Mechanical Engineering Expert’s Opinions on the Shattered French Press Limited
Posted on June 25, 2025 by Expert Witness Profiler
Plaintiff, Wade Mullen, filed a products liability lawsuit against Defendant Bodum USA, Inc. (“Bodum”), claiming damages for injuries he suffered when his Bodum French press shattered and splashed him with hot coffee.
Bodum filed a motion to exclude the testimony of Mullen’s expert Mingxi Zheng, an engineer who inspected the shattered French press.
Mechanical Engineering Expert Witness
Mingxi Zheng has 8 years of experience in the general field of materials science, mechanical engineering design, metallurgy, failure analysis and materials characterization. At her previous engineering roles, she has spent time in both active manufacturing environments and R&D phase companies developing new technologies.
Discussion by the Court
Bodum contended that Zheng’s report should be excluded on reliability grounds, first, because it consisted of analytical gaps concerning what caused the French press to shatter, and second, because Zheng failed to support her analysis of alternative French press designs with any testing of those proposed alternatives.
To be precise, Bodum argued that that Zheng’s report consisted of four analytical leaps: First, Zheng assumed that the scratches and stainless steel residue found on the inside of the French press’ glass carafe came from the metal coil on the press’ plunger “without having done any testing or analysis to determine if in fact there was any contact between the stainless steel plunger component and the glass.”
Second, Zheng concluded that the defects resulting from the contact between the plunger’s metal coil and the carafe could cause the French press to shatter without testing to determine whether such contact “could actually cause a scratch of sufficient dimension and depth to reduce the tensile strength to the point where thermal stress could cause a fracture.”
Third, Zheng conducted no testing to support her conclusion that thermal stress from the hot coffee caused the French press to facture. And fourth, Zheng did not rule out alternative causes for the French press’ shattering.
Causation
The Court considered Zheng’s opinion that, “more likely than not,” the plunger’s protruding coil is what scratched the press’ glass carafe and created crack initiation sites. Zheng reached this conclusion after she analyzed the metal residue observed by the crack initiation site and found it to be made of stainless steel, the same material contained in the press’ plunger and the protruding coil. Zheng concluded that thermal fracturing occurred after observing (1) Wallner lines perpendicular to the carafe’s surface, (2) cracks that formed near-ninety-degree angles with the edge and surface of the glass, and (3) the absence of branching crack patterns, all of which are indicators of thermal stress.
While Bodum objected to Zheng’s failure to conduct additional testing to determine what level of thermal shock would be necessary to shatter the French press given the existence of the imperfections observed on the glass carafe, the Court held that testing is not an ‘absolute prerequisite’ for an expert’s theory of causation to be admissible in a design defect case.
The Court also refused to exclude Zheng’s opinions for failing to rule out fracture due to any other alternative cause.
Alternative Designs
In her report, Zheng explained that she reviewed Bodum’s website and found, based on the company’s image of the replacement part containing the metal coil, that a protruding coil “is the normal condition for this part.” She extrapolates that the protruding coil is a design or manufacturing defect because it allows for the extra pressure of a sharp exposed edge to scratch the inner surface of the glass carafe, which may bring about “compromising contact.”
Based on this finding, Zheng concluded that three alternative design options exist that would reduce or eliminate glass fracture. First, Bodum could—and has—designed a French press with a non-glass carafe. Second, Bodum could use silicone or a different temperature-resistant soft material on the edge of the plunger to act as a barrier between the plunger’s steel components and the carafe. And third, Bodum could weld or tack down the protruding coil to prevent it from scratching the glass.
The Court held that Zheng’s opinion on alternative designs is not grounded in the methods and procedures of science but is rather a conclusion that any lay person could draw by visually inspecting the plunger and searching the internet for varieties of French presses.
Held
The Court granted in part and denied in part the Defendant’s motion to exclude the testimony of Plaintiff’s expert Mingxi Zheng.
Key Takeaway:
The law is clear that an expert need not rule out every alternative in forming an opinion. Although the most convincing expert reports are likely to address alternative causal mechanisms, for an opinion to be admissible, an expert need address only obvious alternative explanations for the phenomenon in question. Here, there is no obvious alternative that could explain the French press’ shattering, nor did Bodum propose any.
Case Details:
| Case Caption: | Mullen V. Bodum USA, Inc. |
| Docket Number: | 1:23cv1166 |
| Court Name: | United States District Court, New York Southern |
| Order Date: | June 23, 2025 |
