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Product Design Considerations and General Guidelines
Wall thickness: Thicker the wall, higher is the material cost and cooling time.
Optimum wall thickness for adequate strength and stiffness is desirable. In case of
the double walled parts the walls must not be too close to avoid webbing and voids
formations.
Minimum outside wall separation gap of 5x wall thickness
Minimum inner wall separation gap of 3x wall thickness
Wall thickness is important for corner radii too. Large outside as well as inside radii
are preferable to small radii which allows for more even flow of material. However
due to material flow characteristics outside corners tend to be thicker whereas
projections into the moulding tend to thin out.
Sharp corners: must be avoided as it creates stress point in the product. Provide
minimum radius of 3/16" at corners and generous radii to joining walls (45° and
above).
Wall Thickness tolerance: it is common to specify a minimum wall thickness rather
than a nominal wall thickness. For general purpose parts as tanks and outdoor toys,
the typical wall thickness variation is ±20% whereas ±10% is considered as precise
tolerance.
Draft Angles: These are required to remove the piece from the mould. On the
outside walls, a draft angle of 1° may work (assuming no rough surface or holes). On
inside walls, such as the inside of a boat hull, a draft angle of 5° may be required. This
is due to shrinkage and possible part warping.
Moulds Pieces: The Mould must have least possible number of pieces, ideally a two
piece mould which is easy, convenient to open-close having least maintenance
saving and flat parting line
Parting lines: The less the parting lines on product better it is. For this the number of
mould pieces be minimum which in turn decreases irregularity of parting lines. Also
parting line location is important, to ensure that it does not leave a visible mark on the
moulded part.
Ribs: Ribs provides stiffness to product and are also economical as the wall section
uses less material and is quicker to mould. Further solid rib sections are not possible
and hence ribs are designed as hollow sections similar to corrugated sheet. They can
be raised or embossed.
D=4XT T

W=5XT

Good average proportions for ribs is 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. Further increasing
depth will bring difficulty in moulding and part removal whereas decreasing the width
increase chance of material bridging off between the two walls and not fully filling the rib.

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