Robin Huggett, Alan Harrison, University
MSc, PhD,a Alkiviades Zissis, DDS,b BDS, PhD,e and Amanda Dennis, BSc, MScd of Bristol, Dental School, Bristol, England
Proponents of injection molding systems have claimed a number of benefits over conventional press-pack dough molding systems. The aim of this study was to evaluate a recently developed injection (dry heat) procedure of processing compared with press-pack dongh molding utilizing three curing cycles. The dimensional accuracy and stability of acrylic resin bases produced by the two molding procedures were compared. Dimensional changes were assessed over a period of 4 months using an optical comparator. The results demonstrate that baseplates produced by the injection molding procedure exhibit less shrinkage than those produced by the conventional press-pack procedures. (J PROSTHET DENT 1992;68:634-40.)
imensional accuracy measurements of the consplit-mold press-pack dough molding system are venti well documented and have been presented in a review.r Techniques using closed molding systems (pour resins and injection molding) have been used for many years, but have failed to achieve universal commercial acceptance.2-8 Proponents of injection molding systems claim the advantages of the elimination of flash and the consequent “raised bite” improved dimensional form and reduced porosity. Claims are also made for reduced production time and costs as well as reduced skin contact and vapor inhalation of the methyl methacrylate monomer. Several studies have compared dimensional accuracy and stability of injection and conventional processing of acrylic resins.5, 6,8-1QThis study evaluates a recently developed injection method of processing compared with the press-pack method. It also compares the dimensional accuracy and stability of acrylic resin denture bases produced by the two methods.
A brass master die was machined to simulate the shape of an edentulous arch in such a way that it could be used for either mandibular or maxillary jaw simulation. Symmetrically located index marks were incorporated by drilling small holes in specific locations; however, only those labeled A, B, C, were used in tlnis study. These were in the
Supported by a grant from the Bristol and Weston Health Authority Medical Research Committee. aLecturer, Department of Prosthetic Dentistry and Dental Care of the Elderly. bVisiting Lecturer. CProfessor, Department of Prosthetic Dentistry and Dental Care of the Elderly. dStatistics Consultant, Computkg Department.
Fig. 1. The brass master die, permanent silicone elastomer mold, and a stone cast illustrating the index marks.
central incisor and left and right posterior regions (Fig. 1). A Dublisil permanent silicone elastomer mold (Panadent Ltd., London, England) was made of the master die. Forty casts were prepared by pouring Kaffir D (South Western Industrial Plasters, Chippenham, England) stone into the mold, ensuring that each cast was produced using the same powder-water ratio and construction technique. Following the conventional dental laboratory procedures, a 3 mm thick wax sheet was adapted and sealed to each cast. Acrylic resin baseplates were then constructed using each of two processing procedures.
Ten baseplates were constructed by use of an injection molding system described by Knott? and by Huggett et OCTOBER1992
Percent linear change of four curing methods
Fig. 2. Significance of the percentage of linear change of the four curing methods (MI to M4) using Tukey’s HSD test.
Table I. Analysis of variance Greenhouse
Between specimens Method 1 Error Within specimens Time Time x method 2 Error Dimension Method X dimension 3 Error Time x dimension Method X time X dimension 4 Error Total
10.96980 6.93993 0.56044 0.50427 4.38697 3.09413 1.94202 12.15919 0.05451 0.58726 7.53067 48.72919
39 3 36 440 3 9 108 2 6 72 6 18 216 479
Huynh Feldt prob.
0.18681 0.05603 0.04062 1.54707 0.32367 0.16888 0.00909 0.03263 0.03486
SS, Sum ofsquares;MS, mean square;prob,probability.
a1.12The polymer and monomer mix was introduced into a closed mold under a pressure of 6 atm at the “stringy” stage. The injection polymer was held under pressure during the thermostatically controlled polymerization process to ensure a flow of polymer to compensate for any contraction. The denture base material used was Meadway (Meadway Dental Supplies Ltd., Old Woking, England) poly (methyl methacrylate) heat-curing resin in a polymermonomer ratio of 3.2:1. Dry heat was used for polymerization using the curing cycle of room temperature to 100’ C in 1 hour and then holding for 30 minutes under 6 atm pressure. This procedure is coded IM.
