Сraig. Dental Materials

to increase the internal stresses in the impression if cooling is too rapid. The exact time required for proper cooling varies with the size of the impression and the particular compound selected.

Effect of Wet Kneading When the compound is softened in a water bath, kneading the material with the fingers is customary to improve the handling qualities. This wet kneading increases the flow of both the softened compound and the hardened impression; this increased flow is believed to be caused by the incorporation of water, which acts as a plasticizer, in the compound.

Wet kneading can modify the flow of the hardened compound to an extent that it exceeds the 6% permissible in the specification (Table 12-14). By kneading for 1 to 3 minutes, the flow of compounds may be more than doubled.

Clinically, excessive wet kneading can increase the flow qualities of the hardened material at mouth temperature to a point at which distortion may occur on removal. Once incorporated, the water remains in the compound for long periods, and subsequent reheating with further kneading has a cumulative effect of increasing flow values.

Accuracy and Dimensional Stability

The optimum accuracy and dimensional stability of compound impressions can be ensured by

*SpecificationNo. 3 is now obsolete.

careful preparation and handling of the material and paying attention to the details of the clinical technique in use. Softening the compound by a method that does not adversely affect its physical properties by overheating or prolonged heating is important. Equally important is that adequate flow is developed during softening to allow close adaptation to the tissues and a minimum of internal stresses in the impression. The tray, copper band, or other container used to convey the compound to the mouth must be strong, rigid (inflexible), and stable. In the mouth, adequate cooling of the compound is essential to avoid distortion when the impression is removed. When the impression is obtained, the cast or die should be made as soon as possible to avoid inaccuracies caused by the release of stresses that produce warpage, which may occur on standing. One source of inaccuracy not within the control of the operator is the thermal contraction that the compound exhibits on cooling from mouth to room temperature.

Thermal Contraction The linear contraction of an impression compound on cooling from mouth to room temperature is approximately 0.3%. This quality is inherent in the material and can result in inaccuracy unless it is recognized and adequate compensation is provided.

Tray Compounds The special compounds for making impression trays are similar in composition and working qualities to impression compounds, except that the temperature at which they soften is higher and the property of flow at mouth temperature is minimal. The ANSVADA Specification No. 3 requirements for tray compound are listed in Table 12-14. Tray compounds are used primarily to make individual trays for corrective wash impressions. The trays are made when softened tray compound is adapted to a study model and the border of the denture area is trimmed. Compound trays lack strength and dimensional stability; and have been replaced to a large extent by trays made in a similar manner from acrylic resins cured at room temperature.

3 for Den- specification No. 3 has established certain limits for the

desirable physical properties of impression compound and tray compound. Impression compounds are required to be homogeneous and to show a smooth, glossy appearance after the surface has been passed through a flame. When trimmed with a sharp knife at room temperature, the cut margins must be firm and smooth. The manufacturer is required to indicate in the package the method of softening, the working temperature, and a curve or data showing the shrinkage of the compound from 40" to 20" C. Two physical tests are required by the specification. One is to test the percent flow at 37" C and at 45" C, and the other is to check the reproduction of the details of a test impression block. The acceptable values of percent flow for impression and tray compounds are shown in Table 12-14.

DIE, CAST, AND MODEL

Dental stones, plaster, electroformed silver and copper, epoxy resin, and casting investment are some of the materials used to make casts or dies from dental impressions. The selection of one of these is determined by the particular impression material in use and by the purpose for which the die or cast is to be used.

Impressions in agar or alginate hydrocolloid can be used only with a gypsum material, such as plaster, stone, or casting investment. Compound impressions, on the other hand, can be used to produce dies of plaster, stone, or electroformed copper. Various rubber impression materials can be used to prepare gypsum, electroformed, or epoxy dies.

DESIRABLE QUALITIES OF A CAST

OR DIE MATERIAL

Cast and die materials must reproduce an impression accurately and remain dimensionally stable under normal conditions of use and storage. Setting expansion, contraction, and dimensional variations in response to changes in temperature

must be held to a minimum. Not only should the cast be accurate, but it should also satisfactorily reproduce fine detail and have a smooth, hard surface. Such an accurate cast or die must also be strong and durable and withstand the subsequent manipulative procedures without fracture or abrasion of the surface. Qualities of strength, resistance to shearing forces or edge strength, and abrasion resistance are therefore important and are required in varying degrees, according to the purpose for which the cast or die is to be used. For example, because it will not be subjected to much stress in use, a satisfactory study cast might be formed from dental model plaster in which the aforementioned qualities are at a minimum. However, an elastomeric impression used to produce an indirect inlay could be copperor silver-formed or poured in highstrength stone, thereby producing a die in which these qualities are sufficient to withstand the carving and finishing procedures that are a part of this technique.

