By Design: Part design 204-GussetsBy Design: Part design 204-Gussets

In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on issues important to design engineers and the molding industry.  During my university days, the word “plastic” was not heard in the school of engineering. When I started designing plastic components in 1957, I simply drew the functional detail and then wrapped a wall around those elements to create a finished part. The only thing that saved me was that everything was prototyped in order to provide parts for clinical evaluation. When the parts didn’t function as required, they were modified until they worked well enough to get by. This trial-and-error procedure taught me what not to do.

   Back in those days, there were a few textbooks and some excellent design brochures published by the resin manufacturers. The only problem was that I didn’t know that this literature existed. Fortunately, the company that I worked for practiced a form of early supplier involvement. Plastics processors, materials salesmen, and toolmakers routinely visited the new product development department.Figure 1. Advancements through the years in plastics processing have made gussets easier to mold. Designed properly, gussets make a part stronger without increasing cycle time.     These were actually sales calls, but they were also learning opportunities. At that time these suppliers were the experts in the industry. They would look at my preliminary designs and make recommendations for tooling, cost, and quality improvements. They would say such things as, “The way you have this part designed, it will have a  sink mark in the center, and it will warp along its length.” Sure enough, the prototype parts warped and had a sink mark in the center. How did they know that by just glancing at the drawing?The Learning CurveAfter all these years, I still recall an embarrassing time when my favorite molder refused to take some drawings to quote until I redesigned the parts to include molding draft angles. I worked like a demon in order to have all of those parts redrawn before his next visit. That experience taught me a lesson that has served me well ever since.   Regrettably, that very beneficial method of working came to an end in the 1960s with the latest “flavor of the month” management scheme. The new philosophy decreed that purchasing could be a profit center. Thereafter, all suppliers called on the buyers and not the design engineers. Fortunately for me, I had, by that time, made so many mistakes that I had learned enough to limp along on my own.   On the negative side, new designers joining the product development department were deprived of a wonderful learning opportunity. They became dependent on “experienced” designers like myself, but I was still a novice. Today, involving suppliers in the initial design phase is touted as a good way of increasing quality while reducing part cost.Gusset IntegrationAnother memorable experience from those early days was the use of gussets. As a recent graduate, I had the advantages of gussets clearly in mind. My first plastic part design made liberal use of these triangular-shaped reinforcing elements (Figure 1). Much to my surprise, however, the molders, and especially the moldmakers, were not impressed with my use of gussets. They said plastic parts didn’t need gussets. They told me that gussets were old-fashioned structures that went out of vogue with welded fabrications and riveted bridges. I had such respect for these suppliers that I immediately stopped specifying gussets. That was another one of my mistakes.   What these suppliers were actually saying was not that gussets were obsolete or bad, but that they were difficult. Gussets, by necessity, have to be thin, and the narrow tooling grooves required to construct gussets were difficult to mill or grind into a cavity. The narrow grooves were also difficult to polish.    Finally, in 1954, EDM was invented in Russia, and by the 1970s the moldmaking industry had accepted this manufacturing process. Thin grooves could be easily and economically burned into cavities. But, by that time several generations of plastic part designers had been taught to avoid the use of gussets. It actually took me years to realize that gussets were no longer difficult or expensive.   Gussets are nothing more than short reinforcing ribs. Their basic function is to spread a load over a broad part area. With more material to absorb the load, the part becomes stronger. One good feature is that gussets achieve this advantage through the use of only a small amount of plastic material. Properly designed, they do not increase cycle time.Gusset GuidelinesAs shown in Figure 1, gussets can be used to stiffen the junction between two walls. In this location, they prevent the part’s side wall from flexing or warping in or out. They also supply more strength than can be provided by a properly radiused corner.   Gussets can also be used to strengthen the junction between hollow or solid bosses and the nominal wall of a part. Heavily loaded stiffening ribs can be prevented from buckling with the use of one or more gussets. The stiffness of a snapfit latch can also be increased by adding a gusset plate. In some cases, this can eliminate the need for thickening the part or changing to a stronger and more costly resin.   Gussets are just another type of projection. They are proportioned the same as other projections, as described in By Design, August 2000 IMM.    The height, space between, and thickness requirements of gussets are identical to an elongated stiffening rib. Thickness must be 50 to 75 percent of the part’s nominal wall thickness, depending on whether the part is molded in a high- or low-mold-shrinkage material, respectively.  If you are like me and most other designers, you probably do not take advantage of the benefits offered by gussets nearly as much as you should. The strength advantages of gussets far outweigh the amount of additional plastic material they use.