Thanks pf. I spent a few minutes on google and wasn't quite satisfied that I understood the significance and the big picture of how it plays into vpi process.
I can understand the fluid like property is good for resin penetration and the egel like property is good for resin retention.
Very briefly, how is the fluid-like property activited during the vpi process?
I believe this is for random wound machines (where there tend to be larger voids to fill), not form-wound where my experience is mainly with Epoxylite 347. This becomes very much less viscous at elevated temperature (it is introduced into the VPI tank at about 60deg C), I don't know if thixotropic resins do likewise. Incidentally, one way of keeping non-thixotropic resins in the winding after VPI is to rotate cure.
I meant to add that it looks like you give thixotropic resins a good stirring up before introducing it into the tank, and they stay thin for long enough to get them into the winding.
We utilize a thixotropic resin here to contain crossover junctions on a PMDC motor and as stated by UKpete, the material is highly viscous at room temp, but becomes very much less at the cure temperature allowing for high wicking properties.
Hello Pete....I just got a chance to jump on line and check out eng-tips, I wanted to add to all of the above good explanations. We use cc118-LV Dolph's class H epoxy resin in our VPI system. One part, low viscosity, thixotropic, impregnating epoxy compound. Has past Navy total submersion testing per MIL-M-1760E. Impregnates and encapsulates in one cycle.
Thixotropic-High and low viscosity with the same resin.
Low viscosity after agitated penetrates multiple layers of mica tape.
High viscosity at rest, with complete encapsulation.
Very expensive epoxy resin, but worth every drop, it must be good stuff if the Navy uses it...
I didn't explain very well as requested in the above post on how this material works in the vpi process. Place the unit in the vacuum chamber and apply dry vacuum for 20 minutes at approximately 4mm Hg pressure. During the dry vacuum cycle, agitate the resin in the reservoir for 15 minutes. This procedure will reduce the viscosity of the resin allowing for maximum impregnation.
Immediately after mixing, transfer the resin to the vacuum chamber, allowing it to flow up from the bottom of the chamber to cover the unit by a depth of at least 1" Maintain vacuum for 60 minutes. Release vacuum and apply pressure of 90 to 100 psi for 30 minutes and release pressure. Remove the unit slowly from the resin. A rate of 4" per minute is recommended. Cure in bake oven at 300f for 8-10 hours for maximum resin retention and encapsulation....
Suggestion to the previous posting: It may be necessary to rotate the unit to preserve a somewhat uniform layer of the resin in certain parts of the process. Also, the unit motion must be accomplished by an appropriate support.
motorhead, is it necessary to warm the resin or the winding beforehand?
What I've noticed with non-thixotropic resins is that in the oven the resin initially goes through a very low-viscosity stage before it starts to gel, and this is when a lot of run-off occurs. I guess there is better retention with the thixotropic resin.
It is recommended before placing the unit in the vacuum chamber, preheat the unit to a temperature of 250f-325f., Cool to a temp of 130f to 150f. The time required will depend on the size of the stator or unit. This process is also to ensure and remove any residual volatile particles or moisture incured in the winding process. Cured resin develops 10,100 psi tensile strength to provide mechanical stability to the winding which helps to withstand centrifugal force on rotating windings, as well as inrush current stress on both rotating and stationary windings, form or mush.
