Re: [Emc-users] polyurethane resin casting

[Erik F. <er...@ae...>]

Did a vacuum pour test.  Complete failure as far as I am concerned.  The
small amount of air introduced in the connection, as well as the small
amount of air in the mix becomes an instant mini volcano.

One option that could work for me is a double pour.  Does polyurethane
water tight seal on "cold" joints?


On Sat, Sep 28, 2013 at 7:43 AM, Erik Friesen <er...@ae...> wrote:

> The current resin I use has a 50 second gel time.  Its a little like
> flying rc, if it gets ahead of you you are in trouble.
>
> I wonder if a person could make a inlet control valve with a piece of
> tubing and clamp, or special clamp?  Something that would take about 15 psi
> forward pressure to open the clamp.
>
>
> On Sat, Sep 28, 2013 at 7:16 AM, Erik Friesen <er...@ae...> wrote:
>
>> For the high speed vacuum, why not just use an external tank, with
>> plumbing valve or equivalent to switch to whichever has highest vacuum?
>>
>>
>> On Sat, Sep 28, 2013 at 12:13 AM, Gregg Eshelman <g_a...@ya...>wrote:
>>
>>> On 9/27/2013 6:29 PM, Erik Friesen wrote:
>>> > Do you think vacuum alone would pull into every nook and cranny?  I
>>> have
>>> > looked for other people that have tried something similar, but didn't
>>> find
>>> > much.  By vacuum, my idea is to in some way place the mold under
>>> vacuum,
>>> > and fill it using atmospheric pressure, perhaps using some type of
>>> funnel
>>> > and valve.  If you do calculations on a 4 sq in mold at my pump rating
>>> of
>>> > 10pa, it calculates to a .07" cubed volume at atmosphere of 101kpa.
>>> >
>>> > Whats the demold/depressurisation time for pressured polyurethane?
>>>
>>> It depends a lot on the resin. Fast cure ones like you're using could
>>> come out in a few minutes to an hour, just to be sure. Most of what I
>>> use is Smooth-On Crystal Clear and I leave it under pressure 24 hours. I
>>> also post-cure at 145F with convection heat so if not left under
>>> pressure long enough I've had problems with small bubbles forming.
>>>
>>> I've also used some of Smooth-On's faster resins like Onyx and have been
>>> able to take out of pressure in about 30 minutes. That one cures hard
>>> enough it doesn't need post curing.
>>>
>>> Think of using pressure like what a deep diver experiences, except for
>>> with the resin and silicone keeping it under pressure long enough
>>> ensures the gasses never get back out.
>>>
>>> You can speed up the cure by shining a halogen work light on the
>>> pressure tank or putting the tank in a very warm room. I've cast
>>> silicone molds under pressure in the middle of winter in a poorly heated
>>> shop, with a halogen lamp shining on one of my larger tanks. Most resins
>>> and silicones need to be at least 75F to cure properly.
>>>
>>> Therin lies a useful property that can be exploited some times. Chill
>>> the components prior to mixing to stretch out the pot life, then after
>>> pouring into the mold, apply heat to speed up the cure.
>>>
>>> However, some resins can fail to cure properly if they're not at the
>>> proper temperature or higher when mixed. In such a case, mix the resin
>>> then fill the mold in a cold room, then transfer to a warm room to get
>>> some more working time.
>>>
>>> If you have a bunch of leftover silicone after casting a mold, freeze
>>> the leftovers. It'll bring the cure nearly to a halt. I've frozen mixed
>>> silicone up to a week and still been able to use it to make molds. It
>>> will be thick but will still flow.
>>>
>>> Another trick I've used to alter curing times is mixing resins from the
>>> same product line with different cure times. I had to cast some large
>>> but thin parts. The longer cure time resin wouldn't properly set in such
>>> thin castings and the short cure time that would set in thin castings
>>> would kick too fast to be able to get the mold filled. So I mixed the
>>> two in a 50:50 ratio and the properties came out somewhere between,
>>> stretching the time long enough to be able to fill the mold and get it
>>> in a tank, while still generating enough heat from the chemical
>>> reactions to fully cure. I told Smooth-On what I did. No reply but they
>>> later added another resin in the Crystal Clear line with cure properties
>>> between the 202 and the longer cure time number. I think they've also
>>> altered the 202 because I can use it as-is in the molds that I couldn't
>>> when I was mixing the two, and even if I do heat the tanks or set them
>>> on the concrete in front of my shop on a sunny day, the castings still
>>> need post curing.
>>>
>>>
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>>
>>
>