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optimising my foam molding

Hi Again,

Well, my PU foam saga continues and I'd like to thank you all for helping me this far. My questions today relate to optimising the foams I am currently making. As some of you already know I'm making a water TDI based foam but what I may not have mentioned before is that I am trying to cast these foams in 'dogbone' shapes for tensile etc testing. I am using a 2 half teflon mold that is clamped together. The procedure I currently use is to mix the polyol and prepolymer with a mechaincal stirrer (although doing it by hand doesn't seem to make any difference) and then pour into the mold - at room temp it take only about 1.5 mins to begin foaming.

What I'm getting is slightly sub optimal. The foams produced have some large bubbles dispersed throughout them and in general the foams look heterogenous. I have tried degassing the ingredients but to no avail. So I started to read about foam mixing heads and realised how poorly controled my procedure is.

I then tried to place the mold into a chamber pressureized with N2 at about 2.5atm - but to my surprise the defect bubbles go bigger!!!!! (could the N2 be reacting with the PU?)

Anyways - my questions:

1) For molding foam, since I don't have access to advanced processing gear (ie: mixing under pressure) am I destined to make crap foams ? 2) Which is more important for molding pu foam, mixing under pressure or foaming under pressure ? 3) Currently I half fill the mold and allow the foam to fill as it expands - should I be aiming to fill the mold completely before the foam starts to expand (thus causing the foams own expansion to provide the internal pressure within the mold ?

Thanks in advance - I will update my 'non-foaming PU' question later.

Best wishes,

Gillian H

Reply to
GillianH
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hi!!! i think i can help u ..infact i am doing these days some working on polyurethane (rigid-insulation) foams . i have some answers for u ..

1.., > mixing under pressure does not effect much because we need homogenized mixing .importance is to mix, if u r using a drill machine alonge with a stirrer its ok ..it does not matter ..but major point is which type of foam u want ..what properties u need ,,,if u produce foam under pressure it will increase its densty and degree of cross linking ...and u will get density more then free rise density ...under pressure = core density ,,,,cup test = free rise

2>>> u dont need to fill the mould completly ,,coz when blowing agent will release during foaming it will automatically expand and foaming is exothermic reaction ..at the same time blowing gas producing co2 and the present oxygen will be heated due to heat produced during foaming.it will become lighter and allow the foam to expand .

ok .. i have many answer still ,ok next time wish u best of luck .. bye

Reply to
farry

Industrial processes produce free rise foams under atmospheric pressure, depression or compression. In general, moulded foams are produced under slight overpressure to optimise mould filling - the overpressure being obtained from the compressed foam gas (usually CO2 in flexible foams). Some moulded foams may also be produced using a vacuum assisted process. You can easily make polyurethane flexible foams with acceptable appearance and properties- close to those of industrial foams, with perhaps slightly coarser cells - by mixing 100-500 g of raw materials by hand or using a simple "drill mixer" as I described before. You then pour the liquids into:

- a square paper or cardboard box to represent slabstock production.

- a metal mould (greased and warmed) or a plastic mould (PE, PP, PTFE, or even silicone, built so that it can withstand internal pressure without deformation) to represent moulded foam production. then let the foam cure. You should select the size of your mould according to:

- the amount of liquid you can mix comfortably

- the "free rise" density that you expect (this can be estimated by assuming adiabatic conditions in the core part of the foam - a reasonable approximation) and therefore the volume you can produce

- the % of compression - if any- that you need to fill the mould. Simple moulds may be filled with as little as 10% compression, but complex moulds may need above 50% compression (i.e. free rise density 100 kg/m3 =>

moulded density 150 kg/m3). Once you have obtained a lump of foam, you simply cut the desired shapes for physical testing (this can be done by first cutting slices using a very sharp knife or, better, a household meat cutter, or, still better, an industrial foam cutter. Dogbones, etc. may then be stamped from a slice of the foam, or cut using a razor blade, eventually with the help of a wooden template. This procedure minimises defects in your test samples, because you can select a portion of foam which is free from large bubbles, and preferably away from the edges, which is impossible if you mould the "dogbones" directly. Hoping that this answers your 3 questions, Good luck, PS: I have no idea why you get larger bubbles when the foam is made under N2 pressure...

"GillianH" a écrit dans le message de news: snipped-for-privacy@p10g2000cwp.googlegroups.com...

testing.

Reply to
infinitum

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