Academy Society Spotlight: Of Rubber and Resin

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By Stefan Knauss, MAMS, CPO

Create a Socket With a Seamless Connection Between HCR Silicone Rubber and Hard Resin Laminate

 

To conjoin materials as dissimilar in their properties as elastic rubber and rigid resin laminate is questionable from an engineering standpoint. Yet with a certain fabrication approach, along with a sound mechanical transitional connection, an apparently seamless, durable transition from resin to rubber is achievable. This article describes how to create a hybrid socket with conventional hard laminated resin along with a special kind of silicone rubber.

To date, our practice has only focused on upper-limb applications for this technique. However, this experience opens a rich territory of new socket and appliance designs for orthoses and prostheses. Moreover, when one realizes that products with this combination of materials are relatively easy to produce, such applications may gain widespread acceptance.

The essential processing technique for high consistency rubber (HCR) silicone is beyond the scope of this article, but a basic fabrication manual on HCR silicone is available from the supplier we use, Factor II, Lakeside, Arizona. For additional hands-on classes in HCR fabrication, contact the International Center of Education for the Advancement of Silicone Prosthetics at www.iceasp.com.

Epoxy-acrylic resin and silicone do not have good properties for chemical bonding. Therefore, they require using another strong fabric material, Spectralon, during fabrication to create a bond. Spectralon is incorporated into the lap-type joint, which can withstand the high local stress associated with unavoidable bending at the joint. The fabric has a progressively terraced insertion into the rubber to distribute forces acting across the joint over a large area. Resin, wicking into these strata, penetrates somewhat into the interior of the rubber at these layers to enhance the mechanical connection.

HCR rubber is used because of its extraordinary strength. A variety of durometer formulations that can be simply formed into precise shapes are available. HCR rubber is catalyzed and conditioned by means of a calendering tool. The calendering tool is essentially a two-roller mill and may be difficult to find, but it is available from Factor II.

Materials and Tools

With the exception of the calendering device and the HCR silicone, all the materials needed to create the hybrid socket are readily available and common in a prosthetic fabrication lab.

Materials

• Plaster cast

• HCR silicone

• Silicone pigment

• PVA cast sealant

• Polyester parfilm

• Clear plastic bags

• Spectralon hose

• Sheer nylon hose

• Epoxy/acrylic resin

• Additional laminating fabric of choice

 

Tools

• Knurled hand tool and smooth cylinder for rolling

• Calendering two-roll mill

• Low-temperature oven

• Vacuum pump

• Conventional laminating supplies

• Pinwheel

Building the Connection

Begin by making the rubber part of the joint first. Use a smooth, dry cast for this step. Draw the outline of areas that are intended to be made of rubber on the cast. Then coat the cast with two thin coats of PVA solution, and when it is dry, spray lightly with polyester parfilm. HCR silicone is clay-like so that it can be rolled into thin sheets and placed onto the cast within the lines of the outline. Take care to produce orderly marginal boundaries and prevent bubble entrapment beneath the silicone. The PVA coating should still allow gas permeability through the cast under vacuum so that during bagging and evacuating air at high vacuum, small trapped air bubbles will be drawn out from the interior surface of the silicone that is in contact with the cast.

Next, don a sheer nylon hose over the cast, pulling it snugly to overlap the silicone margin. This nylon hose will be tacked into the silicone and trimmed to have a ¼-inch overlap. Apply a second nylon hose and then a thin plastic bag, such as a produce bag, to generate a vacuum. After a strong, steady vacuum is achieved, remove and discard the outer bag and the second nylon. The second nylon ensures that the first nylon is neatly drawn down and wetted into the silicone while under vacuum. The first nylon, which runs out of the silicone margin, is designed to give the future lamination a nice interior surface and secure the edge of the rubber against the lamination, however fine that transitional edge may be.

Lay a thin layer of silicone over the trimmed edge of the nylon, leaving approximately 1⁄8 inch of the original ¼-inch margin still exposed. The second layer of silicone can be perforated with a pinwheel prior to vacuum bagging to draw out bubbles that may have become trapped between layers.

The process of embedding and sandwiching fabric into the silicone may be repeated two more times using Spectra hose. Each fabric layer should overlap the previous reaching farther into the rubber form to produce the terraced entry of fabric into the rubber form.

When you have completed sculpting the rubber to the final form, place the cast into an oven for curing. Curing occurs first in a low-temperature oven below 110°F to prevent the expansion and appearance of hidden bubbles and continues for many hours until the material has rubber-like properties. Post-curing for at least one hour at 190°F will bring the rubber to its full elastic strength properties without harming the fabric fibers.

The rubber/fabric creation can be separated from the cast but left on the cast for the creation of a removable cutting strip made of HCR. Use a PVA solution or parfilm on the cured rubber before applying the uncured rubber strip to ensure later separation of the two. Cure the strip in place in the oven. This strip will allow you to cut the lamination along a defined line after the lamination is complete. It is helpful to add one or two strips lengthwise to aid in removing the extra overlying lamination. A cast saw can scar the underlying rubber.

The rubber/fabric assembly is now ready for full removal from the cast. Prepare the cast for lamination with an internal PVA bag. Don the assembly again and add additional laminating fabrics to suit. Complete the lamination by conventional means.

Trim along the aforementioned cutting strips. Be careful to avoid driving glass and carbon fiber dust into the rubber, which will result in an itchy product. High-speed tools may throw fibers into the exposed rubber; therefore, use files and emery boards to finish resin edges.

If the socket has been fabricated correctly, there should be a completely smooth interior transition from one material to the next. Furthermore, by sending the resin through the fabric into the rubber terraces, there may be an unseen level of mechanical penetration (a blending transitional zone below the surfaces) which accounts for the unexpected strength of such a joint.

Hybrid HCR/hard laminated sockets, in contrast to the flexible thermoplastic socket with a rigid external frame, are lighter and less bulky. Another desirable result of this design approach is that elastic brims, as opposed to merely flexible brims, follow the patient's movement and can produce a suction suspension seal. Rubber also offers the prosthetist the freedom to create aggressive supracondylar features that are compliant, comfortable to adjacent bony landmarks, and permit easy donning.

Stefan Knauss, MAMS, CPO, is co-owner of Aesthetic Prosthetics, Pasadena, California, and a member of the American Academy of Orthotists and Prosthetists (the Academy) Fabrication Sciences Society.

 

Academy Society Spotlight is a presentation of clinical content by the Societies of the Academy in partnership with The O&P EDGE.