Scratch-Building a Model - Anyone Worked with Sintra / Foamed PVC?

[Mako]

I'm thinking about working on a spacecraft model, or two, and am considering using either thin styrene sheeting, and/or Sintra / Foamed PVC, the latter of which is apparently listed under a few other names as well, e.g. expanded foam PVC, etc., etc..

Not sure which would be best, or easiest to work with, but the poster-board prototype I came up with seems to be working reasonably well.  I'd like something a bit more robust, and durable though, and would like to be able to paint and detail it, which is why I'm considering the plastic materials.  I might want to be able to drill in some small detail holes too, and/or to cut out a cockpit windscreen, etc., etc., also, and the plastic would seem to be able to permit that as well.

I'm also not sure which thickness(es) to use, for the various materials, so have bought a few pieces, and plan on playing with them.  I need to order some of the thin Sintra material, which apparently comes in 1mm, 2mm, 3mm, 6mm, and 12mm, IIRC.

From what I've read on-line, the styrene sheeting can be glued/welded with acetone, possibly, and/or with other, specialty plastic glues (read much more expensive - like $30 for a pint of the stuff, in some cases).  Want a good, sturdy bond, so I'm not sure the thick modeler's glue, and/or the liquid stuff will be strong enough to bond permanently.  Thinking about using the wicking process to glue the two pieces together, once they've been clamped together.

Hot glue is out, since the working time for it is too short, and it's not strong enough.  Not sure epoxy will work on the plastics, either - might, but may not be as strong as the special glues for the various plastics.

Basically, one of the models may have a complex, curved shape, which I'd like to maintain, with edging that comes to a very fine point, like on the SR-71's fuselage, so the glue needs to be sturdy enough to keep the thin plastic in place, once the clamps are removed.  For another model, it may have sharp, thin, edging too, but at 30 - 60 degree angles, so again, without a lot of reinforcing, I'll need a decent adhesive up to the job of holding things in place.

For the Sintra / Foamed Styrene, I'd like the same strong bond, and ordinary PVC glue seems to be the ticket for that, from what I've read, and been told by people that sell the stuff.

I'd like to be able to score some thin, shallow, panel lines in the pieces, and want some rigidity when bent into shape, but also need decent flexibility too.  My guess is 1mm - 1.6mm may be too thin; 2mm may be just about right; and 3mm may be too rigid.  Thinner, and the panel scoring may weaken the pieces, possibly.

Ideally, I'd like to get away without having to add in interior bulkheads in the models for strength (would be an excellent idea, I suspect), if that's possible, but not sure it will be.  I was thinking of using some rigid foam pieces inside, possibly, instead, since those seem like they'd be easier to create, and install, rather than having to cut bulkheads to exact size, which will no doubt need more precision.

I have seen where people also create their own glues from the acetone, and then adding in either very small bits of styrene, and/or PVC, as appropriate, in order to apply to the interior of joints, in order to reinforce them, which makes sense too.

So, any thoughts on the above?

Have any of you worked with the Sintra, and also styrene sheeting, and can compare/contrast the advantages of each when building models from scratch like this?

I'm also interested in advice on working with either, even if you haven't worked with both materials.  Any tips will be greatly appreciated.

[Connectamabob]

From what I've read on-line, the styrene sheeting can be glued/welded with acetone, possibly, and/or with other, specialty plastic glues (read much more expensive - like $30 for a pint of the stuff, in some cases).  Want a good, sturdy bond, so I'm not sure the thick modeler's glue, and/or the liquid stuff will be strong enough to bond permanently.  Thinking about using the wicking process to glue the two pieces together, once they've been clamped together.

