aja540i 1906 Report post Posted October 25, 2023 (edited) Expanding on an earlier post, i need to replace a 45 year old dust seal for the steering rack on the E21, the part is badly perished and not available as a single part, so i thought i would have a crack at making a new one from scratch. Step one, machined up an aluminium mould, 3 pieces that bolt together with an opening at the top to pour the goop in to, took about 9 hours all up but i'm not a machinist so i am just happy that i didn't throw it out of the lathe half way through!! Bought some casting plastic, mixed it and poured first attempt... Had a few issues, first up i bought the wrong product! the part came out ok but ended up being ridgid plastic rather than flexible so no good for a seal, other issue was a few bubbles on the edge that seals against the steering rack housing, so ordered new product and had a second go... This piece is better, although it could have had more pigment added, nice and flexible, size and shape are ok, but still has a few bubbles, Not worried about the small cosmetic ones in the center but the outside edge needs to be smooth and continuous as it forms the seal,, so next step will be to play around with purging the mould so there is less moisture to react with the urethane and cause bubbles, if that doesn't work i will look at vacuum de-gassing.... Under side of original part... Assembled mould ready to pour... Edited October 25, 2023 by aja540i 5 Quote Share this post Link to post Share on other sites
KwS 2425 Report post Posted October 25, 2023 Very interesting. I wonder if you can put it in a bag as it sets and pull a vacuum on it, if it would get rid of all the bubbles? Quote Share this post Link to post Share on other sites
aja540i 1906 Report post Posted October 25, 2023 I think the main issue will be that the shape of the mould directs the bubbles to the edge where I don't want them to be, I have a cunning plan to try and solve that but I may also have to vacuum it if that plan doesn't work out.... the other factor is that the product only has a 4 minute pot life, so no time for mucking about! Quote Share this post Link to post Share on other sites
aja540i 1906 Report post Posted October 26, 2023 (edited) okay, had another go, rigged up a contraption to hold the mould on an angle and rotate it slowly while i poured the product, hoping to get a nice smooth sealing edge. Also purged the mould with argon before pouring to try and displace air / moisture. Unfortunately none of that helped.... i did get the colour right this time though! Bugger. Will have another go with this setup and a slightly different pouring technique to try and get it right, if that doesn't work i will investigate de-gassing... Edited October 26, 2023 by aja540i 2 Quote Share this post Link to post Share on other sites
jom 98 Report post Posted October 27, 2023 Past experience with polyurethane resin castings of model cars shows that air bubbles are difficult to remove, even with a vacuum. You need to get the resin as mobile as possible, and a 4 minute cure time doesn't help. Would heating the resin make it more fluid? Or maybe look for a resin with a longer cure time? Quote Share this post Link to post Share on other sites
aja540i 1906 Report post Posted October 27, 2023 Recommended working temp for this product is 23 degrees so heating it probably won't help, the issue with vacuuming it is that the bubbles have no clear path out of the mould, they are trapped, so I might try to add some risers to the mould to give them somewhere to escape to. Ideally would like to stop bubbles from forming in the first place but that is a challenge when the urethane reacts to the moisture in the air, and I can't afford a seat in Elons space Uber! I have also ordered some more sensitive scales so I can mix smaller batches of product and not waste so much, the kitchen scales from Briscoes ( they were having a sale!) aren't the best for this. Quote Share this post Link to post Share on other sites
Allanw 1071 Report post Posted October 29, 2023 On 10/27/2023 at 4:25 PM, aja540i said: Briscoes ( they were having a sale!). 🤣 That hardly ever happens! Quote Share this post Link to post Share on other sites
