Journal of Dentistry Vol. 24, Nos 1-2, pp. 99-103, 1996 Copyright 0 1996. Elsevier Science Ltd. All rights resewed Printed in Great Britain. 0300-5712/96 $15.00 + 0.00 ELSEVIER
Sprue design in removable partial denture casting C. A. Burnett and H. Maguire Division of Restorative Dentistry, School of Clinical Dentistry, BT12 6BP, UK
The Queen’s University of Belfast, Grosvenor Road, Belfast
ABSTRACT Objectiws: Correct sprue design is a major factor in the reduction of defects in castings of cobalt-chro-
mium alloy. The purpose of this study was to investigate the sprue arrangements decided by a group of dental technicians, for a series of proposed removable partial denture castings, in order to determine if there was any consistency between them which would suggest the application of criteria in their design. Methods: Information was gathered by postal [email protected]
with recipients asked to indicate their sprue designs on diagrams of patterns for proposed cobalt-chromium partial denture castings. Results: Replies were received for 260 proposed castings from dental schools, technical colleges and commercial laboratories which gave an overall response rate of 52%. Results displayed a wide range of responses in relation to sprue number and dimension, attachment site and the intended direction of metal flow. Conclusion: It was concluded that the group of technicians studied arbitrarily chose sprue arrangements for the castings proposed rather than following definite design criteria that seek to minimize potential problems during casting procedures. KEY WORDS: Casting, Cobalt-chromium J. Dent. 1996; 24: 99-103
23 June 1994; reviewed
partial denture 15 November
INTRODUCTION Cobalt-chromium is the most widely used casting alloy in the construction of removable partial dentures. Faults in casting related directly to the manipulation of the alloy can cause incomplete castings or porosityl. The former is usually easy to recognise but requires that the procedures to produce the casting be repeated which is costly in terms of both time and materials. Porosity is seen as surface pitting on the casting or is revealed within the cast during finishing or polishing procedures. Sub-surface porosity will only be apparent on examination by radiography’. Porosity in castings is one of the major causes of failure of cobalt-chromium castings in service3. Porosity in cobalt-chromium castings has been previously investigated1J4-7 and the main factors implicated are shrinkage of the alloy, gas evolution within the mould, and the composition of the alloy1J4,5. In relation to shrinkage of the alloy, correct sprue design is a major factor in the reduction of porosity7. It is inevitable that there are small but generalised areas of
Correspondence should be addressed to: Mr C. A. Burnett, Division of Restorative Dentistry, School of Clinical Dentistry, The Queen’s University of Belfast, Grosvenor Road, Belfast BT 12 6Bp, UK
1994; accepted 27 January 1995)
porosity throughout a casting due to the dendritic mode of crystallisation of cobalt-chromium alloy as it solidifies. However, careful sprue design reduces the potential for developing massive interdendritic porosity, which produces obvious sites of weakness. A sprue is a wax or metal channel which, when removed from an invested pattern, leaves a passage from the crucible to the pattern, through which the molten metal flows to create the casting. Sprues also act as reservoirs, or heat sinks, allowing the solidifying casting to draw on further molten metal until it has completely frozen. Success in any casting procedure depends upon applying criteria to sprue design with regard to number and dimension, attachment site to the pattern and producing a particular direction of flow to allow the molten metal to fill the mould rapidly, smoothly and completely. The application of the correct criteria in the design of sprue arrangements with respect to each of these factors must reduce the risk of producing faulty castings. The purpose of this study was to investigate the sprue arrangements, for a series of proposed removable partial denture castings, decided by a group of dental technicians in order to determine if there is any consensus which might suggest compliance with established criteria in their design.
