The Effect Different Keeper Tray Materials Burn-out Incineration on Casting Investment during Burn-out Procedures.

 

T.Kogiso, Y.Nakamura, Y.Ohno, R.Kanbara, K,Syoji, T,Masuda K.Yoshihara, Y.Tanaka

 

Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University

Introduction

Fabrication methods of the magnetic attachment keeper component for restorative placement have included casting or cementation procedures.^(1-4) A cementation method, previously reported, utilizes a ready-made plastic pattern for investment burn-out and casting in a dental alloy. This technique does not subject the keeper surfaces to casting procedure roughness or deformations. The keeper is then directly cemented to the cast coping holder and is not subjected to casting procedure heat distortion and is thus a recommended method.Cast keeper coping housings require good casting accuracy as errors will prevent accurate positioning and seating of the keeper to the housing. Laboratory or chairside adjustment of these castings may be occasionally required (Fig. 1).

A new prototype keeper pattern using a keeper tray has been reported. The casting precision of these prototype keeper patterns has been found to be superior to available proprietary preformed keeper housing patterns (Fig. 2).

Fig.1　Adjustment of cast keeper

Fig.2   The influence of keeper tray materials on casting precision

 

Objective

The purpose of the present study was to compare investment material use and casting surface roughness for different samples of cast keeper housings from burn-out to casting, to elucidate the casting problems and error onset and to investigate casting defects different coping patterns.

 Materials and Methods

1.    Materials

Commercially-available ready-made pattern GIGAUSS C 600 KB (GC) was used. The chief component of GIGAUSS C 600 KB  pattern is acrylic resin.

The prototype pattern had the same shape as GIGAUSS C 600 KB (Fig. 2), but the chief component was polyethylene resin.

2.    Analysis items

2.1 The influence of incineration time on casting investment

Casting investments at 10, 20, and 30 minutes after sample incineration were observed, and surface roughness was measured.

2.2 The influence of wax application quantity on casting investment burn-out procedures.

Four samples with different quantity thickness of dental wax were fabricated. Each castinginvestment was qualitatively evaluated, and the surface roughness was measured.

3.    Methods

3.1Samples

The following 4 samples were fabricated for prototype and commercially-available patterns, respectively (n=5) (Figure 3)

(1)                A plastic pattern without inlay wax (Wax 0)

(2)                A plastic pattern with 1 mm inlay wax (Wax 1)

(3)                A plastic pattern with 2 mm inlay wax (Wax 2)

(4)                A plastic pattern with 3 mm inlay wax (Wax 3)

Fig.3　　4 Samples

 

3.2Fabrication method

(1)                The influence of incineration time on casting investment

The ratio of a keeper tray and wax thickness was 1 : 2 in the Wax 1. The sample was used to investigate the influence of incineration time on casting investment.

(2)                The influence of wax application quantity on casting investment

�@Investing procedure

The four samples were sprued with ready casting wax R 15 (GC), and placed on the investing ring base, and invested following manufacturer instructions using Cristobalite investment material (Cristoquick II, GC) (Fig. 4).

�ACasting investment incineration

Four samples were transferred to a furnace, and removed after 30 minutes.

�B Casting investment section

Casting investment removed from the furnace was air cooled, and the base was trimmed to expose the investment mold inner surface of the keeper tray mold(Fig. 5).

Fig.4    Investing procedure

 

Fig.5    Casting investment section

3.3Evaluation items

(1)                Qualitative evaluation of investment casting mold surfaces using stereoscopic microscopy.

Casting investment mold surface (Keeper tray) inner surface mold of each sample was evaluated.

(2)                Surface roughness quantitative evaluation of casting investment surface.

Surface roughness of casting investment was measured using a digital microscope VHX 500 (KEYENCE), and then observed three-dimensionally. Vertical interval of the surface was defined as surface roughness. One-way analysis of variance and Turkey test were performed at 5% significance level.

 Results

1.    The influence of incineration time on casting investment

1.1Evaluation using a stereoscopic microscopy

Corner casting investment demonstrated surface degradations at 10, 20, and 30 minutes periods after incineration in the acrylic resin. In contrast, no surface degradations were seen for all post incineration time periods for the polyethylene resin samples (Fig. 6).

Fig.6   Evaluation using a stereoscopic microscopy

1.2Surface roughness measurement

There was a significant difference in measured surface roughness between acrylic samples tested and the polyethylene resin samples at the 10, 20, and 30 minutes periods. No significant difference was observed between 10 and 30 minutes for the acrylic resin samples (Fig. 7).

