Moulds, Machinery & Fittings
Machinery
Machinery for rotational moulding comes in two basic categories, with the most efficient method using a heated oven. Whether the cycle occurs in or out of the oven, the rocking action necessary to cover the internal surface of the mould is typically described as either bi-axial or rock ‘n roll. Bi-axial rotation involves full rotations around two axes, offering flexibility in the moulding process. Rock ‘n roll rotation is limited to partial rotations on two axes, with the rocking angle crucial for ensuring the powder pool covers the entire inside surface of long, narrow parts.
For specific applications, open flame machinery can offer a good return on investment. While cycle times are usually longer and parts can be more affected by ambient temperature, open flame equipment is a popular alternative when oven cycles are not feasible.
Clamshell machines are single-arm machines that run one part at a time. They are more efficient than open flame machines but may not match the production capacity of multi-arm machinery. Clamshell machines require the least amount of factory space.
Carousel machines usually feature three or four arms on which the moulds are mounted. These arms rotate the parts bi-axially and move through various stations—loading, heating, cooling, unloading, and reloading for the next cycle. Carousel machines are excellent for energy-efficient, high-production manufacturing but require the most floor space.
Shuttle machines typically have two carts on either side of the oven, each carrying mould arms. These carts move in and out of the oven alternately, allowing one cart to heat while the other is unloaded and reloaded. Shuttle machines need less factory space than carousel machines but may not produce as many parts.
Moulds
Most moulds for rotational moulding are currently made from metals such as sheet steel, aluminium, cast aluminium, or CNC machined metals. These moulds are thin, shell-like structures designed to allow rapid heat transfer from the heat source through the metal into the powder.
Large, simple moulds are generally fabricated from metals, while highly detailed parts requiring a high-quality finish use cast metals or nickel electroformed moulds. Since the process doesn’t rely on pressure to form the part, the mould only needs to be strong enough to support itself. As the material cools, it naturally shrinks away from the mould's surface, so mould design must account for this to create an accurately sized final part.
Most moulds are made in two pieces, but for complex designs, three- and four-piece moulds can be used. The area where the mould pieces connect is known as the parting line, which can have a complex curvature. The parting line is crucial because the mould sections must remain tightly clamped together during the heating and cooling process. Careful handling is essential to avoid damaging the parting line.
Moulds are typically mounted to frames in halves for placement on the machine. Sometimes, multiple moulds are placed on the same frame, known as a spider, which can significantly reduce demoulding and charging times, thus reducing cycle times.
Moulds must be vented to allow heated air to flow freely in and out during the process. If the venting tube is too small or poorly designed, the mould can experience pressure issues, potentially causing it to explode or implode. The vent tube must also be deep enough to project through the powder pool to prevent material from flowing out during rotation. A good guideline is a 13mm vent tube for each 1m³ of mould volume.
Moulds can also be surface-treated to provide various textures on the final products. The outside of the mould can be treated with methods such as adding pins, roughening, or using black paint to increase cycle times.
Mould Release
Mould release agents reduce the natural tendency of the polymer to adhere to the mould surface by lowering the surface tension. While some adhesion is necessary to ensure the part forms correctly, excessive sticking can damage or ruin the part, so it must separate easily from the mould at the right moment.
There are various types of release systems, including permanent treatments applied to the mould, reactive and conventional chemical systems, and hybrid systems.
Release agents can be effective for a single cycle or multiple cycles, and they can influence the part's shrinkage rate and finish quality. Over time, release agents build up on the mould, and the rate of build-up depends on the type of release system used. Moulds need regular cleaning to remove any build-up when using temporary systems.
Fittings and Inserts
Rotational moulding can incorporate various inserts and components into the final product design. These can be added after the main part is produced or included as permanent fittings, known as inserts.
Both plastic and metal inserts can be used, with the choice depending on the part's dimensional tolerance, wall thickness, and mould design. During manufacturing, metal inserts can overheat and affect the final part, so it may be necessary to shield them from heat to prevent overcooking the surrounding material.
Handles can be designed to form from the material or added similarly to inserts. Metal plates can be added for identification, and threads can be included. These specialised design elements rely on the correct flow of the powder material to ensure a crisp, clean part. Good design and a high-quality mould are essential, but the additional mould cost is usually offset by producing a one-step part with minimal post-production work.
Various fittings can be added post-production, including threads, taps, faucets, screws, and other features to aid in the final part's assembly.
