I try to write about relevant issues in this column. I take problems and issues we face at my company, and share both the problems and the solutions with other finishers in the hopes that we can all learn from it, and improve our operations. Some problems are simple cause and effect. Something changes and the resulting problem is traced to the change, and…problem solved. Some problems are more subtle and require some detective work to identify the culprit and solve the case. Some problems are just part of the job, and cannot be solved. The best you can do is try to minimize the effect. The outgassing of aluminum sand castings is just this kind of problem. We face it daily, and I would like to share how we handle it.
First, let’s go over some basics. Most cast products are made by pouring the molten metal into a mold. The molds are usually made of either sand, which must be re-molded for each casting, or metal, as in die-casting or permanent mold casting. The molds have cavities or voids that become the cast metal part after the mold is filled with molten metal and the metal has cooled. All metals, when molten, contain some degree of impurities and gasses. The impurities are usually removed as a floating “slag” prior to pouring. The gas, however, often accumulates and creates an effect called “porosity”.
Both aluminum and iron castings can have porosity problems, but the physical properties of the aluminum make it more likely for this porosity to become an issue when the parts are powder coated. Also, because sand castings are usually made with a moistened sand mix, the combination of red hot metal and wet sand releases gasses which tend to further increase the porosity in aluminum sand castings. Though aluminum die-castings can also have issues with porosity, this article will stick to sand castings.
If you were to take an aluminum sand casting and cut it in half, close examination would show microscopic bubbles of trapped gas within the casting. They are the result of any number of things. Poor quality materials in the metal, casting design, moisture content in the sand, and even the relative humidity can have an affect on porosity. The point is that two identical castings can have very different outcomes when they are powder coated and cured, and the problem is due to something called The Ideal Gas Law.
The Ideal Gas Law: PV=nRT
P=Pressure V=Volume n=Molecules R=Gas Constant T=Temp
The Ideal Gas Law is an equation in physics that describes the relationship between pressure, temperature, and volume in gasses. This is relevant to the powder coater because we obviously require higher temperatures to cure our powders, and we know that some of our aluminum sand castings have gas in them. The gas law states that as the temperature of a gas increases, its volume will increase proportionally. If the gas is trapped in a closed container such as a pore in a casting, the pressure of the gas will increase because it cannot expand. If the pore is close to the surface of the casting, the gas will eventually force its way out of the surface to be released into the atmosphere.
Unfortunately, this escape of gas usually happens right about the time the powder coating is flowing. The escaping gas leaves a small crater in the coating, which in itself may not be a cause for rejection. Due to the fact that porosity tends to many pores, and not a single pore, there tend to be small fields of these bubbles, which often lead to rejected product.
So now you have some aluminum sand castings and they are ready for powder coating. You know that there is a chance that there may be an issue with outgassing. What do you do? My first suggestion is to set a minimum acceptable threshold and run a baseline production run. If you have a problem and the number of rejected parts is too high, then move to the next step. There are probably many ways to combat the problem, but here are several more common strategies for dealing with outgassing, most of which we have tried. None of them are, in my opinion, the fail-safe-go to-affordable method, however all have shown success to some degree. They are as follows:
- Degassing. The concept here is to heat the parts above the temperature required to cure the powder. If the powder will cure at 350F, you would preheat the parts to 400F. The theory is that the gas in the pores will expand to a larger volume at 400F, so when the parts are sprayed and cured at 350F the gas will not expand to an escape volume, there by leaving no outgas craters. The problem with this concept is that the parts are generally not sprayed at 350F to 400F. They are cooled to ambient prior to spraying. This creates a vacuum in the pore, which will do its best to equalize the pressure. Once equalized, the problem begins anew when the temperature is increased to 350 to cure the powder, and the result is usually some outgassing but not as much. The problem is better but certainly not cured.
- Anti-gas Primer. The concept of the anti-gas primer is to coat the casting with a coating that is formulated to form a strong skin film early in the curing process. The skin will resist the release of gas bubbles and keep them from rising to the surface of the coating. This sounds good in theory however; in practice it requires the additional cost of material and labor to apply it. We have had castings requiring three and four coats of anti-gas primer that we sanded down between coats because there was so much outgassing on the part. Even after all of that we still had outgassing through the final topcoat. This process is somewhat effective, but can be costly.
- Epoxy Impregnation. This process is often used on aluminum castings that are used in the natural gas industry. To ensure that natural gas cannot migrate through the porosity and escape the containment of the casting, it is put in an epoxy or similar catalyst cured liquid and put under extreme vacuum. This forces the liquid to be drawn into the pores and cured, thus filling the pores and eliminating the gas. This process can be very effective as long as the material filling the pores does not break down or give off gas when heated to 350-400F for the curing of the powder coating. The down side to this process is that it is expensive and time consuming, but it is effective.
- Low Cure Powders. By constantly pushing the envelope of the chemistry, most powder coating companies have developed their own line of low cure powders. Some of these powders can be cured at 285F, and possibly lower. Because most all powders follow a time/temp curve, lower cure temp means longer cure cycle. This will reduce oven temperatures and energy consumption, but will also slow down production cycle times. Also, there is usually a trade off made in reduced performance to allow lower cure temps. Most epoxies, hybrids and polyesters are available in low temp, but higher performance powders and polyurethane generally are not. The low temp powders do help reduce the problem of outgassing as the lower temperature reduces the amount of change in volume of the trapped gas. If the gas does not expand as much, not as much can escape the casting, and the problem is reduced. Because not all coatings are available in low temp, this may not be a viable option for the customer.
- Infrared Pre-Flow Ovens. In some cases, using infrared ovens at the entrance of a convection bake oven can help push the powder flow cycle past the gel stage before the actual metal gets up to temperature. This allows the powder to flow and begin to cure before the gas has a chance to expand and escape. We have not tried this method, but I have been told it helps. The downside is the purchase, installation and operation of electric or gas fired oven panels, but if it works it may well be worth the investment.
These five methods of fighting the problem of outgassing of aluminum sand castings all have their positive benefits, but they all must be applied to every casting if they are to work. Because you can not tell by looking if a casting will have an outgassing issue, you really have no choice but to treat them all the same. A better scenario would be to insist that the casting supplier provide low porosity castings. One thing is for sure, you can generate a lot of rejected product in a very short time, and there is not much you can do about it. In many cases, the cost to strip and recoat the parts costs more than the part itself, so the cost of outgassing can indeed be considerable. When the customer is hesitant to change the powders they require on an aluminum sand casting, all you can do is be sure to educate them as to why there will be a high rejection rate and a high cost.
Outgassing of aluminum castings during powder coating is just part of the job. Try some of these methods to reduce the fall out, and see if they help you. Work with your powder suppliers to use the materials and powders they recommend to help minimize the fallout. A change in casting design, alloy materials, or molding process by the foundry may help. In the mean time, your castings will be passing gas, and no matter how bad it stinks, there is not much more you can do.