Solvent Welding (
Pins method)
Per the post above regarding Overage and Fillets (not nearly as mouth-watering as
Filets), it's time to explain the Pins method.
At the very basic essence, the Pins method is simply inserting a pin (wire) into the joint to force a very small separation of the 2 surfaces to be joined, filling this separation with solvent, waiting a certain amount of time (referred to as
soak time) and then pulling the pins out. It sounds very simple, and it really is that simple. However, there are always methods of implementing this process that work best.
Here's my list of supplies for doing the Pins method:
1) ~4" wide strips of 3/4" MDF (various lengths)
2) Campermount adhesive-backed foam, 1.25" x 3/16" thick. Brand matters, the Home Depot stuff sticks to the panels and sucks royal big time.
WJ Dennis brand from Menards does not.
3) Shims. The thinner, the better. Standard ones are OK but sometimes you need really really thin shims that don't have a steep taper. I found these in the form of cedar shakes, $15-$30 for a bundle. You have to sort through them but you can easily end up with 50 small shims that you will use over and over. I have a cup full of 3-4" long, 1/2" wide shims. Stick the rest in your closet for that fresh mountain getaway feeling every time to go to get clothes, ahhhhhhh!
4) Wire; I use
28 gauge picture hanging wire, you can use just about anything, even stripped & cleaned twist ties
5) some kind of angle bracket
6) clamps
I probably missed something but #1-#4 are the important ones, your clamps and brackets will vary based on how many times you are going to build something, you can improvise for a single build.
MDF Strips & why they are important.
Start by applying the foam to the MDF strips; 2 on each side like so:
You will want something like this to span under the length of the joint. These boards do many things:
- They help remove irregularities in the work surface
- They will absorb excess solvent and prevent your work from sticking to the work surface
- Solvent won't cause the paper backing to stick to the foam
- Provides you with locations to shim in a general as well as a localized manner
- Helps spread out the effect of shimming
- Provides a cushion to push against for adding pins
- Makes you look like a pro when you post build pics
Here's an example of pretty much all of these in action:
Big shims are in use under the MDF boards because the table in the pic may look awesome, but it sucks for big builds (very irregular in certain areas) so I have to make big adjustments to get the joint tight, then I use smaller (thin) shims between the tank and the MDF strips to make fine adjustments to the joint.
Wire
I cut the wire into pieces about 4-6" long, then I bend one end into a loop and twist it to create a "handle" that will be used when you pull the pins out. For the other end (that is inserted into the joint), you will want to clip this with a good quality wire cutter before using it. You need a clean cut! The cutter that usually comes with the wire is OK for cutting to length, but it leaves a barb on the end that can dig into the material and make it difficult to pull out of the joint. I use and
end cutting nipper, which is a tool you can get anywhere pretty cheap - it makes a very clean cut.
You can (hopefully) see the pin/wire here:
Right Angle Bracket
I made a few of these myself out of particle board. You can't see it, but the corner there the 2 perpendicular parts of the bracket come together is beveled off
The downfall of the ones I use is that you can only access one side of the joint. I could fix this easily by cutting out the inside corner of the bracket on a table saw...one of these days.
You can also use a standard right angle bracket with the corner cut out. Whatever you use, you need the corner of the bracket that is right in the joint beveled or cut away so that it doesn't bond to the material when you run the solvent
Process
I'm going to skip over a few steps such as stripping the paper and cleaning & blowing off the bonding surfaces, but those you would obviously need to still do. So this starts with the assumption that you have are ready to set the joint up for bonding (surfaces cleaned and prepped).
Step 1) Lay foam boards under horizontal part.
You will want to place one board parallel with and directly under the joint, such that the 1.25" wide strip of foam will roughly align with where the second (vertical) panel will be placed. Then, place additional strips such that the entire horizontal panel will be supported. You don't want the panel to sag, as this will put an angle on your joint. Use as many foam boards as you need to get the job done.
Step 2) Place the vertical panel in position and clamp to right-angle bracket.
