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Subject 2 : Rotational moulding Increased strength

Ribs in Rotational Moulded Products

Some thermoplastics used in rotational moulding are not inherently stiff materials e.g. PE, EBA.  In order to increase product stiffness, ribs are used extensively without great increase in part weight.   The correct use of stiffening ribs results in stiff lightweight parts that can be produced more economically because  the wall section uses less material and is quicker to mould.

Stiffening ribs in rotationally moulded parts cannot be designed as solid sections (as with injection moulded or compression moulded parts) – the relevant sections would not fill out.  Instead the stiffening ribs are designed as hollow sections similar to corrugated sheet. They can be raised or embossed.

The diagram below shows good average proportions for ribs where the depth D is at least four times the wall thickness T and the width W is at least five times the nominal wall.  Increasing the depth D increases the stiffness but it also increases the difficulty in moulding and part removal.  Decreasing the width W makes moulding more difficult increasing the chance of material bridging off between the two walls and not fully filling the rib.

Section through rotational Mould rib

Round ribs shown below are often specified as they are much easier to mould however they do not provide as much of an increases in stiffness as there is less depth D lying perpendicular to the wall.

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