DESIGN FOR MOULDABILITY (PART 3)

RIBS

Ribs can be used to increase the stiffness or strength of a component without increasing the overall wall thickness and so increasing the weight of the component.  However, ribs can increase the risk of warping and appearance problems.  For successful rib design the following guidelines should be followed:

     To reduce sink marks on the surface the rib thickness should not exceed 50% of the adjoining wall thickness.

     To reduce stress, filling and ejection problems the height of the ribs should not exceed three times the adjoining wall thickness.  When more strength is required more ribs are recommended rather than an increased height.  A deeper rib may buckle under load plus it is difficult to machine into the mould and may cause the part to stick in the mould.

     A minimum radius of 25% of the adjoining wall thickness should be incorporated at the base of the ribs.

     Ribs are most effective when placed down the length of the area subjected to bending.

     Rib spacing should be at least twice the nominal wall thickness.

     A draft angle of at least 0.5 degrees on each side should be incorporated in order to facilitate release from the mould.

Rib Guidelines

Component Section = S; Draft per Rib Side = A = 0.5 - 1.5 Deg; Rib Height = H = <5 x S (usually 2.5-3 x S);

Radius = R = >0.25 x S - 0.4 x S; Rib Thickness = X = 0.4 x S - 0.8 x S; Rib Spacing = 2 x S - 3 x S

SUPPORT RIBS

Support ribs may be used as reinforcement for corners, side walls or bosses.  The following guidelines should be adhered to wherever possible:

     Thickness should be between 50% and 70% of component wall thickness.

     Minimum distance between faces of ribs should be twice component wall thickness.

     Minimum length of support rib face attached to component wall should be twice wall thickness.

     Generous radii should be incorporated at rib ends.

     Minimum of 0.5 degree draft should be incorporated.

     Minimum length of support rib face attached to a boss should be four times wall thickness.

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