Viscosity vs Hardness of Ear Impression Materials


Silicone ear  impression materials have two primary properties that hearing aid and hearing conservation professionals must take into account for each patient, depending on the texture and elasticity of the outer third, cartilaginous portion of the ear canal. These two oft-conflated impression material properties are the viscosity (“thickness”) of the impression material before it cures, and the durometer (“hardness”) after it cures …And even most supply houses & earmold labs conflate these two properties as well, hence this article to explain the difference, and why it is important.

There are actually several other properties of silicone impression materials, such as lubrication, shrinkage, and thixotropy; but these shouldn’t really concern the clinician, as the manufacturers all take these into account to produce good materials that easily release from the ear, and easily flow through the mixing tip when gun-type injectors are used.¹

Brookfield viscometers and various assorted spindles

Brookfield viscometers and various assorted spindles
Click to enlarge in a new window

• Viscosity is a measure of a fluid’s resistance to flow, describing the internal friction of it while moving. For example, imagine a styrofoam cup with a hole in the bottom. If we then pour honey into the cup we will find that the cup drains very slowly. That is because honey’s viscosity is large compared to other liquids’ viscosities. If we fill the same cup with water, the cup will drain much more quickly.

The SI physical unit of viscosity is the Poise, which is in pascal-seconds (Pa•s), which is equivalent to N•s/m² or kg/(m•s); and in the lab it is measured with a Brookfield viscometer, which measures the resistance of a spinning disc or drum — See images at right 


Durometer image medley. Top: Durometer Shore A and Shore D measurement probe dimensions. Middle: Various laboratory Durometer test stands. Bottom: Handheld Durometer measuring a motorcycle tire

Top to bottom: Durometer Shore A and Shore D measurement probe dimensions; various laboratory Durometer test stands; handheld Durometer measuring a tire
Click to enlarge

 • Durometer (or more accurately, Shore durometer) is a measures of the “hardness” of a given material; and is defined as a material’s resistance to  indentation. The “durometer” of the material is measured on various Shore scales: The two most common ones, using slightly different measurement systems, are the ASTM D2240 type A and type D scales. The A scale is for softer plastics, including the “silicone” (vinyl polysiloxane) used for ear impressions and earmolds, while the C and (mostly) D scales are used for harder, more rigid plastics.


 • Impression material types There are two basic types of materials used to make ear impressions: The obsolete methacrylate powder-and-liquid, and RTV Silicone (room temperature vulcanizing vinyl polysiloxane). Silicone comes in two basic varieties: Two-part alkoxy condensation cure, and and two-part additive cure, with most of the material used today being of the additive cure variety. The way to tell the difference is that condensation cure materials use a small amount of hardener from a tube, while additive cure materials use an equal “A+B” mix of materials. All mixing gun materials use additive cure silicone.


• Shrinkage and deformation: All impression materials shrink after curing to some degree; however while silicone shrinkage is for all intents and purposes nil, the same cannot be said for methacrylate, as it shrinks 5-10%, making it difficult for the earmold lab to guess how much wax to add to get a comfortable fit. In addition, powder-and-liquid impressions must be trimmed, sprayed with clear coating, and glued to the bottom of the impression box before shipping to the lab, as they are subject to deformation if they toss about in the box. Worse, in warm weather, the canal portion can droop, causing the impressions to be unusable.

The Hearing Blog recommends against using methacrylate “powder-and-liquid” impression materials for any patient ear impression: If your audiologist pulls out the pill bottle of powder and the vial of liquid, get up and walk out: If s/he is cutting this corner, how many other corners are also being cut?


 NOTE: This article is short a couple of photos; however we needed to publish this for the AudiologyOnline class on IIC’s on 6 August 2013


1) Thixotropy is a third property of silicone impression materials that manufacturers take into account in their formulations; and this goes to why not all silicone materials lend themselves to injector gun mixing tips.  Thixotropic liquids become less viscous over time when shaken, agitated, or otherwise subject to shear stresses. This is an important property for materials that are injected through a mixing tip, as without this temporary “loosening up” it would be very difficult to pull the trigger on the gun.

Another example of a thixotropic liquid is nail lacquer: While in the bottle for weeks on end, the tendency is for the pigments — Especially the white titanium dioxide used for “cream” shades — to settle out, looking ugly in the bottle. However, if the polish were that thick while applying it to the nail, it would result in poor flow, leading to an uneven surface with brush marks. This is solved by compounding the nitrocellulose base to have thixotropic properties, where once you get the stainless steel ball inside the bottle moving, it literally “loosens up” the liquid, allowing for a smooth flow while applying. Good nail lacquer lines generally use two viscosities: A thinner base for transparent to translucent colors, and a thicker base for opaque, creme and metallic shades, preferably using two agitator balls.

2) The Rockwell hardness test scale is similar to the Durometer scale, except it is used on metals. It is defined in the ASTM E18 specifications.

3) References:



← Guest Article: Notched Sound Therapy as a Treatment for Tinnitus: A Guide for Hearing Professionals Made for iPhone hearing aids & CI's: Android catches up with Bluetooth 4.0 Low Energy support →

About the author

Dan Schwartz

Electrical Engineer, via Georgia Tech


  1. annc
    August 6, 2013 at 8:03 pm

    Interesting article on ear impression materials and their properties.

    What you don’t make clear is, is this thek ear impression made in the ear from which an earmold is made for a hearing aid user or is it the earmold itself?



    • Dan Schwartz
      August 8, 2013 at 3:14 pm

      Ann, it is the impression material squirted in your ear. Once it has cured in 3-5 minutes, it is sent to the lab where it has the otoblock removed, trimmed, air bubbles and otoblock thread lines filled in, and then dipped in wax. Then, it is placed, canal pointing up, on a spike, a form about 2 inches in diameter is placed around it, and hot hydrocolloid poured to create the “investment.” Once the investment has cooled, the form and impression are removed.

      Once the investment has been made, the earmold or shell is poured using silicone, dental acrylic, or (Not Recommended) plasticized polyvinyl chloride (PVC); and then allowed to cure, where it is then removed and finished; and the hydrocolloid cut up & tossed back into the hot pot.

      Alternately, very low viscosity liquid RTV silicone can be used to pour the investment, but it’s expensive because it’s single use, and also it must be vacuum degassed before pouring to remove the entrained air bubbles for a smooth finish~

  2. Roy F. Sullivan, Ph.D.
    March 12, 2014 at 10:36 am


    I may have left an “l” off the VO website url. My L key sticks. Also congratulations of 10^5 hits for your blog.

    It took me a long time to reach that mark with my VO website – it now approaches 5*10^5 hits. I created it using Wordpad as my HTML 4 editor at a time where there were virtually no websites with “audiology” in the URL. I wrote that code concurrently with writing the prototype AAA website code.



Leave A Reply

Time limit is exhausted. Please reload CAPTCHA.

%d bloggers like this: