Laminating with E-R Resin
Learning to laminate with a resin that you haven't used before can be intimidating and confusing because there are a lot of unknowns about what you can and can't do with various types of resins. I'll explain some of the variations in the properties of E-R Resin and discuss methods of laminating with it.
E-R Resin is a true two-part epoxy resin. You have to mix 100 grams of part A to 40 grams of part B for it to work properly. You cannot add more hardener to make it set faster; only by applying heat can you help control the speed at which it sets. It is also thicker than other resins, which is one of the properties that provides its strength, and may take time and practice to adjust to working with if you are used to a thinner resin. It is also important to keep part A at room temperature, as it thickens even more if it's allowed to get cold. You may wonder if this type of resin is suitable to laminate a KAFO. Since I laminate 40-inch-long leg braces for thoroughbred horses with it, I can confidently say that it is.
In researching this resin, I learned a lot about resin in general. For instance, when a resin sets too fast, it's classed as a brittle resin. So with other resins, to make our layups strong, the O&P industry traditionally makes them very thick. Therefore, to make a strong resin without the need for thick layups, I had to accept that it would take longer to cure fully. However, technicians tell me they can't wait 24 hours for a lamination to cure. I agree. That's why my team at Prosthetic Research Specialists, Bushnell, Florida, created the heating process to work with E-R Resin, allowing you to take the lamination off the mold in just over an hour. Using heat cures the resin to about 70 percent of its final strength, which, when it's used to fabricate a socket, I've found to be strong enough for me to trim, sand, polish, fit it on the client, and set it up for walking. However, if it is not necessary to produce the socket as quickly, I simply leave the layup on the cast overnight or laminate first thing in the morning and take it off in the late afternoon.
This resin can save money as it doesn't require using composite materials. My basic lamination for a 200-lb. patient with a transtibial amputation who is at a K2 or K3 level is simple: I use a four-layer nylon stockinette and a four-layer Nyglass stockinette. For every 25 lb. of additional weight, I increase the layup by one layer of each. When I started doing this, I asked fabricators around the country how much money they were spending to create an average-size transtibial socket with no special added products. Typically, their raw materials cost included the expense of sealing a cast and composite material. Using the nylon and Nyglass stockinettes with E-R Resin, the cost is reduced by about 23 percent from the lowest estimates I received. Delivery charges are also reduced because it ships as nonhazardous, unlike vinyl ester resin, for example. In addition, it cleans up with soap and water instead of acetone.
When I want to use composites with E-R Resin, I don't hesitate if they are necessary. However, because of its strength, which has been tested and verified by an independent laboratory according to American Standard Testing Methods (ASTM), I have the option to choose when I want to include them.
Another benefit is the ability to make a laminated elevated vacuum socket without needing to use an inner plastic liner. Yes, it can be done. Because E-R Resin bonds to itself during the exotherm stage, it seals the socket, unlike vinyl ester resin, which follows the strands of the fabric and will leak vacuum. After I laminated three sockets for a client using this resin without an additional inner plastic liner, the sockets were extensively tested and passed all tests. (Author's note: To learn more about how to laminate a socket that will hold vacuum, visit www.oandp.com/link/275)
As with any product, to be successful with E-R Resin you must use the appropriate methods of working with it. The following tips will help you when laminating with this resin:
- It is not necessary for the cast to be sealed. You can laminate directly over a wet cast; moisture does not affect the resin. Just make sure the inner bag doesn't break. It may be helpful, however, to pull some of the moisture out of the wet cast first to make the PVA bag stronger.
- Make sure the cast is around 70 degrees Fahrenheit or above. The warmer the cast, the faster the resin will cure; the colder the cast, the longer it will take to set. If the temperature of your workspace is cool, it will affect the temperature of the cast, so you can preheat the cast itself if needed.
- Pull a PVA sheet or bag with a sealed end over your model.
- Do not pull vacuum on your inner bag unless you have a really deep cut in the cast. The fabric and outer bag will do this for you in most cases, and it makes for a smoother finish on the inside of your socket.
- Pull your layup over the model. One of the first things to note here is that because of the strength of the resin itself, you can eliminate a lot of composite or hybrid materials in your layup.
- Pull your outer bag down over your layup and dry the bag. It makes it easier to string.
- Mix the resin, pouring part B into your cup first. Then add part A. This makes it easier to blend. Stir together until the resin starts to clear, add pigment if desired, and stir again.
- When you pour the resin into the bag, pull maximum vacuum. I use 20-27 inHg every time. Don't touch it for a minute or so. Allow it to soak into the distal end before you start pulling it down.
- When you start pulling the resin down, do so gradually and equally all the way around your model. This will chase the air out of the fabric and allow the resin to saturate evenly. I do not use a string until I'm about two thirds of the way down. I squeeze a little down, go to the next side, squeeze a little down, and continue around my model, repeating this process until I feel the need to string.
- Stop when you reach the level you want. The resin is thick enough that it should not creep down much further.
- If you apply heat methods to help cure the resin, you can demold in one and one quarter hours. If you choose to let it sit, allow about three to four hours for it to set.
- If you have any leftover resin in the PVA bag, twist to close off the bag, clamp it with a Yates-style clamp, and pour out the excess. Any pool of resin has a tendency to emit gas. To disburse the gas, wait until the resin starts to set, then place a fan so it blows cool air over the model until the resin hardens. Then turn the fan off and let it continue setting.
Learning to use a different resin can take time and practice; learn the varying nuances before you try to laminate a difficult project. And remember, if you have questions about how to work with any unfamiliar product, call and ask the product's manufacturer to eliminate confusion and give you the best chance of success.
Finally, in determining which products will work best for your clients, I recommend laminating a set of sample sockets to keep on hand. Laminate three to six different sockets with different layups, allow them to cure, and number them. Remember to keep track of the steps you take to fabricate the initial layups. This way, when a practitioner orders a socket, he or she can actually feel the samples and choose which one he or she wants. Then you can simply recreate it.
Ronnie N. Graves, BOCPO, LPO, CO, CTP, is the owner of Prosthetics Research Specialists, Bushnell, Florida, and has worked in the O&P industry for 35 years. He specializes in hard-to-fabricate devices and teaches fabrication techniques. Graves is also president of Florida Disaster Animal Response & Transport, Bushnell, a nonprofit organization that specializes in assisting in animal rescue, particularly in disaster situations.