J. Dent. 1996; 24: Nos 1-2
METHOD Questionnaires, including a diagrammatic representation of patterns for four maxillary removable partial denture castings, were posted to the senior dental instructor or technician at each of the 16 dental schools in the U.K. and the Republic of Ireland, all 14 colleges offering dental technology courses in the U.K., and to 16 large commercial laboratories located throughout the U.K. The latter were chosen at random from advertising in dental technology journals and intended to give a similar demographic distribution to the dental schools and technical colleges. The designs (Fig. I> were all for tooth supported prostheses, and the four major connectors were a horseshoe, a palatal plate, a palatal strap, and anterior and posterior bars as a skeletal design. It was indicated that no portion of the castings was to be greater than 2.0 mm thick. The thickest section of such proposed castings would be the junction of the clasps with the major connector, and the demarcation line between saddles and major connector, the thinnest section being the major connector itself. Each respondent was asked to indicate on the drawings of the patterns the number and position of the sprues to be used to produce a cobalt-chromium casting. The respondents were also asked to record their proposed position of crucible placement, the dimensions of the sprues, in millimetres, and their shape (round or flat) and if they were to use air vents or additional reservoirs. Four questionnaires were sent to each dental school for completion by dental instructors and senior hospital
laboratory technicians, if required. Two questionnaires were sent to each of the technical colleges and commercial laboratories. There were 65 completed replies, 44 from the dental schools, 10 from the technical colleges, and 11 from the commercial laboratories, with sprue designs for 260 partial denture castings, which gave an overall response rate of 52%.
RESULTS Design 1: Horseshoe The number of sprues used varied from one to six (P’&. 2u), with 54% electing to use three sprues. The vast majority of respondents, 86%, elected to use round sprues in all the designs, and henceforth, sprue size will be quoted as a diameter. The sprue size varied from 2.5 mm to 5.0 mm (F&. 3~) with a 3.0 mm sprue most b) Plate
a) Horseshoe Percent
1,. 20 30 40 1.
c)strap Fig. 7. Diagrammatic castings as included
d) Skeletal representation of the four in the postal questionnaire.
Fig. 2. Number of sprues chosen each of the proposed castings.
Burnett and Maguire: Sprue design in removable partial denture casting
c) Strap PW.X?nt
Fig. 4. Sprue attachment sites, x, chosen for each proposed casting.
technique”; however, 49% attached their sprues to the major connector, which was the thinnest section of the casting. Thirty-one percent used air vents, with 5% using reservoirs.
Design 3: Palatal strap 2.5
Fig. 3. Sprue size, (diameter in mm) chosen by percentage of respondents for each of the proposed castings.
commonly used. Fourteen separate sprue attachment sites were indicated (Eg. 4~). Seventy-five percent of respondents sprued through the model, the “inverted sprueing technique”, and attached the sprues directly to the major connector. Five percent of those who elected to sprue from above the model also attached the sprues to the major connector, giving an overall total of 80% attaching sprues to the thinnest section of the casting. Twenty-five percent used air vents, with 5% using reservoirs.
Design 2: Palatal plate The number of sprues used varied from one to five (Fig. 2b), with four employed most often. The sprue size varied from 2.5 mm to 4.0 mm (Eg. 3b) with 62% using the 3.0 mm sprue. Twelve separate sprue attachment sites were indicated (Fig. 4b). All respondents elected to sprue from above the model, as it was impossible in this case to use the “inverted sprueing
The number of sprues used varied from one to four (Fig. 2c), with 48% using two sprues and 34% using three sprues. The size of sprue used varied from 2.5 mm to 5.0 mm (Fig. 3c), with the 3.0 mm sprue again being the most popular. Twelve separate sprue attachment sites were indicated (Fig. 4~). Forty percent of respondents used the “inverted sprueing technique”. Twenty percent of those who elected to sprue from above the model attached the sprues to the major connector, giving an overall total of 60% attaching the sprues to the thinnest section of the casting. Twenty-five percent used air vents, with 5% using reservoirs.