The color of a cast or die can facilitate manipulative procedures, such as waxing inlay patterns, by presenting a contrast in color to the inlay wax. The ease with which the material can be adapted to the impression and the time required before the cast or die is ready for use are of considerable practical significance. A contrast in this respect is seen between dental stone, which can be easily vibrated into an impression to produce a cast ready for use within an hour, and a copper die, which requires electroforming and usually won't be ready until the next day.

DENTAL PLASTER AND STONE

The chemistry and physical properties of dental plaster, stone, and high-strength stone are discussed in Chapter 13. These gypsum materials are used extensively to make casts and dies from dental impressions and can be used with any impression material. Stone casts, which are stronger and resist abrasion better than plaster casts, are used whenever a restoration or appliance is to be made on the cast. Plaster may be used for study casts, which are for record purposes only.

374 Chapter 12 IMPRESSION MATERIALS

Hardening solutions, usually about 30%- silica sols in water, are mixed with stone. The increase in hardness of stone dies poured against impressions varies from 2% for silicones to 110% for polyether, with intermediate increases of 70% for agar and 20% for polysulfide. The dimensional change on setting of stones mixed with hardener is slightly greater than when mixes are made with water, +0.07% versus +0.05%. In most instances the abrasion or scraping resistance of mixes of stone made with hardening solutions is higher than con~parablemixes made with water. A range of effects in the abrasion resistance of surface treatments of stone has been reported. Model and die sprays generally increase the resistance to scraping, whereas lubricants can decrease surface hardness and resistance to scraping.

High-strength dental stones make excellent casts or dies, readily reproduce the fine detail of a dental impression, and are ready for use after approximately 1 hour. The resulting cast is dimensionally stable over long periods and withstands most of the manipulative procedures involved in the production of appliances and restorations. However, procedures involving the bending, adapting, or finishing of metals can be accomplished only to a limited extent on highstrength stone dies and are better fulfilled on a metal die.

When wax patterns constructed on highstrength stone dies are to be removed, some separating agent or die lubricant is necessary to prevent the wax from adhering. Oils, liquid soap, detergents, and a number of commercial preparations can be used. Oils are generally to be avoided because some are wax solvents and soften the surface of the wax pattern. In addition, oil on the surface of the wax increases the difficulty of painting the investment on the pattern in subsequent steps of the casting process. The lubricant is applied liberally to the high-strength stone die and allowed to soak in; usually several applications can be made before any excess accumulates on the surface. The excess is blown off with an air blast before proceeding to make the wax pattern.

DIES FORMED BY THE ELECTRODEPOSITION OF METAL

Electroforming Impressions The essential equipment for the electroforming of impressions to form indirect dies for inlay, crown, or bridge restorations is a source of direct electric current and an electrolyte. The electric current may be supplied by storage batteries with a small variable resistance and an ammeter to indicate the energy in the system. Often the alternating current of 110V is converted to direct low-voltage current suitable for plating. In this case, a transformer and rectifier with some fixed resistance are used, with the same variable resistance and ammeter described for storage-battery equipment. A small container for the electrolyte, with inexpensive wire electrodes and a bar of pure copper or silver for the anode, represent the necessary equipment.

A common electrolyte used for plating copper indirect dies is an acidic copper sulfate solution. Silver electrolytes contain silver cyanide in an alkaline solution. Because of the highly poisonous nature of cyanides and shipping restraints on electrolytes, copper plating is more commonly used.

When solution of the copper sulfate occurs in the water, the salt dissociates to give cupric and sulfate ions. During electrolysis, the positive ions are drawn to the negative electrode or cathode by electrostatic attraction. The negative sulfate (SO=,) ions have gained electrons and move toward the positive electrode or anode. Neutral or nondissociated molecules do not move under the influence of an electric field. Hence, if a nondissociated substance such as glucose is present in the electrolyte, it will inhibit only the migration of the copper or sulfate ions, because the molecules are large but are not influenced by the electric current.

