ELASTOMERIC IMPRESSIONMATERIALS

Four types of synthetic elastomeric impression materials are available to record dental impressions: polysulfides, condensation silicones, addition silicones(polyvinylsiloxanes), and polyethers. Polysulfides were the first synthetic elastomeric impression material introduced (1950). Condensation silicones were made available to dentists in 1955, polyether in 1965, and addition silicones in 1975. Polysulfide and condensation silicone impression materials are described on the website.Polyvinylsiloxanes and polyethers form the vast majority of elastomeric impressions used worldwide today. Changes in recent years have provided greater choice of consistency and new mixing techniques.

ConsistenciesElastomeric impression materials are typically supplied in several consistencies (viscosities) toaccommodate a range of impression techniques. Addition silicones are available: extra-low, low(syringe or wash), medium (regular), monophase, high (tray), and putty (extra-high) consistencies. Polyether impression materials are now available in low, medium, and high consistencies.

Mixing SystemsTwo types of systems are available to mix the catalyst and base thoroughly before taking the impression: static automixing and dynamic mechanical mixing. A very popular means of mixing the catalyst and base is with a so-called automixing system. The base and catalyst are in separate cylinders of the plastic cartridge. The cartridge is placed in a mixing gun containing two plungers that are advanced by a ratchet mechanism to extrude equal quantities of base and catalyst. The base and catalyst are forced through the static-mixing tip containing a stationary plastic internal spiral; the two components are folded over each other many times as they are pushed throughthe spiral, resulting in a uniform mix at the tip end. Because one cylinder may be filled slightly more that the other, the first part of the mix from a new cartridge should be discarded. The mixed material can be extruded directly into an injection syringe or into the impression tray. Intraoraldelivery tips can be placed on the end of the static mixing tip, and the mixed material can be injected into and around the cavity preparation. The tip can be removed, and additional mixed material can be extruded into the impression tray. The automixing systems have been shown to result in mixes with many fewer voids than hand mixes. Although for each mix the material left in the mixing tip is wasted, the average loss is only 1 to 2 mL, depending on the manufacturer’s tip, whereas three to four times this much is wasted in a hand mix as a result of overestimatingthe amount needed. Initially, automixing was used for low consistencies, but new designs of guns and mixing tips allow all consistencies except putty to be used with this system. Addition silicones and polyethers are available with this means of mixing. The second and newest system is a dynamic, mechanical mixer, illustrated in Figure 12-9. The catalyst and base are supplied in large plastic bags housed in a cartridge, which is inserted into the top of the mixing machine. A

new, plastic mixing tip is placed on the front of the machine, and when the button is depressed, parallel plungers push against the collapsible plastic bags, thereby opening the bags and forcing material into the dynamic mixing tip. This mixing tip differs from automixing in that the internal spiral is motor driven so it rotates. Thus mixing is accomplished by this rotation plus forward motion of the material through the spiral. In this manner, thorough mixing can be ensured and higher viscosity material can be mixed with ease. The advantage of this system is ease of use, speed, and thoroughness of mixing, but more must be invested in the purchase of the system compared with hand and automixing. In addition, there is slightly more material retained in the mixing tip than with automixing, but less than that wasted when mixed by hand. Polyether and addition silicone impression materials are available for mixing with this system. One variation in mixing is with the addition silicone two-putty systems mixed by hand. Scoops are supplied by the manufacturer for dispensing, and the putties are most often kneaded with fingers untilfree from streaks. The putty materials that have a liquid catalyst are initially mixed with a spatula until the catalyst is reasonably incorporated, and mixing is completed by hand. It should be noted that latex gloves may interfere with setting of addition silicone impression materials, as discussed later.

