If the load carrying ability or the stiffness of a plastic structure needs to be improved it is necessary to either increase the sectional properties of the structure or change the material. Changing the material or grade of material, e.g. higher glass fiber content, may be adequate sometimes but is often not practical (different shrinkage value) or economical.
Increasing the sectional properties, namely the moment of inertia, is often the preferred option. As discussed in other sections, just increasing the wall section although the most practical option will be self-defeating.
- Increase in part weight and costs are proportional to the increase in thickness.
- Increase in cooling time is proportional to the square of the increase in thickness.
If the load on a structural part requires sections exceeding 4 mm thickness, reinforcement by means of ribs or box sections is advisable in order to obtain the required strength at an acceptable wall thickness.
The efficiency of a ribbed structure can be illustrated with the following example:
Solid plate vs. ribbed plate in terms of weight and stiffness.
Although ribs offer structural advantages they can give rise to warpage and appearance problems, for this reason certain guidelines should be followed:
The thickness of a rib should not exceed half the thickness of the nominal wall as indicated in the figure below.
In areas where structure is more important than appearance, or with very low shrinkage materials, ribs with a thickness larger than half the wall thickness can be used. These will cause sink marks on the surface of the wall opposite the ribs. In addition, thick ribs may act as flow leaders causing preferential flows during injection. This results in weld lines and air entrapment.
Maximum rib height should not exceed 3 times the nominal wall thickness as deep ribs become difficult to fill and may stick in the mold during ejection.
Typical draft is 1 to 1.5 deg per side with a minimum of 0.5 deg per side. Generally draft and thickness requirements will limit the rib height.
At the intersection of the rib base and the nominal wall a radius of 25 to 50% of the nominal wall section should be included. Minimum value 0.4 mm. This radius will eliminate a potential stress concentration and improve flow and cooling characteristics around the rib. Larger radii will give only marginal improvement and increase the risk of sink marks on the opposite side of the wall.
Recommendations for rib dimensions.