The ideal way to specify the wall thickness on a Rotationally Moulded part is to specify the NOMINAL wall thickness and also the MINIMUM wall thickness that can be allowed anywhere on the part. Since moulding is done with a female tool only – with no matching male portion of the tool – accurate wall thickness is not attainable.
A tolerance of +/-20% should be considered as a commercial tolerance, whereas +/-10% would be precision and therefore more expensive to maintain.
A. Inward projections thin out.
B. Outside corners thicker
Although generally the process produces relatively even wall thickness, it is possible to locally increase the wall thickness, if necessary.
A typical wall thickness at Amber ranges from 3mm – 10mm.
An advantage of Rotational Moulding over other processes is that products can be moulded with no draft angle at all, since the tool is of female form with no internal core for the moulding to shrink into. As the product cools it shrinks away from the walls of the mould, making it easier to remove. It should be noted, however, that textured surfaces, moulded-in logos, etc. may necessitate taper even on a female shape.
The draft angles below are recommended to be incorporated, unless it interferes with the functional requirements of the part:
Minimum – 1 degree
Recommended – 2 degrees
The flatness of product surface is subject to the design and cooling process.
Typical flatness tolerances for polyethylene would be 5% ideal with 2% as a commercial tolerance and 1% as a precision tolerance.
If at all possible, the design of parts to be rotationally moulded should avoid large flat areas. If absolutely necessary they should be broken up by reinforcing ribs (see next section) or possibly have a gentle curvature on them. Moulded in detail, such as lettering, logo’s etc. can also break a flat surface up visually so that lack of flatness is disguised.
Corners and their radius can affect the flatness of adjacent surfaces as differential cooling rates can cause the corner angle to distort. This effect is minimised by careful design of the corner, the tooling and cooling process.
If the design, of the product, is not done with the rotational moulding process in mind certain areas can prove troublesome to mould. These include small corner angles, walls very close together and parts with undercuts.
The first two of these result from material bridging over the space between the two walls of the moulding when they are too close together. If bridging occurs the bottom part of the moulding will not fill out
To be sure of proper fill the width should be a minimum of five times the wall thickness.
If an angle is too acute we have a similar effect. If possible the angle should be kept to a minimum of 30 degrees and all radii should be kept as large as possible.
An undercut is any wall projecting inwards or outwards, parallel to the parting line of the tool, which makes the removal of the moulding difficult if not impossible.
Since the rotational moulding process uses hollow tools, with no male core, it is sometimes possible to use the shrinkage during moulding to enable small undercuts to be removed.
Hence in the diagram if the undercut at A is small enough (and even better if there is a generous radius at A) then the part shrinkage will enable the part to be removed. If the undercut is too big then, the split line must be moved to XX and the inward projecting boss has then to be moulded using a removable loose piece. Thus the part has to be designed with product removal very much in mind.
As previously mentioned; it is difficult to hold rotationally moulded parts to tight tolerances. The outside dimensions of a rotationally moulded plastic part are free to draw away from the inside surfaces of the tool as the plastic cools and shrinks.
It should be noted however that where the tool form represents a male form and the moulding shrinks on to the mould it is possible to hold tolerances much more closely.
It should also be noted that since the inside surfaces of the part are formed only by flow of the plastic and not against a males core it is very difficult to control these to any repetitive degree of accuracy.
Although each product design is a special case which must be given individual consideration; in general, a tolerance of 2-3% would be considered a commercial tolerance and 1% to be precision. As with all tolerances the best is the broadest tolerance that will satisfy the end use requirement of the part.