The anode is made of pure copper, and during electrolysis copper atoms give up two electrons (2e) and become CuCCions. The metallic copper of the anode therefore regenerates the solution as the plating process occurs with the removal of copper at the cathode. The Cu++ion is attracted to the cathode (the impression), where it gains

2e and is deposited as metallic copper according to the equation Cut+ +2e +cue. As long as there

is free copper at the anode, the solution will maintain a constant composition.

The action at the electrodes is summarized by the following equations:

Anode cuO- 2e -+ Cu*

Cathode Cuu + 2e + c u O

Similar reactions occur in the electroforming of silver.

Anode Ago - e -+ Agi

Cathode Agi + e + Ago

Metal dies can be made by copper-plating compound or silicone (but usually not polysulfide impressions, where silver-plating is preferred). Such a die is tough and has good strength characteristics, and metal inlays and restorations can be finished and polished satisfactorily on these dies.

A copper-forming apparatus suitable for dental use consists of a transformer and rectifier to reduce the voltage of the domestic supply and convert the alternating current (AC) to direct current (DC), which is needed for electroforming the impression. The low-voltage DC passes through a variable resistor, which is used to regulate the current and modify the rate at which metal is deposited, and a milliammeter, which indicates the current passing through the plating bath. At the anode is attached a copper plate, and at the cathode is the impression to be plated, both of which are immersed in the electrolyte.

The anode is made of electrolytically pure copper and is immersed in the plating solution to the extent that the area of copper immersed is approximately equal to that of the impression to be plated. Copper anodes containing a trace of phosphon~sare superior to pure copper.

The plating bath contains an acid solution of copper sulfate, and a number of formulas have been advocated. One acceptable example is given in Table 12-15.

The copper sulfate is the source of the copper, the sulfuric acid increases the conductivity of the

solution, and the phenolsulfonic acid assists the penetration of copper ions to the deeper parts of an impression and improves the "throwing power" of the solution. Additives other than phenolsulfonic acid (e.g., dextrose, alcohol, phenol, and molasses) are suggested in some formulas.

The surface of the impression is coated with a conductor of electricity before it is attached to the cathode lead wire.

When the impression is in compound, a colloidal dispersion of graphite is painted on the surface to be plated and allowed to dry before the impression is placed into the plating bath. When the impression is a silicone rubber, finely divided copper powder is brushed on the surface to be plated before placing the impression into the bath.

About 15 rnA is a suitable current to start plating a single-tooth impression. Once a thin layer of copper has covered the entire surface of the impression, the current can be increased to as much as two or three times the initial current. If too high a current setting is used, the copper deposit will be granular and friable in nature, and the die will be unsatisfactory. High current densities also produce a heavier deposit on the areas of the impression nearest the anode, and sometimes rapid plating fails to adequately cover the deeper areas of an impression. Plating is allowed to proceed for 12 to 15hours; overnight is usually a convenient period.

The quality of the deposit obtained with a freshly made plating solution is often not as good

as that achieved when the solution has been in use a short time. Loss of water from evaporation should be replaced to maintain the correct concentration of the electrolyte. The sulf~~ricacid is slowly decomposed when the solution is in use, and the addition of a few milliliters of acid is required after a few weeks of use to maintain the quality of copper deposit. A sediment or sludge consisting mainly of fine particles of copper may accumulate on the floor of the bath; the solution should then be filtered. When anodes containing a trace of phosphorus are used, the formation of sediment is considerably reduced.

In copper forming, the distance between the anode and the impression to be plated is an important factor in relation to plating the deeper areas of an impression; the greater the distance, the more even is the quantity of copper deposited and the more readily are the deep areas plated. About 15 cm is a suitable distance in practice; if the distance is shorter, there is a tendency for an excess of copper to be deposited on the more superficial areas of the impression, leaving the deeper areas inadequately plated.

The hydrophobic (but not the hydrophilic) addition-silicone impression materials can be satisfactorily copper-formed. The technique followed is as described in the previous paragraph; however, the impression surface is made conductive by the application of finely divided silver powder. The increased cost of silver and the increasing difficulties in obtaining cyanide-silver plating solutions because of transport restrictions have resulted in the return to copper electroformed dies.

