looking for advice on a good reference.
looking for advice on a good reference.
(OP)
i need to design a part that will required a lot of ribbing and i will need to know its deflection. Can anyone suggest a good reference book that covers the theory of ribbing and the deflection of ribbed parts?





RE: looking for advice on a good reference.
Ribbed structures are tricky to analyze by hand unless the ribbing is simple. The ribbed parts I've worked on did not lend themselves to hand calculations, and we relied on finite element analysis, both structural and mold-filling analysis. Keep in mind that there is more to it than stiffness. The base of the rib will have a corner, which is a stress riser. Intersections of the ribs with walls and each other ribs are also corners. Often the advantage gained with the ribs is negated by this. Make sure you radius these corners. This may, however, make sink marks more noticable. Also, ribs may be dificult to fill. Consider that the rib thickness is generally on half to two thirds of the nominal wall thickness, and that a draft (typically 2% per side) is required to demold. The rib get thinner as it gets deeper, and it is harder to fill. This limits rib dimensions and effectiveness. I've seen ribs fill from the ends and create a weld line in the middle that splits open when the part is loaded. The weld was visible in virgin parts, and predicted by Moldflow. You dont describe the part, but unless the ribing is fairly simplistic, yoi should run moldfilling analysis to check for weld line locations and structural FEA to check stresses and deflections.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: looking for advice on a good reference.
the part is shelf kind of like thing that constant pressure and can not defect more then an 1/8" over a 3' span. so, since the part is flat, I am assuming that the ribbing will be rather simple. Also the ribbing will only need to increase stiffness and not strength the stress consideration will not be a problem.
RE: looking for advice on a good reference.
What process are you using to make this part? I assumed injection molding, but "3' span" sounds like a large part that would require a large costly tool. Extrusion or thermoforming might be indicated here, especially if production volume is not high. Ribbing is awkward in a single sheet thermoform, so if you are looking at extrusion, your ribs will be parallel and in one direction. The analysis of such a part will be simplified by this.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: looking for advice on a good reference.
And as a side note thanks for your advice and do you really work at Argonne? Because in the summer I run around there literally (Waterfall Glen) after work all the time, at least when the weather is nice.
RE: looking for advice on a good reference.
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: looking for advice on a good reference.
RE: looking for advice on a good reference.