Injection Molding
Skills: Injection Molding, Tensile Testing, Factorial Design, ANOVA
Spring 2022 - Summer 2022
Objective
The purpose of this experiment was to analyze how changing the operating parameters in the injection molding process, would affect the tensile properties of a HDPE/LLPDE polymer. This gives further insight into the injection molding operation, along with optimizing the process using mechanical properties and factorial design.
Machinery used for the molding
Design Process
Since there are a variety of operating parameters surrounding the process, our starting point was to determine which parameters we would be analyzing. These were determined to be the injection pressure, mold clamping force, and melting temperature.
Injection pressure affects how tightly the polymer granules are packed within the mold. Mold clamping force affects the amount of force applied to the mold by the clamping component to keep the mold closed during the molding process. Melting temperature affects how easily the melted plastic flows around inside the mold, which can affect the general quality of the final plastic product.
The factorial design method states that to examine the effects of all the parameter changes, there would need to be 2^3 treatments. ANOVA analysis was also used, which is an alternative statistical method that uses a probability value to either accept or reject the null hypothesis that certain variables affect each other.
The experimental procedure was as follows: the preliminary changes in operation variables were input to the injection molding machine, 3 polymer samples were then collected, and finally the samples were analyzed using a tensile test machine.
Design Parameters of Treatments in 2^3 Factorial Design
Sample stress/strain curve for Treatment 1
To observe the effects, the fundamental theories of tensile stress, engineering strain, and Young’s Modulus were applied, using a tensile test machine that generates a stress/strain curve for each polymer sample.
Lastly, plots of the linear portion of the stress/strain curve were collected for each treatment, and p-values for the ANOVA method were calculated, indicating the most effective treatment for the polymer.
Results and Reflection
It was found that Treatment bc gave the highest Young’s modulus value and yield strength, while Treatment 1 gave the highest tensile strength at break. Due to the trends of the former resembling pure HDPE and the latter resembling pure LLPDE, it was concluded that, based on the changes in operating variables, the results were anticipated. Furthermore, it was also determined that the most significant factors were the injection pressure and melting temperature.
The most important takeaway from this project was using factorial design and the ANOVA method to draw conclusions. The factorial design method does a great job of explaining how combinations of different variables affects output while the ANOVA method statistically shows how different variables affect each other. These methods of drawing conclusions are extremely valuable in a variety of engineering applications and I can see them being relevant in many design/prototyping scenarios.
Overview of the injection molding process