Silver-Formed Dies With the advent of the polysulfide impression materials, silver forming was used to make metal dies. Although it is possible to copper-form polysulfide impression materials, consistent results are not always obtained, and the much easier procedure of silver forming lends itself to routine use. The alkaline silver baths used in silver forming soften the surface of impression compound, and silver forming cannot be used. Silicone and polyether impressions can also be silver-formed. A pure silver anode is required, with a silver-cyanide

plating solution, an example of which is given in Table 12-16.

This solution is poisonous, and extreme care should be taken that the hands, workbench area, and clothing not become contaminated. This operation should not be used by inexperienced personnel. The addition of acid to the solution produces hydrogen cyanide, an extremely poisonous gas, and for this reason copper-forming solutions should be kept well away, as should any acid. The plating bath should have a cover that can be in place at all times to control evaporation and dissipation of fumes.

The impression is made conductive when the surface is brushed with powdered silver, which adheres well to rubber impressions. Alternatively, dispersions of silver powder in a volatile liquid vehicle are available, and they are painted on and allowed to dry.

A current setting of approximately 5 mA is suitable to start the plating of a single-tooth impression. Once a layer of silver has been deposited over the surface, this current can be doubled or tripled. With larger impressions involving several teeth and the adjacent soft tissue areas, a current of approximately 10 mA/tooth is suitable for initial plating. This can be doubled or tripled once the surface is covered, and usually about 12 to 15hours of plating produce a suitable thickness of silver.

Distilled water should be added to the electrolyte to replace any loss from evaporation, and the solution should be filtered from time to time. A polysulfide impression that has been silverformed is shown in Fig. 12-30, A. The model made from this silver-formed impression by pouring stone and removing the impression after the stone has set is shown in Fig. 12-30, B.

Problems in Metal-Forming Among the problems encountered in metal forming are the obvious effects caused by current failure, either in the domestic power line or from some defect in the plating apparatus. However, even if the working apparatus is satisfactory, there are problems related to the procedure itself that are sometimes puzzling.

Faulty Conduction The ammeter may show a current flow, but the impression does not plate or plates irregularly or very slowly. This difficulty is caused by a short circuit through the electrolyte, usually because of exposure of the conducting wire to the solution.

Exhausted Solution Plating is very slow, and the deposit is discolored. The solution should be discarded and replaced with a fresh solution. How long a solution lasts depends on the degree of usage and whether contamination occurs. A well-filtered solution protected from contamination and kept to correct concentration by adding distilled water when required gives the longest life. Cyanide solution? must be disposed of carefully.

OverconcentratedSolution Sometimes the ammeter reading drops rapidly to zero after the impression is placed in the bath. Resetting of the current regulator to establish the correct ammeter setting is followed by another drop of the ammeter reading to zero. This effect is caused by

an overconcentrated solution. Adding the required amount of distilled water and rinsing the metal anode in distilled water solves the problem. An overconcentrated solution may also soften the surface of the rubber and discolor any stone areas of the cast.

Metal Anode Too Small An anode that is smaller in area than the impression or impressions to be plated leads to slow and irregular plating.

Friable Metal Deposit If the metal deposit is granular and friable, although of correct color, the current setting is too high.

EPOXY DIE MATERIALS

Until recently, epoxy materials were supplied in the form of a paste to which a liquid activator (amine) was added to initiate hardening. Because the activators are toxic, they should not come into contact with the skin during mixing and manipulation of the unset material. Shrinkage of 0.1% has occurred during hardening, which may take up to 24 hours. The hardened resin is more resistant to abrasion and stronger than a high-strength stone die. The viscous paste is not as readily introduced into the details of a large impression as high-strength dental stone is; a centrifugal casting machine has been developed to assist in the pouring of epoxy resins. Recently, fast-setting epoxy materials have been

supplied in automixing systems similar to those described for automixing addition silicones and shown in Fig. 12-31. The epoxy resin is in one cartridge, and the catalyst in the other. Forcing the two pastes through the static mixing tip thoroughly mixes the epoxy material, which can be directly injected into a rubber impression. A small intraoral delivery tip may be attached to the static-mixing tip if desired for injecting into detailed areas of the impression. The fast-setting epoxy hardens rapidly, so dies can be waxed 30 minutes after injecting into the impression. Because water retards the polymerization of resin, epoxy resins cannot be used with watercontaining agar and alginate impression materials, and thus are limited to use with rubber impression materials.

