There was a way to fix the issues with this tricky part, but sometimes avoidance is best.
Injection molders claim that one way or another, they can mold any shape a designer envisions. In general this is true, but some shapes are more troublesome than others.
One-piece hollow parts are best made by other molding processes. Deep undercuts in rigid materials cause problems. Some products, such as cell phones, require walls too thin to be injection molded. Another troublesome type of part is one where a thick wall surrounds a thin section. A part of this type is referred to as a diaphragm-shaped part. These parts can be square or round, but the classic example is an O ring with a thinner wall across its inside diameter.
Figure 1: Five problems can plague diaphragm-shaped parts: nonuniform melt flow, mold shrinkage, gas traps, moldedin stress, and distortion. The distortion in the flat bottom of this one needed an arbitrator to resolve.
For some unexplainable reason, the problems created by diaphragm-shaped parts have not been covered in available literature. The five primary problems caused by these parts are 1) nonuniform melt flow and 2) mold shrinkage, which results in 3) gas traps, 4) molded-in stress, and 5) distortion. All of these problems are present with the part shown in Figure 1.
My first encounter with this part was as a consultant hired by a molder to arbitrate a dispute between him and his automotive manufacturing customer. We met at the molder’s plant to inspect the tool and perhaps tweak the molding to eliminate the problems.
This was an average-sized molder with reasonably modern molding machines. It had a full-service quality group and a better-than-average mold repair toolroom. I recall noticing how spotlessly clean everything was. I still have a mental image of the fluorescent lights reflected in the sealed concrete floor.
The mold that was open on a bench in the toolroom turned out to be a straightforward two-plate, ejector-pin-type mold with an edge gate on the flange at the largest diameter. After this quick look around we met with the customer to fill me in on the project.
Defining the problem
Figure 2: Another diaphragm-shaped part also exhibits distortion caused by nonuniform shrinkage.
The customer explained that the part was a round tote bin (shown upside-down in Figure 1). The part’s function was to protect, locate, and convey a metal part through a series of automated machining operations. The problem with the part was that the flat bottom wall was distorting inward. This prevented the metal part from properly seating into the tote bin to be located by the tapered side walls. When this happened, the blind robot could not accurately pick up the metal part and locate it in the machining center.
I took one look at a sectioned part and blurted out, “This is a diaphragm-shaped part. The only way you can achieve a flat bottom is by eliminating the relatively thicker section at the closed end of the part.” Here’s how the rest of the conversation went:
Customer: “I can’t do that.”
Me: “Why not?”
Customer: “This is a tote bin and I need that thick bottom wall.”
Me: “Why?”
Customer: “As a tote bin it is bounced down conveyors, dropped on its bottom, dragged across cement floors, run into with lift trucks, and I need that thicker wall.”
Me: “What you need is strength and not thickness. You can get the required strength by changing to a stronger material or by ribbing.”
This tote bin is typical of all round diaphragm-shaped parts. During the cooling portion of the molding cycle, the thick section at the closed end of the part took longer to cool and shrunk more than the thin, horizontal wall. The thick wall was stronger than the thin wall, so as the thick wall shrunk, it became smaller in diameter and that forced the thinner, weaker wall to deform.
A solution
The best way to resolve this problem would have been to first core out the thick section to produce a part with a constant wall thickness and uniform mold shrinkage, and then to strengthen the closed end of the part with a series of ribs. However, this approach would have required part design and mold modifications. These changes would have resulted in a loss of time and an added tooling cost that the molder would have to absorb.
Further discussion confirmed that the metal part seated against the tote bin’s tapered side walls and not on the horizontal wall. Doming of the horizontal wall only prevented the metal part from fully seating into the tote bin. A thorough understanding of the tote bin function provided an opportunity for a quick fix.
A compromise
The first and only run of the tote bins had been made with both halves of the mold at the same temperature. A few molding experiments revealed that restricting cooling in the large core pin allowed the horizontal wall to shrink more on the surface molded against that hotter core pin. Increased shrinkage on the hotter side of the horizontal wall encouraged that wall to warp away from the large core pin when compressed by the thicker wall surrounding it. With the horizontal wall distorting in the opposite direction, there was no restriction to the metal part seating into the tote bin. It is believed that this is the technique used to mold the tote bin thereafter.
The customer was happy because there was no additional delay in receiving usable tote bins.
The molder was happy as he avoided the cost of a mold revision.
The consultant was happy because the molder and the customer thought he was a very clever fellow.
I did, however, leave the project knowing that the tote bin was not as good as it could have been. There was still an unnecessarily thick wall around the closed end of the part. That thicker wall required that every part ever molded would use more plastic material and have a longer molding cycle than necessary. The difference in mold shrinkage between the thick wall and the thinner horizontal and vertical walls would result in molded-in stress at the junction between these walls. This residual stress was not known to have caused any problems. Perhaps the closed end of the tote bin was not as heavily loaded as the customer thought.
Not all diaphragm-shaped parts are round
Another example of a round diaphragm-shaped part is shown in Figure 2 (p. 40). The core pins that form the thin wall in the center of the part are flat. The distorted shape shown here is created by the same nonuniform shrinkage as that on the tote bin. The part drawing, or data base, may have tight flatness specifications but that will not prevent a part of this shape from distorting.
Round is the most common, but diaphragm-shaped parts can also be square or irregular in shape. A five-sided machine cover with a thicker tongue-and-groove fitting around its open edge is a diaphragm-shaped part. Round parts always distort into a dome shape. A square part will warp as though it had been subjected to a twisting force applied in opposite directions on two sides of a part. Warpage of this type is difficult or impossible to control by adjusting the molding conditions.
The best way to handle diaphragm-shaped parts is to avoid them.
About
Glenn Beall Beall owned Glenn Beall Engineering, a plastics product design and development company, from 1968 to 1993, and after retiring from that company, he established and is currently president of Glenn Beall Plastics, Ltd., a plastics consulting business.
