[PSUBS-MAILIST] (no subject)

Thu Aug 29 21:21:10 EDT 2019

The apex isn't really loaded and I wouldn't expect much deformation there at all even if it did get hot. If you used a pedestal to actually bear on the ID at the apex in order to counter the material weight elsewhere, you could do more harm than good right where you need the best optical clarity in the finished dome. Where you will see the most load is at either edge of the bearing surface where the cumulative weight of material above in conjunction with a local hotspot could lead to local deformation. You could lower your temperature a bit to make sure that you're never in trouble, but you'll get the best anneal with the highest possible initial soak temperature, so might want to look at maybe making a ring just to provide some radial constraint. If you accidentally get close to a forming temperature locally anywhere, just think about how it would want to deform. The base is the only place on the part that is not supported in three dimensions by adjacent material. Anything metal will conduct better than the acrylic, so no worries about insulating the heat transfer with a metal support. Again, none of this may be necessary. It depends how confident you are in creating an even temperature with your convection, and measuring / controlling it appropriately.

Sean

-------- Original Message --------
On Aug. 29, 2019, 18:51, hank pronk via Personal_Submersibles 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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