You only (ever) need to clamp the vertical panel to the right-angle bracket. Do not clamp the bracket to the horizontal panel. You want the panel + bracket to act like one piece that you can move around. Use simple hand-squeeze clamps, no need for bar clamps or C-clamps (in fact, those would be problematic, so you don't want to use those). Make sure that the bottom edge of the vertical panel and horizontal part of the bracket are co-planar. I usually do this by holding the bracket down lightly and then bringing the vertical panel to it, letting loose on the panel a bit, then clamping them. Put the vertical panel back in position and loosen the clamps one at a time, just long enough to allow the panel to "settle". You may have to repeat this in Step 3 depending on the gap.
Step 3) Snug up the joint.
This is a "dry fit" step. Your goal here is to make those 2 pieces touch all the way along the joint, because this is the position the 2 pieces will be in after you set the joint. I find the best way to view the joint for a gap is to look along the joint from one end with decent lighting, and if necessary, hold a small flashlight and focus the beam right on the joint and run it back and forth. You will be able to see where the material is touching and where there is a gap.
You will also be able to see if you are leaving an even amount of overage across the joint. In the case of bonding an end panel to the front, this is important, because uneven overage means your panel will not end up vertical, and the finished edge of the end panel will not be perfectly flush with the top/bottom panel (which could allow air to intrude). This is usually not a huge deal unless you're just way off. In which case, what were you thinking? Geez.
Use shims as necessary to tighten the gap between the horizontal panel and the vertical panel sitting on top of it. Sometimes you will have no gaps. If you have no gaps, then congratulations, you don't need to shim...yet.
Step 4) Pinning and shimming
Remember that overage? Here's where this comes in again. Hold the pin in one hand, with the end of the pin in contact with the joint. Now press down on the overage. Pin should slide right into the joint. Boom.
Pins should be inserted every 6" or so. Too close, and it can make shimming a total pain. Too far and you might end up with not enough of a gap. You want enough of a gap to allow the solvent to wick into the joint, without it being too little which can result in not enough squish-out for the fillet.
Pins only need to be inserted far enough into the joint to cause separation;
do not insert the pins such that they extend through the joint as this will form a dam and prevent solvent from wicking past the pin! Try to insert the pins about the same distance for every pin.
The longer the joint, the higher the odds that pins will get loose. You can place a shim under pins that are loose upon insertion, or under other pins that become loose as you insert new pins. But sometimes it's better to let a loose pin fall out and come back to it later, as it may not need as much shimming after the joint is fully pinned. This is especially the case for top/bottom joints where pinning 2 adjacent sides will result in loose pins that will tighten up after you put pins in the other 2 sides.
Once you have pins in place, start tugging. you don't want to tug hard, the point is to search for pins that really come out easily. A light tug should not pull a pin out, if it does, add a shim and move to the next one. Keep going until you can't tug out a pin with a full trip around the joint. This means if you have a bottom joint with 20 pins and the 11th pin is loose and requires a shim, you check all 20 pins again. That one additional shim or even a simple adjustment to a shim can actually cause a pin in a seemingly nonsensical and completely unrelated location to fall out.
Step 5) Blow out the joint
After all pins are set, blow out the gap with canned air - same stuff you blow out electronics with. It's nice and static free, just don't tip a full can too far off vertical as it will blow out the liquid, forcing you to take the joint apart to clean it off (it may look OK but, no, it's not). Sometimes the canned air will blow loose a pin and you have to go back to Step 4.
Step 6) Secure the assembly
This is optional but it helps sometimes. If you have a right-angle bracket, you can add a small amount of weight to the horizontal part to keep it in place. Don't clamp though, at least, I don't.
Step 7) Run the solvent & pull the pins
Again, I'm going to skip the specifics of applying solvent and leave that to another post as there are many pointers related to this and joint preparation, avoiding bubbles, fixing bubbles, etc...so this step assumes you understand how to apply solvent using a squeeze bottle & needle.
I usually get all ready to run the solvent, then I set the bottle down and take a moment to realize that this is it. This is the moment when you could screw the whole thing up. Then I clear that out of my head, take a couple breaths and now it's go time. Blue 42, set, HUT!
Start a mental stopwatch in your head, starting with the time you first start applying solvent to the joint. You want to allow a minimum of 15 seconds of soak time before pulling the pins, and depending on the solvent you are using, it can be as much as 30 or even 45 seconds. Much longer past that and you might have problems, depending on the material thickness (thicker material/joint, longer working time).
