Ribs and profiled structures

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.
Parallel ribs should be spaced at a minimum distance of twice the nominal wall thickness; this helps prevent cooling problems and the use thin blades in the mould construction. Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots. Ribs.
Ribs should be orientated along the axis of bending in order to provide maximum stiffness. Consider the example in the figure above where a long thin plate is simply supported at the ends. If ribs are added in the length direction the plate is significantly stiffened. However, if ribs are added across the width of the plate little improvement is found. Ribbing is typically applied for: 1. Increasing bending stiffness or strength of large flat areas 2. Increasing torsional stiffness of open sections Adding corrugations to the design can stiffen flat surfaces in the direction of the corrugations (see figure below). They are very efficient and do not add large amounts of extra material or lengthen the cooling time. The extra stiffness is a result of increasing the average distance of the material from the neutral axis of the part, i.e. increasing the second moment of inertia. Corraguations.
Flat and open areas.
Ribs and box sections increase stiffness, thus improving the load bearing capability of the molding. These reinforcing methods permit a decrease in wall thickness but impart the same strength to the section as a greater wall thickness. Dimension image with chart of case 1-6.
Comparison of profiles in terms of torsional rigidity and bending.
The results demonstrate that the use of diagonal ribs have the greatest effect on the torsional rigidity of the section. The change from an I section to a C section helps in terms of horizontal bending terms but not in torsional terms. As double cross ribs (option 6) can give tooling (cooling) problems option 8 is the recommended solution for the best torsional performance. Depending on the requirements of the part the acceptability of possible sink marks at the intersection of the ribs and profile wall need special consideration. For maximum performance and function the neutral lines of the ribs and profile wall should meet at the same point. Deviation from this rule will result in a weaker geometry. If, due to aesthetic requirements, the diagonal ribs are moved slightly apart then the rigidity is reduced 35%. If a short vertical rib is added to the design then the torsional rigidity is reduced an additional 5%. See figure below. Torsional rigidity and resistance to torsional stress as a functionof the way in which the ribs are connected to the profile.