How to get the best from your impregnation plant

The main quality control is built around the US MIL Test Ring. This is an excellent test to show how well the system is working. If the system is faulty, the ring can give some indication as to what could be the problem. Bear in mind that not all sealants are the same, meaning that some may perform better than others. It is a useful tool when considering alternative impregnants. The ring is precise and will show even marginal differences between comparable products.

To start with, some questions:

1. What type of sealant are you using?
2. Is your sealant US MIL-I-17563C approved?
3. Do you have an effective quality audit?
4. Are you satisfied with the sealing performance?
5. Do you use a test ring to evaluate process performance?
6. Do you regularly impregnate more than once?

Gel Time

Gel Time of the sealant is all important. Unless treating virgin components, the porosity is likely to be contaminated with a cocktail of cutting oils, water, emulsifiers etc., some of which contain anti oxidants. The latter can poison the sealant, especially if the sealant is low in catalyst, thus making it impossible for the sealant to cure within the porosity. The quality of sealant you have chosen affects the type and percentage of catalyst you can use, so please check with your supplier if you feel it necessary to exceed the recommended level of catalyst.

Catalyst strength is sometimes kept to a minimum because of the fear of the sealant curing in the autoclave. A cure in the test tube will only tell you that the sealant will cure in the test tube, but this is quite different to the cure in the porosity in both size and the risk of contamination in the latter.

Test tube cured sealant benefits from exothermic reaction during curing. In comparison, it is very unlikely that any exothermic action would occur within the porosity due to the minute amount present. Therefore non-cure within the porosity is the greater risk. Note that once a sealant has been subjected to a cure cycle and remains uncured, it is unlikely to cure even if reheated. Even worse, the uncured sealant within the porosity becomes a contaminant for further impregnations, rendering the casting scrap.

Always have the maximum allowable catalyst present in the sealant. The illustrations above show that with little catalyst in the sealant, the slug in the test tube will appear stress free see Fig 1. On the other hand, sealant with the correct amount of catalyst may be found to be crazed and fractured – See Fig 2/3. This should be ignored, as this condition does not relate to the sealant deposit within the porosity. Note how the crazing in the test tube diminishes as the test tube gets smaller as with no crazing as with Fig 4.

Sealant Storage

When working with higher levels of catalyst in the sealant, it is important to ensure that the temperature control system fitted to the sealant storage tank is working correctly to maintain permissible storage temperature to supplier's recommendations. A chilled sealant chills the casting resulting in a greater heat rise to the ultimate curing temperature. To over cool the sealant will encourage condensation of moisture from the atmosphere, reducing sealant performance.

Sealant Condition

Ensure that your sealant is clean and free from contaminants such as swarf and similar debris. Autoclave gel-ups start with debris build up at the bottom of the autoclave/storage tank, so keep the system clean.

Check clarity of sealant to ensure no water/oil is present.

Some sealants are more prone to contaminants than others. If there is discoloration or a translucent appearance to the sealant it ask your sealant supplier to test a sample for contamination. Some contamination of oil may not unduly affect sealing performance, but could affect temperature and/or chemical operational performance. Trace where the contamination is coming from and eliminate it.

Check that sealant is being correctly degassed before use.

Degassing cycles are often overdone on the basis that the more you degass, the better the sealant. This is not the case. The sealant requires dissolved air as this is part of the stability mechanism. Removing this by extended periods of degassing will make the sealant anaerobic and cause it to cure.

Absorbed air (as distinct from dissolved air) is the free air that is taken in from the atmosphere whenever the sealant is sitting at atmospheric pressure. This needs to be removed from the sealant before impregnation takes place, or this free air causes sealant to exude from the porosity during the curing cycle. The result is poor sealing.

This air can also inhibit the cure within the porosity. Link this with low catalyst level and you have a major curing problem. Free air generally comes off the sealant fairly quickly below 50 mbar and can normally be accommodated within the normal cycle time.

This applies to dry vacuum transfer systems only. Wet impregnation systems should have a separate degass cycle before impregnation commences.

Check that correct temperature is being applied for curing

May aging hot cure tanks in use today have no means of circulating the hot water during the cure cycle. This can lead to cold spots within the tank and particularly within the batch of components. It is also possible that the heat input could be down for a variety of reasons such as caking of heating elements etc.

Temperature is a fundamental part of the impregnation process. No matter how good a sealant is, without heat, it will not cure. It's either liquid or solid. The cure takes place in seconds, but that short period is critical and the heat must be maintained to achieve polymerization. If the sealant fails to cure when first heated, it is then unlikely to cure even at a higher temperature.

First ensure that the heating is adequate for the load of components going through the plant. If not, reduce the load accordingly. Maintaining a large load and extending cure times does not work. Run the water cure at maximum temperature and ensure that the temperature during the cure cycle does not drop below 90°C. Fit a powerful centrifugal pump with filter or other device to give heat circulation. Do not fit aeration as this removes heat.

