Building a foam plug

[mrdebian]

Hi all,

I made 8 molds in the past all using wood as this was the only one that knew how to work with but is a very slow process.
I now have a new design (for a 17.5 foot kayak) which has been converted to a 3D.
My idea is to use a CNC to print it in foam and then make the mold from it.
As I haven't use this method before I've got a couple of questions which I will really appreciate your thoughts on them.

1. They use locally a kind of white foam which looks quiet fragile and was wondering what materials I can add to it to make it stronger before making the final sand/paint on it or if there is any other type of foam that I can use?
   
2. Does the normal waxing procedure works on that material as well? I have 100% success with my waxing method under other materials like wood and fiberglass.
3. I read somewhere that there is a kind of foam that saves you lots of sanding because of using less filleting; I'm not sure if that makes any sense the way I write it (sorry not native English)
4. I need to use the same foam plus to make 2 molds out of it so ideally it has to be one piece after the first release.

What is the procedure that you are using when making a foam plug?

Thanks a lot in advance

 

[comfisherman]

Toured a facility a few weeks ago that did cnc foam molds. It was foam sprayed over osb wood forms, the router came and cut the foam back a bit undersized, then they shot glass over the top. Then the cutter came back in and cut the glass down. It was then gel coated and sanded to whatever the desired quality needed to be.

 

[wet feet]

Do you have ready access to a CNC with at least an 18 foot X or Y axis?What type of joint do you intend to use for the hull and deck and how much of the joint feature needs to be incorporated in the plug?Strictly speaking,you can use just about any foam that will withstand the forces created by the moulding process.The choice then becomes one of whether to use a heavy coating of a primer/filler type of coating or whether to screed the entire surface with a filler that can be rubbed down for paint.You might feel it better to machine undersize and add a layer of filler all over,as described above.It works but the total machining time starts to add up and the cost reflects this.The plug will need to have a flange added to hold the correct shape at the line where the sections join,has this been added to the 3D file you would give the CNC company?
The idea of using the same foam for the second half of the boat may involve a bit more than you imagine.It will need enough reference points to allow the CNC to establish a G54 reference in all 3 axes.Which almost certainly will require a flat surface on the underside.

 

[ondarvr]

The foam was EPS, it's relatively cheap and easy to shape, but lacks durability. It's cousin is XPS, it's a bit more durable.

Neither can really be used as a mold without a coating, which can help with durability and protect them from solvents. Except for epoxy, most resin systems will dissolve them rapidly, but there are polyester coatings that don't, Styroshield is a primer and Styrosafe is a resin that can be used over either one. For one-off shapes the primer alone, or with a top coat can work. The resin is for when you need a more durable shape that may need to last longer, or even be a permanent mold.

This is something I just did yesterday for our companies 40th birthday.

EPS foam CNC cut and then coated with Styroshield primer. For demonstration purposes I sprayed, rolled and brush it on for comparison, then lightly sanded it to show the difference in surface profile of each method. I will continue to record the process of turning them into finished parts or plugs and molds.

The carbon you see is from making a serving tray with Sunshield, this is for cupcakes during the next CAMX show in Atlanta.

 

[mrdebian]

Do you have ready access to a CNC with at least an 18 foot X or Y axis?What type of joint do you intend to use for the hull and deck and how much of the joint feature needs to be incorporated in the plug?Strictly speaking,you can use just about any foam that will withstand the forces created by the moulding process.The choice then becomes one of whether to use a heavy coating of a primer/filler type of coating or whether to screed the entire surface with a filler that can be rubbed down for paint.You might feel it better to machine undersize and add a layer of filler all over,as described above.It works but the total machining time starts to add up and the cost reflects this.The plug will need to have a flange added to hold the correct shape at the line where the sections join,has this been added to the 3D file you would give the CNC company?
The idea of using the same foam for the second half of the boat may involve a bit more than you imagine.It will need enough reference points to allow the CNC to establish a G54 reference in all 3 axes.Which almost certainly will require a flat surface on the underside.

I designed the kayak in 2 dimensions and mention a few more changes along the process to a naval architect who is currently doing the 3D conversion. The CNC will make the foam in two pieces as is 3 meters. so 2 pieces the deck, 2 the hull and then will be joined.
Flange is already added to 3D.