Impression Techniques Three common methods for making impressions for fixed restorations are a simultaneous, dual-viscosity technique, a single-viscosity or monophase technique, and a putty-wash technique. In nearly all cases, impression material is injected directly on and into the prepared teeth and a tray containing the bulk of the impression material is placed thereafter. After the impression is set, the tray is removed. The simultaneous, dual-viscosity technique is one in which low-consistency material is injected with a syringe into critical areas and the high-consistency material is mixed and placed in an impression tray. After injecting the low-viscosity material, the tray containing the higher-viscosity material is placed in the mouth. In this manner, the more viscous tray impression material forces the lower-viscosity material to flow into fine aspects of the areas of interest. Because they are both mixed at nearly the same time, the materials join, bond, and set together. After the materials have set, the tray and the impression are removed. An example of an impression using this procedure is shown in Figure 12-10. In the single-viscosity or monophase

technique, impressions are often taken with a medium-viscosity impression material. Addition silicone and polyether impression materials are well suited for this technique because both have a capacity for shear thinning. As described in Chapter 4, pseudoplastic materials demonstratea decreased viscosity when subjected to high shear rates such as occurs during mixing and syringing. When the medium viscosity material is forced through an impression syringe, the viscosity is reduced, whereas the viscosity of the same material residing in the tray is unaffected. In this manner, such materials can be used for syringing and for trays, as previously describedfor the simultaneous, dual-viscosity technique. The mechanism for shear thinning is discussed in the later section on the viscosity of impression materials.The putty-wash technique is a two-step impression procedure whereby a preliminary impression is taken in high- or putty-consistency material before the cavity preparation is made. Space is provided for a lowconsistency material by a variety of techniques, and after cavity preparation, a low-consistency material is syringed into the area and the preliminary impression reinserted. The low- and high-consistency materials bond, and after the low-consistency material sets, the impression is removed. This procedure is sometimes called a wash technique. The putty-consistency material and this technique were developed for condensation silicones to minimize the effects of dimensional change during polymerization. Most of the shrinkage during polymerization takes place in the putty material when the preliminary impression is made, confining final shrinkage to the thin wash portion of the impression. Care must be taken sothe wash material can freely escape via vents in the putty material when the wash impression is made. If not, the wash material can compress the putty in the second-stage impression, inducing permanent distortion and inaccuracies to the impression. The putty-wash technique was extended to addition silicones after their introduction, even though their polymerization shrinkage is significantly lower. Manufacturers add coloring agents to the accelerator or base as an aid in determining the thoroughness of the mix. Normally a different color is used for each consistency of a particular product line so one can distinguish the wash (low) consistency from the tray consistency in the set impression. Retarders may be added as well to control working and setting time of the products.

Composition and Reactions

The next two sections describe the general composition and setting reactions of addition siliconeand polyether impression materials. The following section describes their physical properties, permitting a more direct comparison of the various types and their properties.Addition SiliconeAddition silicone is available in extra low, low, medium, heavy, and very heavy (putty) consistencies. A representative product line of addition silicones is shown in Figure 12-11. The base paste of this class of impression materials contains a moderately low-molecular-weight polymer (polymethylhydrosiloxane) with more than 3 and up to 10 pendant or terminal hydrosilane groups per molecule (see formulas below and addition silicone formula 1 [AS1]).The base also contains filler. The accelerator (catalyst) and the base paste contain a dimethylsiloxane polymer with vinyl terminal groups, plus filler. The accelerator also contains aplatinum catalyst of the so-called Karstedt type, which is a complex compound consisting of platinum and 1,3-divinyltetramethyldisiloxane. Unlike the condensation type, the addition reaction does not normally produce a low-molecular-weight byproduct, as indicated in the reaction shown below (AS2). A secondary reaction can occur, however, with the production of hydrogen gas if —OH groups are present. The most important source of —OH groups is water (H—OH), the reaction of which under consumption of Si—H-units is illustrated above (AS3). Another possible source of hydrogen gas is a side reaction of the Si—H units of the polymethylhydrosiloxane with each other, under the influence of the platinum catalyst, also shown above (AS3). Not all addition silicone impression materials release hydrogen gas, and because it is not known which do, it is recommended that one wait at least 30 minutes for the setting reaction to be completedbefore the gypsum models and dies are poured. Epoxy dies should not be poured until the impression has stood overnight. The difference in the delay with gypsum and epoxy is that gypsum products have much shorter setting times than epoxy die materials. Some products contain a hydrogen absorber such as palladium, and gypsum and epoxy die materials canbe poured against them as soon as practical. Examples of high-strength stone poured after 15 minutes against addition silicone, with and without a hydrogen absorber, are shown in Figure 12-12. Latex gloves have been shown to adversely affect the setting of addition silicone impressions. Sulfurcompounds that are used in the vulcanizationof latex rubber gloves can migrate to the surface ofstored gloves. These compounds can be transferredonto the prepared teeth and adjacent soft tissuesduring tooth preparation and when placing tissueretraction cord. They can also be incorporateddirectly into the impression material when mixingtwo putties by hand. These compounds can poison the platinum-containing catalyst, which results inretarded or no polymerization in the contaminatedarea of the impression. Thorough washing of the

