Azer HandiMix JD 2008

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Effect of mixing methods on the physical propertiesof dental stones §

Shereen S. Azer * , Ronald E. Kerby, Lisa A. KnoblochDivision of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, Columbus, OH, USA

1. Introduction

Dental casts and die materials, especially for xed prostho-dontic procedures, are required to accurately reproduce theimpressions they are made from, providing all the ne details,in addition to being dimensionally stable and resistant toabrasion. 1,2

Several materials that closely fulll these requirementshave been used to fabricate dies. Among these products aredental stone, epoxy resin, as well as dies electroplated with

metals like copper and silver. 3–6 Additionally, a blend of stoneand investment material has been proposed to fabricaterefractory dies. 7

Improved dental stones, however, have been by far themost popular in fabricating working casts and removable dies,because of their reasonable cost, ease of manipulation, andability to produce consistent results, especially high strength/high expansion (ADA type V) stone. 2,8,9 These products arecommonly mixed either by hand or mechanically undervacuum. Recently, a new type V stone product, HandiMix

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a r t i c l e i n f o

Article history:Received 13 February 2008Received in revised form12 May 2008Accepted 13 May 2008

Keywords:HandiMixDental stoneGypsumCompressive strengthDiametral tensile strength

Setting timeSetting expansionSurface porosityMicrohardnessHand-mixing Vacuum-mixing Shake-mixing

a b s t r a c t

Objectives: This in vitro comparative study evaluated the effect of different stone mixing methods on material properties of four dental stones. Two ADA type IV stones (Silky-Rockand Snap-Stone), one type V high expansion stone (Die Keen), and one recently introducedtype V specialty stone (HandiMix) were chosen for this study.Methods: Forty cylindrical specimens (25 mm 12.5 mm) were cast for each of the ninestone sub-groups and bench dried at 23 2 8 C for 1 and 24 h. Specimens were then tested inan Instron in tensile and compression modes at crosshead speeds of 0.5 and 1.0 mm/min,respectively. Four rectangular-shaped specimens (30 mm 15 mm 15 mm) of each stonetype were cast and bench dried for 48 h. Knoop microhardness measurements wereobtained from dened areas on each specimen for surface hardness testing using 200 g load and 20 s dwell time. A 12.6 mm 2 area was then delimited in the center of two sides of each specimen and photographed under low power magnication (40 ). The average porenumber per area was then determined for each specimen for surface porosity testing. Thesetting time and setting expansion for each stone type was recorded as well.Results: ANOVA (P < 0.001) and Ryan–Einot–Gabriel–Welsh test ( P < 0.05) showed signicantdifferences between diametral tensile strengths and pore numbers for both stone types andmixing methods.Conclusion: Within the limitations of this study, the newly introduced mixing method didnot appear to have an effect on the physical properties of HandiMix stone.

# 2008 Elsevier Ltd. All rights reserved.

§ Presented at the 85th General Session of the IADR, 21–24 March 2007, New Orleans, LA.* Corresponding author at : Division of Restorative and Prosthetic Dentistry, The Ohio State University College of Dentistry, 305 West 12th

Avenue, #191, P.O. Box 182357, Columbus, OH 43218-2357, USA. Tel.: +1 614 292 7467; fax: +1 614 292 9422.E-mail address: [email protected] (S.S. Azer).

a va i l a b l e a t www.s c i e nc e d i r e c t . c om

j ou r n a l h om e pa ge : www. i n t l . e l s e v i e r he a l t h . c om / j ou r na l s / j de n

0300-5712/$ – see front matter # 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2008.05.010

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http://dx.doi.org/10.1016/j.jdent.2008.05.010

mailto:[email protected]



(Whip Mix Corp. Louisville, KY) was introduced to the market.This material is hand mixed by shaking the pre-weighedpowder and liquid for 20 s in a supplied disposable plasticcontainer. The stone is then poured into the impressionwithout the use of additional equipment.

