What adding ribs adds up to Let's have a look at what the effect of adding ribs really is. Model A represents a flat plastic surface without ribs. Model B is the exact same surface, with three added ribs. Model C is, again, the same surface but now as thick as the wall thickness and the height of the ribs combined. Model E is identical to Model B, but with the ribs in the other direction. Model F has the same dimensions as model B, but with “zigzag” or sheetpiling type ribs. (Yes, there's no model D)
When is a part strong enough? In order to determine if a part is strong enough, one must determine how much stress is induced by the expected load or the expected deformation.
Blue is low stress, green medium and red indicates a high stress.
First, we are going to compare how the three models behave under the same load. We fixate the models on one end and apply a downward force on the other end. Green arrows = Fixation
Model A. Case1
Model BCase 2
Model C Case 3 First conclusions
Model B, reversed load Case 4 Same conditions as above, but now the force is pushing upwards. The resulting stresses and the displacements are the same. Compression stress vs tensile stress But in this case the highest stresses are compression stresses. In practice under compression a plastic tends to be a bit stronger, because cracks are less likely to be initiated.
Model E Case 5 Obviously, the direction of the ribs matters. Perpendicular to the induced stress they are not of much use. This model E behaves almost identical to model A In other words, adding these ribs does not help at all.
A much stronger version of ribs Model F Case 6 In this version the ribs have normal wall thickness and alternating the wall is moved to the top of the ribs.
Comparing how the three models behave under the same displacement. In these calculations we fixate one end and push the opposite end down to a predetermined displacement. Often a construction needs to have a certain flexibility rather than withstand a certain force.
Model A Case 7
Model B Case 8 We subject this model to the displacement that Model A had in case 1. The resulting stress is very high. Adding the ribs in this case would make matters worse and could indeed lead to failure.
Model C Case 9 In order to get it to bend as far as Model B in case 2, a very much higher force is required.
Conclusions. When a part is submitted to a certain load, a weight for example, adding ribs can be very beneficial. It can be used to add strength and stiffness. In order to be effective ribs must be placed in the right directions. When a part is submitted to a certain displacement, as a rule ribs are counterproductive. This means ribs must be added with care and understanding. For various reasons, sheet piling type ribs are more suitable to the injection molding production process. When design circumstances allow, they should be considered. (model F)
Main cases All calculated cases