gloves with detergent and water just before mixingsometimes minimizes this effect, and some brandsof gloves interfere with the setting more than others.Vinyl and nitrile gloves do not have such aneffect. Residual monomer in acrylic provisional restorationsand resin composite cores has a similarinhibiting effect on the set of addition silicone materials.The preparation and adjacent soft tissues canalso be cleaned with 2% chlorhexidine to removecontaminants.PolyetherPolyethers are supplied in low-, medium-, andheavy-body consistency. The base paste consists ofa long-chain polyether copolymer with alternatingoxygen atoms and methylene groups (O—[CH2]n)and reactive terminal groups (see polyether 1 [PE1]).Also incorporated are a silica filler, compatible plasticizersof a nonphthalate type, and triglycerides. In thecatalyst paste, the former 2,5-dichlorobenzene sulfonatewas replaced by an aliphatic cationic starter asa cross-linking agent. The catalyst also includes silicafiller and plasticizers. Coloring agents are added tobase and catalyst to aid in the recognition of differentmaterial types. Examples of polyether impressionmaterials are shown in Figure 12-9.The reaction mechanism is shown (PE2) in a simplifiedform. The elastomer is formed by cationicpolymerization by opening of the reactive terminalrings. The backbone of the polymer is believed to be a copolymer of ethylene oxide and tetramethyleneoxide units. The reactive terminal rings open underthe influence of the cationic initiator of the catalystpaste and can then, as a cation itself, attack and openadditional rings. Whenever a ring is opened, the cation function remains attached, thus lengtheningthe chain (see PE3). Because of the identical chemicalbase, all polyether consistencies can be freely combinedwith each other. A chemical bond between allmaterials develops during curing.

Setting PropertiesTypical values of the setting properties of elastomericimpression materials are presented inTable 12-3. The temperature rise in typical mixes ofimpression materials was pointed out in the previoussection, but Table 12-3 illustrates that the temperaturerise is small and of no clinical concern.ViscosityThe viscosity of materials 45 seconds after mixingis listed in Table 12-3. As expected, the viscosityincreases for the same type of material from lowto high consistencies. Viscosity is a function of timeafter the start of mixing.A shearing force can affect the viscosity of polyetherand addition silicone impression materials, aswas mentioned in the section on impression techniques.This effect is called shear thinning or pseudoplasticity.For impression materials possessing thischaracteristic, the viscosity of the unset materialdiminishes with an increasing outside force or shearingspeed. When the influence is discontinued, the