Don't use acetone IMO. In my experience it does not act as aggressively as one would need on polystyrene to make a good bond. What you want is MEK (Methyl-Ethyl Ketone) or/and Dichloromethane (also known as methylene chloride). Depending on where you live, you may be able to get these from a paint supply place (or hardware store paint thinner/stripper isle). In regards to the pro cements, look for local plastics suppliers instead of online: the price difference is huge. If I buy that stuff locally, I can get it for like 1/3 or 1/4 the cost of online, which is actually better by volume than the hobby stuff. I suspect the online stuff gets marked up because of special shipping hazards, but am not sure.

Hobby model cements used to be made of these solvents, but I think they've been getting phased out in favor of less toxic chemicals over the last 10 years or so. Modern model cements I cannot vouch for, as I stopped using them when it started getting hard to find the good stuff. The less/non toxic solvents are not very good, in my experience.

In general though, solvent-welded joints are the best bond you can get (short of heat welds). If you use the good stuff (MEK or Dichloromethane), the result will be as good as if it were cast as one piece. Stick with the liquids mostly. The gels are just liquid solvents with some plastic pre-dissolved in it. Still decent, but the bond isn't as strong as with the pure liquid. Gels IMO are good for when you need to tack something in place with a some positioning time before finalizing the join with liquid. If you have liquid solvent, you can make your own gel by dissolving sprue or cut-offs in it. This is better than pre-bought gel, as you can be sure the filler plastic is the exact same kind as the plastic you are gluing.

Basically, one of the models may have a complex, curved shape, which I'd like to maintain, with edging that comes to a very fine point, like on the SR-71's fuselage, so the glue needs to be sturdy enough to keep the thin plastic in place, once the clamps are removed.  For another model, it may have sharp, thin, edging too, but at 30 - 60 degree angles, so again, without a lot of reinforcing, I'll need a decent adhesive up to the job of holding things in place.

A lot of this sort of building has moved away from sheet-on-frame construction to resin-on-foam instead. It's simpler and easier to manage. Though the skill involved is admirable, I don't think I'd personally want the headaches of making something with complex curves using sheet on frame these days.

I'd like to be able to score some thin, shallow, panel lines in the pieces, and want some rigidity when bent into shape, but also need decent flexibility too.  My guess is 1mm - 1.6mm may be too thin; 2mm may be just about right; and 3mm may be too rigid.  Thinner, and the panel scoring may weaken the pieces, possibly.

2mm is actually pretty rigid. Good for bulkheads and other flat cut pieces, but I wouldn't want to do compound curves without thermoforming, or unless the curve was really gradual. You're probably thinking about how easy it is to bend with your hands, but keep in mind that every flex is going to create tension on the seams, and that can add up a lot. If you're just flexing the sheet instead of thermoforming, it's way better to plan the model so the skin isn't load bearing, that way you don't have to sacrifice flexibility for strength or vice-versa.

That said, thermoforming is easy. Get yeself a heat gun and try it!

Ideally, I'd like to get away without having to add in interior bulkheads in the models for strength (would be an excellent idea, I suspect), if that's possible, but not sure it will be.  I was thinking of using some rigid foam pieces inside, possibly, instead, since those seem like they'd be easier to create, and install, rather than having to cut bulkheads to exact size, which will no doubt need more precision.

The simplest would be to fill the model with rigid expanding foam. Though you want to choose the foam carefully, and practice it on a disposable box assembly or something first, as if done incorrectly expanding foam can inflate walls and burst seams.

Another option is to pour a liquid resin (like an epoxy) into the model and roll it around to coat the inside in a solid layer. I've done this before, and it works really well.

I have seen where people also create their own glues from the acetone, and then adding in either very small bits of styrene, and/or PVC, as appropriate, in order to apply to the interior of joints, in order to reinforce them, which makes sense too.

Again, I don't recommend acetone. It's cheap, easy to get, and not as toxic as other options, but it doesn't dissolve aggressively enough to make for good bonds in my experience. It also doesn't work on PVC IIRC. PVC is tough to solvent weld, as there aren't many solvents is its PH zone (which is one of the reasons it's popular pipe material, actually: it's pretty chemically resistant). I think the PVC cements out there are a cocktail of several solvents formulated to attack it from multiple directions at once, PH-wise.