Design 4: Skeletal The number of sprues varied from two to six (&g. 2d), with half of the respondents choosing to use four sprues. The size of sprue varied from 2.5 mm to 5.0 mm (Fig. 3d), with 47% using the 3.0 mm sprue. Twenty-two separate sprue attachment sites were indicated (Fig. &I!). Fifty-eight percent of respondents used the “inverted sprueing technique”. Twelve percent of those who elected to sprue from above attached the sprues to the major connector giving an overall total of 70%
J. Dent. 1996; 24: Nos 1-2
attaching the sprues to the thinnest section of the casting. Twenty-eight percent used air vents, with 5% using reservoirs.
DISCUSSION The principal considerations in sprue design relate to their number and dimension, attachment site and the intended direction of metal flow. Criteria in relation to these factors can only be appreciated and adhered to if there is an understanding as to how they relate to the production of satisfactory castings.
2.0 mm at the junction of minor and major connectors. Sprues must therefore be attached to the thicker areas of the intended casting and avoid direct attachment to thin major connectors. Attachment to the thickest section of the casting also reduces the risk of distortion of the casting during sprue removal. In this investigation, regardless of the direction of sprueing, 65% of all sprues were attached to the major connectors, the thinnest sections of each proposed casting. Attaching the sprues to a thicker region of the casting, such as the junction of major and minor connectors, makes the finishing process more difficult and time consuming. Placement of sprues posterior to a major connector renders removal and finishing a relatively easy process.
Sprue number and dimension In any casting, the greater the number and thickness of the sprues, the more readily the metal will fill the mould. However, the greater the number and size of the sprues, the more time consuming it is to remove them from the completed casting, and the greater the risk of distortion of the casting as this is being performed’. Also, the point of attachment of sprues is a common site for defects,l and so an excessivenumber should be avoided. Two sprues, and, in many cases, a single sprue8 should, if correctly designed, allow production of satisfactory cobalt-chromium partial denture castings. The majority of respondents in this investigation elected to use three or more sprues in all designs, except the palatal plate. Round sprues produce a better flow of metal than flat sprues and were used by the vast majority of respondents. A preformed round wax sprue pattern is available commercially in a variety of dimensions. A sprue should be of sufficient dimension to allow the metal forming the denture base to freeze first; by doing so, it creates a molten metal reservoir, or heat sink, to allow the cooling metal contracting adjacent to it to draw further alloy from it to prevent porosity. The most popular diameter of sprue used by the respondents in this study was 3 mm, but the utilisation of a range of sprue sizes, from 2.5 mm to 5.0 mm, was demonstrated.
There was a wide range of separate attachment sites chosen for sprue attachment, from 12 in the plate and strap designs, to 22 for the skeletal design. This variation is explained by the majority of respondents attaching their sprues to sites on major connectors. A correct attachment site should produce a flow of metal from the thickest to thinner to thinnest portions to provide a unidirectional system of solidification as this reduces the tendency for porosity to occur, in the narrow region, as a result of premature solidification5. The cross-sectional thickness of partial denture castings may vary from 0.5 mm, in plate major connectors, to about
Direction of metal flow Sprues must carry the molten metal to the mould by the shortest route to reduce the risk of premature solidification as the metal cools. The flow of metal from the crucible must not be interrupted by sharp angles, to prevent turbulence in the stream’. Also, the route should not reverse on itself as the flow of molten metal should move in line with the centrifugal casting force, and not against it. There are two methods of attaching the crucible, via the sprues, to the wax pattern, either from above the model or from below it. Attaching the crucible above the model, “top sprueing”, allows straightforward access to the thickest section of the casting in the case of maxillary castings, as these sections are closer to the crucible, without the sprues having to change direction. Placing the crucible below the model, “inverted sprueing”, generally attaches the sprues to the major connector, as this is often in close proximity. If an attempt is made to bring the sprues up past the major connector to attach them to thicker sections, then the metal must flow back in a reverse direction to form the connector producing the risk of