Open-cast technique for metal molds used in constructing facial prostheses

Open-cast facial

technique

for metal

molds

used in constructing

prostheses

Oscar N. Guerra, M.S.P.H., D.D.S.,* and Ken Canada** Ellis Fischel State Cancer Hospital and Cancer Research Center, Columbia, MO.

M

eta1 molds are used extensively by prosthodontists engaged in treating maxillofacial patients. In recent years, the use of epoxy resin/metal combinations and other resins in the fabrication of molds has been advocated. However, these resins have been used with specific systems and do not adapt readily to other systems. Metal and plastic or artificial stone molds seem to have a wider range of use with the various prosthetic materials. Metals, ‘particularly Linotype metal, offer several unique advantages. The metals have a high degree of accuracy and the ability to duplicate minute details of sculpted prostheses. Metals have a greater compressive strength and are more ductile than artificial stone, thus permitting greater compressive stress to be placed while the maxillofacial material cures in the mold. Heat is used with some of these materials, and the metal molds transfer heat much more effectively than do the epoxy resin/metal combination and plaster-type molds. In addition, metal molds can be used repeatedly with no loss of detail or accuracy. The fabrication of metal molds has been described in the literature.l, ’ The common method of fabricating metal molds utilizes the “lost wax” technique. This technique has the following disadvantages : It requires (1) large amounts of investment material, (2) large amounts of wax for construction of patterns, (3) galvanized or sheet-metal rings for investing purposes, and (4) multiple steps which prolong the metal-mold construction process for varying lengths of time. The open-cast technique to be described eliminates these disadvantages. CONSTRUCTION

OF INVESTMENT

CASTS

Construction of metal molds for a nasal prosthesis will be described, although this procedure can be used for all categories of facial prostheses. As with other types of metal casting procedures, investment casts are obtained first. Impressions are made, This National *Chief,

investigation was supported Institutes of Health. Department

**Maxillofacial

by General

of Maxillofacial Prosthetics

Research

Support

Grant

FR-05618

from

the

Rehabilitation.

Technician. 421

422

Guerra and Canada

J. P1osthet. October,

Dent. 1976

and the resultant dental stone cast is prepared for construction of two investment casts: one for the facial side and the other for the tissue side. For the facial side, the sculpting is carried out in either wax or clay, and the margins of the sculpted prosthesis are smoothed and sealed on the cast. The cast is trimmed to the dimensions desired on the final mold or, generally, 1.5 to 2 cm. beyond the margin of the prosthesis. Several V-shaped slots are cut into the master cast along the border to act as keyways. The indexed master cast with the sculpted prosthesis is then boxed with gummed brown paper tape. The adhesive on the tape is moistened prior to wrapping it around the cast. Irreirersible hydrocolloid* material is then poured into the boxed cast and, allowed to set. A ratio of one part powder to two parts ice-cold water is used to facilitate pouring the impression material. When the impression material is set, bent paper clips are imbedded into the irreversible hydrocolloid, and a layer of plaster of Paris/slurry water mixture is flowed onto the surface and allowed to set. This plaster backing stabilizes the impression material. The brown paper tape is removed, and the master cast with the prosthesis in place is separated from the impression. The impression with the paper backing is carefully wrapped with paper tape so as not to distort the irreversible hydrocolloid, and investment material is poured into the boxed impression so that a model approximately 3.0 cm. thick is obtained. The tissue-side investment cast is constructed in the following manner. The master cast, with the sculpted prosthesis removed, is again boxed with brown paper tape. The irreversible hydrocolloid impression material is poured into this boxed master cast, and using paper clips, a plaster of Paris backing is applied. The back of this plaster of Paris is marked No, 1. The paper-tape boxing material is removed and the master cast separated from the impression. This impression with a plaster of Paris backing is once again boxed with brown paper tape, and an impression is made of impression No. 1 in the manner described above. It is not necessary to use any separating medium during this procedure. A plaster backing is again used, and this impression is marked No. 2. The impressions are separated. The impression in plaster backing, marked No. I, is discarded, and impression No. 2 with its plaster of Paris backing is boxed with paper tape and poured in investment material. When the investment material is set, the paper tape is removed. Similar procedures have been described for the construction of investment casts for orbital prostheses.l, z The open-cast technique requires the same types of investment casts used in other casting procedures. The modification in technique occurs after the investment castshave been obtained. OPEN-CAST TECHNIQUE Boxing. This new technique depends upon the use of gummed asbestostape? which can be purchased at a hardware store. The 3 inch width of tape is ideal for this technique. The asbestostape is dipped in water to thoroughly moisten (Fig. 1). The tape will be used for the boxing procedure. The initial layer of tape around the investment

cast must be measured

and cut in such a way that it ends in a butt joint

*D-P elasticimpression cream, D-P Company, Glendale, Calif. fGum-Bestos, Grant-Wilson, Inc., Chicago, 111.

