The mystery of the cracked stadium seats

Editor’s note: Peter Lantos is president of The Target Group, an industrial consulting firm located in Erdenheim, PA. The following story describes his company’s recent experience helping a molder solve a product failure.

Not long ago a Japanese businessman in the U. S., having seen an attractive baseball stadium seat in the states, arranged to have 30,000 such seats produced at an injection molder in Japan for installation at a stadium in Nagoya, a city 200 miles from Tokyo. Within a week of installation, a large number of seats began to crack, and the entire lot had to be replaced with freshly molded seats produced by the same molder. These held up without cracking. The businessman blamed the molder for the product failure and turned to our company, The Target Group, to find the cause of the problem.

The Target Group was given 12 seats—six with cracks, six without. We started our examination by evaluating factors that might cause cracks.

We eliminated design as the problem, as it was identical for both sets of seats. In fact, the same mold was used to produce all seats. Seat design was also deemed basically sound: It was rigid, had a curved contour that provided good load support and impact resistance, and it contained no sharp corners.

Installation was also eliminated as a factor, as was seat location. The cleaning process was the same for both good and bad seats. Finally, all seats endured similar end-use conditions—it seemed unlikely that the bad seats all had the misfortune of encountering crack-causing, overweight spectators. The seats (Figure 1) were rectangular, about 12 by 15 inches, and gated at the center. Substantial cracks, about .125 inch wide at the base and 7 inches long, emanated radially from the gate, along the flow lines. This suggested a problem in the molding process. Unfortunately, the molder and the stadium businessman had become adversaries; thus no information was made available about either the molds or molding conditions.

The Testing
We decided first to conduct resin tests. The seats were supposed to have been molded from orange-pigmented HDPE of .965 density and 5.5 melt index, and tests confirmed this. Ground-up resin samples were tested at Springborn Laboratories (Enfield, CT). Results are in Table 1. The data also showed that there was no significant difference in density or melt index between resin from good and bad seats. We concluded that neither the wrong resin had been used, nor was there a difference in the moldability of the resins.

Chemical testing was performed by Jordi Assoc. (Bellingham, MA). Resin samples from each seat were frozen in liquid nitrogen and ground up, extracted with toluene and filtered, after which the liquid was evaporated. The residual solids were dissolved in isopropyl alcohol and analyzed via reverse phase high-performance liquid chromatography. The results (Table 1) indicate that identical stabilization systems had been used in each resin. But there were also trace amounts of byproducts showing fluorescence in the bad seats. These byproducts are associated with resin degradation, suggesting that some degradation and/or oxidation of the HDPE had taken place. No such byproducts were found in the resin of the good seats.

Also, resin taken from each kind of seat was ground up and test bars were injection molded. Izod impact-strength tests (Table 1) showed that cracked seats had roughly half the impact strength of the good seats. This was significant and we hypothesized that the resin of the cracked seats had encountered excessive heat in the molding process.

We performed additional investigations. Resins were examined at Structure Probe (West Chester, PA) using light microscopy (LM) and transmission electron microscopy (TEM). The results of TEM indicated that the pigment in each resin was dispersed equally well. Thus the low impact strength was not caused by poor pigment dispersion. The LM results, however, showed a major difference between the two resins. The good resin had a spherulitic crystal structure of relatively large size. The bad resin was made up of fine crystal spherulites, which could be caused by the presence of a nucleating agent, or by rapid cooling resulting in rapid crystallization. No nucleating agent was detected, and pigment concentrations (pigment can act as a nucleating agent) were identical. Thus it seemed the fine crystal structure must have been the result of rapid cooling.

We hypothesized that the bad seats had been heated excessively and then cooled too quickly. This would then have set up internal stresses in the part, resulting in cracking during use.

We decided to test the hypothesis by checking for residual stress. Polarized light microscopy would have been the best method, but for that parts have to be transparent or translucent, and these weren’t. So we placed the seats in an oven to see what would happen. Table 2 shows that 15 minutes in an oven at 95C (203F) caused the cracks to become significantly enlarged. Corner warpage of the cracked parts, significant to start, became worse. Corners on good seats had no initial warpage and did not warp while in the oven. This movement of sections of the cracked seats was taken as further evidence of locked-in stresses, while the good seats apparently were stress-free.

The Answer
Internal stresses resulted from the molding conditions: excessive melt temperature and too-low mold temperature. Reduced impact strength resulted from the excessive heat encountered by the resin during molding. Judging from the fact that cracking took place soon after installation, and that the cracks were large, we determined that the molding conditions were quite bad.

There is a lesson here: Overheating a resin to improve flow, and running a mold too cold, may speed the cycle in the short term. But in the long term part quality suffers, and the cost to the molder exceeds any initial savings.