introducing turbulence in the stream and also increasing the length of route. Other than for the palatal plate design, which could not be sprued through the model, the “inverted sprueing technique” was the more popular for the designs proposed in this study. The ease of the technique, in which manufactured formers are used to produce a crucible as an integral part of the refractory cast upon which the wax pattern is laid, may encourage its high incidence of use. Attaching the crucible former above the model, by the sprues themselves, although more time consuming, allows the operator to attach the sprues to the nearby thicker sections of the proposed casting and to produce the desired pattern of flow. Nonetheless a substantial number of respondents in this investigation who elected to sprue from above also chose to site their sprues on the major connector. It is the authors’ suggestion that maxillary castings should correctly be sprued from above the model and that the “inverted spruing” technique be
Burnett and Maguire: Sprue design in removable partial denture casting
reserved for mandibular castings with a bar major connector which is sufficiently thick for direct sprue attachment. Correct sprue design alone will not eradicate porosity as gas within the mould, and as a reaction of mould products, may be incorporated in the casting”. The use of air vents will allow some of this gas to escape ahead of the incoming molten metal. In this study, air vents were incorporated in only 28% of the proposed designs. Gaseous porosity may also be minimised by avoiding overheating of the alloy during fusion and casting in the shortest possible time’. The large number of respondents, 95%, using 3.0 mm or larger diameter sprues, corresponds with the lack of use of additional reservoirs, with the sprue being large enough to act as a reservoir itself. Most cast cobalt-chromium partial dentures will be no thicker than about 2.0 mm at their thickest cross section. Efforts to improve the quality of cobalt-chromium partial denture castings require that definite criteria be adhered to when deciding the sprue arrangements. It is clear from this investigation that such criteria are not being applied, particularly with regard to the site of sprue attachment and direction of sprueing. Sprueing technique has been described as an art that is not well understood or practised I1. Many technicians have developed their sprueing techniques under the guidance of instructors or technicians in a purely empirical manner. Although individual techniques may produce castings that appear to be sound, porosity defects, external or internal, may be present, and, when delivered to patients, constitute a potential cause of failure in service and are ultimately the clinician’s responsibility. A limitation of any research based on surveys is the potential bias introduced by the sampling method. No attempt was made in this study to identify differences between the respondents and non-respondents. However, the authors believe that the sample size is large
enough to validate the results and justify further research. It is intended that this study be the basis for a laboratory-based project designed to investigate how each of the factors addressed in relation to sprue design quantitatively affects the quality of cobalt-chromium castings. It was concluded from the diversity in results that established criteria for the design of sprue arrangements were not applied by the group of dental technicians investigated. References 1. Lewis AJ. Porosity in basemetal partial denture casting alloys, related industrial alloys, and pure metals. Aust Dent J 1977; 22: 208-211.
HargreavesJ and Hobkirk JA. Castingdefectsin subperiosteal implants. J Dent 1982; 10: l-6. 3. van Noort R and Lamb DJ. A scanningelectron microscope study of cobalt-chromium dentures fractured in 2.
service. J Dent 1984; 12: 122-126. 4.
LewisAJ. The effect of variations in mould temperature, metal temperatureand mould sizeon the developmentof internal porosity in caststructures.Aust Dent J 1977; 22:
LewisAJ. A metallographicevaluationof porosity occurring in removablepartial denture casting. Aust Dent J
243-246. 1979; 24: 408-411.
CompagniR, Faucher RA, and Youdelis RA. Effects of spruedesign,castingmachine,and heat sourceon casting porosity. J Prosthet Dent 1984; 52: 41-45. 7. Matin KA and Manderson RD. The influence of sprue designon cobalt-chromium alloy castingdefects.J Dent
1984; 12: 175-182.
RoydhouseRH and Skinner EW. The accuracyof large castings.J Dent Res 1961; 40: 1057-78. 9. Young HM, CoffeyJP, and CaswellW. Spruedesignand its effect on the castibility of ceramometalalloys.J Pros-
thet Dent 1987; 57: 160-164.
10. Lewis AJ. The influence of the refractory investmenton the developmentof porosity in caststructures.Aust Dent J 1977; 22: 455-457.
11. PrestonJD and Berger R. Somelaboratoryvaluesaffecting ceramometalalloys.JProsthet Dent 1977; 21: 717-721.