DESIGN FOR STIFFNESS

When a part requires a certain degree of stiffness what needs to be considered are operating temperatures and length of time of the load.  Also, is the load frequent or infrequent?

Material selection is important, possibly filled material – talc, glass, etc.  Stiffness is mainly increased in the direction of the glass fibres, so design with flow direction in mind.

But, design is also important.  A key issue is whether the part is to be subjected to tensile or compression loading.  For stiffness ribs should be added not additional section.  Think of steel girders with ‘I’ and ‘T’ sections which are almost as rigid as solid beams but at a fraction of the weight and cost.  It is also necessary to consider how many ribs are required to achieve the desired result.  Ribs should not be too close together, especially if they are deep, as this will make it difficult to cool the core that forms the rib pattern.

Side walls may be strengthened by the addition of buttress ribs.  The same rules apply as for conventional ribs as detailed below if sink marks on the outside of the part are to be avoided.

Stiffness can be increased by use of the following features: 

     Ribs – most commonly used.  For a part that may bend ribs should be positioned perpendicular to the point of bending.  For parts under tension, diagonal ribs are the most effective at increasing stiffness.  In all instances deep ribs are more efficient than thick ribs.

      V-grooves – incorporated where significant increases in stiffness are required.  However, often not used because they give uneven top and bottom surfaces.  V-grooves should be perpendicular to the bend.

      Corrugation – Similar to V-grooves.

For a straight beam loaded in the middle corrugation is the stiffest followed by ribs then V-grooves assuming the same amount of material is used in each case.  However, for a panel that could be loaded in different directions, say at right angles to each other, a rib form would then be the stiffest option.

For information on rib design click here.

The use of ribs may result in surface blemishes (sink marks) which may be disguised in a variety of ways. 

Contact us for a copy of our free Design Guide

More Design Advice for Injection Moulding 

More Rutland Plastics Newsletter Articles