COMPARISON OF IMPRESSION

AND DIE MATERIALS

High-strength stone dies may be from 0.35% larger to 0.25% smaller than the master, depending on the location of the measurement and the impression material used. In general, occlusogingival (vertical) changes are greater than buccolingual or mesiodistal (horizontal) changes. The shrinkage of the impression material toward the surfaces of the tray in the horizontal direction usually results in dimensions larger than the master. In the vertical direction, shrinkage is away from the free surface of the impression and

toward the tray, and dimensions slnaller than the master are obtained.

Invariably, metal-formed dies show more vertical change than high-strength stone dies, with the differences being between 0.25% and 0.45%, depending on the impression materials, whereas the horizontal changes are not significantly different for the two die materials. The accuracy of rubber impression materials is in the following order from best to worst, regardless of whether stone or metal dies are used: addition silicone, polyether, polysulfide, and condensation silicone.

Epoxy dies all exhibit some polymerization shrinkage, with values ranging from 0.1% to 0.3%, and as a result the dies are undersized.

Ranking materials by the ability of an impression-die combination to reproduce surface detail produces different results than does ranking by values for dimensional change. If a release agent is not needed on the surface of the impression, epoxy dies are best for reproducing detail (10 pm), followed by metal-formed dies (30 pm), and high-strength stone dies (170 pm). However, polysulfide inlpressions require the use of a release agent with epoxy dies, and their reproduction of detail is comparable to that obtained with high-strength stone. The siliconeepoxy combination produces the sharpest detail, although not all epoxy die materials are compatible with all silicone impression materials.

Resistance to abrasion and scraping should

also be considered. Metal-formed dies have superior resistance to abrasion, epoxy dies have

good resistance, and high-strength stone dies have the least resistance.

I SELECTED PROBLEMS

Problem 1

A mix of alginate was made, and the setting time was shorter than previously experienced with this brand. The material was too stiff at the time of insertion to obtain adequate seating and surface detail. What factors should be considered in the correction of this difficulty?

Solution a

The most common cause for the shorter setting time is too high a temperature of the mix water. The setting reaction is a typical chemical reaction that is accelerated by increases in temperature; a 10" C increase in temperature almost doubles the rate of the reaction. In many operations, a single water faucet is present with a mixing valve to control the temperature of the water. Gradual failure of the mixing valve can inadvertently result in the use of water substantially hotter than the 21" C usually recommended.

Solution b

The setting time can also be shortened by incorrect dispensing resulting in too high a powder/liquid ratio. If the alginate powder is not aerated before each dispensing, the weight of each scoop will be too high, because the apparent density of the powder is higher after standing. The increased amount of powder causes the setting or gelation time to be less than normally experienced.

Solution c

Aging of the alginate powder in a warm, humid atmosphere can affect the setting by

reducing the effectiveness of the retarder, resulting in a shorter-than-normal setting time.

Problem 2

A mix of alginate was made, and it appeared thicker than normal. After the impression was taken, its surface seemed grainy and lacking in surface detail. What precautions should be taken to avoid this condition when the impression is retaken?

Solution

A thick consistency can result from lack of aeration of the powder before dispensing, or from an incorrect number of measures of powder or water. However, a thick, grainy mix can occur from inadequate mixing of the correct proportions of powder and water. Inadequate mixing may be caused by casual rather than vigorous spatulation or by not mixing for the full time recommended. Hot water used in combination with the two factors just mentioned accentuates the problem of thick, grainy mixes. If the next mix is carefully dispensed, the temperature of the water checked, and the length of mixing timed, a smooth, creamy consistency should result.

Problem 3

An alginate impression was taken of a patient with a fixed appliance, and tearing of the impression occurred in critical areas as a result of severely undercut areas. What can be done to improve the next impression, because alginate impressions are supposed to be elastic?

380 Chapter 12 IMPRESSION MATERIALS

Solution

It is true that alginate impressions are flexible, although they are not entirely elastic (they exhibit incomplete recovery) and have rather low resistance to tearing compared with elastomeric impression materials. Several precautions can be taken to improve the chances of an acceptable impression. The severely undercut areas can be blocked out, thus placing less stress on the alginate during removal. Recall that the strength of the alginate improves quite rapidly for 5 to 10 minutes after setting, and the impression can be left in the mouth a few extra minutes before removal. Also, remember that the tear strength is a function of the rate of removal of the impression, and rapid rates enhance the chances of an acceptable impression. Finally, mixes with high powder/liquid ratios have higher tear strengths; however, to obtain a smooth consistency that will record the desired level of surface detail, the ratio must not be increased beyond the limit.

