07/22/2022
By Sokny Long

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a doctoral dissertation defense by Matthew Bont on “Factors Affecting Anisotropic Shrinkage in LSR Injection Molding.”

Ph.D. Candidate: Matthew Bont
Defense Date: Wednesday, Aug. 3, 2022
Time: 1 to 3 p.m.
Location: ETIC 445, North Campus

Committee Chair (Advisor): Stephen P. Johnston, Professor, Department of Plastics Engineering, UMass Lowell

Committee Members:

  • Carol M.F. Barry, Professor & Chair, Department of Plastics Engineering, UMass Lowell
  • Margaret Sobkowicz-Kline, Associate Professor, Department of Plastics Engineering, UMass Lowell

Brief Abstract:

This research on liquid silicone rubber (LSR) injection molding includes a literature review of LSR injection molding process variables and modeling. Topics include LSR materials, primary injection molding variables and their effects, and factors affecting shrinkage. The review also covers current LSR injection molding models and simulation work, followed by a discussion of relevant case studies.

Then, anisotropic shrinkage in LSR injection molding was investigated by varying mold temperature, shear rate, cross section thickness, and fill time. Optical measurement of physical trace markers was used to measure process driven shrinkage of the samples. These showed linear shrinkage ratios between the shrinkage in the flow direction against shrinkage in the crossflow direction from above 1.30 to below 0.92. Anisotropic shrinkage increased with increasing average degree of cure across the thickness with increasing mold temperature, increasing fill time, and decreasing wall thickness. An inflection point was found near 0.6% average simulated degree of cure, where anisotropy starts to increase more rapidly. Shrinkage anisotropy was found to generally increase with increasing shear rate in regions where curing was significant.
Finally, simulation and shrinkage predictions were investigated in a blind validation study. Linear part shrinkage was predicted using a correction factor model as a function of average degree of cure and simulated local cavity pressure fit with experimental data applied to the simulation results. The validation study was first conducted using a typical gasket geometry with an average shrinkage difference of 0.35% cross 4 regions from experimental shrinkage. The corrected predicted final shrinkage was an improved average of 0.1% shrinkage difference from experimental results. Second, a diaphragm geometry showed an average shrinkage difference of 0.25%. However, the corrected predicted shrinkage decreased in accuracy due to differences in radial flow effects and an underestimation of cavity pressure. Further improvements could be realized by further material and flow pattern characterization and cavity pressure instrumentation.
The results demonstrate that LSR does in fact display anisotropic shrinkage behavior that is being driven by the cured in molecular orientation during filling/curing. These results demonstrate the possibility of designing more accurate molds at reduced cost/time by incorporating an improved understanding anisotropic shrinkage into existing simulation models.

All interested students and faculty members are invited to attend the defense.