Effect of viscosity on the accuracy and stability of elastic impression material

Accuracy of different putty-wash impression techniques with various spacer thickness

One of the most important steps is accurate impression making for fabrication of fixed partial denture.

The two different putty-wash techniques that are commonly used are: (1) Putty-wash one-step technique, (2) putty-wash two-step technique.

A uniform wash space is needed for an accurate impression. Nissan et al recommended the use of two-step technique for accurate impression making as there is uniform wash space for the light body material to polymerize.

The aim of the present study was to compare the accuracy of stone casts obtained from different putty-wash impression techniques using various spacer thickness.

The critical factor that influences the accuracy of putty-wash impression techniques is the controlled wash bulk which is absent in one-step putty-wash impression technique and with polyethylene spacer was used.

How to cite this article: Chugh A, Arora A, Singh VP. Accuracy of Different Putty-Wash Impression Techniques with Various Spacer Thickness. Int J Clin Pediatr Dent 2012;5(1):33-38.

Measurement of the viscosity of irreversible hydrocolloids

Three techniques have been used to assess the shear rate dependence of the viscosity of mixed alginate impression materials prior to setting. Two methods used a cone-plate viscometer, and one method was a capillary extrusion technique. Data from the three sets of experiments have been critically compared. It has been shown that an unset alginate material exhibits Bingham behaviour.

In situ replication techniques: II. Quantitative methodologies for replicate materials

Because replicate materials have requirements different from those of recording or impression materials, quantitative methodologies were sought using commercial impression materials. Two satisfactory objective techniques resulted, a laser-scattering and a capillary flow test. Using high-resolution gratings to stimulate tooth detail (less than 1 micron), the reproduction quality of 36 two-stage replicas was determined in diffraction, reflection, and in an unblazed state. Using precision bore glass tubes (0.25, 0.5, 1, and 2 mm diameters) to simulate the high-energy surface of enamel, the flow characteristics of nine elastomers (the first stage replicates) and four epoxies (the second stage replicates) were determined at isobaric conditions. Because the laser spot size was relatively large (0.6 mm) and the pressure differential was small (25 mm Hg), both the global resolution and the low shear rate characteristics could be measured. Of the commercial materials tested, Reprosil Light had the best combination of fluidity and resolution, regardless of which positive material was used. Although Permagum Low, Silene Wash, and Xantopren Blue scored high in one of the two tests, none of these materials could compare to Reprosil Light within the context described herein.

Time-dependent accuracy of elastomer impression materials. Part I: Condensation silicones

1. Although condensation silicone elastomers can be used to produce multiple dies from a single impression, one can expect changes in dimensions of dies produced from successive pours. 2. Putty/wash systems exhibit variable behavior among different brands. Some products produce a series of dies of increasing diameter over a 4-day period, while other brands may produce a series of dies whose diameters decrease as they are successively poured. 3. Light-body wash silicone impressions made in adhesive-lined custom acrylic resin trays produce dies which increase in diameter with time. 4. Putty/wash system, in general, show a greater eccentricity of dies than is seen with custom-tray/adhesive/wash systems, but there were some significant exceptions among the brands tested. 5. There does not appear to be any difference in behavior between single-wash and double-mix techniques in the condensation-silicone systems. 6. For maximum accuracy, the condensation-silicone systems should be poured as soon as possible after making the impression.

A comparison of six impression syringes

Five manually operated, and one air-activated, syringes were evaluated and compared. When used with polyether or polysulfide, some disposable syringes were found to require in excess of 100 pounds of force to operate. The metal reusable syringe required only 13 and 21 pounds, respectively, with the same materials; the air-activated syringe required none. There was no significant difference among the syringes with regard to bubble entrapment during impression making. There was a significant difference found in this regard among the five experienced dentists. The air-activated syringe was most easily controlled, as evidenced by the significantly smaller number of tracing errors made with it.

The rheological characteristics of elastomeric impression materials

An indirect extrusion viscometer has been used to assess the shear rate dependence of viscosity of a range of pastes supplied for elastomeric impression materials. A suggested specification for these pastes has been proposed.

Apparent viscosity of materials used for making edentulous impressions

The viscosity of commercially available impression materials used for making edentulous impressions was determined as a function of time and rotational speed. The impression materials studied showed large differences in viscosity and could be ranked into three groups. The values for one group--Plastogum, Ackerman cement (1:1 powder-to-liquid ratio), and light-bodied Permlastic--ranged from 23,800 cp to 57,200 cp. The viscosity values for a second group--Unilastic, syringe Elasticon, Luralite, and Coe-Flo--ranged from 92,200 cp to 107,000 cp. For a third group--Omniflex, Jeltrate, and Ackerman cement (2:1 powder-to-liquid ratio)--the viscosity values ranged from 239,000 cp to 257,000 cp. Light-bodied Permlastic, Ackerman cement (1:1 powder-to-liquid ratio), Jeltrate, and syringe Elasticon functioned as Newtonian liquids immediately after mixing. All other materials were non-Newtonian in behavior. The viscosity of most impression materials increased dramatically with time; the only exceptions were the zinc oxide-eugenol impression materials.

An apparatus for measuring the rheological properties of dental materials

An indirect extrusion capillary viscometer has been developed. This has been tested for nonsetting Newtonian fluids and was found to give results close to, but slightly lower than the actual viscosity. The same apparatus has been successfully applied to a non-Newtonian fluid to determine the dependence of viscosity on shear rate. The technique described should meet the requirements for assessing the rheological characteristics important in the mixing and setting of dental materials. The developed viscometer must be coupled with a sensitive mechanical testing machine capable of an adequate range of crosshead speeds that can be changed rapidly. By obtaining force vs time curves at different shear rates for setting materials, viscosity can be calculated as a function of time. Also, the viscosity at any given time during the setting process can be calculated as a function of shear rate. This chould be of aid in the interpretation of changes in rheological properties during setting of dental materials.