Controlling your density is probably one of the hardest things to do in the world of spray foam. It will require the correct material for you (get into that later), correct spray technique, correct substrate preparation, correct mixing pressures, correct temperatures, as well as correct gun maintenance. If all of these things are perfect, the result will give you less then 2lb (1.8-1.9) density foam. Let's break down each of these things, and how they can affect your density.
Correct Spray Technique:
There is no 100% rule for spray technique. Everyone sprays different. Up, down, side to side, swirl, and anything else you can think of can all get the same results, IF done properly.
What you need to keep in mind while spraying, is a few simple things. These will help keep your density as low as possible.
When spraying you want to make sure you do the maximum thickness of pass, which you are allowed to do (in Canada this is two inches, in the USA there is a product that can be sprayed 3 inches with no problems). Spraying the maximum thickness will eliminate pass lines within your finished product. Each pass line is a thin layer of very dense material. This can account for upwards of an extra .1-.2lbs per cubic foot. You can see how this adds up and can easily take a good density foam and quickly make it a bad density.
The next thing to keep in mind is to always spray 90 degrees to your substrate. If you do not, you can be "pushing" the foam as it rises, this can create waves in the foam, which have the potential to have skins on them. These lite skins can add up and increase the density (they can also damage cell structure).
While spraying at 90 degrees you want to work your way down (or up) the cavity. While applying the wet foam you want to cover the fresh product by 80% and have 20% going on fresh substrate. You want to be sure to move back and forth quickly so your material is just laying into the surface of the fresh rising foam, and not actually pushing it around, or penetrating it. It's better to go back and forth many times to build up thickness, then it is to go slow and create waves in the rising product.
In a perfect world you would love to do the cavity in one motion from top to bottom (or vice versa) "riding the wave" at a consistent speed all the way down the cavity.. This would eliminate and breaks, where a skin will be created. Doing this will take a lot of practice, and chances are, will still need a few touch ups around the edges.
Correct Substrate Preparation:
There are a few rules when it comes to spraying on something. You want to make sure it is clean, as warm as possible, dryer the better (less then 18% humidity), and free from oils.
Foam needs heat so it can react. Having a nice and warm material is a good start, but having a warm substrate is the next thing you need, to get great density. A cold substrate (like concrete) will act as a heat sink. It will pull away the heat from the reaction. I'm sure many of you have seen the foam stay "wet" and get poor rise from it when spraying on a cold surface, that is the reason why. To prevent this there are a few tips. One easy one is following the sun. Spray onto the walls the sun it hitting. They will be warmer then the walls in the shade. Another trick is to put heaters in the place where you are spraying.
This leads us to the next part... keeping the area dry. It's important to remember to spray on dry materials. Water and foam hate each other. You will have foam falling off the walls if the substrate is to damp. Wood and concrete can hold moisture for a long period of time. It's best to check them with a moisture meter to make sure they are less then 18% humidity. If they are over, you can not spray. Dehumidifies may be a good investment if you run into this problem a lot. You need to be careful when you are heating a building. A lot of people use propane heaters. This is terrible. Propane gives off a "wet" heat. This creates a lot of extra unwanted moisture in the spray area. Its best to use diesel heaters. They give off a very dry heat.
Correct Mixing Pressure:
It is important that you have foam that is being mixed properly. Poorly mixed foam can relate in a lot of issues, one of which is bad densities. Most chemical manufactures tell you that your best mix of material will come at 800-1000 psi at the gun. So your machine should be set to around 1150-1300, because of the pressure loss in your hose. At these pressures you should be able to eliminate any "marbling" that can take place in your product. All high pressure foam systems are mixed using direct impingement. This means the two sides are colliding at high pressures within the mixing chamber then being forced out the tip, on to the substrate. A good chamber will give the foam a little swirl within the chamber, to get better mix. If you feel you gun pulsing as its spraying or you are getting raw material shots, you chamber may be worn out, or machined incorrectly.
If all you seals are holding and working correctly in both your machine and your gun, you should be able to easily maintain the 1:1 mix ratio you need for good foam. If they are bad, and you have leakage and you may be actually getting off ratio foam. Most of the newer machines in the market (within the last 8 years or so) will shut you down before this gets to bad. Once the pressure get to far off, it will shut down letting you know there is an issue somewhere.
