Considerations in the Lamination Process
October 2012 Issue
There are many factors to take into consideration when laminating a socket. I typically laminate six different layups and share those samples with practitioners who wish to have Prosthetic Research Specialists (PRS) laminate their sockets. The practitioner then has several samples from which to choose and decide what will work best for each patient. Here are several considerations that I take into account.
- Strength. You need to make the socket strong enough to do the job while minimizing weight.
- Heat relievable. Pure carbon will delaminate if heat is applied, so avoid its use in areas of the socket that might need to be heat relieved to make fit adjustments.
- Stress tolerance. The socket must be able to take the action of repeated stress caused by energy-storing prosthetic systems.
- Cost containment. Measure the raw material costs to make a socket. This will provide a guideline to keep costs under control while maintaining quality.
Not all resin systems are equal. Learning the capabilities of each type of resin will put you on the road to learning which system works best for your clients. A resin that sets fast is usually more brittle. The more brittle a resin is, the more composite fabrics you have to use to make it strong enough to ensure that it won't break. A resin that takes up to 24 hours to set, while impractical for our industry, is much stronger, and it creates a lighter-weight socket. There are happy mediums, however. Even most longset resins can be cured quickly, but you have to learn how to do it correctly.
It is possible to make sockets strong enough without using carbon braid. However, there are ways to incorporate a carbon product and still be able to heat relieve. Being able to heat relieve a socket allows a practitioner to adjust the fit of the socket. The practitioner can just heat an area and push it out or in as appropriate to make the patient more comfortable.
Every manufacturer should be able to tell you what the flexural and tensile strengths are for its product with a standard layup. Take the time to test a few layup variations with different resins to see which one works best for your lab.
The viscosity of the resin also impacts the socket's performance under stress. If you use a thin, watery-type resin, it can easily "starve out" under vacuum, which creates a weaker socket. A thicker resin will stay where you put it and yield a stronger socket with a smoother finish. When using a thickerviscosity resin, allow sufficient time to saturate your fabric as you pull from the top to the bottom.
Lamination costs include more than just the resin. Let's start with the cast. If it's a wet cast, you need to seal it; whether you use a casting balloon or a chemical coat, it costs money and labor to apply. A moisture-resistant resin can save this expense. Figure out your PVA bag costs and add about 10 percent for broken bags. Now measure exactly how much composite material you use in your layup to build the socket and how much resin is needed for the lamination. Calculate the cost of the composite material that is used and the cost per gram of resin plus a proportionate share of the delivery charges for your materials. Consider that using a cheaper resin requires an increase in fabric expenses to make a final product, while a higher-cost, higher-strength resin will allow you to see a reduction in composite costs and therefore an overall reduction in your costs to build a socket. Also, the ability to heat relieve your sockets will greatly improve your bottom-line laminating costs because you can adjust the fit rather than make a new socket. After you add all of this up, you will start to get an idea how much raw materials cost.
Ronnie N. Graves, BOCPO, CO, LPO, RTP, is the owner of Prosthetics Research Specialists, Bushnell, Florida, and has been in the O&P industry for 33 years. He specializes in hard-tofabricate jobs and teaches fabrication techniques. Graves is also president of Sumter Disaster Animal Response Team, Bushnell, Florida, a non-profit organization that assists in animal rescue, particularly in disaster situations.