I got some Nylon 12 samples back from the laser sintering process today. They look great - by far the smoothest of all the samples so far. Both the spigot holes are a smidge too small though. So now I'm going to measure them up to see how different they are from the CAD model, then tweak the CAD model accordingly before having any more made.
I'm also planning to sacrifice one in the oven to see how they cope with heat. The chap that made them for me said the melting temperature is just over 180 degrees, and they come out of the machine at about 140 degrees. I can't see the working temperature in the carb getting that high as the nearest heat source is the coolant in the choke. But I'll stick one in the oven and take it to 150 degrees for instance, if it's still rigid enough to do its job then I think we'll be good to go.
Last edited by CJH on 03 Dec 2015, 20:16, edited 1 time in total.
"I'm a man of means, by no means....King of the Road!"
melting temp is not important, rated service temp is, if you exceed the rated temp for long periods the material will degrade and crack, sound familiar?
Ask for a material data sheet
matweb is graeat for data.
Thank you all for your previous assistance with my van’s choke problem. I’m just about to get our van round to our local garage, who are going to pull the carb off the engine, and take photos of the bits that have broken. Could I post the photos here on the forum, so you all can see what I need, please? Hopefully, someone out there will then be able to tell me where to locate new / replacement parts…… Thanks!
Just to note My interest, if they work out a sensible price I'll take four or five for stock, we have three vans running DG carbs and I have at least two carbs sitting on the shelf for rebuild ..... Someone pm me when you get to the Guinea pig testing stage, Incase I don't get by this thread at the right time....
( every time I get ten mins to log on here I end up with a hour of admin stuff to see to and then don't get any time to investigate !!)
Ian's post this afternoon reminded me that I hadn't done either of the things I said I'd do to finish off this experiment. So this evening I tested one of the laser sintered nylon parts in the oven. It was a pretty crude test - just a test of what I could do to the part with a couple of pairs of pliers after 10 minutes in the oven at a given temperature. At 100 degrees I couldn't tell the difference from room temperature. At 120 degrees I could maybe detect a bit of extra flex in the part. At 150 degrees a good hard squeeze with the pliers left the part permanently deformed, but I doubt the forces on this part would ever get up to that kind of level. I don't know how else to test the suitability of this Nylon 12 material. I don't really think it's likely to get to much above 100 degrees in operation inside the carb anyway.
Itchy's suggestion of Matweb gives a long list of materials with 'Nylon 12' in the name. And actually the properties of the parts that I had printed aren't important - it's the properties of the material that digits2widgets uses that's important, so I've emailed them for more information.
The other thing I was going to do was compare the 'as printed' dimensions to the specified dimensions in my CAD model, as both of the spigot holes turned out a bit too tight. I've done that and I now need to tweak the CAD model so that the finished items come out as planned.
"I'm a man of means, by no means....King of the Road!"
I agree water cooled manufold between the engine and carb probably won't get much above 100.
Would be worth using a thermocouple on a run on the carb to prove it.
Put my name down for a few.
E D I T, petrol boils at 95 degrees so carb must surely not be higher or the petrol would boil in the float chamber, I suppose it's being replaced by cool fuel but In traffic there won't be much flow.
itchyfeet wrote:
E D I T, petrol boils at 95 degrees so carb must surely not be higher or the petrol would boil in the float chamber, I suppose it's being replaced by cool fuel but In traffic there won't be much flow.
Yes, I imagine the petrol has a cooling effect. Don't forget there's a return flow back to the tank, and even when the engine isn't using much fuel the flow rate from the pump is probably enough that there's flow back to the tank. I can't remember where that returned fuel comes from though - does it come from the float chamber?
"I'm a man of means, by no means....King of the Road!"
Mocki wrote:and there is no petrol at the carb at all if you are on lpg, so no cooling effect if there was any ....
I didn't mean that, what I meant is a designer must aim for a carb temperature to be kept below the boiling point of the petrol, probably the reason for the thick insulator spacer between carb and inlet manufold,
The flow of petrol will mean the petrol in the float chamber will not be there for long so it may be that the carb could be slightly above the boiling point and it still won't boil.
I would have thought that the volume of air passing through a good conductor is more than enough to cool a carb.
If you take out the squeeze bang and blow.
Two litres of air times 250 revolutions per minute approx
Where those cranks are is right next to a venturi, and a constant cooling from the carb body
I think I'd agree. I can't recall the carb ever being too hot to touch, but that means very little as I'm not in the habit of poking around in there when the engine's hot.
The information from digits2widgets is that the material is something called PA 2200. The melting point is between 172 and 180 degrees, and the Vicat softening temperatures are 163 (B/50) and 181 (A/50). That B/50 test means that a 1mm2 flat ended needle will penetrate to a depth of 1mm under a load of 50N (~5Kg) at 163 degrees. The A/50 test is the same test but with only a 10N load.
The finished parts are apparently slightly porous so they advise sealing them if they're being used with fluids, but in this application the part is supposed to remain dry.
I think based on this it's worth having a batch made, although I might look into how I can measure the carb temperature first.
"I'm a man of means, by no means....King of the Road!"
Had a few mins to look at one of the carbs on my shelf today , this bit was broken on one of them
I took it from the scrap carb I have , wasn't the easiest bit to change !
Perhaps we should look at some of these if the other bits are successful ?
I'm pretty sure I have another carb with the same fault too , when I get chance to look