The dimensional accuracy of cast and die materials hasbeen the subject of several in vitro investigations over the past

decade, with some conicting ndings. Chaffee et al. 10

reported that improved dental stone provided a similar degreeof dimensional accuracy in reproducing a complete arch whencompared to epoxy resin. However, other investigators foundthat epoxy resin exhibited considerable shrinkage compare togypsum products, and suggested that technique modicationswere required to obtain castings that would adapt to toothpreparations if epoxy resin were to be used as die material. 9,11

Heshmati et al. 12 measured the linear expansion of 6 ADAtypes IV and V improved dental stone materials and reportedthat all stone products showed higher mean linear expansionvalues at 120 h compared to 2 h (ADA recommendation).

Other major desirable characteristics of die materialsinclude surface hardness and abrasion resistance. Ghahre-mannezhad et al. 13 reported that applying one coat of cyanoacrylate adhesive as a die hardener to type IV dentalstone increased the surface hardness by 150% and theabrasion resistance by 48%. In addition, the application of surface hardeners was shown to create a less porous gypsumsurface. 8 Other investigators however, found that die hard-eners reduced the surface hardness of gypsum die stones. 14

Furthermore, no signicant difference concerning surfaceabrasion and wear resistance was found when conventionaltype IV dental stone was compared to resin-impregnated typeIV stone. 15

The compressive and tensile strengths have been the mostcommon laboratory testing modalities to characterizemechanical and physical properties of dental stone. 16 Jørgen-sen and Kono 17 showed that vacuum mixing increased thecompressive strengthof dental stone by 20% owing to reducedgypsum porosity. Some authors reported that the diametraltensile strength of type IV stone increased when allowed to dryin a microwave oven compared to bench top, 18 whereas,others found that the microwave drying method reduced thecompressive strength of type IV stone. 19

The aim of this in vitro study was to evaluate the inuenceof the various mixing techniques on some physical propertiesof dental stone, as well as to compare the newly introducedHandiMix stone to three other commercially available dental

stones.The null hypothesiswas that thephysical properties of dental stone would not be affected by the mixing methodutilized.

2. Materials and methods

Whip Mix Corporation, Louisville, KY 40209, USA and HeraeusKulzer, Inc., Armonk, NY 10504, USA supplied the four stonematerials used in this study. Material testing and evaluationwere done according to the American National StandardsInstitute/American Dental Association (ANSI/ADA) standards,specication #25 for dental gypsum products. 20 The materialswere divided into nine groups according to the method of mixing (Table 1 ). HandiMix is a new dental stone product thathas been recently introduced to the market. This material ishandmixed by shaking the suppliedpre-measuredpowderandliquid in a special disposable plastic container for 20 s. The fastset stone can be separated from the impression after 10 min.

2.1. Compressive and diametral tensile strength testing

For each of the nine study groups, 40 stone cylindricalspecimens (25.0 mm in length and 12.5 mm in diameter) 16

were fabricated for a total of 360 cylinders. All materials weremixed by the same investigator for standardization purposes,and poured in a special split-metal mold to obtain the desireddimensions. Additionally, HandiMix stone was shake-mixedfor 20 s according to the manufacturer’s directions.

2.1.1. Compressive strengthFor each stone group, 20 cylinders were tested in compressionin a Universal Testing Machine (Instron 4204, Canton, MA) inopen air under 1000 kg (10 kN) load at 1.0 mm/min crossheadspeed until fracture.Ten cylinderswere tested after 1 h, and10after 24 h from the setting time. Compressive strength testing was done on the height of the cylinders with moist lterpaperpadding 0.5-mm thick placed between the specimens and theloading platens. Compressive strength ( C) values werecalculated by the formula C = P /p r2 , where P is the load tofailure and r is the specimen radius. 16,18,19

2.1.2. Diametral tensile strengthThe same setup for each stone group was used; 20 cylinderswere tested in tension in the Instron Universal Testing Machine in open air under 1000 kg (10 kN) load at 0.5 mm/

Table 1 – Materials used in the studyMaterial Mixing method Abbreviation Batch # Manufacturer

