Thanks David. I am afraid I did not use the triangular blocks of wood, as I thought the purpose was purely to save volume/cost of material and I did not realise there was another reason, to put a limit to thickness. Some parts of my casting are thicker than 1/2" so perhaps this was my mistake - though I saw no evidence of increased temperature, none at all.

Interesting. The swelling seems to have deflated now and although some distortion remains, I can force the casting back into the cavity, so all is not lost and I think the casting will still do the job satisfactorily. 

I can not think of any further explanation. There may have been some other error on my part of which I was not aware. I am tempted to buy some more, and see if I can do a better job next time, and learn something, but as you say, it is expensive stuff.

It has been a worthwhile job, if a little unsatisfying not to know exactly what should have been done better. Thank you for your advice and for taking an interest, I do appreciate that.

As an adjunct to my 15^th May posting I have to report a further failure of my urethane casting. On 15^th May I reported that the results were good except for the last of the four castings in which I ran out of material and used dregs from the containers, probably no longer the correct mixture and possibly not stirred enough as I was panicking a bit at that stage. Unsurprisingly that casting did not cure properly and parts of it remain sticky and soft.

Now I have found a problem with the second of the four castings: over a period of a few days a kind of gaseous reaction must have taken place as a large blister is appearing, which has also had the effect of distorting the casting out of shape. See photographs here.

Now, I am sure the proportions were correct, as I was not even measuring at this stage but mixing complete 2-part packs. And I am also pretty sure I did stir it well. The usage time is given as about 4 minutes for this product, and I am pretty sure I was manually mixing quite thoroughly for about two minutes before pouring. The first casting I did, which remains deliberately bonded into the tabernacle, seems to be good. The double photograph shown is of the second casting I did. Now the only difference in procedure between the two pours, is that the first one was done in one shot, what was left over was poured into the next mould but was not enough to fill it. So I went away and mixed up the next batch and after a few minutes competed the pour. I noticed that the previous pour was still liquid or soft at that stage, but in any case I had been told that it would be OK to pour in two stages. I am not sure now if that was good advice, because that is the only difference in procedure that I can see between a good casting and one which some days later is showing a defect.

In this case I am not too worried as I only need two good castings – and even that one which has now gone a bit out of shape I can probably release that gas and bend it back into shape – these blocks are going back into the cavity they came from, locked in and bolted up and they will still do an OK job. Also I am still in a position where I can cast another one if necessary. I still think the method is good where a little flexibility is useful, eg packing at the top of a tabernacle. And if there is any fault it is mine.

However I do want to let David know, and to add this to the bank of information we have. My personal conclusion is I will only use this stuff where there is good reason to prefer it over the familiar old epoxy – be very, very careful about proportions and thorough mixing – and unless a better explanation comes forward, try to calculate the volume required correctly and do the pour in one shot.

Anonymous wrote:

I have "personally" used cast delrin bushings, e.g. on motor vehicle suspension, also machined it for other purposes. Indeed, I have a set about 3' away from me as I type and some raw bar in the next room. Have you?

Yes. In a lifetime of designing and making things, I've machined most of the common engineering materials, including the plastics. Your Delrin bar is extruded, not cast. 

The reason I mentioned castable is that I understood the original poster wanted a square block rather than a cylinder (or rod) as it is usually supplied. I don't know of anyone supplying larger square blocks.

I buy my engineering plastics from https://www.directplastics.co.uk , and they list several materials in 50mm thickness of sheet. I would choose https://www.directplastics.co.uk/pvc-sheet for this application, for a suitable balance of material properties and economy. Still, the price is high, at £72.85 + carriage + VAT for a square block 250mm x 250mm x 50mm, and a big bandsaw is going to be needed to cut the hole for the mast. All in all, casting a 2 part polyurethane material is the better way for those without access to a well-equipped workshop, if a plastic is preferred; but a stack of plywood squares, epoxied together, is also a workable solution.

Graeme Kenyon wrote:

Pouring urethane:  some feedback for David

David Tyler is also your 'Go-to' man about solid plastics, having used them in his self-steering gears, rudders and leeboards in the past.  In your neck of the woods, of course, there are a number of industrial plastics specialists.  I used to deal with one in Nelson, but they don't appear to be there any more: I tried to find the name.  But they were very helpful.

BTW, your comment:"Failure to do so does not just result in delayed curing and possibly slightly lower quality (as would happen with epoxy)" is not quite correct as applies to epoxy.  Incorrect mixing ratios and insufficient stirring means that epoxy will not cure to full strength and may well be considerably weaker.  I have had it happen twice on this project: in the first place because I hadn't correctly appreciated the ratios a set of pumps mixed at and the second because one of the pumps had a blockage that I only noticed when I came to mix up another batch.  The first one I remedied (I can hardly say corrected) with mechanical measures, the second I took apart and glued again.

Being a female of the species, I have a tendency not only to read the instructions, but also the spec sheets.  Always worth it with expensive, two-part products!

It's great to hear that you are making good progress, again. I'm looking forward to seeing your boat come sailing up the river in the not-too-distant future!!