We use Crystal Clear resin to encapsulate lighting components and circuit boards. Occasionally, we encounter bubbling issues with the resin and we don’t know where the bubbles are coming from.

Smooth-On sells different Crystal Clear resins worldwide to companies that provide lighting devices for commercial and military airports (runways and terminals), mining, military facilities, public buildings (emergency lighting), and more.   

What is unique about this application is that the Crystal Clear resin is poured over metal components, and the outcome can vary depending on several variables.

We occasionally see bubbling as a phenomenon and can relate the following experiences. 

1. We worked with a customer to determine that they were not measuring and mixing accurately, and ended up with excess Crystal Clear 200 - Part A in their poured material.  When the resin was at the end of its cure cycle, it was at its peak exotherm, and the viscosity was quite high.  The residual Part A generated excessively large bubbles that did not quite make it out of the resin and rested on top of the pour, creating these cavities.  We introduced them to a gram scale and a vacuum pump/chamber.  We also showed them the double mix technique before vacuuming.  This solved the issue.
2. Thermal Shock—occasionally, we see customers working in a cool environment (less than 60°F / 15 °C). Components in their workspace are also cool. The Crystal Clear resin that has been mixed and poured over components exotherms quickly, and the cold metal warmed by the resin expands, creating air. Again, the nature of the resulting bubbles that rise and come to rest on the surface is relatively large. The remedy is to pre-warm components before pouring resin.
3. A customer complaint was received regarding large bubbles/cavities appearing on the surface of the encapsulated lighting fixtures, including circuit boards using Crystal Clear 200.  Bubbles and surface delamination from the circuit board seem to be concentrated around the highest heat-generating areas (the LED and the 222 SMD resistor). Those resistors dissipate current and typically run at 50–70 °C, while the LEDs usually operate at 40–50 °C.

Crystal Clear 200 has an HDT of around 49 °C when fully post-cured. Since the resin encapsulates the components, heat buildup is more pronounced. Once the resin reaches its HDT, it softens slightly and can “move.” If microbubbles were trapped during mixing and pouring, they would remain invisible under pressure casting. However, once the part is demolded and reheated in service, those bubbles expand—similar to gas escaping when you open a seltzer bottle. This results in a delayed bubbling effect. Those bubbles are trapped inside and are always looking to come out.

Vacuum de-airing before pouring would help minimize microbubbles, and vacuum casting (rather than just pressure casting) could further reduce trapped air, especially beneath components on the circuit board. Still, we recommended switching to Crystal Clear 220, which has an HDT of ~80 °C to eliminate the issue. That higher HDT means the resin remains rigid under operating conditions, keeping any microbubbles permanently trapped.  It is our opinion that this is not a moisture issue.

The above are some scenarios that Smooth-On technicians have encountered with customers casting Crystal Clear resin over lighting fixtures and circuit boards, which is a very specific application. If you have questions about your project, don't hesitate to contact Smooth-On Technical Service in the USA at +1 (484) 546-0466.