Die Keen (DK) Vacuum DKV 0606151 Heraeus Kulzer, Inc.Hand DKH

Silky-Rock (SR) Vacuum SRV 085080602 Whip Mix Corp.Hand SRH

Snap-Stone (SS) Vacuum SSV 062060503Hand SSH

HandiMix (HM) Vacuum HMV P: 21070601 L: 06131 AFHand HMH

Shake HMS

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Although vacuum mixing for HandiMix stone resulted infewer porosity counts, the newly introduced shake mixing method showed no signicant differences in pore countscompared to vacuum- or hand-mixing for that material.Regardless of the mixing method, HandiMix stone exhibitedsignicantly reduced surface porosity than hand-mixed SilkyRock and Snap-Stone ( Fig. 3).

Similarly, no signicant differences in surface hardness(KHN) were found between the different stone study groups,especially when comparing the shake-mixing method to othertraditional vacuum- and hand-mixing methods ( Fig. 4).

3.3. Setting time and setting expansion

There were no signicant differences found between thedifferent mixing methods regarding the setting time for eachindividual stone material ( Fig. 5). Die Keen exhibited thelongest setting time of all stones tested and Snap-Stone theshortest. However, a signicant differencewas found betweenHandiMix and Die Keen (type V stones) regarding setting expansion. The hand-mixing method for Die Keen stoneshowed a signicantly higher expansion mean than thevacuum-mixing method. Additionally, both mixing methods

for Die Keen showed signicantly higher setting expansionmeans compared to all the other stone groups ( Fig. 6).

HandiMix stone, regardless of the mixing method, showedno signicant differences in setting expansion compared totype IV stones (Silky Rock and Snap-Stone).

4. Discussion

The null hypothesis in this study (that the physical propertiesof dental stones would not be affected by the mixing methodutilized) was accepted. The present investigation wasdesigned to evaluate a newly introduced method for mixing dental stone, which reduces time and equipment involved, bycomparing it to traditional mixing techniques in relation tosome of the physical properties of the resultant stone. Andalso to comparethe newly introduced dental stone (HandiMix)to other commercially available stones. It is worthy to notethat the stone materials used in this study meet therequirement values set forth by the ANSI/ADA specication#25 for gypsum products.

It has been reported that forpractical purposes, stone castsshould be separated from impressions after approximately

Table 2 – Mean compressive and diametral tensile strengths in MPa of the stone materials for the various mixing methods

Material Mixing method Test Time (h) N Mean S.D.

Die Keen (DK) Vacuum Compressive 1 10 43.5 7.424 10 48.4 8.8

Diametral tensile 1 10 5.3 0.824 10 7.1 1.7

Hand Compressive 1 10 32.8 6.024 10 52.9 11.1


Silky-Rock (SR) Vacuum Compressive 1 10 34.4 2.124 10 37.8 3.1




Snap-Stone (SS) Vacuum Compressive 1 10 32.2 3.424 10 45.0 4.6




HandiMix (HM) Vacuum Compressive 1 10 31.9 4.224 10 53.1 8.6




Shake Compressive 1 10 36.7 5.1

24 10 32.9 2.7Diametral tensile 1 10 4.5 0.624 10 3.7 0.4




45 min to 1 h, 21 and that dental stone would reach sufcienthardness after 24 h. 22 In this investigation, and in accordancewith the compressive strength testing described by Tunceret al. 19 , stone specimens were tested in compression andtension after 1- and 24-h from the start of mixing.

Earlier investigations suggested that vacuum-mixing pro-vided a denser and more homogeneous stone mix with lessentrapped air compared to hand mixing. 17,21,23 In the presentinvestigation, however, there was no signicant difference

found in compressive strength values between both methodsafter 24-h, except within the HandiMix stone sub-group,where vacuum mixing showed a higher compressive strengthfor this material. In contradiction to previous reports, hand-mixed Die Keen stone showed the highest diametral tensilestrength after 24-h compared to all other stone groups. Theseinconclusive ndings may suggest that the method of mixing of dental stone might not necessarily affect the strength of theset stone.