You want to keep your pressures as close as possible. I would say anything within 250 psi of each other you are okay. If it gets worse then that, I would start looking at why.
I say 250psi because you are using mechanical gauges which have a tolerance on them, as well as heating up two materials with different viscosity (that's a different conversation).
This is a fun one to try to perfect. You may be tweaking temps all day in some cases, other days you can set it and forget it. Every manufacture will give you guild lines for where to start to set your temps. Regardless of what you are told, it's up to you in the field to find the best temps to run at.
You will know when you found the best temperatures because when you spray your foam on a warm substrate, it will stay "wet" looking for almost a second and then forth up and look like the finished product.
If the foam is staying wet looking longer then that, or is dripping, it is too cold.
Now if you pull the trigger and are seeing foam that is already the color of your finished product and never looked wet, it's too warm. Your foam is reacting before it hits the substrate. This will created small balls of foam that will punch through the rising foam damaging cells, as well as increasing density. Your gun will also start to plug up a lot, and loose it's pattern, if the foam is too warm.
Some days finding that sweet spot is hard to do, and will change your set points through the day. If you can find the sweet spot, you will thank yourself for it once your set is finished, and you get your paycheck.
On a side note, adjusting temperatures can also balance out your pressures a little bit. Some people like to run their resin 5-10 degrees hotter then the ISO side. The theory behind this is that it will thin the resin a little more allowing the viscosity to become closer, allowing the back pressures to be more equal. Keep in mind the hose heat temp is red from the ISO side, and the hose should never be used as a heater, just a device which maintains a temperature. Saying that it is not a good idea to have the hose temperature warmer then theniso preheater...
Correct Gun Maintenance:
Being able to maintain a good pattern is key. If your pattern is all over the place it can be hard to control from where the foam is going. You may be wasting it by putting it on the studs or adding extra skins at the sides or just creating a lot of over spray. Drilling out your tip and having a clean air cap in good condition is key for this.
Correct Material for You:
This is a very touchy one. Everyone has their favorite material because of a supplier, sales guy, tech support, kick back, cost, or based just on the performance of it.
It's important to work with a material that you can control. being able to control it is a huge advantage for you. It will make it nice and easy to dial in your set points on a daily basis, as well as control the rise, so you do not get unwanted skins.
If you are buying (using random numbers) the material for $2400 a set but it is a pain to work with and you are averaging 2.7 lbs for your density, is it really that cheap? Let's find out. Spraying at that density will give you 4444 board feet a set, which works out to 54 cents a board foot.
Now let's say you have a material that is $2800 a set and easy to work with. Because of that you are getting an average of 2.2 lbs for you density. That's 5454 board footage per set. That works out to 51 cents a board foot. The "more expensive" foam is actually 3 cents cheaper to spray per board foot! So if you sprayed 5454 board feet at 54 cents, the set would cost you $2945.15 to get the same yield out of it. Even though you are paying $400 more a set to start with. In the end not only will you save money, but you get a better, more predictable product (not to mention less clean up) which you can make more more money off of a single set. Also keep in mind a happy sprayer will be more productive then one that hates the material he is working with.
When choosing you foam it's important to keep a few things in mind.
1) Actual board footage costing (what you actually get, not what a supplier tells you).
2) Ease to work with
3) Support. You want to have a company that will work with you so you can get the best densities possible from their product. If they can't show you a low density product, chances are their product is difficult to work with, which in the end will cost you money.
A good company will send someone to see you, so they can teach you how to use their product. Also when times get rough, you want to make sure your supplier can get you the answers you need. You want to have someone in your corner that can get you weird information customers/architects/inspectors demand.
Another thing a lot of times chemical companies will pass issues off as equipment problems and not their issue. Equipment companies will pass problems off as material problems and not their issue. Having a supplier that say he will look after you means they have a solid understanding of the equipment as well. They should be able to explain your problem to you and why you need a certain part. A lot of times companies just throw parts at a problem, until they find one that works. This can be veru expensive. Remember you as a customer always have to right to question things. It's always best to question what you do not understand. That is the only way to learn and grow!