Have any of you worked with the Sintra, and also styrene sheeting, and can compare/contrast the advantages of each when building models from scratch like this?

I only have a tiny bit of experience with Sintra (I'd love to use it, but it's really expensive where I am). It sands and carves really, really nicely, though as it is a foam inside, it tends to leave a rough surface. Haven't tested glues with it, though I suspect that PVC pipe cement is best for gluing it to itself. for gluing it to styrene, I'd want to consult with a plastics supplier for a solvent glue, or just use a good epoxy. Since it's a foam, it might not react well to any solvent aggressive enough on PVC to form a good bond (acetone is kinda rubbish for bonding solid styrene, but will still dissolve foam styrene like nobodies business B/C of the surface area to volume ratio).

[Johnnytodd]

Hi,  I've built a few models from scratch and may be able to help.  Firstly, how big is your spaceship gonna be?  I havn't worked in Sintra or foam but have worked in Styrene, wood, putty and paper.  Plastic (styrene) sheet is ideal if you are planning lots of small detail.  I use PLASTRUCT products https://plastruct.com/  (available at US hobby stores) their cement is Dichloromethane (about $5 2oz)- basically this stuff melts/welds the Styrene so you get a very strong bond .  Buying all the various shapes, sizes, thickness of Styrene can get expensive though....  

For small areas with compound curves my first choice is to use two-part epoxy putty https://www.avesstudio.com/ this stuff sticks to anything and cures rock hard - a little goes a long way.  

If you search my builds here you'll see I'm a fan of solid basswood construction - its less expensive and less toxic than plastic - and generally robust.  Ideal for the complex curves of aircraft.  If I were making a spacecraft with detailed interior I would probably use model airplane plywood as the skin over a scrap wood bulkhead inner frame (plywood bends just like your poster board but much stronger). Plywood comes in 1/64"-1/32" and bonds with wood glue.  Clamping, pinning, gluing wood requires a bit more patience than plastic.  Buy wood here: http://www.nationalbalsa.com/

Feel free to PM me if you have questions

[Mako]

Thanks for the detailed replies, tips, and suggestions.

I really appreciate them.

That's what I was afraid of on the glue/plastic solvent(s), so I won't bother with the acetone, excepting perhaps for small, minor assemblies, that don't need a lot of strength.

I don't think MEK is currently available here, so will need to try the other solvent you've suggested.

I'm going to do some smaller vessels, for 15mm / 1/100th scale, initially, and then may try some larger ones, if that goes well, e.g., perhaps 6" x 6" - 8" x 10", or so, and then moving up to 10" - 12" x 20", eventually.

[Connectamabob]

Word of fair warning about Plastruct: their plastic stock comes in several different formulas. You want to make sure you get specifically just plain polystyrene, and not their ABS or other formulas. Their core market was architectural models, and their ABS is actually deliberately formulated to to bond poorly with both glue (including solvents) and paint, so that models can be taken apart and their materials reused. I mostly use Evergreen for styrene stock, as I got tired of having to sift through Plastruct stock to eliminate the "gotchas" (and often finding the size/shape I need wasn't available in not-ABS).

Note that this applies only to Plastruct brand ABS. Regular ABS that you'd get at a plastic's supplier plays as nicely with paint and solvents as styrene, and is a good alternative if you need something that behaves like styrene but with better stiffness and/or impact resistance.

Plastruct's liquid cement is one of the good ones, but it isn't as good as some of the other old good ones like Tenax or Ambroid (both of which are impossible to find anymore), as it has a small amount of plastic pre-dissolved in it.

Soft wood like basswood and balsa are good economy materials. Somewhat similar to working with cereal box cardboard, only sturdier. Basswood is great for plank or sheet on frame construction, and balsa is a viable alternative to foam when it comes to carving and skinning. In fact one big advantage balsa has over foam is you can use it to make thermoforming bucks.