Fig. 3 – Mean surface porosity counts.

Fig. 4 – Mean Knoop hardness number (KHN). Similar letters are not statistically significant.




Specimen dimensions for compressive strength testing were prepared in accordance with ANSI/ADA standards aswell as with previous similar investigations which consis-tently exhibited 2:1 length to diameter ratio. Some usedcylindrical specimens 40 mm in length 20 mm in dia-meter, 18–20 while others used cylinders of 12 mm inlength 6 mm in diameter 16 or 10 mm in length 5 mm indiameter. 24 In the present study cylindrical stone specimens(25 mm in length 12.5 mm in diameter) were prepared.However, for the diametral tensile strength testing, previousinvestigators used varying geometrical specimen designs.Some used at cylindrical specimens 10 mm in diame-

ter 3 mm in height, 16 while others used cylinders of equaldiameter and height (5 mm 5 mm). 24 In a recent publicationby Hersek et al. 18 the same geometrical design used forcompressive testing was used for diametral tensile strengthtesting (40 mm in height 20 mm in diameter). The presentstudy followed the model set forth by Hersek et al. 18 being themost recent.

Although not recommended by the manufacturer, andonlyfor comparison reasons in this investigation, the fast setting Snap-Stone was vacuum-mixed for 5 s using the Vac-U-Spat(Whip MixCorp.,Louisville, KY). Winkler et al. 25 conducted thesame technique for 10 s, but in the current study, and after

Fig. 5 – Mean setting time in minutes for the various stone study groups. Similar letters are not statistically significant.

Fig. 6 – Mean setting expansion for the various stone study groups. Similar letters are not statistically significant.




multiple pilot trials, 5 s were found to be the optimum timelimit to vacuum-mix and pour the fast set stone into the splitmold before hardening. Perhaps this short mixing time undervacuum was not enough to prevent the formation of voidswithin the set stone, as evidenced by the large number of porosities found in that sub-group comparable to the handmixing (Fig. 3). However, the surface hardness of this stone

was not statistically weaker than the other stone groups(Fig. 4). This might be explained by the almost completereaction of the fast setting stone within the early minutes, asseen by Winkler et al., 25 who found that the microstructure of Snap-Stone, as observed by SEM at 10 min, was essentiallyunchanged after 24 h. Additionally, in the present investiga-tion, hardness testing was conducted after 48 h. Previousinvestigators reported that after 48 h air dried stone productsexhibited their greatest surface hardness. 26 The results of thepresent study probably conrm such reported ndings, asnone of the stone mixing methods showed statisticallysignicant difference between the mean KHN values of thetested stone groups ( Fig. 4).

Furthermore, the various stone mixing methods, in thisstudy, did not seem to affect the setting time or setting expansion of each of the tested dental stones. Interestingly,vacuum-mixed Snap-Stone (not recommended by the man-ufacturer) was not statistically different from the hand-mixedproduct.

The introduction of new and innovative mixing techniquesfor restorative and prosthetic dental materials is certainly anencouraged effort, especially if the time involved is reducedand procedures become more efcient. The limitations of thisstudy included comparing only four dental stones, as well ascomparing a mixing method that was designed by themanufacturer for only one type. The authors attempted tostandardize the study as far as was applicable in order toobtain meaningful statistical results.

5. Conclusions

This investigation evaluated the effect of various stone mixing techniques on the physical properties of the newly introducedmaterial, HandiMix, as well as other type IV and V dentalstones. Within the limitations of this study, the following conclusions are drawn:

1 Mixing methods did not appear to have signicant effect onthe physical properties of dental stone.

2 Shake-mixing for HandiMix stone proved to be an accep-table alternative to traditional mixing methods, withoutaffecting the physical properties of the stone.

3 HandiMix specialtystonewasnot statisticallydifferentin itsphysical properties from ADA Type IV stones.

Acknowledgements

The authors thank Whip Mix Corporation, Louisville, KY andHeraeus Kulzer, Inc., Armonk, NY, for supplying the materialsused in this study.

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