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What is the best filler/cement for a large styrene model?

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  • Member since
    November 2015
What is the best filler/cement for a large styrene model?
Posted by ship69 on Tuesday, February 5, 2019 1:40 PM

Hello

I am working on a fairly large styrene (HIPS/Plasticard) model, and I need to join or "weld" sheets of fairly thick (3mm) styrene together by their edges.

My model has a number of fairly large gaps (c.0.5 to 1.0mm) between the 3mm sheets of styrene. These gaps are quite long (c. 30cm) and of variable width.

The gaps are too large for solvent type cements to penetrate by capillary action.

So I need a filler or cement that:
- Has a low viscosity in order to get into the smaller gaps
- Does not shrink when it dries (in order to avoid multiple applications)
- Gives a good strong bond onto the styrene
- Has some flexibility (to stop it from cracking in use)
- Can be sanded easily.

Any recommendations? Does such a filler/cement exist?

BACKGROUND
I have tried 2-part epoxy glues, but they are hard to work with and they also don't pentrate the narrow gaps very well. I have just ordered some Milliput filler, but reading further, I now fear that it will be rather brittle when cured, take rather a long time to cure and may not be strong enough either (??)

I am currently trying to dissolve some lumps of styrene in Slaters MEK-PAK solvent/cement and then painting that on as a sort of "goo", however it clearly shrinks as it dries and so multiple layers need to be applied, which is time-consuming.

Any suggestions?

J

  • Member since
    September 2012
Posted by GMorrison on Tuesday, February 5, 2019 2:06 PM

Is there a way to get a piece in behind the gap/ joint? Otherwise it's bound to crack back open at some point.

 Modeling is an excuse to buy books.

 

  • Member since
    May 2011
  • From: Honolulu, Hawaii
Posted by Real G on Tuesday, February 5, 2019 2:13 PM

Maybe UV cure glue?  It is acrylic resin, and supposedly adheres well and is strong.  I have been wanting to try it out myself, but have had no luck sourcing it where I live.

BTW, what are you making?  Can you share pics, or is it Super Secret?Big Smile  

“Ya ya ya, unicorn papoi!”

  • Member since
    November 2015
Posted by ship69 on Tuesday, February 5, 2019 2:31 PM

GMorrison

Is there a way to get a piece in behind the gap/ joint? Otherwise it's bound to crack back open at some point.

No, the join needs to be like an invisible join. i.e. It needs to be the same thickness as the base material (i.e. 3 mm thick) and sanded on both sides soas to be flush.

  • Member since
    November 2015
Posted by ship69 on Tuesday, February 5, 2019 2:34 PM

Real G

Maybe UV cure glue?  It is acrylic resin, and supposedly adheres well and is strong.  I have been wanting to try it out myself, but have had no luck sourcing it where I live.

BTW, what are you making?  Can you share pics, or is it Super Secret?Big Smile  



Of course if the resin is TOO hard when cured it will be difficult to sand to a flat finish.

Sorry, the model is commercially sensitive so it can't be photographed at this point.



  • Member since
    August 2005
  • From: Mansfield, TX
Posted by EdGrune on Tuesday, February 5, 2019 4:02 PM

Can think of two approaches:

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush

I have found that UV glues harden sort of rubbery.  Sanding is problematical  May work for nav lights but ...

  • Member since
    November 2015
Posted by ship69 on Tuesday, February 5, 2019 6:42 PM

EdGrune

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

Sounds intriging. How would you actually do that? Is there some clever tool or machine that would allow me to accurately cut a strip of styrene to a fixed width and depth.  Part of my original problem was that I did the original cutting on a bandsaw which then created a wobble of about 1mm in its cut. I have been trying to 'fettle' it with selective filing by hand into a better fit, but it's extremely time-consuming. I also find sanding by hand has a nasty habit of accidentlly removing more material at each the end the strip being sanded...

 

EdGrune
Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush


I'm not sure I really understand what you mean. Yes, I guess cutting a bevel at say 45 degrees would increase the surface area of the join compared to a 90 degrees edge. However I can't see how either round or half-round styrene stock would help create a strong, seamless bond.



  • Member since
    September 2012
Posted by GMorrison on Tuesday, February 5, 2019 8:18 PM

You are trying to do something that's next to impossible to do. Imagine joining two sheets of plywood together edge-to-edge.

Glue bonds things together much better in shear than it does in tension. That's Ed's first suggestion.

In the real world, you would weld the two pieces together if they were metal, and grind the bead. Ed's second suggestion is in that vein.

Maybe if the two pieces are flat, they could be keyed together. That might take the form of a piece of styrene the same thickness, an inch or so long and fairly narrow in width; being glued into a slot in each half.

 Modeling is an excuse to buy books.

 

  • Member since
    November 2015
Posted by ship69 on Wednesday, February 6, 2019 8:18 AM

GMorrison

You are trying to do something that's next to impossible to do. Imagine joining two sheets of plywood together edge-to-edge.

Glue bonds things together much better in shear than it does in tension. That's Ed's first suggestion.

In the real world, you would weld the two pieces together if they were metal, and grind the bead. Ed's second suggestion is in that vein.

Maybe if the two pieces are flat, they could be keyed together. That might take the form of a piece of styrene the same thickness, an inch or so long and fairly narrow in width; being glued into a slot in each hal

 

 
 
 
Interesting.
 
Yes, your "EG 1.0" makes good structural engineering sense. The problem is that I haven't the slightest idea how I could cut the required steps out! Any suggestions?

