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Last Updated: November 10, 2022

Fiber Optic Center NewsIn the fiber optic cable assembly process, working with epoxy generates a lot of questions. When I visit fabrication facilities, we discuss the finer points of mixing the material, loading syringes, minimizing waste, heat-treating crystallized epoxy, and more.

For this article, I decided to share a few answers to some of the questions I hear most often, along with favorite epoxy tips and tricks.

Our technicians might be overmixing epoxy. What advice can you give us?

If you mix too vigorously, you can trap air in the material and introduce bubbles. In the production process, it’s important to dispense a drop of epoxy – not a bubble. So when mixing the material, you want to thoroughly blend Part A and Part B with minimal bubbles. A favorite tip: Instead of mixing for 60-90 seconds, I recommend 25 uniform passes using a consistent mixing rhythm that minimizes bubbles.

Read my article for detailed mixing instructions.  

What’s the easiest way to load a syringe with a mixed package of epoxy?

It’s important to have BOTH HANDS available to do the job. This means your syringes should be clean, ready to use, and positioned for loading. I recommend using a test tube rack to hold the syringes vertically. My favorite tip: If you don’t have a test tube rack, there are 2 ways to improvise.

  1. Tape the syringes to the side of a table or desk.
  2. Fashion a simple fixture from a block of Delrin (wood works ok) with a drilled hole slightly larger than the diameter of the syringe barrel. Drill the hole all the way through the block so any spillage doesn’t collect at the bottom. Also, a larger block offers a more stable base, with room for multiple syringes.

We use extremely small quantities of epoxy. How can we reduce waste?

I suggest you purchase the smallest package available, mix Part A and Part B, dispense all the material into syringes, and use what you need immediately. A favorite tip: As quickly as possible, label the syringes with the remaining pot life and put the syringes with the unused material in the freezer to preserve the pot life. To use this epoxy, remove a syringe from the freezer, let it warm up briefly, and dispense another small quantity.

Read my article for step-by-step instructions.

Since cleanliness counts, how can I improve my epoxy work surface?

The pot life on epoxy starts the moment the two parts are mixed. This means you need to plan your work, and have everything you need at your fingertips. Here’s a checklist:

  • Very clean mixing equipment and any secondary mixing containers, e.g., stainless steel spatulas, polyethylene (PE) and polypropylene (PP) mix containers that have been final-rinsed with electronic-grade isopropyl alcohol (IPA, <0.5% water)
  • Timers
  • A test-tube rack (or something similar) to hold syringes upright, as discussed above
  • Any dispensing equipment reservoirs, positioned to be filled directly

A favorite tip: Make a polyethylene work surface. PE can be cut using woodworking tools, and you can place it in any dedicated space to create an easy-to-clean work surface. PE is gleaming white in color, chemically resistant, and available in 1/8″ or ¼” sheets. You can easily wipe off uncured epoxy with acetone and a Kimwipe and remove cured epoxy by gently scraping it off.

I found crystals in our epoxy. Does this mean it has gone bad?

Crystallization in epoxies can look like salt granules in the otherwise clear liquid. Or the epoxy may look foggy, milky, or grainy. Or it may have solidified. Rest assured, the material has not “gone bad.” Crystallization in epoxies is a common occurrence. It happens because the liquid resin is fighting to return to its natural state, which is a solid form. Once the epoxy has crystallized, it IS usable. You just need to heat-treat the material and return it to the clear liquid state.

Read my article to learn how.

Our epoxy is curing differently and has a different color. Can I use it?

Color variation from batch to batch is a non-issue. Since epoxies are manufactured in batches, the raw materials used by manufacturers to synthesize the epoxy are subject to small variations. These slight variations in raw materials can result in mild color variations.

What about the curing difference? While it is possible for a batch of epoxy to be faulty, this is very rare. Most likely, this points to some variation in the epoxy that is allowed by the epoxy’s specification. Again, epoxies are manufactured in batches, with specified manufacturing windows for viscosity, work time, and other parameters. This means the final product can – and probably will – show some variation from batch to batch. What happens if your cable assembly process has been very specifically tailored to a given batch of epoxy and a new batch has slightly different properties? This slight variation might result in an unwelcome disruption to your production process. However, minor process adjustments can solve the issue and, happily, make for a more robust process.

A favorite tip: Cure the epoxy by itself (not in your application) to ensure it meets the manufacturer’s specifications. This simple experiment may offer clues to determine how to tweak your process. For example, if the epoxy takes longer to cure, you can keep it in the oven a bit longer or raise the temperature to ensure a complete cure.

Our epoxy is past expiration – can it still be used?

Fiber Optic Center recommends that you test the material before using it in your production process. Cure it to see if it sets as expected. However, it’s difficult to determine if it has, indeed, cured and set properly.

A favorite tip: When in doubt, throw it out!

How can I select the right epoxy for a new application?

I’m happy to help you determine the epoxy that will work best for your application. Just send an email to me at and include answers to these questions:

  • Can you tell me something about your application?
  • What substrates are to be bonded?
  • What is the main purpose of the adhesive (e.g., alignment, in the optical path, index matching)?
  • Are you planning UV or heat cure? What is the maximum acceptable temperature for cure?
  • What strength of bond, hardness of material, and viscosity do you need?

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