Design guidance
Rota Design is passionate about ensuring that it’s clients achieve tangible business advantage via product and rotational moulding design. We continually strive to ensure our designs deliver excellent return on investment for our clients. Our design engineers consider the following parameters in design of rotationally moulded components.
mould design and product removal
A critical consideration in rotational moulding design is how the component will be released from the mould. Commonly we design a two piece mould which provides a cost effective and less problematic solution than multiple part moulds. Any deterioration in the manner in which the mould closes could allow gaps and consequential flashing. Flashing has to be manually removed thus adding labour cost. It is common to see a mould deteriorate due to poor design, inappropriate manufacture, misuse and/or poor maintenance.
A number of solutions to ensure continued accurate alignment between the moulds are possible. The most common are flat mating surfaces using steel dowels or keys to provide correct registration. Dowel holes wear after prolonged usage and mating faces deteriorate through inappropriate operator cleaning procedures or from distortion caused by inherent thermal or mechanical stresses.
A more expensive solution is to provide a continuous profile for example, similar to a woodworking tongue and groove used for containment and alignment. Deterioration of the moulds may be minimised through avoidance of parting lines at corners, especially where the line is in vertical orientation. Add small radii to mask the line.
The overall aesthetics of the finished product is assessed within the design brief, for instance, the parting line may not be critical for a product buried below ground level but will need to be masked for say, playground furniture. Where a complex shape dictates a multiple mould (i.e. more than two parts) the design may include a hinged or sliding arrangement for opening the mould in a defined sequence.
The factors affecting user friendliness, productivity, longevity of the mould, maintainability and product function and quality issues are given timely consideration in the product, material, moulding process and mould design. Rota Design considers the “overall system” and not just the individual elements in isolation.
shrinkage, expansion & contraction and dimensional variation
Shrinkage occurs within rotational moulding materials which has the advantage in assisting with removal of complex shaped components from the mould but gives the product designer tolerance problems especially at dimension sensitive locations such as abutments and joints. The other factor to be considered is that during the heating and cooling cycles the mould will expand and contract dependent upon the material being used.
Within the moulding process shrinkage variations occur as a result of ambient temperature fluctuations, process times and machine parameters. Rota Design is aware of these limitations and wherever possible turn a process disadvantage to an advantage by engineering out the problem as the product design evolves.
surface flatness and reinforcement elements
Rotationally moulded products are hollow and it is generally accepted that it is difficult to assure flatness of a plane surface. Where this is critical, it may be possible to counteract anticipated distortion by adding a crown to the face or reinforcing ribs to the product.
Reinforcing or stiffening ribs are added as hollow elements and act in a similar manner to corrugated sheet materials. The greater the height of the rib the greater the stiffness and the sides of the ribs will be designed with a draft angel or be tapered to assist in removal from the mould. It is a commonly used feature in rotational moulding to use two closely spaced walls, which are moulded together to provide additional strength and/or rigidity. This feature is known as kiss-off.
radii and angles
It is vital to understand the nature of viscosity of the rotational moulding material and how any given material flows and forms within the mould. Sharp corners or difficult to access areas may result in porosity. A large radius for example improves the flow of material at corners and assists in the distribution of stress. It is important to design in draft angles at right angle to the parting line to minimise distortion especially where a product tends to toward or around the mould.
venting gasses
Gasses generated during the process require venting to atmosphere so the inclusion of a venting hole or holes is vital. The diameter and location of these holes is a function of rotation, volume, material used and the process cycle.
wall thickness variance
The mould must be designed with due consideration for wall thickness at any point. In some instances, the wall thickness is defined by the design itself. In principle, the higher the internal surface temperature of the mould, the greater the thickness of material deposited becomes. Shielded areas and deep features reduce the heat energy available resulting in thinner walls in those areas.
Where the objective is to achieve uniform wall thickness, the inherent characteristics of the rotational moulding process are used to their fullest extent. Various devices may be used at the moulding stage to cool or direct heat to any given area or feature to achieve an appropriate wall thickness. Also changing the axis speed and adding dwell times within the cycle may be used to modify the finished characteristics of the moulded product.
introducing material into the mould
Pre-measured polymer, in powder, is manually loaded into the open mould cavity. A secondary drop box may be required to provide insulation, multiple colours, foaming, strengthening features or the opportunity to have different internal and external walls.
Summary
We could continue discussing the options available to the product designer, mould manufacturer and rotational moulder for eternity! However, it should now be clear from the foregoing that there is a requirement to consider the design and manufacture as a whole and not each element in isolation.
Consider Rota Design as your competent partner since we act in an independent manner and are experienced in all aspects of the process from product conception to delivery of manufactured components.