The effect of sprue design on the marginal accuracy of titanium castings

424 © 1998 Blackwell Science Ltd

Journal of Oral Rehabilitation 1998 25; 424–429

The effect of sprue design on the marginal accuracy oftitanium castingsD . C . N . C H A N * , R . B L A C K M A N † , D . A . K A I S E R † & K . C H U N G * † *Faculty of Dentistry,National Yang-ming University and Dental Department, Veterans General Hospital-Taipei, Taiwan, R.O.C. and the †Dental School,The University of Texas, San Antonio, Texas, U.S.A.

SUMMARY The purpose of this study was to measure

the effects of sprue number and position on cast

titanium crown margins. Twenty-four complete

veneer crown wax patterns were fabricated on a

stainless steel die with a 30° bevel finish line. Twelve

wax patterns were sprued with one 8-gauge wax

sprue and the remaining 12-gauge double sprued.

All patterns were invested with a phosphate bonded

investment. Castings were made with a titanium

casting machine following the manufacturer’s

instructions and using commercially pure titanium

(. 99·5%) ingots. The castings were than carefully

Introduction

Crown finish lines and margins are often placed

subgingivally, where biologic considerations are of great

concern. Currently, numerous low gold and base metal

alloy systems are available for crown construction. One

alternative to gold or base metal alloys is titanium,

which for the past two decades has mainly been used

in dentistry for the manufacturing of dental implants

(Hansson, Albrektsson & Branemark, 1983; Parr,Gardner & Toth, 1985; Albrektsson et al., 1986).Recently there has been great interest in the use oftitanium for fixed and removable prostheses (Shanleyet al., 1981; Rubeling & Kreylos, 1984; Hruska, 1987;Szurgot et al., 1988; Blackman, Barghi & Tran, 1991;Hruska & Borelli, 1991; Wang & Boyle, 1993).

Margin sharpness is always a primary considerationwhen assessing the accuracy of dental castings. A test

cleansed and the margins were examined withindirect impression technique. Data were analysedwith an ANOVA and the Student’s t-test withconfidence level at 95%. The results revealed thatthe marginal discrepancy for the double sprueinggroup (32·1 6 12·8 µm) has significantly lessdiscrepancy (P , 0·001) than the single sprueinggroup (49·8 6 16·4 µm). There was no statisticallysignificant differences in marginal discrepancybetween locations within the sprueing techniques(P . 0·05). An improvement in the degree of castingaccuracy of titanium crown was indicated by thedouble sprue design used in this investigation.

for the ability of alloys to cast fine margins has been

reported by Mackert, Moffa & Jendresen (1975). The

same test was then used to compare the castability of

the dental casting machines (Eames & MacNamara,

1978). In their investigations, alloy was cast into the

cavity created by a utility blade, and the width of the

meniscus in the solidified metal was used as an indicator

of castability. The main disadvantage of this method is

that the flat casting used for the test differs from a

dental casting in size and configuration, so that the

observed sharpness of the margin may not relate directly

to that of a clinical casting. Whitlock et al. (1981)

and Hinman et al. (1985) suggested using an 18-gauge

polyester mesh pattern as a simple means of obtaining

a castability value and thus a means for assessing alloy

castability. Thereafter, the wide assortment of designs

that have been created and proposed to be the abstract

tests (Barreto et al., 1980; Naylor et al., 1990).

A C C U R A C Y O F T I TA N I U M C A S T I N G S 425

Current titanium casting systems are based on an

electric arc design and melting takes place in an argon

or helium atmosphere to reduce titanium oxidation

during casting. Greener et al. (1986) found that a

vacuum argon/electric arc pressure casting machine

produced only 10 to 20% castability using 1000 µm

and 500 µm mesh with both commercially pure

titanium and Ti-6Al–4 V alloy. Using an argon/electric

arc vertical centrifugal casting machine* produced 100%

castability with 1000 µm mesh and pure titanium. The

casting machine was equipped with a high temperature

heating source (a nonconsumable tungsten arc), an

inert argon atmosphere, and a durable copper crucible.

