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[Alan via Personal_Submersibles]

Hank,
you are making great progress there.
In case this helps, I heated a 1" thick bar of cast acrylic to form it in to a flange
for glueing to a dome for mounting. Even at forming temperatures it was very
hard to bend. It was similar to the hardness of an eraser.
Alan

> On 30/08/2019, at 12:51 PM, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> Sean,
> What about a pedestal in the centre supporting the apex
> hank
> 
> On Thursday, August 29, 2019, 6:44:32 PM MDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> 
> You definitely don't want to block off the interior surface. (Or if you did, would need to slow down your heating and cooling ramp rates sufficiently to avoid a temperature differential from inside to outside). If time isn't an issue, there is that. Ideally you would apply and remove heat evenly all over the part, so your convection should somehow direct air across both surfaces. Constraint may not be strictly necessary with a reasonably thick dome. You just want to discourage any creep under its own weight.
> 
> Sean
> 
> 
> -------- Original Message --------
> On Aug. 29, 2019, 18:31, hank pronk via Personal_Submersibles < personal_submersibles at psubs.org> wrote:
> 
> Thanks Sean,
> I do have a circulation fan installed and there is a vent, but did not think to constrain the base.  I was thinking about placing the dome on glass with the flat edge of the conical seat on the glass to help it maintain its shape.  The worry there is heat transfer through the glass.  Maybe a glass donut so the heat can pass through the hole under the dome
> Hank
> 
> 
> On Thursday, August 29, 2019, 6:21:26 PM MDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> 
> Annealing temperatures are lower than forming temperatures, so you should be shy of the plastic deformation limit, but you need to be wary of the potential for local hot spots that are not necessarily represented by your oven temperature measurement. Hence the requirement for forced air circulation, which should help to even out the applied temperature. You also want to avoid having a completely sealed oven - you want to bleed some air, and have some makeup air draw. Something like two air changes per hour, which will serve to vent off any polymer or contaminant vapors that could otherwise condense on the acrylic surface during cooling. Consequently, you also need to be careful to avoid cold spots as a result of the makeup air. Put some thought into the convection. You don't need 100% support of the dome at annealing temperatures, but it could be useful to constrain the base in order to provide some dimensional stability. Whatever you do, you don't want to interfere with the ability of that heat to soak in and to conduct out though.
> 
> Sean
> 
> 
> 
> -------- Original Message --------
> On Aug. 29, 2019, 18:04, hank pronk via Personal_Submersibles < personal_submersibles at psubs.org> wrote:
> 
> Hi All,
> I am getting close to my first anneal of my acrylic prior to machining.  I have a new radius cutting attachment built to machine the new dome and the oven is done tomorrow.  I also have a detail plan on machining steps.  The only item not resolved is, do I need to support the dome in the oven when I do the final annealing?
> Hank
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