viscosity immediately increases. This property isvery important for the use of monophase impressionmaterials, and is illustrated for polyether in Figure12-13. In the case of polyether, shear-thinning propertiesare influenced by a weak network of triglyceridecrystals. The crystals align when the impressionmaterial is sheared, as occurs when mixed or flowingthrough a syringe tip. The microcrystalline triglyceridenetwork ensures that the polyether remainsviscous in the tray or on the tooth but flows underpressure. This allows a single or monophase materialto be used as a low- and medium-consistencymaterial. Cooling of the pastes results in substantialviscosity increase. Before using, pastes have to bebrought to room temperature.The effect of shear rate (rotational speed of theviscometer) on the viscosity of single-consistency(monophase) addition silicones is shown in Figure12-14. Although all products showed a decrease inviscosity with increasing shear rate, the effect wasmuch more pronounced for two products, Ba and Hy,with about an eightfold to eleven-fold decrease fromthe lowest to the highest shear rate. The substantialdecrease in viscosity at high shear stress, which iscomparable with the decrease during syringing,permits the use of a single mix of material, with aportion to be used as syringe material and anotherportion to be used as tray material in the syringe-traytechnique.

Working and Setting TimesThe working and setting times of addition siliconeand polyether impression materials are listed inTable 12-3. In general, for a given class of elastomericimpression materials by a specific manufacturer, theworking and setting times decrease as the viscosityincreases from low to high. Polyethers show aclearly defined working time with a sharp transitioninto the setting phase. This behavior is often calledsnap-set. This transition from plastic condition intoelastic properties is rather short compared with olderaddition silicones, which was shown in investigationsof rheological properties of setting materials(Figure 12-15).Note that the working and setting times of theelastomeric impression materials are shortenedby increases in temperature and humidity; on hot,humid days this effect should be considered in theclinical application of these materials.The initial (or working) and final setting timescan be determined fairly accurately by using a penetrometerwith a needle and weight selected to suitthese materials. The Vicat penetrometer, as shownin Figure 12-16, with a 3-mm diameter needle and a

total weight of 300 g, has been used by a number ofinvestigators. A metal ring, 8 mm high and 16 mmin diameter, is filled with freshly mixed materialand placed on the penetrometer base. The needle isapplied to the surface of the impression material for10 seconds, and a reading is taken. This is repeatedevery 30 seconds. The initial set is that time at whichthe needle no longer completely penetrates thespecimen to the bottom of the ring. The final set isthe time of the first of three identical nonmaximum penetration readings. When the material has set, theelasticity still allows penetration of the needle, but itis the same at each application.Dimensional Change on SettingThe impression material undergoes a dimensionalchange on setting. The major factor for contractionduring setting is cross-linking and rearrangement ofbonds within and between polymer chains. Impressionscan expand if water sorption takes place and animpression can be distorted if seated after the materialhas set to any degree. Finally, distortion or creepwill occur if the material does not recover elasticallywhen the set impression is removed from undercuts. Imbibition is discussed in the section on disinfectingimpressions, and creep-induced distortion is discussedunder elastic recovery.Addition silicone and polyether impressionmaterials undergo shrinkage due to polymerization.The linear dimensional change between a dieand the impression after 24 hours is listed in Table12-3. The addition silicones have the smallest change,about −0.15%, followed by the polyethers at about−0.2%. The contraction is low for these two productsbecause there is no loss of byproducts.The rate of shrinkage of elastomeric impressionmaterials is not uniform during the 24 hours afterremoval from the mouth. In general, about half theshrinkage observed at 24 hours occurs during the firsthour after removal; for greatest accuracy, therefore,the models and dies should be prepared promptly,

although in air the elastomeric impression materials are much more stable than hydrocolloid products.