 

[mrdebian]

@ondarvr thanks for the details. I do have access to specific polyester resin which works fine with foams. I've done a few tests already.
My question is if I use EPS foam for example and let's say Styroshield primer (I don't know if I can find that in Europe) if that along will be enough when I will have to release the mould from the plug without destroying it?

 

[ondarvr]

How well it survives depends on many factors, but shape is the primary one. A flat panel may survive making 10 parts, a complicated shape that is large would be destroyed while de-molding the first part. And that's if you didn't damage it while applying the first layer of glass.

The primer doesn't add a great deal of strength, and isn't designed to. If you need a more durable shape a layer of glass over the primer is needed.

The primer isn't a great mold surface, but the 707-002 primer does polish to a good gloss, and can be used over the Styroshield.

Think of it like an egg shell

 

[fallguy]

Why not use a pourable structural foam?

Make a wooden box, pour then cut the foam with CNC into half shells inverting with software. Coat the foam with resins and finish and the bottom and top are done.

Foam is the last thing to do cheaply, in my view.

Am I misunderstanding?

 

[wet feet]

Am I misunderstanding something?A plug will probably not be much use after the mould is pulled,so I am a bit mystified by mention of multiple parts.A foam mould is equally lacking in durability unless a glass skin is added and this would need careful consideration in light of the tendency for polyester to dissolve polystyrene.It also largely negates the purpose of using a CNC as extensive hand finishing will be necessary.

 

[mrdebian]

@wetfeet ideally I need to make two moulds from that plug in order to build two kayaks at the same time to speed up things as during summer months (May-October) I don't build any at all due to other work commitments.
It seems less work to release the first kayak from the foam plug and then use that as a plug to make another one.

 

[wet feet]

Considering the size of a kayak,i know I could produce a hull plug in less time than it would take to ship a machined plug from another country.I do have the advantage of having done this sort of thing quite often before.It is easiest with a flat joint plane between the upper and lower parts of the hull.All you have to do is set up a base board and erect the sections on it.Infill with slabs of yellow polyurethane insulation foam (although denser is better) skim the surface to just below where a fairing batten would sit and skim with polyester body filler.the sections need to just appear through the surface while fairing and once there is a skin,they can be ground back to just below the filler-which is then applied to fill the grooves and give a continuous surface.Paint,which you are going to have to do anyway,and very important you make templates of the actual shape at the baseboard.Once the mould is made you have the choice of destroying the plug and reusing the baseboard or starting with another.Where you repeat the process with the deck and use the plan view templates to make certain that the shapes will match.The cockpit rim will probably be as much work as the entire bottom.

The CNC machining route absolutely demands a good 3D CAD model without surfaces that don't quite match in terms of edge continuity and blend quality.The last thing you need is a machined plug that then needs many days of fairing and finishing.Do you have a good CAD file that needs no further work before toolpaths can be created?

 

[mrdebian]

Thanks @wet feet .
I guess my 3D will be perfect as the guy you making it has a lot of years experience in shipping industry.

What is the procedure that you receommed from your experience to follow with the plug. How it looks the following:

Get from CNC the plug, sand it a little bit if it needs and then applying a layer or resin (spefic for foam) to make it a little bit more stable.
Then sand with 320 or higher number to make it smoother and then paint it and afterwards polish it.

Would that be enough to make the mould without sticking to the mould assuming I did a proper waxing etc.
I guess I will have to use for the mould as well the same resin that works with foam.
What about the gelcoat. When it cures won't be meld/damage the plug? I guess not because it doesn't make such an exotherm?

 

[wet feet]

This may be a lengthy post.Simply using the phrase CNC machining can skim right past some of the realities of the process.To begin with you have to be in possession of a 3D surface (or solid) CAD file.the file has to consist of surfaces that don't have edges which don't match and different descriptions may be used to describe this fault.The joined surfaces will need to have normals facing in the correct direction and some CAM programs will check for this and rectify the situation.They can only do this if they can read the file format,which may mean exporting a STEP or IGES file and sending it to the guy with the CAM program.Depending on the tolerances set for each end of the process,those surfaces may no longer appear to be as nicely joined once imported.