Thirty test plates were constructed by the press-pack procedure, and the following three water bath curing cycles were used (10 baseplates for each): (1) rapid curing cycleTHE
100’ C for 20 minutes, bench cooling for 10 minutes, and immersion in cold water (coded RC); (2) overnight curing cycle-room temperature to 70’ C in 1 hour held at 70“ C for 7 hours (coded OC); and (3) overnight curing cycle-room temperature to 70” C in 1 hour, held at 70’ C for 7 hours, followed by 100° C for 3 hours (coded oc+).
Measurements were taken across the dimensions AB, BC, CA (Fig. 1) with a Nikon Optical Comparator instrument (Nikon Profile Projector 6CT, Rank Precision Industries Ltd., Leicester, England) at 10 power magnification. For each dimension, 10 readings were taken and the mean value was calculated. The coefficient of variation for the repeated measurements never exceeded 0.04%. 635
Analysis of linear changewith time
3. Significance HSD test.
of the percentage of linear change with time (T1 to 7’4) using Tukey’s
Analysis of effect of dimension
Fip;. 4. Significance of the percentage of linear change of the three dimensions (01 to 03) us&g Tukey’s HSD test. -
II. Distance AB: Percent change
RC OC 0C-t
0 hr -0.044 -0.293 -0.258 -0.280
-0.083 -0.493 -0.362 -0.371
+0.060 -0.545 -0.294 -0.242
-0.070 -0.441 -0.339 -0.255
IM, Room temperature to 100’ C in 1 hour, held for 30 minutes at 6 atm; RC (rapid cure), 100’ C for 20 minutes, bench-cooled for 10 minutes, immersed in cold water; OC (overnight cure), room temperature to 70” C in 1 hour, held at 70° C for 7 hours; OC+, ~oorn temperature to 70’ C in 1 hour, held at 70° C for 7 hours, then 100” C for 3 hours.
The measurements were taken on five occasions. These were: 1. On the stone cast (considered to be the starting, or zero point, for the dimensional change) 2. On the corresponding baseplate immediately after deflasking (0 hour)
3. After immersion in water at 37“ C for 24 hours 4. After immersion in water at 37” C for 28 days 5. After immersion in water at 37O C for 4 months The data obtained were analyzed using analysis of variance with repeated measures.
RESULTS The results of the percentage of change are summarized in Table I. All three main effects of method (M), time (T), and dimension (D) were significant (p < O.Ol), while none of the interactions was significant. Tukey’s HSD test was used to compare means and revealed that (Qa.s5,se = 3.79; MS error = 0.1928) method 1, the injection method, was different from methods 2,3, and 4, with method 1 showing less change than the other three methods. Method 3 was also significantly different from method 2, with method 2 showing the greatest change (Fig. 2). Analysis of the time effect (Fig. 3) revealed that time 1 was significantly different from times 4 and 2 (Qs.ss.res
0.1 O-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -
Time m m
of change in relation
to time for the dimension
of change in relation
= 3.68; MS error = 0.0406), time 2 showing the greatest change and time 1 showing the smallest change. An analysis of the effect of dimension is presented in Fig. 4. Dimensions 1 and 3 were significantly different from dimension 2 (Qe,e5,7s = 3.4; MS error = 0.1689), and there was no apparent reason for this uneven contraction. All 40 baseplates, under the conditions of measurement used in this study, showed a shrinkage for all three dimensions at each time period. The only exceptions were the injection molded baseplates, which showed a mean expansion for the AB dimension of 0.06 % after immersion in water for 28 days; this difference, however, was not significant as there were no significant interactions. For the dimension AB, in all procedures there was a trend toward uneven contraction after immersion in water (Table II and Fig. 5).
to time for the dimension
Table III. Procedure IM RC oc oc+
Distance BC: Percent change 0 hr
-0.184 -0.453 -0.403 -0.623
-0.218 -0.603 -0.499 -0.659
-0.284 -0.555 -0.454 -0.555
-0.257 -0.602 -0.438 -0.660
Abbreviations as in Table II.
Table III and Fig. 6 show that all the procedures displayed similar behavior in the BC dimension-that is, shrinkage after deflasking and further shrinkage after immersion in water at all time periods. Again, the injection molded baseplate (IM) exhibited less contraction than all other
7. Percentage of change in relation to time for the dimension CA.
0 -0.2 -0.4 -0.6 -0.8 -1 - 1.2 -1.4
I 24 hours
/ 28 days
/ 4 months
Fig. 8. Percentage of change in relation to time for the area ABC.