Mechanical PropertiesTypical mechanical properties of elastomericimpression materials are listed in Table 12-4. Thepermanent deformation (in the current specification,elastic recovery, which is 100% minus the permanentdeformation), strain in compression, and dimensionalchange are properties used in ANSI/ADAspecification No. 19 (ISO 4823) to classify elastomericimpression materials as low, medium, high, or veryhigh viscosity types. The requirements for theseproperties are given in Table 12-5. Further requirementsof the specification for elastomeric impressionmaterials are indicated in Table 12-6. The consistencydiameter is used to classify viscosity by measuringthe diameter of the disk formed when 0.5 mL ofmixed material is subjected to a 5.6-N weight at 1.5minutes after mixing for 12 minutes. Because the settingtimes of elastomeric impression materials vary,the consistency diameter is affected not only by theviscosity but also by the setting time. The classificationof a material by the consistency diameter may bedifferent from that by a true viscosity measurement.Elastic RecoveryThe order in which the permanent deformationof the elastomeric impression materials is listed inTable 12-4 demonstrates that addition silicones havethe best elastic recovery during removal from themouth, followed by polyethers. A material with apermanent deformation of 1% has an elastic recoveryof 99%.Strain in CompressionThe strain in compression under a stress of 0.1 MPais a measure of the flexibility of the material. Table12-4 illustrates that, in general, the low-consistency materials of each type are more flexible than thehigh-consistency elastomeric impressions. For agiven consistency, polyethers are generally the stiffest,followed by addition silicones.

FlowFlow is measured on a cylindrical specimen 1 hourold, and the percent flow is determined 15 minutesafter a load of 1 N is applied. As seen in Table 12-4,silicones and polyethers have low values of flow.Typical elastomeric impression materials apparentlyhave no difficulty meeting the mechanicalproperty requirements of ANSI/ADA specificationNo. 19 (see Table 12-6). Although the flow, hardness,and the tear strengths of elastomeric impressionmaterials are not mentioned in the specification,these are important properties; they are also listed inTable 12-4.HardnessThe Shore A hardness increases from low to highconsistency. When two numbers are given, the firstrepresents the hardness 1.5 minutes after removalfrom the mouth, and the second number is the hardnessafter 2 hours. The low-, medium-, and highviscosityaddition silicones do not change hardnesssignificantly with time, whereas the hardness ofpolyethers does increase with time. In addition, thehardness and strain in compression affect the forcenecessary to remove the impression from the mouth.Low flexibility and high hardness can be compensatedfor clinically by producing more space for theimpression material between the tray and the teeth.This can be accomplished with additional block-outfor custom trays or by selecting a larger tray whenusing disposable trays.A new variation in polyether provides less resistanceto deformation during removal of the impressionfrom the mouth and the gypsum cast from theimpression. To achieve this, the filler content wasreduced from 14 to 6 parts per unit, thereby reducingthe Shore A hardness from 46 to 40 after 15 minutes,and from 61 to 50 after 24 hours. The ratio ofhigh-viscous softener to low-viscous softener waschanged to achieve a consistency similar to that ofthe conventional monophase polyether.

Tear StrengthTear strength is important because it indicatesthe ability of a material to withstand tearing inthin interproximal areas and margins of periodontallyinvolved teeth. The tear strengths listed inTable 12-4 are a measure of the force needed toinitiate and continue tearing a specimen of unitthickness. As the consistency of the impressiontype increases, tear strength undergoes a smallincrease, but most of the values are between 2.0 and3.9 kN/m. Values for very high consistency typesare not listed because this property is not importantfor these materials. Higher tear strengths forelastomeric impression materials are desirable, butcompared with the values for hydrocolloid impressionmaterials of 0.3 to 0.7 kN/m, they are a majorimprovement.

Creep ComplianceElastomeric impression materials are viscoelastic,and their mechanical properties are timedependent. For example, the higher the rate ofdeformation, the higher the tear strength; and thelonger the impressions are deformed, the higher thepermanent deformation. As a result, plots of creepcompliance versus time describe the properties ofthese materials better than stress-strain curves.Creep-compliance time curves for low-consistencypolysulfide, condensation silicone, addition silicone,and medium-consistency polyether are shownin Figure 12-17. The initial creep compliance illustratespolysulfide is the most flexible and polyetheris the least flexible. The flatness or parallelism ofthe curves with respect to the time axis indicateslow permanent deformation and excellent recoveryfrom deformation during the removal of an impressionmaterial; addition silicones and polyethershave the best elastic recovery.The recoverable viscoelastic quality of the materialsis indicated by differences between the initialcreep compliance and the creep compliance value