Your "EG 2.0" is slightly subtle because the extruded/stock square cross-section long thin piece of styrene could be made to bend into the gap between the two base sheets to fill any inaccuracies. The main problems with this approach are
1. That there would still be a gap of unknown size on the left of the diagram where the two base sheets meet.
2. I don't actually have any such square cross-section bars
3. Depending on how far you squeezed the square cross-section bars in, the overall width of the entire piece would vary. This would create inaccuracies in the overall size.
 
I have to say the main reason why I am using styrene at all is because you CAN basically weld two sheets together by apply a solvent cement. And when it dries it seems to end up like a 'weld' and the dried result is very nearly as strong as the original styrene. The only problem here is that you need a really good fit for this approach to work!
 
Cheers
 
J
  • Member since
    August 2005
  • From: Mansfield, TX
Posted by EdGrune on Wednesday, February 6, 2019 8:20 AM

ship69

 EdGrune

Scrape opposite sides of the opposing parts to half thickness to create a half-lap joint 

 

 

Sounds intriging. How would you actually do that? Is there some clever tool or machine that would allow me to accurately cut a strip of styrene to a fixed width and depth.  Part of my original problem was that I did the original cutting on a bandsaw which then created a wobble of about 1mm in its cut. I have been trying to 'fettle' it with selective filing by hand into a better fit, but it's extremely time-consuming. I also find sanding by hand has a nasty habit of accidentlly removing more material at each the end the strip being sanded...

 

 

 
EdGrune
Bevel the joining surfaces, one or both sides.  Glue a piece of styrene stock; round, half round, or square into join. I’d use Tamiya Thin, Tenax, or MEK as the glue.  Scrape/sand flush

 


I'm not sure I really understand what you mean. Yes, I guess cutting a bevel at say 45 degrees would increase the surface area of the join compared to a 90 degrees edge. However I can't see how either round or half-round styrene stock would help create a strong, seamless bond.

Morrison understands

I would build a scraping jig.  The end stopper would be a piece of metal (brass, steel) which is ideally half the thickness of the part, here 1.5 mm.  It should be fairly wide 2.5 cm (1-inch) or so.  You will also need another piece of metal which will be your depth stop.  

Slide the workpiece onto the jig to the end stopper.   Clamp or secure the depth stop on the top of the workpiece at your desired halflap depth.  

Hold a single blade razor blade or scraper vertically on the end stop and flush against the depth stop.  It will be on an angle with the workpiece material to be removed. Drag the across the workpiece removing material until the cut bottoms out.

Repeat using the same setup on the other piece, remembering to flip sides.

Glue and clamp

 

As far as the weld bead bevel,  doing some back of the envelope cipherin' using some of that math stuff you're supposed to remember from school -- the Pythagorean Theorum.  If you bevel at 45 degrees the edge such that the bevel is 3mm deep and extends back 3mm from the edge,  the gluing surface which you are going to fill with a piece of stock (ideally 3mm square stock) will be increased to 4.25mm.   This is over the former 3mm flat gluing surface.

hypotenuse == sqrt(side1**2 + side2**2)

  • Member since
    November 2009
  • From: Twin Cities of Minnesota
Posted by Don Stauffer on Wednesday, February 6, 2019 9:28 AM

For larger cracks I use auto body filler, usually called glazing compoud or glazing putty.  A big tube is expensive, but you get a lot- it is a big tube!

For very small cracks I use paint.  I do not stir the bottle- I dip a toothpick into the thick stuff at the bottom of the jar and apply it with that toothpick.  Sometimes it takes a couple of applications.  I use Testors enamels, and always use flats when I use it as filler, so it dries faster.

 

Don Stauffer in Minnesota

  • Member since
    September 2012
Posted by GMorrison on Wednesday, February 6, 2019 9:40 AM

I think you've got what you need.

The combined thickness of your lap joint can be slightly thicker that the sheet itself, since you plan to sand it a little. But, the closer you are at assembly, the less work later.

I'd definitely work from samples for a bit before you tackle the model. Solvent will make the edges of your sheets distort, it has a habit of running outside the joint.

There's the second oldest carpenter's adage: there's never time to do it right, but somehow there's always time to do it over.

 Modeling is an excuse to buy books.

 

  • Member since
    November 2015
Posted by ship69 on Wednesday, February 6, 2019 10:40 AM

> I would build a scraping jig.

I only half-understand that. I guess you are proposing to use a square razor-blade, yes? Either way, I didn't know that you can scrape styrene like that. Are there any videos on YouTube of this sort of thing?

Either way, how do you end up with a clean, accurate edge to the area that you are scraping? And a clean, accurate edge would surely be pretty important, unless of course you are prepared to fill the gaps with something...

For now I have been filling with layers of a goo made up of off-cuts from my base styrene dissolved in MEK-PAK solvent/cement. Clearly it won't be as strong as what you are suggesting but once the MEK evapourates fully it seems to be fingernail proof which will should do me for now. And if it falls to pieces I can always try cementing it back into place!

Cheers

J

  • Member since
    August 2014
  • From: Willamette Valley, Oregon
Posted by goldhammer on Wednesday, February 6, 2019 11:29 AM

Another idea.....clamp the sheet down with a thin metal straight edge and sand from the sheet edge to the metal edge.  If using 3mm sheet, would suggest a 5 mm or so setback from the edge to the metal straightedge.  Flip the second sheet and repeat. This will give a bigger glue area than just a butt joint, and just have minor sanding/filling on the two seams.  Search "scarf joint"  for diagrams if need be.

Same idea as the half lap joint shown above, without trying to carve a flat joint half the thickness of your sheet (rabbit cut for a half lap joint).  if you have a trim router, could try to cut the rabbit with it and a sharp bit.  On some scrap first, use a straight edge to keep things square and make shallow passes, maybe a couple to get half way on a 3mm thickness.

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