Let's imagine that our man with the CAM system has a file of adequate integrity to proceed.What type of machine is he planning to use for the job?If he has a 5 axis machine,a few more things become possible,but these are scarcer than 3 axis machines so lets go over a few points that crop up on a 3 axis machine.To begin with there is the matter of actually being able to cut the material without the motor body or the collet nut fouling the block.For those who haven't handled them,it may be worth mentioning that an ER32 collet nut is 50mm diameter.So to determine whether the slab may have to be machined in stages it will be necessary to apply some judgement or to draw the tricky section with a cross section of the tool and the collet nut moved to the area of concern to gain some assurance.There are programs which will do this and you may care to ask your chosen machining business if they have them.Worst case,you have a near vertical section and it is deeper than the tool length;oh dear-what do we do now?Easy answer is to limit the depth of the initial block so that clearance is assured.Then skim the block flat and bond another slab in place and repeat the process.Perhaps several times.Eventually we will get our foam plug.

"What about a 5 axis machine?" you may be thinking and it may seem like a better option.I believe it is actually,but not without similar considerations.It does give several options and with a large enough flat bottom tool,the process can be much faster than zillions of passes of a ball end tool on a 3 axis machine.I had a former colleague who wouldn't believe that a flat bottom tool would produce a compound curved surface,even after I asked him to consider it as a ball end tool of infinite radius..... I will admit that for concave recesses it may be better to impose a machining boundary and use a ball end tool for those areas.If you read my preceding ramblings you will have noticed that I mentioned the possibility of the motor body coming into contact with the piece being worked on and if we consider a 5 axis machine,this includes the spoilboard.The solution is to lift the block to a height of half the motor body plus a bit of clearance and the machine operator and programmer will be aware of this-they may even be the same person.If we are being sensible,the blank will be glued up from slabs that are a touch larger than the waterlines on which they are located and the blank will look a bit like the stepped surfaces of the pyramids.A quick roughing cut should remove most of the steps and leave 5-10mm for the finishing cut.

I hope some of this will be useful and will post more later-but first I have a dentist appointment to get to.

 

[wet feet]

(continued)
A bit earlier fallguy suggested pouring a slab of foam.Thats the easy part.The challenges that need to be overcome are the distortion that is highly likely when machining away a considerable proportion of a slab that has undergone an exothermic cure and will have internal stresses and also finding a tool that will work with machining a female shape.It isn't easy to find cutters with flute length of more than 8 times diameter,for that matter it is still quite challenging to find a source of any cutter with a flute length greater than 4 times diameter and a specialist supplier needs to be located.There are special tools with a long shank which will allow a fairly fine tool to extend quite a distance from the collet,but they have their own set of problems in that first they and the special press you need to insert the tools are both expensive hardware and then the diameter of the shank is possibly a bit excessive for a shape with fine ends.It may be possible to create a female shape using the technique,it will have a very poor surface due to the cell size that results from using pourable foam and the fairing and finishing inside a female shape are a lot more difficult than with a male shape.The ends in particular being a real nightmare.You can lose many days in the process and should you persist,you will have a very fragile female tool.It may be very slightly easier to split it down the centre as you would at least be able to machine polish the component's finished surface,which you definitely wouldn't have the space for at the ends of a kayak shape in a female tool.

 

[wet feet]

I'm not seeing any reactions,but in spite of that I will post a bit more.
A machined foam plug will be very fragile.It will not get much less fragile with a few coats of high build primer,but it will be possible to get one good mould off it and you may be lucky enough to get a second.The choice of coating depends on the type of foam used and it would be foolish not to try some of the paint on a small piece of the foam to see if there are compatibility issues.Particular brand advice is best sought from the people who sell the product and they will need to know the foam type to advise on the best product.Before applying the paint it might be best to remove any machining marks with perhaps 150 grit paper.Then apply at least two generous coats to seal the surface.I wouldn't bother with anything finer than 180 grit until all pinholes have been eliminated and I find 3M acryl stop good for minor blemishes.Recoat for the final time and start the flattening process.Once the surface looks even you can get to the finer grits,perhaps beginning at 320 and working your way through to 1200 before breaking out the compound to get a glossy surface.
Do keep in mind that if the plug loses a few fragments while releasing the first mould,you can always use the new mould to create a sturdy laminate that can serve as a plug for future iterations of the same part.In fact in some industries it is normal to make a pre-production plug and to then take a mould from this this to serve as the source for a master plug for the future-once any minor blemishes have been taken care of.I know of one instance where two master plugs were made with epoxy resin and one was stored in a cave beneath a mountain as a permanent and safe way of keeping it available in the event of a catastrophe.