The Importance of Resin Mixing Procedures in the Composite Shop

Fig. 1. The technician opens new boxes of resin and hardener, taking control of the materials from that point on. Photo Credit: Abaris Training Resources

In recent months, the topic of storage, handling, weighing, mixing and deployment of epoxy resins and adhesives comes to the fore in conversations with customers.

The main complaints were about inconsistencies between operators in the workshop and in the controlled contamination area (CCA) at two aerospace suppliers. Upon further investigation, it was determined that the operators were not reference documented protocols but instead, he had followed methods imparted through on-the-job-training (OTJ). Although there were links to written specifications and procedures in operational planning, they were rarely consulted by operators who “already knew what to do”. Our conclusion was that many of the fine details of written procedures had been lost in verbal communications and were therefore overlooked by shop personnel.

The conversations we had and the suggestions we made to tighten controls and implement best practices are described below.

Storage and handling

Resin containers opened in the workshop were the first indication that the written or verbal procedures were inadequate or not being used by the operator(s). In one case, the handling method was addressed in a document on how to handle hazardous materials but was not directly conveyed in the procedure referenced in the planning document.

Manufacturers warranty resin the duration of the conversation for a limited time, ranging from a few months to a few years — until you open the package! This means that the moment the container is opened, the load moves towards the user for further control of the materials (Fig. 1).

This is how resins (especially amine hardeners) have been compromised by untrained operators who left cans open for long periods in the shop. An always open container is more convenient for the operator, but it subjects the resins to carbon dioxide (CO2), water (H2O) or other airborne contaminants – which can affect the properties of the cured resin. Containers should always be sealed when not in immediate use. Better yet, purge each container with nitrogen before sealing to better preserve the contents.

Weigh and mix

With epoxy resins, it is very important to have a accurate stoichiometric ratio from part A (resin) to part B (hardener) to achieve the desired number of reaction sites (crosslinks) and, therefore, full structural properties in the cured resin.

There are two ways to measure resins — using either a parts by volume (PBV) or parts by weight (PBW) as supplied by the manufacturer. Most aerospace manufacturers use PBW methods to ensure accuracy (Fig. 2).

Weigh and mix a two-part resin system.

Fig. 2. This resin has a PBW mix ratio of 100:14 with a total quantity of 130 grams required. This equals 114 grams of Part A resin and 16 grams of Part B hardener. The technician tares the container on the scale and adds the required amount of Part A (left), Part B (center left), combines the parts A+B (center right) and mix the two components for three to four minutes for thorough mixing (right). Photo Credit: Abaris Training Resources

The criticality of this requirement cannot be underestimated. Operators sometimes get careless and “bounce the scales”, accidentally adding too much hardener to the resin. Worse still, they intentionally add more hardener than specified to speed up the reaction in the name of “saving time”, resulting in compromised properties — a problem that goes largely unnoticed until it is.

Appropriate proceduresin the same way recurrent training, can go a long way in reducing the chances of this happening. If the operator knows Why the mix ratio is so important to resin/adhesive performance that they’ll probably be more cautious.

It turned out that the operators in this example also needed instructions on how to mix the resins properly and for how long. To better assess the problem, we asked these questions: How much time do your operators spend mixing? Do they all follow the same procedure? Are these details getting lost in OTJ? Do operators take shortcuts? Responses vary, pointing to insufficient knowledge of written procedures.

Manufacturers recommend mixing for three to four minutes for small quantities of low viscosity materials (less than 60 poise) – longer may be required for larger quantities or high viscosity (thixotropic) resins. We found that most operators mixed small amounts (

Semkit assembly.

Fig. 3. The Semkit consists of a dispensing tube filled with resin, a dasher rod filled with hardener, and a rod to push the hardener into the tube. Photo credit: PPG, Sylmar, CA

Additional mixing problems pre-weighed or volumetric packaging appeared. This includes the use of a Semkit, two-part cartridge or a resin bi-pack.

  • Semkits1 come with part A in a tube and part B in a dasher (piston) rod. Unless you have an automated Semkit mixer, mixing them by hand requires some skill. The right technique: Once the dasher rod is screwed into the top of the tube, push the resin from part A up the tube to make room for part B. Next, retract the dasher rod, then, at the Using the wand, slowly fill the cavity with the hardener from Part B (Fig. 3). Then, holding the red capped side of the tube up, push the impeller plunger up and down several times with the wand still in place, rotating the impeller at least 180º per race. Remove the wand and continue to mix for a specified time or number of strokes as indicated on the Semkit card or label. The dasher rod is then removed and the appropriate nozzle is screwed into the end. Place the tubing in the dispensing gun and extrude a ribbon four to five inches in resin before use in the prescribed application.
  • Cartridges require the operator to check that parts A and B come out of the end of the cartridge before attaching the tip. First, position all cartridges to be used vertically to allow resins to settle and air to come to the top. Install the cartridge in the gun and hold it vertically with the tip up. Push the plunger until both parts A and B are present. Apply the mixing tip and extrude a four to five inch ribbon before use (Fig. 4).
  • Bi-packs contain both parts, separated by both sides of a plastic sleeve – usually separated by a removable divider band or a burst seal. The seal is removed and the resin is moved to the hardener in the pocket and mixed for at least four minutes (Fig. 5).

Figure 4. The technician checks that both parts A and B are dispensed before installing the mixing tip (top). Extrude a small ribbon of resin through the mixing tip before using the material (bottom). Photo Credit: Abaris Training Resources

Medicine cage

For heavy users of resin and adhesive, it is recommended that a centralized “pharmacy” systemcomposed of personnel trained, certified in all aspects of storage, handling, weighing, blending and dispensing, be assigned to prepare sealed kits precisely dosed resins and hardeners necessary to meet the work order requirements of each operator. This system can be automated and linked to production control so that the pharmacy can work ahead of the operator’s request.

Uncommon knowledge

It may seem that all of this is common knowledge. However, based on our findings, what we thought was well understood was not. This highlights one of the problems I often see in composite production facilities, which is that the simplest things get overlooked because they aren’t really monitored. The tiniest and most important details of written procedures can be filtered out over time, like a bad technical phone game. The good news is that we can audit our operations, provide recurring operator training, and refocus attention on the small details. We will all feel better about the quality of the product!

Resin bi-pack procedure.

Fig. 5. Resin bi-pack as supplied (left). The divider is removed and the resin is mixed with the hardener (center left), mixing is complete after four minutes of mixing (center right) and the corner is cut and the mixed resin is ready for use (right). Photo Credit: Abaris Training Resources


  1. Injection-style Semkit PPG package, manual usage instructions available at

Landscape photo credit: PPG, Sylmar, CA

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