However, a major limitation of abstract castability

monitors is the inability to measure both casting

completeness and casting fit. This obstacle was overcome

to some extent by using machined metal stylized dies

that simulate the configuration of prepared teeth. While

the type of preparation varied, most of these studies

evaluated castability in terms of casting accuracy and

relative fit (Myers & Cruickshanks-Boyd, 1982;

Brockhurst, McLaverty & Kasloff, 1983; Bessing, 1986;

Verrett & Duke, 1989; Sunnerkrantz, Syverud & Hero,

1990). In a clinical study of titanium crowns fabricated

by machining, the marginal fit of titanium crowns

was reported to be excellent (Andersson et al., 1989;

Bergman et al., 1990). Blackman, Baez & Barghi (1992)

found that pure titanium copings could be cast, fitted,

and cemented to their dies with acceptable fitting

accuracy measured axially. A mean of approximately

50 µm was recorded for both 45° and 90° margins. On

a basis of surface or margin defects, the 45° margin

design was better than the 90° form. Recently, milled

titanium & cast titanium were reported to have similar

discrepancies, 54 6 65 µm and 60 6 42 µm,

respectively, by microscopic measurement of the sealing

crown margins (Leong et al., 1994).

In crown construction, sprueing is a variable that

affects casting success. Preston & Berger (1977) revealed

that commonly used sprue designs were inadequate

when casting titanium. Sprue geometry was also

identified as one of the major factors producing different

castability and porosity effects with low dentisty

titanium alloys (Chai & Stein, 1995). When casting

titanium, a modified sprue design is imperative to

ensure proper mould filling. The purpose of this study

was to measure the effect of the number of sprues and

*Ohara Co., Ltd, Osaka, Japan.

© 1998 Blackwell Science Ltd, Journal of Oral Rehabilitation 25; 424–429

Fig. 1. Dimensions of the stainless steel die (in mm).

their positions on cast titanium crown margins using

an indirect technique for evaluating marginal definition.

Materials and methods

A stylized die machined from a stainless steel rod,

7.7 mm in diameter, simulated the preparation for a

complete veneer metal crown. The test casting design

represented a complete veneer crown. The core was

6 mm high and 7 mm in diameter, with a uniform sharp

cervical margin of 30° (Fig. 1). Instead of the shape of a

tooth, a cylindrical form of wax pattern was constructed

with 2 mm in thickness of the occlusal surface. The

stainless steel die was conditioned in a 37 °C oven for

10 min to simulate the oral cavity temperature. Wax

patterns were prepared by dipping the pre-heated die

into molten inlay casting wax.† The metal die with wax

in excess of the final wax pattern was placed in a

sculpting device, as described by Philip & Brukl (1984)

and secured with a set screw. A razor blade was

positioned vertically on the sculpturing device and the

screw attaching the movable vertical blade to the base

was gradually tightened. The wax was gently shaved

by the razor blade to obtain a cylindrical external form

as uniform in thickness as possible. The crown margin

†Kerr blue inlay casting wax, Type II; Kerr Mfg. Co., Romulus,

MI, U.S.A.

426 D . C . N . C H A N et al.

Fig. 2. Schematic drawing of a crown pattern positioned in

casting ring.

was then precisely craved to a knife-edge margin of 30°

on the metal die. Margin sharpness was checked under

a stereomicroscope at 103 magnification.

The sprue designs evaluated were referred to as

angular attachments to the patterns. There were two

groups of castings. The first group had one 8-gauge

sprue centred occlusally on the pattern. The second

group had two 8-gauge sprues placed occlusally on

patterns at the occlusal-axial line angles, flush with

external pattern walls, and directed straight toward the

margins. The sprue lengths for both groups were 5 mm

in the straight portion and 10 mm with a 45° terminal

pattern angle attached to a special crucible former

(Fig. 2). An 18-gauge vent was waxed to the pattern

with a 2 mm margin at the top of the pattern and acted

as an opened vent. In the single sprueing group, the

vent was located along with the straight portion of the

main sprue and the vent was located between the

sprues on the wax patterns in the double sprueing

group. The sprue/wax pattern junction was carefully

refined under 103 magnification. Customized rings

were made from plastic sheet for investing the patterns.

The complete wax pattern with sprue former was

invested with a phosphate bonded investment*

according to the manufacturer’s instructions (liquid :

powder ratio, 35 mL : 175 g). The investment was

spatulated under vacuum for 15 s and then slowly

vibrated into the ring which was filled to

approximately 5 mm above the pattern. The vent

remained visible. The location of the crown pattern in

*Ohara Titanium Vest for Crowns; Ohara Co. Ltd, Osaka, Japan.


