looking for advice on a good reference. 1

Structural Analysis of Thermoplastic Components by Trantina and Nimmer is a good general source for plastic part design.

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

Thanks rick for the recommendation and all the advice.

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.

What material are you planning to use? The terms "shelf" and "constant pressure" suggest a long term load, and creep may be an issue. If so, you need to select a material with a low creep rate if the loading is significant. Some materials are more prone to creep than others and this may affect your material choice. The book I referenced has a chapter on creep that discusses the use of an isochronus stress strain curve to determine a long term apparent modulus for use in your deflection calculations.

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

I will look in to the creep issue but it is not suppose to be in permanent use, so hopefully it won’t limit the material options. As for how we are going to mold it still up in the air. Regular high pressure mold tooling is likely going to price it out of the picture, but we do have a few similarly sized molds for low pressure injection mold (the parts use a lot of flowing agent in them to form) that were not to badly priced. Though right now the leader in the club house is a reaction molding process, because the material is harder and molding is likely much cheaper.

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.

Yup, I work at Argonne, until they tell me different.

Rick Fischer
Principal Engineer
Argonne National Laboratory

If you can get a copy of G.E.'s Noryl thermoplastic resins design handout, you would have a reference that may answer several of your inquiries. The copy that I have is dated to 1983, so there may updates. I I have attached the table of content for your review; also, the periodical machine design features, annually, a comprehensive report on material science including plastics. Machine design also have special features on rib design. You'll need to do some digging and if I do find useful info. in my archive within a week, I'll let you know.

Oops, here is the reference