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 direction of 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 are to be avoided on the outside of the part.
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.
As explained previously, ribs can 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 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°
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
Each of the above are guidelines only and may vary according to material and general design of the product. If thicker ribs are required then any resulting surface blemishes (sink marks) may be disguised in a variety of ways. Aesthetic considerations will be covered in a future article.
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