As consumer demands evolve, it’s imperative for companies to become environmentally conscious. As a result, OEMs and manufacturers who use aluminum die castings have adapted quickly, and sustainable manufacturing practices have become increasingly common.
There exists a misconception on testing requirements for vacuum impregnation sealants and vacuum impregnation processes. Testing impregnation sealants for application compatibility and testing the impregnation process effectiveness are grossly different.
A question that is often asked is “Will the sealant used in vacuum impregnation melt?” This is asked by those who’s castings need to withstand heat. In this blog, we will answer this question.
The short answer is no, Godfrey & Wing sealants do not melt under standard operating temperatures. Impregnation sealants are based on polymethacrylate plastics. There are two types of polymethacrylates: thermoplastic and thermoset.
A thermoplastic is a polymer that softens on heating and hardens on cooling. Common thermoplastics are polyethylene and polystyrene.
A thermoset is a durable plastic that once formed or molded, retains its form and shape even if heated or subjected to various solvents. Vacuum impregnation sealants are thermoset.
These two types of plastic are both called polymers. Polymers (“poly” = many from Greek) are very large molecules comprised of smaller molecules called monomers (“mono” = one). Most polymers consist of monomers that are similar to each other, joined together in a straight chain, like a strand of spaghetti. However, chemists have learned to link these chains together, called “crosslinking”. Chemists crosslink thermosets in a 3D network to prevent them coming apart when heated or exposed to aggressive liquids.
Looking at the pictures above, a simple way of understanding this is to imagine a thermoplastic polymer is like a bowl of spaghetti. If you are really careful, then you could separate all the individual strands or chains. This is what happens when a thermoplastic is heated –the chains separate and flow. In the thermoset example, this is impossible.
The crosslinked methacrylate polymers used in impregnation sealants can often withstand temperatures over 392 °F (200 °C). The sealants can also withstand aggressive fluids such as hot automotive fluids without degradation. As a result, vacuum impregnation is the preferred method to seal casting porosity. It is the simple and permanent solution to porosity.
When choosing a vacuum impregnation sealant, you have two sealants choices: thermal-cure, or anaerobic. Choosing one over the other has nothing to do with the quality of the sealant, but it has everything to do with the part material, size of the pore, and leak path.
Curious on what sealant is best for you? The following infographic is a simple snapshot that will help educate and allow you to determine if thermal-cure or anaerobic sealant is best for your needs.
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MIL-I-17563C is the Military Specification for cast or powder metal components. This specification is approved for use by all Departments and Agencies of the Department of Defense and “covers the requirements for impregnants suitable for use in sealing the voids found in cast or powder metal components which cause leaking of contained fluids.”
To answer the question above, it’s important to first understand the 3 classifications within MIL-I-17563C.
Class 1 – Suitable for service temperatures up to 300°F (149°C)
Class 1a – Suitable for use on mortar shell castings up to 300°F (149°C)
Class 2 – Suitable for service temperatures up to 500°F (260°C)
Class 3 – Suitable for use where air pollution requirements apply and compatible with acrylic-nitrocellulose lacquer paint system up to 300°F (149°C)
In general, all organic based vacuum impregnation sealants are qualified to Class 1 and 3. Class 2 was designed specifically for sodium silicate which is in very limited use today, and Class 1a qualifies the sealant to be compatible with TNT (Trinitrotoluene) and Composition B explosives.
To qualify for MIL-I-17563C, the sealant is subjected to various tests including pot and storage life stability. Once activated, the sealant must be stable at 75 +/- 5°F (24 +/- 2.8°C) for one month for thermal cure and 45 +/- 5°F (8 +/- 2.8°C) for anaerobic while under aeration. Un-activated or un-catalyzed material in an unopened container must meet the manufacturer’s original specification at the end of one year from date of manufacture. All Godfrey & Wing sealants are approved for two (2) years.
The sealant reactivity to various metals including aluminum, copper, iron, magnesium and zinc is also tested. Samples must not exhibit any obvious surface defect such as holes, pits and fissures. Samples with copper shall not show a greenish coloration after exposure for 24 hours.
