Rotational moulding: slow cycles, cheap moulds and awkward freight

Oven time sets the pace, and nothing speeds it up

Powder must be heated through the mould, fused against the wall and then cooled before demoulding, which takes a long time by the standards of any other plastics process. Because the constraint is heat transfer rather than machine speed, a plant cannot buy its way out with a faster press. Capacity is therefore added by arms, moulds and floor space. Schedulers group parts of similar cycle onto the same arm so that one slow item does not hold a whole rotation, and mixed-cycle loading is one of the most common causes of quiet capacity loss.

Labour is a larger share of cost than owners like to admit

Loading powder, fitting inserts, releasing clamps, demoulding warm parts, trimming vents and openings, fitting fittings and finishing surfaces are all manual. Automation exists but rarely pays at the volumes this process serves. The consequence is that wage levels, absenteeism and training turnover hit gross margin directly, and that a shop bidding for work in a higher-cost location must win on lead time, design assistance or proximity rather than on price. Many operators only discover their true labour content when they attempt to quote against an importer. A shop that cannot state its labour minutes per part, by product, is not in a position to know which of its customers it is subsidising.

Materials are narrow, and powder quality is not a detail

Most output is polyethylene supplied as a ground powder with a controlled particle distribution. Poorly ground or contaminated powder produces bubbles, pinholes and weak walls that only appear when a tank is filled and tested. Colour is added at the powder stage, so the range a shop can offer depends on minimum grind quantities rather than on customer preference. Additive packages for outdoor durability and, for potable or food applications, material approvals restrict which grades are usable. Chasing a cheaper powder without re-validating the wall is a reliable way to create warranty exposure.

Cheap tooling is the sales argument and the trap

A cast or fabricated aluminium mould costs a fraction of an injection tool, which is why small firms can commission a bespoke tank, planter or housing at all. The trap is that low tooling cost invites proliferation. Shops accumulate moulds for products nobody orders any more, occupying storage and quietly deteriorating, while customers assume no charge applies for keeping them. Sensible operators run an annual tool review, charge storage after a period of dormancy, and require a minimum order before returning a sleeping mould to the rotation. Charging a modest fee for a first article from a dormant tool also filters out enquiries that were never going to become orders.

The delivered cost problem

A moulded tank is mostly enclosed space, so a load fills a trailer long before it approaches a weight limit. Nesting helps for open shapes such as bins and pallets, but closed vessels barely nest at all. This gives regional producers a durable advantage over distant ones and shapes where plants are sited. It also means quoting ex-works misleads customers badly: two suppliers with similar unit prices can differ substantially once haulage is counted. Buyers should insist on delivered pricing to their actual site, and suppliers should quote it before a competitor does.

Frequently asked questions

Why is the unit price so high when the mould was so cheap?

Because the process trades tooling capital for running cost. Each part occupies an oven arm for a long cycle and needs manual loading, demoulding and finishing, so machine time and labour dominate the price in a way they never do in high-speed moulding. The saving on tooling is real and often decisive at low volumes, but it does not carry into the piece price. If your annual quantity grows substantially, ask a moulder to quote the same part for comparison.

Can inserts and fittings be moulded in, or should they be added afterwards?

Both are done, and the choice affects cost and reliability. Moulded-in inserts avoid a secondary operation and can seal better, but they must survive oven temperatures, be positioned accurately and not create stress concentrations as the part cools and shrinks. Post-moulding fitting is more flexible and easier to inspect, at the cost of extra labour and a joint that must be tested. For pressurised or potable applications, agree how the joint will be leak tested before the design is fixed.

How do we assess wall quality on a rotomoulded tank?

Section a sample. Wall thickness varies with geometry, and the meaningful checks are minimum wall at corners and around openings, absence of voids or pinholes through the section, and proper fusion on the inner surface. A poorly fused inner skin looks acceptable from outside and fails in service. Ask for impact testing on cut samples, confirm the grade and additive package used, and for potable applications require the material approval documentation rather than a verbal assurance.