tained at this temperature for 8 hours. The flask assembly is allowed to cool to room temperature in the water in which it was processed. If the water bath has not completely reached room temperature, the assembly can be removed and placed under running water for 20 minutes for final cooling.

(2) Short cure method. The denture flask press containing the flask is placed in the room temperature water bath. The water is slowly heated to 160° F. (71° C.) and maintained at that temperature for 111⁄2 hours. The water is then heated to 212° F. (100° C.) for 30 minutes to complete the polymerization. After polymerization, the flask assembly is removed from the water bath, bench-cooled for 30 minutes and then cooled under running water for 20 minutes. The short cure method is preferred for use with removable partial dentures that have small or thin acrylic resin sections.

c. Procedure for Setting Electric Curing Unit. The Hanau curing unit, Model 2, is the most commonly used type of unit for polymerizing acrylic resin (fig 5-113). It has two timers and two heat regulating controls. The water temperature rises about 2° F. per minute under normal conditions. If the flask assemblies are placed in the water at 75° F., the water temperature will reach 160° F. in about 45 minutes. The heating time must be added to the processing time for each stage of

5-66. Deflasking

When the flask assembly has been cooled, the processed denture can be removed from the flask by either the ejector press method or the malleting method.

a. Ejector Press Method. The cover of the flask is removed with a suitable instrument (A, fig 5-114) and placed in the ejector press. The larger metal plate of the press is centered over the artificial stone mold and the smaller plate is centered over the ejector disc in the lower half of the flask (B and C, fig 5-114). The press is closed and the ejector chisels are passed through the holes in the sides of the press into the slots between the two halves of the flask. The chisels act as levers, the sides of the flask as fulcrums. The handles of the chisels are pushed down and then pulled up, slowly forcing the halves of the flask away from the mold (D, fig 5-114). Then the flask is cleaned, lubricated, and made ready for later use.

b. Malleting Method. The cover of the flask is removed with a suitable instrument to expose the artificial stone cap. The flask is held in the left hand with the cap up. The thumb and index fingers are positioned at the junction of the lower half and the top half of the flask. Pressure is exerted to separate the halves of the flask. While pressure is exerted, the cap is hit with a rawhide mallet. If the inside of the flask was clean and lightly lubricated, the upper half of the flask will come loose. The flask is turned over and the ejector disc on the bottom of the flask tapped with the mallet. This will remove the bottom half of the flask. The ejector disc is removed with a suitable instrument. The flask is cleaned, lubricated, and made ready for later use.

5-67. Removing Denture and Cast From

Investment Material

This procedure must be carried out very carefully so that the denture itself is not marred.

a. First, the artificial stone topping is pried

off from the upper section exposing the denture teeth (A, fig 5–115).

b. With a plaster saw, the outer walls of the investment are cut in the right and left cuspid areas and in the right and left tuberosity areas (B, fig 5-115). These sections are pried loose gently with a plaster knife. If the investment does not come off the cast at this time, it is removed with a plaster knife (C, fig 5-115).

c. The investment is trimmed from the lingual surfaces of the teeth (D, fig 5-115). When the palatal or lingual sections are clear of the teeth they are gently pried up and removed (E, fig 5-115). The dentures on their casts then are ready to be remounted on the articulator to check the occlusion.

5-68. Remounting the Denture

The deflasked denture with cast is remounted on

the articulator before it is trimmed and polished. This is done to check and correct any changes in the contact relation of the occlusal surfaces of the teeth which may have occurred during the processing. The same articulator with exactly the same settings must be used in remounting the deflasked dentures and casts.

a. The stone casts are repositioned on the original plaster mountings using the indices which were cut into the casts before mounting. These indices were protected with petrolatum during the flasking and polymerizing procedures. The casts are sealed in position with sticky wax or plaster of paris.

b. The mountings are attached to the articulator, the condylar elements locked, and the articulator closed.

5-69. Purpose

There are always some corrections to be made in the occlusion of a processed denture even though flasking and processing steps were carried out very carefully. Corrections are necessary because of errors inherent in the materials used. Dimensional changes in the trial denture can cause errors in the finished denture. Expansion and contraction of the acrylic resin during polymerization resulting in shrinkage can cause small alterations in the position of the teeth. (Expansion occurs as the polymerizing temperature is reached. Contraction occurs during polymerization and cooling.) Expansion of the investing material and the pressure of the denture flask press can contribute to tooth movement. That is why it is necessary to correct the occlusion.

5-70. Responsibility

The procedures for correcting the occlusion are accomplished by the dental officer or by the dental prosthetic specialist under the direct supervision of the dental officer. The procedure is very exacting, and it should never be attempted by anyone who does not have a sound knowledge of occlusion. The dental removable prosthetic specialist must be willing to spend the time learning the principles involved and acquiring a systematic method. If a systematic method and sequence is not followed, a loss of occlusal vertical dimension, centric occlusion, and balance will result.

5-71. Selective Grinding

The correction of occlusion for complete dentures is called selective grinding, a procedure which eliminates all areas of interference between the maxillary and mandibular denture teeth. When the occlusion has been corrected, the maxillary and mandibular denture teeth function smoothly together with each tooth sharing proportionately the masticatory load and, in so doing, transmitting the masticatory forces evenly to the tissues.

5-72. Procedure and Sequence for Selective Grinding

a. The procedure for selective grinding consists of placing a piece of articulating paper on the mandibular denture teeth and tapping the maxillary denture teeth against them (fig 5-116). The points of heaviest contact are seen as white spots surrounded by carbon markings indicating where the cusp has cut through the articulating paper. Small mounted carborundum stones are used to grind the areas. Those ground surfaces of the denture teeth are polished with a rubber wheel when the procedure is completed. The procedure for selective grinding will be described in terms of supporting cusps, guiding inclines, balancing cusps, working cusps, and anterior tooth surfaces (fig 5-117).