Pattern or mould making foam?

Cato · 2026-01-01T19:36:13+0000

I need to make a section of tube to bring a ram air duct to an air box. Most of it will be easy enough to work out by measurement with a bit of cardboard aided design, but one section would be difficult and time consuming to do that way. So I'd like to make a form from foam and go from there.

My question then is what do I need to look for and buy that is stiff enough to hold a shape, easy enough to shape with a knife, dreadnought file etc, and has a fine enough surface when cut and shaped to act as a mould or be scanned for a 3D print?

Dimensions wise the finished piece will be about 450mm long, 150mm high and 50mm deep/thick. It's curved so I would think it'd need to start out 150mm deep/thick to then be shaped as a curve. There are other shapes, depressions and protrusions to be included but those are the basics.

wyn · 2026-01-01T19:40:03+0000

Maybe some sort of modeling/tooling board.

Bobtail4x4 · 2026-01-01T19:51:09+0000

find some kingspan in a builders skip. a quick wipe with filler will give a surface to mould,

ajlelectronics · 2026-01-01T19:53:39+0000

Cato said:
or be scanned for a 3D print?

It would be far better drawn in 3D. Is it really too complex a part to be drawn? I would be happy to have a go at it for you otherwise.

Cato · 2026-01-01T20:18:46+0000

ajlelectronics said:
It would be far better drawn in 3D. Is it really too complex a part to be drawn? I would be happy to have a go at it for you otherwise.

I've no idea what's involved in drawing something for printing, so it's possible. I'd still need to make up the form to take measurements from, and when done it'll be a fairly complicated shaped duct.

As a rough description there will be two oval outlets a third of the way in from each end, the ends will be closed and on the bottom side there will be a single oval inlet about 150 wide that drop down and turns through 90 degrees in the opposite direction to the outlet. The main body itself will be a banana shape with the outlets in the concave side. The convex side will have 3 (from memory) shallow depressions in it, and 2 protrusions that locate in the frame.

So to me with my lack of knowledge or experience in 3D printing and CAD it would seem that scanning was the way to go?

I'll grab some pictures of the location it sits when I'm back in the workshop tomorrow which might help.

Cato · 2026-01-01T20:20:35+0000

wyn said:
Maybe some sort of modeling/tooling board.

That looks like it'd work, and I could build up a layer or two in the required areas rather than carving stuff off of a big chunk.

Keith 66 · 2026-01-01T20:28:46+0000

Bobtail4x4 said:
find some kingspan in a builders skip. a quick wipe with filler will give a surface to mould,

I have used kingspan / celotex for plugs & it is fast & easy to carve but too soft for anything with detail. It takes body filler ok but the filler is much harder & if you go through it you will hog the foam down instantly leaving low bits.
You can buy Sikablock modelling foam in various densities from East coast fibreglass, the .70gm one is quite hard similar in density to cured bodyfiller, with the .24 gm version being quite a bit faster & easier to shape.

sako243 · 2026-01-01T23:36:26+0000

Does it need to be foam? Can you not just use clay?

Dapph2 · 2026-01-01T23:59:15+0000

Cato said:
That looks like it'd work, and I could build up a layer or two in the required areas rather than carving stuff off of a big chunk.

Hard blue closed cell foam insulation sheeting is also good as it is available on 40 mm or so thicknesses . Builders merchant's often have damaged sheets for sale or in their skips .
Here it is at Amazon in handy small sheets for about £15 .

Sanding or carving it with a new Stanley blade it works well if your gentle .

https://www.amazon.co.uk/TAIANJI-Polystyrene-Rectangle-Foamboard-Modelling/dp/B0DKNQRYG9/ref=pd_lpo_d_sccl_1/521-7883890-8459362?pd_rd_w=OBDlo&content-id=amzn1.sym.bb13d3fc-af40-4fff-a822-e0e4c415da96&pf_rd_p=bb13d3fc-af40-4fff-a822-e0e4c415da96&pf_rd_r=HQ7PGPJ83GK9S3ZQP4EF&pd_rd_wg=ros96&pd_rd_r=6439c447-9abb-4f20-a2b7-bc3cab3c690a&pd_rd_i=B0DKNQRYG9&th=1

Cato · 2026-01-02T06:49:53+0000

sako243 said:
Does it need to be foam? Can you not just use clay?

I think clay would be problematic as whatever I use has to be self supporting on both sides due to the voids.

This is where it's going, in between the airbox and the frame at the top and pick up the two airbox inlets.

Then the lower section will come down and forward to pick up airflow from the front with a section of ducting connecting to the new part from below the fairing/headlamp. That will either be a 'Y' piece to come from a single opening on the new part splitting to go either side of the radiator, or I may use two openings on the new part and run a tube either side of the radiator.

Whichever way the air enters, it's got to be picked up in front of the radiator so that it's picking up clean cool air.

daedalusminos · 2026-01-02T07:31:28+0000

The correct way to model parts like this is to scan that section of the bike including all connecting points (and obstacles), bring that model into CAD and design the custom part to fit.

oldbikedavey · 2026-01-02T07:47:41+0000

What is the intended final material ?

RichardM · 2026-01-02T08:04:30+0000

Foam carving blocks, blue builders insulation is good too.

Cato · 2026-01-02T08:06:07+0000

oldbikedavey said:
What is the intended final material ?

I'm not 100% but the existing stuff (airbox and ducting) is ABS plastic which stands up well and from what I read is easier to print than nylon etc. So I reckon sticking with ABS would be the best way forward.

Morris · 2026-01-02T08:28:16+0000

Another vote for high density blue foam.

Cato · 2026-01-02T08:35:32+0000

daedalusminos said:
The correct way to model parts like this is to scan that section of the bike including all connecting points (and obstacles), bring that model into CAD and design the custom part to fit.

While that may be possible I don't think it's practical for a one off like this.

indykid · 2026-01-02T10:16:02+0000

daedalusminos said:
The correct way to model parts like this is to scan that section of the bike including all connecting points (and obstacles), bring that model into CAD and design the custom part to fit.

If you're just scanning a physical part that has been made and fits in the frame so you can extract the surface and print it (I appreciate that's wildly oversimplified), I'm not sure I see the necessity to scan the envelope.

If you wanted to do some iterative design optimisation, it would definitely be the right approach, but if it's just a case of 'a 3D print of this foam plug, please', the scanning is just a modern approach to taking the plug and asking a fibreglass place to make you one.

Pigeon_Droppings2 · 2026-01-02T10:20:07+0000

With my boat repairs I've been using PET foam core for bulkheads...that stuff is easy to carve and also very dense too. You could even fibreglass it if you wanted!

The down side is it's quite expensive.

Tangledfeet · 2026-01-02T10:37:07+0000

@Cato What sort of size (LWD) do you reckon you'd need..? We use blue styrofoam for design stuff at school and I might be able to smuggle some out and post it to you....

This stuff: https://www.km-wholesalesuppliers.co.uk/RANGE728-297-Craftfoam-Blue

Cato · 2026-01-02T11:04:01+0000

Tangledfeet said:
@Cato What sort of size (LWD) do you reckon you'd need..? We use blue styrofoam for design stuff at school and I might be able to smuggle some out and post it to you....

This stuff: https://www.km-wholesalesuppliers.co.uk/RANGE728-297-Craftfoam-Blue

That's kind of you but given the amounts involved it won't be expensive to buy what I need. :thumbup:

Pattern or mould making foam?

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