Table 1. Casting mould preparation

Phase Time

Burnout

20–800°C 60 min

Hold temperature 30 min

Processing cycle

800–950 °C 15 min


950–1200 °C 30 min


Cool to 25 °C 80 min

the ring was marked on the investment so that the

rings could be properly orientated for casting. The rings

were cast with the pattern located downward in the

lower outer quarter of the trailing half of the casting

mould. The investment was allowed to bench set at

room temperature (µ 25 °C) for 45 min before

beginning burnout procedures.

Burnout procedures and temperature setting followed

manufacturer’s recommendations, outlined in Table 1.

The wax was eliminated and moulds were heat-socked

in a conventional burnout furnace. These refractory

moulds were then transferred to a high-temperature

processing furnace* for mould expansion. After heat

processing, the refractory moulds were allowed to cool

in the furnace until they could be picked up with

unprotected hands and placed in the casting machine.

Castings were made using an argon/electric arc vertical

centrifugal casting machine following manufacture’s

instructions.* A 7 g titanium (. 99·5%, commercially

pure) ingot was used to cast each specimen. The

refractory mould was secured in the casting machine

with the pattern mould orientated to the casting arm

motion according to the recommendations reported by

DeWald (1979). The casting procedure was repeated for

each of the specimens with the shortest possible delay

between castings. Twelve castings were made for each

group providing a total of 24 complete veneer crown

castings.

The castings were allowed to bench cool, divested

manually, air-braided with 50 µm aluminium oxide

abrasive to remove residual investment and then

ultrasonically cleaned in distilled water for 2 min. No

grinding, deburring, or polishing was attempted and no

effort was made to reseat the castings on the original

die. The margins of the resultant casting were examined

by an indirect method using an impression technique


Fig. 3. Schematic drawing of an impression with the crown

removed, and cut into six sections named a to d.

(Brockhurst et al., 1983). An impression of the entire

casting margin was made with a 15 mm diameter metal

ring filled with impression material.* The circular

margin was centred in the ring. After setting, the casting

was separated. The impression was removed from the

ring and placed in a matching tubular jig that guided

cuts dividing the cylindrical impression into six equally

sized segments (Fig. 3). Each recorded 1/6 of the test

casting margin at 60° intervals. Each section was

numbered according to its position in the jig. Castings

with the double sprues were aligned in the impression

ring with sprues at positions marked ‘a’ and ‘d’ on

the metal ring. Vents were located midway between

positions ‘b’ and ‘c’ for all castings. The sections were

examined under a microscope at 503 magnification.

The marginal discrepancy was then determined by a

scale calibrated with a stage micrometer. The 30° margin

impression was lined up on the scale. Slides were then

taken for all sections and the image further enlarged

10 times for a total magnification of 5003. The distance

from an actual casting edge to a potentially perfect

margin was then recorded as the marginal discrepancy

(d), in µm as shown in Fig. 4. The marginal discrepancy

was recorded for each of the six sections per casting.

Thus, 72 sections were measured for each group. Data

were statistically analysed both by an analysis of

variance procedure and the Student’s t-test between

the means for each position and group of castings,

respectively, with confidence level at 95%.

*Express, light-body; 3 M Dental Products, St. Paul, MN, U.S.A.


Fig. 4. Schematic drawing of the measurement of one section of

impression. d: marginal discrepancy.

Table 2. Results of marginal discrepancy measured in this study

Marginal discrepancy (µm)

Position a b c d e f

Group Range 30–75 15–60 15–75 30–75 30–90 30–75

I Mean 56·3 41·3 47·7 45·8 53·8 47·1

s.d. 14·5 11·5 14·3 11·7 17·6 13·2

Group Range 15–50 10–45 10–35 15–60 15–60 15–60

II Mean 32·5 32·9 25·8 36·3 34·2 31·3

s.d. 11·8 11·6 9·5 14·5 15·9 12·8

I, Single sprueing group; II, Double sprueing group; s.d.,

Standard deviation.

Table 3. The results of the analysis of variance test of the single

sprueing group

Sum of Mean

Source square d.f. square F-ratio P

Position 1 707·29 5 341·46 1·29 0·28

Error 17 464·58 66 264·62

Results

Marginal discrepancy values varied around the

circumference of all castings. The ranges, means and

standard deviations of marginal discrepancies for tested

groups are listed in Table 2. There were no statistically

significant differences in marginal discrepancy between

the six locations within the groups (P . 0·05). Analysis

of variance results are listed in Table 3 & 4. The overall

mean and standard deviation of marginal discrepancies

for the single sprueing group and the double sprueing

group were 49·8 6 16·4 µm and 32·1 6 12·8 µm,

respectively. The double sprueing group had

significantly less marginal discrepancy than the single

sprueing group (P , 0·001).