Sometimes, I will run a quick second pass of solvent right before pulling pins, moving at 2x speed or faster. I'll do this if I think it took a bit too long from start to end of joint, just to make sure there is enough squish-out.
Start pulling pins, starting with the same location you started applying solvent. Pulling pins without causing the vertical panel to slide is the trick - it's hard to explain, it's just something you have to learn.
You don't want to "hold" the vertical panel, because when the pin "pops" out, you have a natural tendency to compensate for this by pushing on the vertical panel. Yanking on the pin can work depending on how far you inserted the pin and how heavy the parts are (for a top/bottom joint, yanking works great until the last few pins).
What you can do it put your thumb or fingertip against the edge of the horizontal panel right next to the pin, and pull the pin with your other hand. The horizontal panel prevents you from pushing on the vertical panel upon the "pop". Or, you can lightly press down on that now ever so handy overage, the same way you did to put the pin in.
If you practice enough, you can grab the pin between your thumb and pointer finger and put your thumb knuckle on the vertical panel and "roll" your thumb backwards a bit, pulling out the pin without the "pop" jarring things badly.
All will be right with the world until you go to pull out the last pin. Sometimes when you pull that last pin, that's when things start to slide. You would be amazed how slick the joint is. I've had a 110 gallon 36" tall build made with 3/4" walls slide on me after pulling the last pin on the bottom panel joint. But that's what the next step is for.
Step 8) Making Adjustments
For an end panel or baffle, you have about 15 seconds to make adjustments before the solvent sets up enough that you won't be able to move it. During this time the critical alignment point, IMO, is the bottom joint. What I do it run my fingernail up and down (pointing it toward the direction of travel) so I can feel if there is a difference between the alignment of the 2 panels, making adjustments as needed. Again, this is a "feel" thing, and it doesn't have to be dead-on perfect. Now don't forget the other side (top joint). And don't forget to check to make sure the overage is still even! Basically this is 15 frantic seconds checking to make sure things are lined up right, followed by the realization that you can't change it now, and you just have to hope that you didn't just screw the pooch.
For a bottom/top panel, it's a much longer joint and it's easy to forget that you need to check alignment all the way around. You can pull that last pin and sit there on that end making adjustments until it looks great, only to realize that the other end slid 1/4" and part of it is off the panel. Not like that's happened to me, nope. Never happened to me. Never never never. Except for that one time.
Step 9) Loosen clamps
For a vertical panel bond (not a top/bottom joint), after you have the joint in position and it has firmed up enough so that you can't move it, perform this step.
If you placed any weight on the right-angle bracket, remove it before doing this step!
Put very light downward pressure on the top edge of vertical panel, then carefully release each of the clamps one at a time - just for a fraction of a second. The thought being that while your dry-fit Step 3 should have closed the up the joint, there is a fraction of a tiny bit of material on each side that got dissolved, and you want to now close that gap. I know, we're talking nanometers here, but hey guess what air is smaller than nanometers so enough of your backtalk, keep that up and you get no dessert kiddo. Wow that was random
Step 10) Add weight
This is a good step especially for top and bottom joints, as well as the second end joint (end to back) where you have a flat surface on top so you can easily place something across the top of the assembly and add weight. Any weight will do. I've literally used a 2x4 to span across, then put 12 packs of beverages, camping chairs, soccer balls, even small dogs. Ok maybe not dogs.
For end panels or baffles (first joint) where there is no surface to lay a weight across, if you want to you can use small sandbags as long as they don't cause the panel to tip. Personally, I just put the weight back on that was removed in Step 9 and call it good.
Step 11) Check shims
I say "check" and not "adjust" because you may not need to adjust anything. Some recommend pushing all the shims in a little bit...but, if you only needed shims on 1/4 of the pins, this would actually cause the areas with no shims to loosen up. The combination of the 2-sided MDF+foam boards plus all that prep plus weights should be enough to ensure that the joint is snug as a bug in a rug at this point. But if you want to walk around and give each shim a little push to make sure it's snug, knock yourself out.
Step 12) WAIT.
Don't touch anything. Let the joint cure. The longer it sits there without any movement, the better joint strength.
