Since the core / coil assemblies are somewhat different between manufacturers, the following recommendations should be used as a starting point in developing a consistently repeatable casting process. Crosslink Technology Inc. manufactures a number of casting systems, which are finely tuned to achieve excellent results in terms of crack resistance, partial discharge and accuracy.
Depending on the design and construction, whether or not cushioning is employed, some devices may require more or less rigid compounds.
- Keep the number of sharp edges to an absolute minimum (none is preferred) by cushioning. Sharp edges can cause micro-cracks in the encapsulant during the curing process.
- Eliminate protruding sharp edges from the winding insulation.
- Recommended minimum wall thickness for the encapsulant in any area is 3.17 mm (1/8”).
Recommended Casting Steps (Hand processing):
- Dry the core / coil assembly at 125°C to eliminate any residual moisture, (this is best accomplished in a vacuum oven but a regular oven is sufficient in most cases). Make sure that the assembly itself reaches and remains at 125°C for at least 1hr.
- Make sure the mould is thoroughly cleaned and is free of any residue.
- Pre-heat the mould to 125°C. Leave in the oven until the pour is ready
- Thoroughly mix the resin in its container, (this is necessary to re-mix any settled fillers and create a homogenous mix)
- Thoroughly mix the hardener in its own container, (to obtain a homogenous mix)
- Mixing precautions:
- If mixing with a drill mounted blade, keep the blade below the surface of the material to minimize air inclusion.
- If mixing by hand, use a dry steel spatula or equivalent and minimize turbulence during mixing. Occasionally scrape the sides and bottom of the container to insure a homogenous mix.
- Never use wooden sticks since wood contains moisture from the surrounding air
- Pre-heat the resin and hardener to the recommended temperature (usually 80°C)
- Lightly stir each component
- Weigh each component into a clean steel container according to the stated mix ratio within +/- 1%. (i.e.: in the case of CLR 1837 / CLH 5185 -100 parts of CLR 1837: 30 parts of CLH 5185).
- The container should be large enough to allow the mix to rise during de-airing without overflowing the container.
- It is best to weigh the resin first and weigh the hardener second.
- Thoroughly mix the resin and hardener together, using the above described technique, to minimize air entrapment during mixing.
- Place the mixed material into a suitable vacuum chamber and de-air under 1 - 4 Millibars of vacuum for 5 minutes or until the coarse bubbles subside.
- Please note that it is not always possible to vacuum mixed material until bubbles disappear. This is because some hardeners contain volatile components, which will continue to be “vacuum stripped” after all the air has been removed.
- Remove the pre-heated mould from the oven and apply mould release. (We recommend a silicone based mould release like our CLA 8000 or equivalent. If possible, apply the mould release via spray to ensure complete coverage of hard to get to areas.)
- Assemble the part to be cast into the mould, carefully positioning inserts such as connection pads, to avoid any surfaces to be embedded in the epoxy from contact with the mould release. (Mould release contaminated inserts will not adhere to the epoxy and could be a source of corona).
- Check the mould temperature and put back in the oven if necessary to make sure the whole assembly is at 125°C prior to pouring.
- Pour the mixed casting compound into the mould, into one corner only, slowly, letting the material rise pushing the air ahead of it. (It is critical to continue pouring into only one spot. Turbulent flow or material “folding over itself” causes excessive air entrapment.)
- Place the filled mould into the vacuum chamber and de-air under 1 – 4 Millibar for 5 minutes. The vacuum may have to be released periodically to prevent the material from over flowing. Continue vacuum until all coarse bubbles disappear.
- Transfer the mould into a curing oven, pre-heated to 125°C.
- It is important that the mould is transferred into a pre-heated oven immediately after the de-airing is finished. This is necessary to maintain the "gellation profile" of the encapsulant. DO NOT PLACE THE TOOL ON A HEAT SINK, such as a concrete floor or steel workbench, because any cooling will change the “gel profile” and alter the resultant shrinkage.
- Periodically check the tool in the oven and top up with mixed material as necessary to replace any shrinkage. (Most mould designs incorporate a "reservoir" as part of the tool for this purpose)
- Post cure as per technical data sheet.
- For best results, pour under vacuum. This is done by placing the assembled mould into a vacuum chamber and drawing vacuum on the mould itself. The mixed material is then introduced into the chamber (poured through a suitable hose or pipe) to fill the mould. Once the mould is full, the vacuum is released, applying atmospheric pressure to the material.
- For even better results, cure the material under 20 KPS (80 PSI) pressure. In this case, pressure is applied during the gellation phase, compressing any residual gas bubbles in the casting. Compressing such bubbles will increase the corona inception value.
Key points to remember:
- We want to make sure that the core / coil is totally dried out before casting. This is especially important if the cushioning materials used are prone to moisture pick up (i.e. paper)
- The unit has to be above 100°C to drive off moisture. This may take several hours.
- We want to make sure that all sharp edges are cushioned so we don’t get “micro cracks” in the casting.
- We want to make sure that both resin and hardener containers remain closed until we are ready to use them (some materials pick up moisture from the air)
- We want to make sure that all fillers are properly mixed in and our mix is homogenous.
- We want to make sure our temperatures are correct. (Do not rely on the oven controller. They are notoriously inaccurate. Measure the actual temperature inside the oven.)
- We want to make sure that we pour into one corner, slowly, letting the material rise on its own.)
- We want to make sure that we vacuum all the air out of the poured material.
- We want to make sure that we maintain the proper gellation profile through accurate temperature control.
Many of our customers have been able to achieve low enough corona levels to meet corona free status on their metering transformers, which is a requirement in North America.
Further improvement using SF6 gas.
Further reduction in corona level is achievable by introducing a quantity of SF6 gas while pouring under vacuum using the following method:
- Place the mould containing the part to be encapsulated into the vacuum tank.
- Draw vacuum on the mould (down to 1 – 4 Millibars)
- Connect a bottle of SF6 gas to the venting valve (normally used to release the vacuum)
- Open the venting valve for 30 seconds. This action will reduce the vacuum to some degree in the chamber but, instead of air, the space will be filled with SF6 gas. Provided the chamber was not allowed to come up to atmospheric pressure, the remaining vacuum will be strong enough to de-air the material as it is introduced through the filling spout into the mould within the chamber.
- Disconnect the SF6 from the vent valve so it can be used as normal to release the vacuum from the chamber at the end of the pour.
- Open the filling valve and slowly fill the mould.
The idea behind this process is to mix SF6 gas into the atmosphere inside the vacuum chamber thus increasing the dielectric strength of any residual air that may remain trapped in the casting. This is an effective method of reducing corona due to trapped air but the gas is expensive. In light of this, it is advisable to reduce the size of the vacuum chamber by displacing as much volume as possible using inert materials (i.e. cast epoxy sheets etc.).