Procedure IM RC oc oc+
Distance CA: Percent change 0 hr
-0.119 -0.602 -0.234 -0.300
-0.061 -0.814 -0.300 -0.386
-0.131 -0.641 -0.271 -0.239
-0.194 -0.585 -0.415 -0.360
Abbreviations as in Table II.
baseplates and for all time periods. The third considered dimension, CA (Table IV and Fig. 7), exhibited the same changes. Again, the injection molded baseplates exhibited the least shrinkage.
There was a range of changes from -0.284 % to +0.060 % for the injection molded baseplates (IM), from -0.293% to -0.814% for the rapid curing procedure (RC), from -0.234% to -0.499% for the overnight procedure (OC), and from -0.239 % to -0.660 % for the overnight procedure with a terminal boil (OC+). Since there were no significant interactions, the various methods may be considered to have similar effects over time and dimension. Further consideration was given to the changes in the area limited between the index marks (A,B, and C). This area, defined ABC (A), is a triangle with irregular sides. Using the mathematical formula A = $S(S - A)(S - B) (S - C), where S = A + B + C, and A, B, and C = length of the sides of the triangle, the changes in the area can be de-
Area change of four curing methods
Fig. 9. Significance of the percentage of area change of the four curing methods (Ml to M3) using Tukey’s HSD test.
Analysis of area change with time
10. Significance of the percentage of area change with time (77 to T4) using Tukey’s
of variance Greenhouse
Between specimens Method 1 Error Within specimens Time Time X method 2 Error Total
13.46259 9.66226 0.70982 0.31495 6.17332
38 3 35 117 3 9 105 155
0.23661 0.03499 0.05879 30.34917
Huynh Feldt prob.
Abbreviations as in Table I.
termined.13 Table V shows that both the main effects of method and time are significant (p < O.Ol), with a nonsignificant method by time interaction (it was not possible to determine the area of one of these specimens).
From Table VI and Fig. 8, it can be seen that the changes that occurred were minimal in the injection molding procedure for all four periods of time (range of changes, -0.234 % to -0.319%). The changes that occurred with the
VP. Area ABC: Percent change 24 hr
as in Table II.
rapid curing procedure (RC) were the largest (-0.971% to -1.274%). The changes that occurred with both overnight curing cycles OC (-0.590% to -0.716%) and OC+ (-0.671% to -0.933 %) were smaller than those of the rapid cure procedure. Tukey’s HSD test shows similar results. Method 1 was again significantly different from methods 2, 3, and 4 (Qa,os,es= 3.85; MS error = 0.2761), and method 2 was significantly different from methods 3 and 4. Method 1 showed the smallest change, and method 2 showed the greatest change (Fig. 9). Fig. 10 shows that results similar to the above were found with the time effect; time 1 was significantly different from time 2 (Qo.os,1os= 3.68; MS error = 0.0588).
The results demonstrate that the injection molding procedure used produced baseplates that exhibited statistically significantly less contraction than the three conventional processing cycles with the press-pack procedure. There have not been any other reports concerning the dimensional stability of baseplates produced by the injection molding procedure described; however, the findings of other studies concerning other injection methods and using the same measuring and testing processes are noteworthy. Goodkind and Schulte3 reported that there were no significant changes in dimensional stability of bases produced by either the pour technique or with conventional autopolymerizing resin. Murphy et a1.,gAnderson et al.,1° and Freilich et a1.14reported that the SR-Ivocap injection system (Ivoclar-Vivadent, Ltd., Lekester, England) exhibited less polymerization shrinkage than a conventional press-pack system. Therefore from this and previous reports it would seem that the injection molding procedure
has the advantage, when comparing dimensional accuracy and stability, over the conventional press-pack method. CONCLUSION With a newly developed injection molding procedure (Isomate Accord system ‘I, I’), it has been shown that baseplates can be produced that exhibit less shrinkage than those produced by the conventional press-pack procedures. These findings were statistically significant, and are in agreement with those of previous studies. We thank Mr. N. J. Knott for the useof the IsomateAccord inmolding system.
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