses of up to 25mm.
For most products a wall thickness of 4-6mm is considered normal.
What plastics or other materials can be rotomoulded?
The most commonly moulded plastic is linear polyethylene. This is available in Linear Low
Density (LLDPE), Linear Medium Density (LMDPE) and High Density (HDPE) forms.
Crosslinked Polyethylene (XLPE) is used where exceptional low temperature impact
strength is required in moulded parts.
Ultra Low Density Polyethylene (ULDPE) can be used for parts requiring extreme flexibility
and low temperature performance.
Other materials include Polypropylene (PP), Nylon (PA) and Fluoropolymers – Poly Vinyl
Difluoride (PVDF), Ethylene Chlorotrifluoroethylene ECTFE. These are more expensive
materials than polyethylene.
Fluoropolymers are typically 30-60 times the price of polyethylene and are used only where
extremely high chemical and mechanical performance is required.
What colours can rotomoulded products be made in?
Francis Ward provides a standard colour range but any colour can be formulated if a
specific colour match is required.
Special effects are also possible including stone effect and metallic effect.
With customised mould textures, and post mould finishing, wood effects and rock effects
can be reproduced.
How are plastics coloured?
The best method of colouring rotomouldable polymers is compounding. This involves
melting the polymer granules, adding a pigment Masterbatch powder, extruding the
molten mixture, cooling the extrudate and pelletising.The coloured pellets are then ground
to powder for moulding.
Compounding provides a homogenous mix with pigment well dispersed and encapsulated.
The pigment when added to the polymer in this way has little or no effect on the base
properties of the polymer.

2. Alternatively, natural powder can be ‘dry-blended’ directly with coloured pigment, usually in
a high speed mixer. This can be slightly cheaper than compounded colour, but reduces the
need for stock of many colours of compound.
However, compared to compound colour, the individual particles of dry-blended colour
produce a weaker product and one that has a colour that is less light-fast. For these
reasons, Francis Ward never recommends the use of dry-blend in rotomoulding.
Francis Ward commonly uses a compound blend technique which involves the addition of a
percentage of compounded powder to natural powder in the mould. Mixing is achieved
during moulding and the resultant parts have good colour with good mechanical
properties, and the cost of coloured parts is reduced.
What radiuses can be used on rotomoulded products?
The use of correct radiuses on the corners of rotomoulded parts is critical.
Sharp corners with tight radiuses are stress concentration points in moulded products and
commonly are impact fracture propagation points.
The correct radius to adopt will depend on the part to be moulded. Large parts may
require greater radiuses. Preferably a minimum of 5mm should be used.
This is particularly important on any concave internal corners in the product.
Why are rotomoulded plastic products more expensive than blowmoulded?
Rotomoulding is a slow process in comparison to blow moulding and is therefore ideally
suited to low volume production of large parts.
It is also a very manual process compared with blowmoulding, which is largely an
automatic process. It can be economic to produce small parts in high volumes, if multiple
moulds are employed, or if small parts can share a moulding cycle with larger parts.
The advantages of rotomoulded parts over blow moulded parts are chiefly design
flexibility, no seams or welds, and very low inherent built in stresses in moulded parts.
Wall thicknesses can be much greater in rotomoulding than blowmoulding. Also
rotomoulding set-up and mould costs are a fraction of the cost of equivalent
blowmoulding alternatives.
How much do moulds for rotomoulding cost?
The cost of moulds varies according to the size and complexity of the moulded part.
A simple steel mould for a 20-litre part, may cost as little as £1500, and a simple 200 litre
steel tank mould will cost £4000-£6000.
At the other end of the scale, large complex steel moulds with multiple pieces may cost in
excess of £100,000.