obtained by extrapolation of the linear portion ofthe curve to zero time. As a result, addition siliconeshave the lowest viscoelastic quality and require lesstime to recover viscoelastic deformation, followed bythe polyethers.Detail ReproductionThe requirements of elastomeric impressionmaterials are listed in Table 12-6. Except for thevery high-viscosity products, all should reproducea V-shaped groove and a 0.02-mm wide line in theelastomeric. The impression should be compatiblewith gypsum products so the 0.02-mm line is transferredto gypsum die materials. Low-, medium-, andhigh-viscosity elastomeric impression materials havelittle difficulty meeting this requirement.

Wettability and Hydrophilization of Elastomeric Impression MaterialsWettability may be assessed by measuring theadvancing contact angle of water on the surface ofthe set impression material or by using a tensiometerto measure forces as the material is immersed andremoved (Wilhelmy technique). The advancing contactangles for elastomeric impression materials arelisted in Table 12-7. Of all the impression materialsdiscussed in this chapter, only alginates can be consideredtruly hydrophilic. All of the elastomeric impressionmaterials possess advancing and receding contactangles greater than 45 degrees. There are, however,differences in wetting among and within types of elastomericimpression materials. Traditional additionsilicone is not as wettable as polyether. When mixes ofgypsum products are poured into hydrophobic additionsilicone, high contact angles are formed, makingthe preparation of bubble-free models difficult.Surfactants have been added to addition siliconesby manufacturers to reduce the contact angle,improve wettability, and simplify the pouring ofgypsum models. This class with improved wettingcharacteristics is most accurately called hydrophilizedaddition silicone. Most commonly, nonionic surfactants

have gained importance in this area. These moleculesconsist of an oligoether or polyether substructure asthe hydrophilic part and a silicone-compatible hydrophobicpart (Figure 12-18, A). The mode of action ofthese wetting agents is believed to be a diffusioncontrolledtransfer of surfactant molecules from thepolyvinylsiloxane into the aqueous phase, as shown,thereby altering the surface tension of the surroundingliquid. As a result, a reduction in surface tensionand therefore greater wettability of the polyvinylsiloxaneis observed (Figure 12-18, B). This mechanismdiffers from polyethers, which possess a high degreeof wettability because their molecular structure containspolar oxygen atoms, which have an affinity forwater. Because of this affinity, polyether materialsflow onto hydrated intraoral surfaces and are thereforecast with gypsum more easily than are additionsilicones. This affinity also allows polyether impressionsto adhere quite strongly to soft and hard tissues.By observing water droplets on impression surfaces,it has been shown that hydrophilized additionsilicones and polyethers are wetted the best, and condensationsilicones and conventional addition siliconesthe least. Wettability was directly correlated tothe ease of pouring high-strength stone models of anextremely critical die, as shown in Table 12-7. Using atensiometer to record forces of immersed impressionspecimens (Wilhelmy method), polyether was shownto wet significantly better than hydrophilized additionsilicones for both advancing (74° versus 108° C)and receding contact angles (50° versus 81° C).To evaluate the ability of impression materials toreproduce detail under wet and dry surface conditions,impressions were made of a standard wave patternused to calibrate surface analyzers. The surfaces ofimpressions were scanned for average roughness (Ra)after setting to determine their ability to reproduce thedetail of the standard, the value of which is shown witha double line in Figure 12-19. From a clinical standpoint,most impression materials produced acceptabledetail under wet and dry conditions. Polyethers produced

slightly better detail than did addition silicones,and were generally unaffected by the presence of moisture,whereas detail decreased some for addition siliconesunder wet conditions, even if hydrophilized.