Godfrey & Wing currently has five sealants approved on QPL-17563 and are listed below:
95-1000 – Thermal curing
95-1000A – Thermal curing
95-1000AA – Thermal curing
95-1000AC – Anaerobic
95-1000ACP – Anaerobic
Conditioning testing under various test media, time and temperature are shown in Table 1.
TABLE 1 – Conditioning Tests
| Impregnation Class | Material | Media Specifications | Time | Temperature |
|---|---|---|---|---|
| 1 and 3 | Carbon Removal | P-C-111 | 30 minutes | 73.4 +/- 3.6 °F (23 +/- 2 °C) |
| 18% Sulfuric Acid | O-S-809 | 2 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Turbine Fuel | MIL-T-5624 | 48 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Fuel | ASTM D-910 | 48 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Diester Grease | MIL-G-23827 | 48 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Stoddard Solvent | P-D-680 | 48 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Ethyl Alcohol | MIL-E-463 | 48 hours | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Hydrocarbon Fluid | TT-S-735 | 14 days | 73.4 +/- 3.6 °F (23 +/- 2 °C) | |
| Water | N/A | 14 days | 212 °F (100 °C) | |
| Oil | MIL-H-17672 | 14 days | 210 +/- 5 °F (99 +/- 2.8 °C) | |
| Hydraulic Fluid | MIL-F-17111 | 14 days | 210 +/- 5 °F (99 +/- 2.8 °C) | |
| Lubricating Oil | MIL-L-7808 | 48 hours | 255 +/- 5 °F (121 +/- 2.8 °C) | |
| Ethylene Glycol | MIL-E-9500 | 14 days | 300 +/- 5 °F (149 +/- 2.8 °C) | |
| Ethylene Glycol | MIL-E-9500 | 14 days | 397 +/- 5 °F (197 +/- 2.8 °C) | |
| Thermal Resistance | N/A | 14 days | 300 +/- 5 °F (149 +/- 2.8 °C) |
One of the most common questions in vacuum impregnation is how much does it cost to impregnate a part, or in other words, how much sealant will a part consume? Parts consume sealant in two ways as part of the normal vacuum impregnation cycle.
The first way that parts consume sealant is that sealant actually penetrates and gets trapped inside the part where the sealant then solidifies and seals the part. The second way sealant is consumed in the vacuum impregnation process is the design waste. This is the excess sealant on the outside of the part that ultimately gets washed off and discarded. If you know the amount of sealant that gets trapped (picked up) inside the part during the impregnation cycle, then you can do a rough estimate of the overall cost simply by multiplying the “pick up” amount by 4.
This calculation not only takes into consideration the sealant cost but also compensates for compressed air, electricity, water, labor, and design waste associated with the overall vacuum impregnation process. It does not, however, include any factor for profit, depreciation, freight, general or administrative expenses, selling costs, or any other non-impregnation process costs.
With that in mind, we can provide some basic information to help you establish sealant pick up rates for both powder metal (PM) parts and traditional aluminum-based parts. This information is based on historical data and is only meant to provide a ballpark understanding of typical sealant consumption. Given that every part is unique and different your application may or may not adhere to these guidelines. As for PM parts with a typical density of 6.8, you can assume an average pick up rate of 1 gram of sealant for every 40 grams of compacted metal. So, if a compacted part weighs 160 grams it will usually pick up about 4 grams of sealant.
For aluminum, let’s assume a density of 2.8 grams/milliliter. If you know the weight of the aluminum part in grams you can simply divide that by the density to get the volume of the part in milliliters. By dividing that by 1,000 you can convert the volume to liters. Once you have the volume you can simply multiply that result by a factor of 8 to get the estimated sealant pick up. For example, if a part made from aluminum weighs 1,500 grams, divide that by 2.8 to get 535mL. Divide 535 by 1,000 to convert to liters which will yield .535. Multiplying the .535 by 8 will provide an estimated sealant pick up of 4.3 grams. This information can then be used to help establish your annual sealant volume or daily sealant consumption.
