Material Testing for Surgical Caddies

Medin Technologies manufactures caddies for surgical equipment. But due to the high cost in the current materials they are using, the company is searching for alternative materials. This project tests polypropylene and qualifies the material as suitable.

Problem: Surgical instruments are boiled in water to sterilize them. Once boiled, they are kept inside precisely molded or manufactured caddies. Because of the change in temperature, the caddies can easily deform.

Goal: To qualify an alternative material (polypropylene) for potential cost savings and contingency planning. The analysis of the data would help to determine whether it is possible to use the new material without a compromise in quality or performance of the product.

Tools: Design of Experiment, ANOVA, SPC charts, minitab, material testing

Conclusion: Polypropylene is a good material for manufacturing surgical caddies. The expansion and contraction of this material is within the tolerance for making these containers.


To qualify materials for the manufacture of surgical caddies, porosity, temperature stability, impact resistance, machinability, discoloration, dimensional stability, and surface tension need to be measured.

Our focus was on dimensional stability analysis. Length and width of polypropylene blocks of 3 different heights, each of two colors (black and white), were measured using a digital vernier calipers (to determine volume of the material). These dimensions were measured at two different temperatures: 250 °F and 65 °F.

To make sure that the material was stable, the width to length ratio, and density needed to be constant. Control charts for width, height, width to length ratio, and density were plotted. They showed that the dimensions were within the control limits.

To determine the variance and interaction, Design of Experiment (DOE) was performed for density and Analysis of Variance (ANOVA) was performed for density and volume of the blocks respectively.

Control Chart for W/L Ratio

Control Chart for Length

Control Chart for Width

Control Chart for Density

Analysis of Variance:

  • Factor 1 (5 levels): Height (1”, 0.5”, 0.375”, 0.25”, 0.1875”)

  • Factor 2 (2 levels): Temperature (65F, 250F)

  • Response is Volume

  • The following were inferred from the plots:

    • The response is almost normally distributed

    • The residuals are similar to the ones obtained for density

    • There is a significant interaction between height and temperature

    • The volume remains almost constant for both temperature

    • The volume increases with height

Main Effect Plot for Volume

Normal Probability Plot

Interaction Plot for Volume

DOE (Statistical Experiment):

  • 2 factors: Weight and Temperature

  • Response: Density

  • Levels for weight (grams): 5 (18, 23, 34, 46, 91)

  • Levels for temperature (Fahrenheit): 2 (65, 250)

  • Number of replications: 8

  • All p-values are less than 0.05: null hypothesis is invalid

  • Highest F value is for weight, second highest is for Temperature: They play the major role in the response. The F value for the interaction weight*temperature, is very low. It doesn’t contribute much to the response.

  • R squared adjusted value: 98.34% (this is very good and showed that the plot is normal and there is very little variance)

  • Residual plots showed equal variance and little to no correlation.

Contour Plot

Interaction Plot for Density

Main Effect Plot for Density

Versus Fits

Normal Probability Plot for Density

Versus Order