Disinfection of Elastomeric ImpressionsAll impressions should be disinfected uponremoval from the mouth to prevent transmissionof organisms to gypsum casts and to laboratory personnel. Several studies confirm that addition siliconeand polyether impressions can be disinfectedby immersion in several different disinfectants forup to 18 hours without a loss of surface quality andaccuracy.

Relationship of Properties and Clinical ApplicationAccuracy, the ability to record detail, ease of handling,

and setting characteristics are of prime importancein dental impressions.Silicones generally have shorter working timesthan polysulfides but somewhat longer times thanpolyethers. Single-mix materials have some advantagein that, as a result of shear thinning, they havelow viscosities when mixed or syringed but higherviscosities when inserted in a tray. The time of placementof an elastomeric impression material is critical,because viscosity increases rapidly with time asa result of the polymerization reaction. If the materialis placed in the mouth after the consistency orviscosity has increased via polymerization, internalstresses induced in the impression are released afterthe impression is removed from the mouth, resultingin an inaccurate impression.Thorough mixing is essential; otherwise portionsof the mix could contain insufficient acceleratorto polymerize thoroughly or may not set at thesame rate as other portions of the impression. In thisevent, removal of the impression would cause lesselastic recovery and result in an inaccurate impression.Automixing and mechanical mixing systems produce mixes with fewer bubbles than hand mixing,save time in mixing, and result in a more bubblefreeimpression.Polymerization of elastomeric impression materialscontinues after the material has set, and themechanical properties improve with time. Removaltoo early may result in high permanent deformation;however, excessively long times in the mouth areunacceptable to the patient. The manufacturer usuallyrecommends a minimum time for leaving theimpression in the mouth, and this minimum is usedfor testing the materials according to ANSI/ADAspecification No. 19.Dimensional changes on setting can be compensatedfor by use of a double-impression or putty-washtechnique. When using a double-impression technique,a preliminary impression is taken in the highorputtylike-consistency material, providing some

space for the final impression in low-consistencymaterial. The preliminary impression is removed,the cavity prepared, and the final impression takenwith the low-consistency material, using the preliminaryimpression as a tray. In this way, the dimensionalchange in the high consistency or puttylikeconsistency is negligible, and although the percentdimensional change of the low-consistency materialis still large, the thickness is so small that the actualdimensional change is small. The double-impressiontechnique is suitable for use with a stock impressiontray, because the preliminary impression serves as acustom tray. With the monophase and simultaneousdual-viscosity technique, a slight improvement inaccuracy results when a custom-made tray is usedbecause it provides a uniform thickness of impressionmaterial. Several studies have shown, however,that relatively stiff stock plastic or metal trays yieldnearly the same accuracy.Clinical studies have shown that the viscosity ofthe impression material is the most important factorin producing impressions and dies with minimalbubbles and maximum detail. As a result, thesyringe-tray technique produced superior clinicalresults in the reproduction of fine internal detail ofproximal boxes or grooves.The accuracy of the impression may be affectedwhen the percentage of deformation and the timeinvolved in removing the impression are increased.In both instances, permanent deformation increases,the amount depending on the type of elastomericimpression material.Because elastomeric impressions recover fromdeformation for a period after their removal, someincrease in accuracy can be expected during thistime. However, polymerization shrinkage is alsooccurring, and the overall accuracy is determined bya combination of these two effects. Insignificant elasticrecovery occurs after 20 to 30 minutes; therefore,dies should be prepared promptly after that timefor greatest accuracy. Addition silicones that release

hydrogen are an exception to this guideline.Second pours of gypsum products into elastomericimpressions produce dies that are not quite asaccurate as the first, because the impression can bedeformed during the removal of the first die; however,they are usually sufficiently accurate to be usedas a working die.

Sumber:

Ronald L. Sakaguchi, John M. Powers. Craig’s RESTORATIVE DENTAL MATERIALS. THIRTEENTH EDITION. Mosby ELSEVIER. 2012