428 D . C . N . C H A N et al.

Table 4. The results of the analysis of variance test of the double

sprueing group

Sum of Mean

Source square d.f. square F-ratio P

Position 812·50 5 162·50 0·99 0·43

Error 10 825·00 66 164·02

Discussion

In this investigation, castings were made with

commercially pure titanium having a low density and

a cold mould casting technique. Rapid cooling and loss

of fluidity of molten titanium would be expected as it

enters the cold mould. This may reduce the ability of

metal to consistently wet the surface of the mould

and completely fill the mould space. The extreme

temperature differences between mould and metal also

shorten the time for the gases inside the mould to

escape (Hero, Syverud & Waarli, 1993). However, the

disadvantage of casting lower density metals with

centrifugal casting machines could be counteracted by

higher rotational speed or by using longer sprues

(Brockhurst et al., 1983). The results of previous studies

(Young, Coffey & Caswell, 1987; Verrett & Duke, 1989;

Peregrina & Schorr, 1990; Leong et al., 1994; Chai &

Stein, 1995) influenced the specific sprue design used

in this study to resolve the mould filling problem. Large

angulated sprues were used with the open vents in this

investigation instead of conventional straight direct

sprues. This compromised the conflict between rapid

solidification of the molten metal and an inadequate

mould gas escape mechanism during casting. It is also

possible that vents close to the margin acted as an

additional reservoir of molten metal during the

solidification process and chill sets for the rapid

elimination of heat from the casting (Rawson, Gregory

& Lund, 1972; Wright, Grisius & Gaugler, 1980). Lower

discrepancy values adjacent to the vent at positions ‘b’

and ‘c’ confirmed the positive effect of venting in this

sprueing design.

In order to distinguish between the property of

castability of an alloy and the deficiency of a test casting

representing a dental crown, the potential of an alloy

to cast fine margins is often expressed by the minimum

value, whereas the quality of castings produced is

indicated by the marginal discrepancy. The extent of

the results shown in Table 2 indicated that the double


sprueing group had marginal discrepancies as low as

10 µm compared with 15 µm for the single sprueing

group. A clinical study conducted by Dedmon (1982)

disclosed that most dentists would not detect a 50 µm

marginal step with an explorer tip. Brockhurst et al.

(1983) measured the castability of various dental alloys

in terms of clinical requirements. The quality of casting

margins was expressed by means of the term ‘deficiency’

between the edge of the casting and the theoretically

sharp edge. On the basis of theoretical calculations of

the clinically tolerable marginal openings, they proposed

a value of 50 µm including 25 µm for the cement layer

and 25 µm for the deficiency of a casting margin.

In the present study, the term deficiency could not

be used because it was obvious during measurement

that castings varied with regard to the 30° angle of the

margin. According to the previous study conducted by

Brockhurst et al. (1983), the definition of deficiency as

2·70 multiplied by the radius of the rounded edge of

the margin requires a 30° margin, therefore the term

diameter is used instead of deficiency. For comparison

the diameter corresponds, approximately, to a factor

of 0·75 multiplied by the measured discrepancy. For

example, a 25 µm linear discrepancy multiplied by 0·75

would have an edge diameter of approximately 20 µm.

Bessing (1986) used a method similar to that proposed

by Brockhurst et al. (1983) to evaluate the marginal

accuracy of four different alternative crown and bridge

alloys. His results showed that the castability of the

silver–palladium alloys was inferior to that of gold-

based alloys. The diameters of the crown margin edges

for the most accuracy group ranged from 3 to 81 µm,

with mean and standard deviation of 32 6 12 µm. The

mean and standard deviation values obtained from the

double sprueing group of titanium castings in this study

(32·1 6 12·8 µm) matched reported by Brockhurst et al.

(1983). Casting titanium crown margins were shown

to be equal to or, in some cases, even better than the

type III gold alloys and silver–palladium alloys castings.

The design of this study showed much improvement in

the degree of casting accuracy than the results

determined from similar procedures described recently

(Syverud, Okabe & Hero, 1995).

Acknowledgment

The authors would like to thank Dr Peter Almquist for

assistance in casting of the samples.


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The effect of sprue design on the marginal accuracy of titanium castings

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