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Subject 2 : Rotational moulding Technical Requirements

Wall Thickness

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 be more expensive to maintain.

It should also be noted that because of material flow characteristics outside corners tend to be thicker that nominal wall {usually an advantage since outside corners are frequently heavily loaded} whereas projections into the moulding tend to thin out.  These are generally not regarded within the general wall tolerance.

 

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.

View – Corner Radii

Taper and Draft Angles

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.

INWARD PROJECTIONS
Minimum   –  1 degree
Recommended   –  2 degrees

Flatness of Rotational Moulded Surfaces

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.

View – Ribs in  Rotational Moulded Products

Difficult to Mould Geometry

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.

Undercuts on Rotational Mouldings

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.

Tolerance of Rotational Moulding

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.

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