In an earlier blog article, we discussed the most common ways that sealant can get into vacuum pumps during the impregnation process.As a reminder, the list included:
The next step is to discuss how to defend against the problem of sealant getting into vacuum pumps. Even if an impregnation system is properly set up and maintained, accidents can happen that can lead to this problem. The best defense is a trap that the sealant can be “caught” in prior to reaching the pump.
Depending on the level of protection desired, traps can be as simple as a loop in the vacuum hose/line or as sophisticated as a mechanical trap with sensors.These traps are located between the impregnation vessel and the vacuum pump.The purpose of a trap is to give the sealant an alternative place to go (get trapped) versus into the vacuum pump.
The simplest approach, which is effective when sealant rarely enters the vacuum pump, is to loop the vacuum hose/line in such a way that gravity alone stops sealant before the pump.In this case, the weight of the sealant cannot overcome gravity and therefore cannot reach the peak of the vacuum hose, thereby “trapping” it in the line, typically to drain back into the vessel.
The next level of technology is a mechanical trap that is either maintained manually or set up for semi-automation (Fig 2 below). With these types of traps, there is a level sensor that sends a signal to the PLC, indicating that it is full and needs to be drained. A manual drain at the bottom allows maintenance personnel to easily drain the system.
The highest level of technology is a fully automated trap that allows for continuous running of the impregnation system (Fig 3 below). These traps also have level sensors to indicate when they are full. In addition, the traps are connected to pneumatic valves that are programmed to open between cycles, draining any sealant in the trap back into the sealant reservoir.
The interior of the mechanical traps can be of various design. Those requiring the least maintenance will have steel baffles that the air/sealant would have to navigate to get to the pump. Designed correctly, these are very effective and pulling any sealant out of the air flow.
Another option is to have filter elements (typically in the 5 micron range) that the air can pass through, yet the sealant will be trapped in.
Depending on the size and performance of the system, a cost-effective Vacuum Sealant Trap can be designed and installed. The additional protection for the vacuum pumps are worth the investment so as to reduce downtime and maintenance costs.
For more information on this topic, contact Godfrey & Wing.
There are a couple of possibilities that would require a double cycle impregnation.
In the vacuum impregnation industry one of the more frequent topics of discussion surrounds the difference between anaerobic and thermal curing impregnation sealants.
It is well known that thermal cure sealants, like Godfrey & Wing’s 95-1000A and 95-1000AA, are more widely used than anaerobic sealants (95-1000AC and ACP). Many will say the reasons for the increased use of thermal cure sealants are acquisition cost of the sealant or equipment. Others will cite ease of maintenance while still others will say it is due to the elimination of a host of failure modes.
In reality the decision for choosing a specific style of sealant lies with the part to be impregnated. Some parts are just more likely to be sealed in one versus the other. It has nothing to do with the quality of the sealant, but it has everything to do with the materials being impregnated and the size of the pore or leak path.
| Thermal Cure | Anaerobic | |
|---|---|---|
| Curing | Polymerizes when heated to 195°F | Polymerizes in the absence of air over 24-48 hours |
| Pressure Testing | Immediately after impregnation | Need to wait 24-48 hours after impregnation |
| Maintenance | Continuous refrigeration | Continuous refrigeration and aeration |
| Packaging | 5 gallon, 50 gallon, 250 gallon | 4-5 gallon carboy |
| Applications | Castings, aluminum, iron, steel, zinc | Powdered metal, electrical applications, cables |
Powdered metal applications have large, through porosity and work best when using an anaerobic sealant. Why is this? First, once impregnated into the part, the sealant may be activated in a chemical bath to seal the pore at the surface, allowing the remaining sealant trapped in the porosity to cure over time. This reduces the chance of sealant migration from the pore which could be found when using a thermal curing sealant with large open porosity.
Likewise when sealing micro-porosity in a casting, a thermal cure sealant will perform better than an anaerobic as the process used to impregnate thermal cure sealants is more robust and thorough. Also since the sealant is forced to cure when it reaches the 195°F cure temperature, impregnated parts may be tested immediately after impregnation. Thermal cure sealants also will be less reactive and have a much longer pot life than anaerobic sealants.