Your tank should be regularly checked with a portable digital immersion probe in a number of areas, especially close to the bottom, and compared with the resident sensor.

Check orientation of component – porosity position

This is often the greatest source of failure to seal. First, know your component. Know where it leaks. Orientate the component in the basket so there are no possible air locks around where the porosity is located. Do not make sealant draining the first priority; this can always be recovered in the drain tray. Failure to orientate the component correctly will generally cause the sealant not to penetrate or if it does, to be pushed out of the porosity during washing and curing.

Condition of the component prior to impregnation

It is important to know what processes, if any, the component is going to be subjected to ahead of the point of carrying out the impregnation process. The cocktail of chemicals trapped within the porosity can have a dramatic effect on whether the sealant cures or not. See the diagram to the right.

Stress relieving from the foundry can be important. Some heat treatments call for oil quench, some call for hot water. The latter is preferred. Avoid oil at all cost – porosity saturated with oil is a non-starter. Once the oil has penetrated the porosity, it cannot be removed. This means that if oil testing is required, it should be carried out after impregnation.

Preparatory Drying

Oven drying has always been the traditional method of preparing the component by, cooking the components at around 150°C for one to two hours. Often this procedure is the first thing to be dropped when operations or operators are under time constraints. There is no to tell whether a component needs drying or not. It isn't worth the risk: do it!

Vacuum drying has benefits over oven baking. The presence of both temperature and vacuum allows more scope in removing highly volatile matter such as that in cutting fluids. The dynamic effect of differential pressure across the cross section within which the defect is sited also has the effect of forcing out liquids from the cavity that would not otherwise budge by the oven method.

By far the better and recommended method for drying is by vacuum. As an example:

To vacuum dry a batch of components preheated to 80°C and vacuumed down to 20mb equates to 340°C in an oven at atmospheric pressure. Desirable results can be obtained at lower vacuumed temperatures. With aluminum and plastics playing an even greater part in our technical advancements, an oven temperature of 340°C would cause havoc and be unacceptable to component requirements.

There are instances where the volume of water within a cavity exceeds the latent heat stored in the surrounding metal or composite material and vacuum drying would have the effect of freezing of the entrapped moisture. In such cases, oven drying would be more appropriate. This should not necessarily be a problem with powdered parts as liquids can and should be avoided with effective production planning.

Vacuum for impregnation

As distinct from vacuum drying, the vacuum cycle (dry vacuum) for impregnation can be short especially when carried out at elevated room temperatures. This is specially so for virgin components that have been kept clean and dry.

Generally a vacuum of between 20 and 10 mbar is adequate for most applications. This ensures that the free air has been removed. Anything below 10 mbar would appear to have little effect other than removing absorbed air and monomer from the sealant.

Using test rings in routine processing

If your sealant supplier is approved to US MIL-I-17563C, the most effective tool for quality audit is to pass a US MIL-I-17563C Test Ring through your plant. Check with your supplier for availability. Obtain certified rings if you can. See below for an alternative.

Identifying Correct Test Rings

It is possible that there will be limited availability of the US Mil Test Ring. Due to their composition, they are difficult to manufacture and therefore expensive to use for routine testing.

A substitute ring having identical porosity levels has been produced in recent years that has proved most useful as a Quality Assurance Tool. Ask your supplier if he can supply these. Alternatively, you can purchase them online from the X-Seal online store.

Using the US MIL Standard Test Ring to test overall plant performance.

Placing the test ring or several test rings in different parts of the process basket and passing it/them through the plant may also reveal some interesting results. It allows you to know more about your plant and where possible problems lurk – forewarned is forearmed. It is the one certain standard against which performance can be judged. It may not be able to tell what is wrong with the process application but at least you will have the knowledge whether the process is working correctly or note.

Testing with US MIL-I-17563C Test Rings

1. If possible, first weigh the test ring to 0.1g precision and record.
2. Secure the ring to a component about to go through your plant.
3. Carry out a normal impregnation cycle, washing and curing.
4. Remove ring and re-weigh and record. Well applied high quality impregnant will increase the weight of the ring by as much as 1 gram.
5. Clamp into pressure test fixture and apply air pressure of approx 3.5 bar. DO NOT SUBMERGE IN WATER AT THIS STAGE.
6. Observe the ring when pressure is applied and note if anything exudes – this is likely to be uncured sealant.
7. If there is no such exudation, immerse in water and observe any leakage.
8. If the part should leak:
   1. Compare rate of leakage with the leakage chart shown below and record.
   2. Return the leaking ring to the plant and carry out one further impregnation, test and observe again as above.

In accordance with the US MIL-I-17563C approval, the ring should be sealed completely. The US MIL spec allows for a second impregnation to seal. A second impregnation should only be necessary where the component is of a poor grade or the system is failing. If the sealant is of good quality and the process is correct, you should get a seal from one impregnation.

Leakage Chart

Note: Even high quality sealants will fail if the method of processing does not meet the laid down specification.