There’s more to the potting process than simply pouring a liquid resin onto an assembly. To learn more, we spoke with Matthew Griffin, an engineer with Scheugenpflug Inc. USA in Kennesaw, GA.
There’s more to the potting process than simply pouring a liquid resin onto an assembly. To learn more, we spoke with Matthew Griffin, an engineer with
Scheugenpflug Inc. USAin Kennesaw, GA.
For help with your potting application, call Scheugenpflug at 770-218-0835.
ASSEMBLY: I realize that potting materials vary depending on the chemistry and application, but in general, what attributes do these materials have in common?
Griffin: There really are no common attributes. They can cure with heat, moisture, ultraviolet light or a hardener. Their viscosities range from low (salad oil) to high (mustard). They can be formulated for water- or solvent-resistance, elasticity, and electric or thermal conductance or insulation. Depending on the application, the resins may have different fillers that influence several aspects of equipment design.
The common point is that the dispensing equipment needs to match the material, whether it be epoxies, silicones or polyurethanes. That includes the dispensing head, material preparation, and different seals and gaskets. The viscosity of the resins and hardeners varies greatly, which influences our designs dramatically.
ASSEMBLY: If I needed to pot a small automotive sensor in a high-volume assembly operation, what equipment would you recommend and why?
Griffin: In this case, the customer requirements state that the machine needs to have a very good repeat accuracy. The quality of all the parts has to be guaranteed.
The mixing ratio, the dispensing position and the amount dispensed are the fundamental parameters. We can position the dispense head to within a tenth of a millimeter.
In such a case, we would recommend a fully automatic multifunctional cell. For high processing reliability, a highly automated system is needed to prevent faults or malfunctions due to human intervention. A piston metering unit should be used so the mixing ratio can be achieved precisely and repeatably.
ASSEMBLY: If I needed to pot a much larger assembly, such as motor windings, in a low-volume operation, what equipment would you recommend and why?
Griffin: A semiautomatic system for lean production. The lean concept requires the perfect balance between manual and automated sequences. Lean production works according to the basic principle of “as much automation as necessary, but as little as possible.” For manufacturing plants with small and medium-sized production volume, it is not worth investing in highly automated equipment. The principle of one-piece flow dictates that the machine should only be in operation when goods are actually in demand. The costs that would arise from production breaks in a highly automated plant would be unsustainable.
This is why the trend is clearly heading towards space-saving machines that operate economically with high-quality output. They must, of course, be inexpensive, which is a true challenge for many machine manufacturers.
When we speak of low-volume production, we need to also think about changeover requirements. It should be possible to design the production line so it can be adapted to subsequent products. This is where a modular system is critical.
ASSEMBLY: Most potting compounds are two-component materials. How accurately do the materials need to be mixed on ratio?
Griffin: The correct hardening and performance characteristics of the material are only possible if the two components are mixed homogeneously, in the right proportions, and without the risk of separation, which is possible if the materials have different polarities. The material will continue separating until mixed correctly.
ASSEMBLY: When dispensing dots or beads, the accuracy of the shot size is very important. Do you have more leeway in potting? How important is volumetric accuracy in potting applications?
Griffin: In both cases, metering must be very precise. If shaped parts are filled, the customer will expect a flush and extremely smooth surface. Here, we must take into account that the materials could shrink and flow, and that this will have to be compensated for during filling. To guarantee a smooth, flush surface while taking into account the part’s tolerances, precision equipment is necessary.
ASSEMBLY: Preventing air bubbles in the potting material is important. First, what are some suggestions for preventing air bubbles in the material before dispensing?
Griffin: I can recommend several suggestions:
* Do not store opened containers of material.
* The material must be prepared with vacuum assistance.
* Make sure that the material does not display deaeration properties while under vacuum.
* Make sure that the parts are dry. Use a preheater if necessary.
* All of the machine parts (lines, pumps, metering units) must be vacuum-tight.
* Dispense the material in a vacuum chamber.
ASSEMBLY: When and how would you apply a vacuum to parts that have been potted? Is it better to apply vacuum as the material is being dispensed or after?
Griffin: Basically speaking, this is not what we recommend. If evacuation does not take place until after dispensing, it will not be possible to ensure that the deaeration will be successful in the deeper areas of the part. The part shape is often responsible for air bubbles being incorporated during dispensing. Furthermore, the air incorporated in the material can only escape via the surface.
ASSEMBLY: Some manufacturers heat their parts before they are potted. Why and how would that be done?
Griffin: There are two reasons to heat the part before dispensing. The first is to dry the parts to remove all traces of humidity. The other reason to heat the parts is to match their temperature to that of the material. Adjusting the temperature optimizes the flowing behavior of the material and improves cross-linking.
Parts can be dried with dry compressed air, an oven, infrared radiation and plasma treatment.
ASSEMBLY: Can you offer any tips, tricks, suggestions or advice to make the potting process go more smoothly?
Griffin: The metering and dispensing tasks should always be viewed as being a joint sequence with the processes performed both upstream and downstream. This is the only way to guarantee a perfectly tuned quality chain. The metering or dispensing machine is not the only influence on the quality of the dispensing process. Other factors, such as resin properties, storage, part design, and the hardening method also play important roles and must be taken into consideration when planning the production process. It’s a good idea to consult a dispensing expert right from the beginning.
We always recommend holding metering trials beforehand, as each metering task is essentially individual. All parameters, such as the part, material, atmosphere, process times and requirements, should be clarified as soon as possible-before the start of the project.
ASSEMBLY: Can manufacturers design their products to facilitate the potting process?
Griffin: Dispensing on electronic substrates can be clean, fast and bubble-free if the part design is clear-cut and straightforward. Well-designed components also help cut cycle times considerably. Therefore, the dispensing process should be taken into account during the design stage.
Dispensing: Perfecting the Potting Process
October 4, 2010