Fig.7     Surface roughness measurement

2.    The influence of wax amount on casting investment

2.1Observations of casting investment using a stereoscopic microscope

Distortions and degradations of the corner casting investment was observed for all samples with additive wax in the acrylic resin. In contrast, no destruction of the casting investment was observed for in the polyethylene resin/wax samples. There was no degradations of the casting investment both in the acrylic and polyethylene resins without additive wax (Wax 0) (Fig. 8)

Fig.8   Evaluation using a stereoscopic microscopy

2.2Surface roughness measurement

A significant difference was observed between acrylic and polyethylene resins of Wax 1 and 3. There was a significant difference between Wax 1, 2, and 3 in the acrylic resin. No significant difference was found both in the acrylic and polyethylene resins of Wax 1, 2, and 3. (Fig. 9)

Fig.9     Surface roughness measurement

 

Discussion

1.    Keeper tray

The magnetic attachment keeper coping fabrication methods and the magnetic attachment keeper abutment fixation technique was developed by Toyota et al. The original recommended method proposed was a casting technique. A ready-made keeper pattern was incorporated into the wax pattern, and cast. This method has been widely adopted in the clinical setting due to ease and availability. However, several problems with this technique  have been pointed out in clinical and basic experimental evaluations. The problems include the excess formation of an oxide film on the mirror-polished keeper surface due to casting investment exposure and high temperatures required. Also, other problems associated with high casting burn-out heat exposure are casting shrinkage keeper deformation, and alteration of magnetic properties and decreased corrosion resistance^5,6,7). These problems may result in a overall decrease in the actual and potential attractive force of a magnetic attachment and therefore cannot be ignored.

Tanaka et al. developed a cementing technique method. In this technique, a separate coping housing excluding the actual keeper was cast, to be followed by the cementing of an unaltered keeper to the cast coping housing^1)-4). Keeper space is required on a coping surface. A ready-made plastic pattern (Keeper Tray^@) was developed to facilitate wax build-up procedures. The plastic pattern is made of castable acrylic resin with 0.3 mm thickness, leaving a space for the cement after casting. The cast coping holder used to fix a keeper to the casting investment is not necessary in this method. Therefore, the mirror-polished adsorption face can be used without removing a holder⁷⁾.

2.    Prototype plastic pattern

Acrylic resin is readily used due to its thermal plasticity. This resin can be compression molded above 150 °C, and offers a greater cost performance⁸⁾. It is used for commercially-available ready-made patterns. However, casting defects caused by the combined use with other materials has been pointed out. The general properties of acrylic resin, the major component of this proprietary ready-made pattern, include resistance to deformation, insulation properties, and water resistance, but poor chemical resistance. ⁹⁾ It was speculated that chemical interaction may occur when acrylic resin is incinerated with dental wax. To avoid such problems, a preliminary experiment was performed to evaluate substitute plastic materials for the acrylic resin.  Polyethylene (PE), polypropylene (PP), polyacetal (POM), and ABS resin were used as samples. PE presented the highest surface accuracy after casting. General properties of PE include milky-white translucency, lighter than water, physical properties similar to dissolved paraffin wax at incineration with a low melting temperature of 130 °C, and acid and alkali resistance related  to higher molecule structure⁸⁾. PE is thus considered to have the least potential adverse influence on casting precision when in combination with dental wax. Polyethylene was therefore selected as the test sample material for use in this preliminary study.

3.    Cast observation

Observation using a magnifying microscope demonstrated burnout degradation of the corner casting investment of acrylic resin at 10, 20, and 30 minutes after incineration. In contrast, no burnout degradation in the casting investment was observed. The surface roughness became coarser with time, demonstrating a significant difference between acrylic resin and PE.

There was no surface roughness influence caused by the wax quantity used with acrylic resin and PE samples.

The results suggested that casting defects found in the fabrication of a keeper coping using a conventional acrylic plastic pattern are not likely caused by the casting process itself, but by casting investment degradation during burnout incineration. The wax amount is not relevant to the casting surface degradations seen. Plastic patterns are incinerated completely at 10 minutes, and the investment mold surfaces becomes coarser with greater burnout time.

The single use of acrylic resin and PE presents great heat plasticity, and high casting precision can be expected.

A casting burr is caused by a crack of a mold. In a single wax-pattern casting, a crack of a mold is formed by an impact to the mold, and rapid and overheating after investment⁹⁾. However, rapid heating casting investment is used in the present study, and there were no such casting defects found as described above. Therefore, it is considered that the cast defect is caused by the simultaneous incineration of wax-pattern and plastic materials.

PE has great acid and alkali resistance, and melts at 130°C, lower than acrylic resin. PE is incinerated and permeated to the casting investment at wax boiling temperature, suggesting no blocking of airflow. On the other hand, acrylic resin demonstrates heat plasticity at 150°C. It was suggested that acrylic resin softened around wax boiling point blocks the airflow, and evaporated wax destructed the mold.

Conclusion

The casting investment procedures of a prototype  keeper mold and commercially-available keeper trays mold at burn-out incineration process were compared. The following conclusions were drawn:

1.       The observation of a casting investment using a stereoscopic microscope showed corner degradations inside the keeper tray surfaces of the acrylic resin mold samples tested. No destruction of the casting investment was observed in PE samples molds.

2.       The surface showed more surface irregularities with increased time of burn-out.

3.       There was no difference in the observed surface roughness for both the acrylic resin and PE samples with amounts of additive wax.

4.       Surface roughness was decreased and lower for the PE samples than acrylic resin samples for all conditions.

The results suggested that the use of a polyethylene mold pattern results in a superior result compared to acrylic resin patterns use for attachment keeper castings.

 

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