Volume Number

36 4

Open-cast

Fig. 1. The first layer in water.

The

tape

of asbestos tape softens considerably

technique

is premeasured, cut, and can be easily

for metal

facial

and thoroughly moistened adapted to the investment

molds

423

by dipping cast.

(Fig. 2). This will prevent leakage of the molten metal which would occur if the tape were allowed to overlap. Four additional layers of moistened, gummed asbestos tape are added around the investment cast (Fig. 3). A rubber band placed around the investment cast and asbestos tape will assure a tight seal of the tape to the model. The superior edges of the asbestos tape are then turned on end, and a pin indicating the height to which the metal will be poured is passed through the asbestos boxing material to provide a visual reference at the time of casting (Fig. 4). Drying. The investment models boxed with the gummed asbestos are placed in a dry-heat oven at 150 C. and allowed to dry overnight. The rubber band may be removed after initial drying (about one hour) hardens the asbestos tape. Shorter drying periods are required with higher oven temperatures. We have been able to cast the smaller molds in as little as 4 hours of drying time with the oven set at 160 C. However, to prevent accidental fracture of the investment models due to steam from the moisture within the model, the longer drying time and lower temperature of the oven are preferred. Two hours prior to casting, the temperature of the oven is increased to 190 C. and maintained until the molds are ready to be poured. Casting. Linotype metal is melted in a cast-iron pot over a gas-air torch. When all the metal has melted (approximately 260 C.) and the slag has been removed, the torch is kept in place and the heated investment models are removed from the oven. The asbestos tape will be quite hard in this dried condition. The investment casts are placed over a suitable insulating material to avoid burning the bench top. Then, the molten Linotype metal is poured into the investment mold to a level indicated by the

424

Fig. will

Guerra

and

J. Prosthet. October,

Canada

2. The first layer of asbestos result in leakage of the molten

Fig. 3. After the first layer layers of moistened asbestos pressure.

tape must Linotype

end in a butt joint. metal as it is poured.

of asbestos is well tape are wrapped

adapted around

Overlapping

to the investment the cast and luted

the

first

Dent. 1976

layer

cast, four additional together using finger

Fig. 4. The asbestos boxing is tightly sealed to the cast with a rubber band. The upper edge of the boxing is turned on end, and a pin indicating the desirable height of the metal is passed through the boxing material.

Fig. 5. Casting of the Linotype metal damage in case of accidental spillage.

takes

place

over

an insulated

bench

top

to prevent

previously placed metal pins (Fig. 5). The metal is kept in a molten state by playing the flame of the torch over the surface for approximately 3 to 5 minutes. The metal is allowed to bench cool for 30 minutes, and it is then plunged into water. The asbestos tape is readily removed, and the investment is easily removed from the metal molds. A disclosing aerosol spray * is used to indicate areas of inter*Detex,

Aerodent

Corporation,

New

Orleans,

La.

Open-cast

Fig.

6. Details

Fig.

7. Note

are accurately the excellent

reproduced fit of the two

technique

on the metal

for

metal

facial

molds

425

castings.

sections.

ference, and adjustments are completed by minor grinding. Minute details are accurately reproduced in the metal molds, and both sectionswill fit closely and uniformly to produce an excellent prosthesis (Figs. 6 and 7). C clamps are employed to maintain pressureduring curing. SUMMARY

A new (open-cast) technique for the fabrication of metal molds has been presented. Its simplicity relies on the use of gummed asbestostape as a boxing agent for the investment models. This technique has the following advantages over the “lost wax” technique: ( 1) it requires approximately one-quarter the amount of investment material, (2) it eliminates the use of investing rings or wax patterns, (3) it eliminates the boil-out step for the wax pattern and reducesthe total time required to make a metal mold, and (4) it affords a high degree of visibility. The only disadvantage is that the larger prosthesesrequire large amounts of Linotype metal which is very heavy. However, this has not restricted the use of this technique since these metals cast well and can be easily placed in large ovens. This technique, becauseof its advantages, offers more efficient use of laboratory time and personnel. References 1.

2.

Chalian, V. A., Drane, J. B., Metz, H. Prosthetics, in Chalian, V. A., Drane, Prosthetics: Multidisciplinary Practice, pp. 283-329. Rahn, A. O., and Boucher, L. J.: Facial and Concepts, Philadelphia, 1970, W. ELLIS FISCHEL STATE CANCER BUSINESS 70 AND GARTH AVE. COLUMBIA,

MO.

65201

HOSPITAL

H., Roberts, A. C., and Standish, S. M.: Extraoral J. B., and Standish, S. M., editors: Maxillofacial Baltimore, 1971, The Williams & Wilkins Company, Prostheses, B. Saunders

in Maxillofacial Company, pp.

Prosthetics 113-150.

: Principles