Shop Essentials: Preparing the Cast, Mixing Plaster, and Pouring the Mold

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By Ronnie Graves, BOCPO/L, CO, CTP

In my travels around the country speaking to a wide variety of O&P fabrication technicians and fabrication business owners, I've found that some of the most back-to-basics skills are often the most overlooked and underappreciated in training. And yet, in the end, they make all the difference when creating a quality device for the patient. In that spirit, I want to revisit the topics of preparing casts, mixing plaster, and pouring molds, to end up with a functional model for the practitioner to work with.

Preparing the cast: Plaster cast

The inside surface of the plaster cast should be smooth and free of foreign materials. Make sure that any fabric that is inside is attached to the surface and not bridging gaps.

Using an indelible pencil, trace over any pencil marks that are on the inside of the cast so that they are well marked, as they are going to transfer to the outside of the plaster mold when you are done (Figure 1).

Figure 1

Figure 1

Figure 2

Figure 2

The next step is to coat the inside of the cast with a releasing agent to make sure the plaster won't stick to the mold. There are several ways to do so. One is to pour a product called Green Soap into the cast (Figure 2). This product is commonly used in the ceramic industry for mold release. Slosh it around until all inside surfaces of the cast are wet. It is essential to use a separator or releasing agent or the plaster will stick to the cast and will be difficult to remove.

Next, place the cast into a sandbox to catch the spillage in case the plaster overflows or the cast splits open-which will definitely happen to you eventually (Figure 4).

Figure 3

Figure 3. Simple hand tools for plaster

Figure 4

Figure 4.

Figure 5

Figure 5.

Figure 6

Figure 6.

Figure 7

Figure 7.

Check the outside surface of the cast to see if the practitioner marked alignment reference lines. If so, place a pipe inside the plaster, matching the same plane as the reference lines. These lines represent the alignment the practitioner wants when it comes time to set up the prosthetic leg. If you pour the cast with the pipe in place, it will make the prosthetist's job of modifying the model easier.

Figure 8

Figure 8.

Figure 9

Figure 9.

Figure 10

Figure 10.

To hold the pipe in place, I lay two paint stir sticks on top of one another and tape the ends together (Figure 5). I tape the ends so that I can spread the middle and slide the pipe into the gap. The gap should be tight enough to hold the pipe in place while the plaster sets. Make sure that the pipe does not rest on the bottom of the cast. If it does, and the practitioner has to shorten the cast for a suction liner, the pipe will interfere with the process.

It is also a good idea to use a hammer to flatten the end of the pipe that is going to be inserted into the plaster (Figure 6). This keeps the pipe from rotating after the plaster sets. I use ½-inch galvanized water pipe so it won't rust. Place the pipe and holder on top of your cast, and check your reference marks again (Figure 7).

An existing socket or clear check socket

The only real difference you face now in doing a fiberglass cast rather than a plaster cast is checking to make sure that every hole in the socket is sealed. If you use masking tape to seal the holes, make sure it wraps around the socket several times (Figures 8-10). Plaster has a habit of finding ways to leak out.

You can presplit the socket using a band saw, and then tape up the splits, to make it easier to remove the socket when the plaster hardens. Remember to sand any plastic burrs and remove grit from sanding on the inside or they will cause the plaster model to have a rough surface. If you choose not to presplit the socket, you will have to cut the socket with a cast cutter after the plaster cures.

After everything is sealed, spray the inside with a thin layer of silicone to act as a separator. This will make the socket slide off the hardened plaster.

In a lot of cases, at this point you will pour the plaster with the socket placed in the vertical transfer jig. Make sure that you protect the jig from the plaster. I usually keep a thin coat of silicone on my jig so that if I do spill, it won't stick. Also place a trash can under the jig to catch any spillage; it beats scraping plaster off of the floor.

After you pour the slurry plaster into the socket, gently tap the sides of the socket to bring all of the air bubbles to the surface. This will eliminate air pockets, making future modifications easier for the practitioner. Allow the plaster to harden prior to taking the socket off of the plaster model. If the plaster is too soft, you can distort the true shape.

Remember that the practitioner is depending on you to transfer to the model both the alignment and the exact shape that he or she has captured from the patient.

Mixing the plaster

Figure 11

Figure 11.

Figure 12

Figure 12.

The importance of mixing plaster properly is underappreciated. If you do not take the time to mix the slurry plaster thoroughly, your mixture will be lumpy. These lumps contain dry plaster and will wreak havoc for the person trying to modify the model later. One mixing method that works well for me is to put the amount of water I need (about two-thirds of the total volume you need to fill the cast) into a bucket that is reused for this purpose (Figure 11).

If setting time is not critical, use cold water. Use warm water if you need the plaster to set faster. Some technicians add salt to the mixture to make it set faster, but if you do this, the practitioner will find that buildups do not stick as well as they should. I do not recommend adding salt unless you are simply duplicating a previous model and are in a hurry for the plaster slurry to set.

After you have the water in the bucket, carefully shake or sift the dry plaster into the water, following the ratio printed on the package. By sprinkling the plaster in slowly, you give the plaster and water a chance to blend. Continue to sift the plaster in until you have a mound above the water. Allow it to sit for a couple of minutes and soak up the moisture (Figure 12).

Be sure to wear gloves for the next procedure. Use your hands to carefully stir and blend the plaster. By doing it with your hands, you can feel for lumps in the slurry. This will also allow you to eliminate a lot of bubbles in the mixture that are caused by using a power mixer. Add more plaster to the mixture if you need to thicken it. Add more water if you want the mixture to be thinner. Consistency is going to depend on the particular project. Thinner is useful if there are a lot of fine details to capture. Thicker sets up faster but can lose fine detail.

Tap the bucket on the sides and bottom to raise the bubbles to the surface. It helps to get them out prior to pouring the wet plaster into the mold. The preparation of the cast depends upon what type of cast you are pouring the plaster into-there are several methods for each type, which are beyond the scope of this particular article.

Figure 13

Figure 13.

Figure 14

Figure 14.

Figure 15

Figure 15.

Don protective eyewear for this procedure. Slowly pour the water and plaster solution into the model (Figure 13). I have seen many technicians who had plaster splash into their face while doing this, which can burn the cornea of the eye.

After filling the cast, I tap its sides until all the air bubbles come to the surface. If you can get the air out, it makes the job of modifying the model much easier. Allow the plaster to set completely before putting it in a pipe vise. If you do it too soon, the plaster will not have set around the pipe and it will rotate or wobble.

Preparing the mold

After the plaster has cured, you can strip off the plaster cast using a knife (Figure 14). Be careful to avoid cutting into the plaster model, which will require repair work later if you dig gouges into it.

Be sure and transfer the patient's name and the date onto the top surface of the model before throwing the cast away. Trace all of the indelible pencil marks at this time. They tend to fade as the plaster dries so re-marking them helps later (Figure 15).

Let the ordering practitioner know that you have the model ready for him or her to modify. On the measurement chart, note the date, time, and who completed this task. Record-keeping is a must and should be practiced at all times.

Some practitioners will want you to wrap the cast in a plastic bag to retain the moisture. With time and experience, you will learn the preferences of the practitioners with whom you work and will soon be producing exactly what they need.

Ronnie N. Graves, BOCPO/L, CO, CTP, is the owner of Prosthetics Research Specialists, Bushnell, Florida, and has worked in the O&P industry for more than 36 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. Ronnie also owns VIP Veterinary Inclusive Prosthetics/Orthotics and for 18 years has been providing prosthetics and orthotics for animals in need.