Problem 4

An alginate impression is to be taken of a patient known to have a problem with gagging. What steps can be taken in materials management to reduce the problem and yet obtain a satisfactory impression?

Solution a

The viscosity of mixed alginates remains fairly constant until just before the setting time because the retardation reaction prevents the formation of the calcium alginate gel. When the working and setting times are known accurately, insertion of the impression material into the mouth can be delayed until just before the end of the working time, but allowing enough time to seat the impression. The impression should then be removed at the earliest possible time after it sets but while it still has adequate properties. Thus the impression is in the mouth the shortest possible time.

Solution b

A fast-setting alginate can be selected that not only sets in a shorter time but has better mechanical properties in a shorter time than regular-setting material.

Solution c

Care should be taken during the loading of the maxillary impression tray to avoid excessive amounts of alginate in the posterior portion of the tray. The posterior part of the tray should be seated before the anterior portion, with the patient's head in a position to avoid excess alginate flowing in a posterior direction on the palate.

Problem 5

The set alginate impression separated from the tray during removal from the mouth, resulting in distortion and tearing of the impression. What can be done to avoid this?

Solution

Two choices can be made. A change in the brand of alginate to one that adheres to the metal tray is one option. However, if a nonadhesive alginate is desired, select a tray with perforations that provides mechanical retention. Secondly, use commercial tray adhesives specifically formulated for retention of alginate impressions.

Problem 6

An inaccurate model is obtained despite the use of the correct mixing technique for the alginate and the proper pouring procedure for the gypsum model. What possible factors might have resulted in the inaccurate impression?

Solution a

The tray selected may have been too small, thus providing too little alginate between the tray and the tissues. During the removal of the set alginate, the percent compression may have been too high when the bulge of the

tooth compressed the alginate in the undercut areas. This excessive compression results in higher than normal permanent deformation. Correct the problem by selecting a tray that provides about 5 mm of space between the tray and tissues. With the same amount of undercut, the alginate is subjected to lower percent compression, and lower permanent deformation results.

Solution b

A second possible cause of the problem could be the premature removal of the impression. The permanent deformation of the impression on removal decreases with the time after setting. Therefore premature removal of the impression, even though the material is no longer tacky, can be the cause of excessive permanent deformation of the impression in undercut areas.

Solution c

A third possible cause of the inaccurate impression might be a too-slow rate of removal of the set impression material. With everything else being equal, a slow rate of removal rather than a snap removal results in higher than normal amounts of permanent deformation.

Solution d

A fourth reason for the difficulty might be overextension of the alginate to areas not supported by the tray. The weight of these overextended areas can deform the impression; if they are not cut away, deformation and permanent distortion of the impression in these areas can occur.

Solution e

Finally, the alginate impression can be permanently distorted if after removal it is stored in such a manner that the impression is deformed. Two possible conditions could cause this effect: placing the impression down with the weight of the tray on it, and wrapping the impression too tightly in a damp towel.

Problem 7

When an impression was taken using polysulfide impression material, a number of voids appeared on the surface of the impressions of the teeth and abutment preparations. What is the reason for such a failure, and how can it be avoided?

Solution

Some air bubbles are always incorporated into the pastes during the manufacturing process, and these bubbles can be observed when dispensing the pastes onto the paper pad. The mixing of the base and catalyst pastes should be done carefully to avoid incorporating air into the mixture. After the two pastes are initially mixed, the material must be spatulated and spread over the widest area possible and flattened to form a thin layer over the surface of the pad. The spatula must be kept in contact with the mixture so that air bubbles are eliminated. The material is gathered up with the straight edge of the blade and redeposited onto the paper pad, then mixing is continued with a wiping, pressing motion until a homogeneous mix is obtained.

Problem 8

On critical observation of an impression obtained with a polysulfide material, it was found that some areas were incompletely reproduced. What is the cause of these large voids, and how can they be avoided?

Solution

The setting time of polysulfide elastomeric impression materials is affected by humidity. Delay in injecting the low-consistency syringe material results in a higher-viscosity material, reducing the flow, and thus a large area is incompletely produced. Also, the impression does not effectively displace saliva from the surface of the tooth.

This type of failure may be avoided by isolating the field with cotton rolls, drying it carefully with air blasts, and then using a