Considerations for designing parts to be potted
Design considerations for parts that will be potted are the following areas (Click to go to the area of interest):
The biggest problem when potting a part is the entrapment of air. The air carries moisture so that if air is trapped inside the potted unit corrosion can occur. This can be a large problem if the unit is a PC board and units have failed 2 years after potting just due to trapped air..
In the design of the unit it is best not to have horizontal surfaces so that air can not be trapped under the surface. If that can not be done then a draft angle will help the situation.
Provide exit places for the air to come out of the part. When the unit is potted the operator normally pours the potting compound over the total top of the unit trapping the air inside and the air has to bubble out through the material as it flows down in to the spaces. To eliminate this, the ideal design is to fill from the bottom of the unit so that the air is pushed out ahead of the resin system. Therefore, a space that will allow the dispensing tube to go to the bottom of the unit is ideal especially if there are horizontal surfaces. Also if spaces are designed around the edges of the part being potting it helps allow the air to escape
If bottom filling is not possible, then design in places which could be poured from the top which go all the way to the bottom of the unit and can then flow across the unit. If there are horizontal surfaces like a PC board, then the board should be designed to have 1/8 inch holes in the board for air release. If the resin system can flow from one side to the other quickly, then the operator will have less tendency to cover the top totally with resin capping in the air in the part.
All potting resins shrink - from 1 to 4%. So how do you reduce the visual shrinkage? The shrinkage that you will see is normally in parts that are not open to the air very much. As the potting materials cure they do so from the inside out. That means the thickest part hardens first and shrinks as it does so. However, the liquid in the thinner areas, which is still liquid, flows in to take up the shrinkage and the liquid level is lowered by the amount of shrinkage. Normal shrinkage is about 4%. Therefore a part that is not very tall vs. it’s length and width will not show much shrinkage. However, this means a horizontal part is the best when it comes to shrinkage but that is not the best for air entrapment.
There are two ways to overcome the shrinkage marks on a vertical potting. First is to use a second pour that is time consuming. After the first pour has gelled make a second pour to “cap” off the unit. The second method is to design in a thin riser, so that it will gel last which allows material to flow in to the part to make up for the shrinkage. After the unit has gelled then the riser will be cut off.
When using plastics, such as in potting shells or cases, choose those that are easy to bond to with the potting material. If the bond is weak, then water can get in the broken bond. The plastics resistant to bonding are the least expensive, polypropylene and polyethylene but also Teflon® is difficult to bond. These plastics can be treated to get a bond but that is another step and expense.
Plastics such as ABS, Nylon, PVC with or without glass filler in them are acceptable. Glass filled polyester, PPO, Polycarbonate and other engineered plastics will work but require special resin systems.
When designing a part which will be potted several factors should be taken into account to keep the cost low. These include the processing speed to pot the unit, the amount of potting compound used, how much automation will be use vs. the labor hourly cost. See the page on costs and on potting hints to get a broader understanding of the cost involved before starting the design process.
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Last Modified: March 28, 2015