3. Where part to part dimensional reproducibility is critical, or high surface finish is required,
aluminium moulds can be produced, these are more expensive than moulds fabricated
from mild steel sheet.
In general moulds for rotational moulding are less expensive than those used for
blowmoulding or injection moulding.
How many products can I make off one mould?
The life expectancy of moulds depends on the quality of manufacture, gauge of metal
used, weld effectiveness etc.
Francis Ward has been able to produce more than 100,000 parts from some of its smaller
steel moulds.
Larger moulds, which experience greater moulding and de-moulding stresses, will produce
far less parts before the need for repair or refurbishment.
A 1000-litre capacity steel mould may produce up to 10,000 parts before needing
significant attention.
Aluminium moulds are not subject to such stresses but they are formed from a softer
metal that can be scored or suffer abrasion, so maintenance of aluminium moulds is
usually more frequently carried out than that carried out on steel moulds. They also require
care when cleaning, particularly on mating surfaces.
Can I have flat surfaces on my products?
It is very hard to produce large flat areas in rotationally moulded parts.
Distortion during part shrinkage and cooling is common and difficult to prevent.
Some jigging of parts and control of cooling will reduce distortion, but it is recommended
that flat areas are not designed into parts.
Straight sides can be achieved by using ribs, but it is always best to consult your
rotomoulder on how to establish the best shape for your product.
How can I make my rotomoulded product stronger?
There are a number of ways to ensure your parts are as strong as possible.
● Your part design should have correct radiuses and wall thicknesses. It is possible to
use computer modelling, such as Finite Element Analysis (FEA) to assess the stress
characteristics of a design and adjust where necessary.
● Select the best material to achieve the performance you need. LLDPE is tougher
than LMDPE which is tougher than HDPE. Rigidity increases with density but
toughness reduces.
● If the parts are coloured, make sure that you are using the best colouring methods.
● Optimise processing conditions. Undercooked and overcooked mouldings will have
reduced mechanical and impact strength.

4. ● Use a competent and experienced rotational moulder who will be able to guide you
through all of the above.
What are the minimum and maximum size of moulds?
Due to the capabilities and machinery available at Francis Ward, we are able to create
moulds that range from 0.5L to 40,000L capacity.
What accreditations should a rotomoulder have?
When selecting a rotomoulder, ideally you should ensure they have ISO 9001, which will
provide you with confidence in their quality management systems.
In addition they should be members of the Association of Rotomoulders (ARM),
represented in the UK by the BPF (British Plastics Federation).
Francis Ward are also proud to be a UN-certified packaging manufacturer.
How is rotomoulding more environmentally friendly than other plastic moulding
processes?
Rotomoulding is more environmentally friendly in a number of ways.
For example:
● There is minimal waste as the raw material is used up in each moulding.
● The most commonly used material - polyethylene - is the most environmentally
friendly in terms of manufacture.
● It can also be recycled / processed to make new rotational mouldings or it can be
used to generate heat through incineration.
How do I go about creating a design for rotomoulding?
We have over 50 years of experience in rotational moulding so are happy to assist with the
production of design concepts based on your requirements.
We also work with partners to create 3D CAD drawings and can then manage the entire
process end-to-end from management of toolmaking, sampling and prototyping, in-house
fabrication and production.
What are the advantages of using a rotomoulded tank for the storage or transportation of
liquids?
There are a number of factors that make a rotomoulded product ideal for the storage or
transportation of liquids:
● Moulds are biaxially rotated at low rotation speed to produce one piece hollow
mouldings.
● Due to the low pressures involved, and the rotation of the powder as it melts and
shapes, you can achieve containers that have no seams or joints, and effectively no
built-in stress.
● Wide range of materials to select from to give product optimum resistance to
liquids, solids, chemicals and weather conditions.
What are the maximum temperatures that a rotomoulded product can stand?

5. This depends on the type of material used in the production of the rotomoulded part, and
also depends on other factors such as length of time exposed to heat and any pressure
that is also being applied.
As a guide however, polyethylene is suitable up to approx 60°C, polypropylene to about
100°C, and nylon 120°C.
How long can I expect a rotomoulded product to last?
This very much depends on the application, however due to the strength and resilience of
rotomoulded products some can last over 40 years.