THERMAL ANALYSIS AND EVOLUTION OF SHAPE LOSS PHENOMENA DURING POLYMER BURNOUT IN POWDER METAL PROCESSING
Open Access
- Author:
- ENNETI, RAVI KUMAR
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 18, 2005
- Committee Members:
- Randall M German, Committee Chair/Co-Chair
Joseph Lawrence Rose, Committee Member
David John Green, Committee Member
Renata S Engel, Committee Member
Ivica Smid, Committee Member - Keywords:
- Powder metallurgy
polymer burnout
stainless steel
shape loss - Abstract:
- Powder metallurgy technology involves manufacturing of net shape or near net shape components starting from metal powders. Polymers are used to provide lubrication during shaping and handling strength to the shaped component. After shaping, the polymers are removed from the shaped components by providing thermal energy to burnout the polymers. Polymer burnout is one of the most critical step in powder metal processing. Improper design of the polymer burnout cycle will result in formation of defects, shape loss, or carbon contamination of the components The effect of metal particles on polymer burnout and shape loss were addressed in the present research. The study addressing the effect of metal powders on polymer burnout was based on the hypothesis that metal powders act to catalyze polymer burnout. Thermogravimetric analysis (TGA) on pure polymer, ethylene vinyl acetate (EVA), and on admixed powders of 316L stainless steel and 1 wt. % EVA were carried out to verify the hypothesis. The effect of metal powders additions was studied by monitoring the onset temperature for polymer degradation and the temperature at which maximum rate of weight loss occurred from the TGA data. The catalytic behavior of the powders was verified by varying the particle size and shape of the 316L stainless powder. The addition of metal particles lowered the polymer burnout temperatures. The onset temperature for burnout was found to be sensitive to the surface area of the metal particle as well as the polymer distribution. Powders with low surface area and uniform distribution of polymer showed a lower burnout temperature. The evolution of shape loss during polymer burnout was based on the hypothesis that shape loss occurs during the softening of the polymer and depends on the sequence of chemical bonding in the polymer during burnout. In situ observation of shape loss was carried out on thin beams compacted from admixed powders of 316L stainless steel and 1 wt. % ethylene vinyl acetate (EVA). The results showed that shape loss primarily occurs by viscous creep during the softening of the polymer. At the onset of burnout of EVA, a recovery in shape loss was observed. The recovery occurred primarily during the first stage burnout of EVA and was attributed to the formation of polyethylene co-polyacetylene which forms with a carbon double bond. The in situ strength was also found to increase during the formation of polyethylene co-polyacetylene. No recovery of shape loss was observed during burnout of polymers (polyethylene and polypropylene) which convert to yield hydrocarbons without forming carbon double bonds. The critical stages for shape loss during polymer burnout were identified in the present study. The shape loss profile and the viscosity evolution during polymer softening was modeled. One implication of the current research would be significantly improved dimensional control (possibly with carbon contamination) would be possible in powder metallurgy by the selection of polymers that form double bond during burnout, such as ethylene vinyl acetate (EVA), vinyl acetate (VA), etc.