How to Hot Mount

How to hot mount

Preliminaries to Mounting

  • Clean the Specimen
    The specimen should be cleaned before mounting in order to improve adhesion to the mounting medium. Use acetone or at least alcohol. Cleaning in an ultrasonic bath could be necessary. Remember to dry the specimen well. Clean specimens should be handled with either gloves or tweezers.
  • Adjust the specimen size to the cylinder size (~ 3.0-5.0 mm distance to cylinder wall).
  • Adjust the specimen height – the final height of the mount should be approx. 20 mm.
  • Select the most suitable hot-mounting resin.

Hot-mounting process

Mounting Process Clips

Hot mounting - process clips

Small, thin specimens can be supported during the hot mounting process by the use of metal fixation clips.

Mounting process cylinder

Hot mounting - process cylinder

The clean and dry specimen is placed in a mounting cylinder in a hot mounting press, and the appropriate mounting resin is added. A temperature of around 180° C and a force of about 250 bar is applied during the embedding of the specimen. Water cooling is used to obtain the shortest possible mounting time.

Two types of hot-mounting resin:

Thermosetting resins

Thermosetting resins

Thermosetting resins cure at elevated temperatures. To avoid porous, non-uniform mounts, it is important that the pressure is always constant and at the right level. Once the mount is cured, the only way to remove the specimen is to destroy the mount.

Thermoplastic resins

Thermoplastic resins

Thermoplastic resins soften or melt at elevated temperatures and harden during cooling. This resin type can be used to mount pressure-sensitive specimens. The mounting medium is first heated and then exposed to force when it is soft. This ensures that the mounting medium is pressed into open pores and cracks.

Thermoplastic mounting media can be melted again.

Preheating

Preheating

In the case of porous and/or pressure-sensitive specimens, such as minerals or electronic parts, it can be a good idea to soften the resin by heating before applying pressure. Preheating is also useful when applying thermoplastic resins.

Using Sensitive Mode, the total heating time is split into two – preheating and heating – where force is applied only during the heating phase and not during the preheating phase.

Temperature Sensitive Specimens

Temperature Sensitive Specimens

The temperature for the mounting process can be reduced to a minimum of 150° C for all resins. This is useful when dealing with temperature-sensitive materials. If the temperature is reduced, the recommended heating time should be increased accordingly.

Chamfered Ram:

Chamfered Ram:

A mounted specimen may have sharp edges that can cut up preparation surfaces. This is countered by using a lower ram with chamfered edges. The chamfered lower ram also eliminates the risk of hard samples damaging the mounting unit cylinder.

Selection of Resin / Selection Guide

The choice of resin/mounting media is very important in order to obtain the required result and fulfill the mounting requirements.

The purpose of this guide is to provide detailed information, hints and tricks about the various hot mounting materials. Go to Hot Mounting Selection Guide

Hot Mounting Sections table

Troubleshooting - hot mounting

Problem

Radial Cracking
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Cause: Insufficient distance between specimen edge/corner and cylinder wall, or the specimen has sharp corners.
Solution:
Increase the cylinder diameter or reduce the specimen size. The distance between the specimen and the cylinder wall must be a minimum of 3.0 mm to avoid cracks in the resin. This is especially critical for specimens with sharp corners.
Shrinkage
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Cause: Incorrect choice of resin.
Solution:
Re- mount a new specimen using a resin with a lower linear shrinkage value.
Blistering
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Cause: Insufficient heating time.
Solution:
Increase the heating time, or increase the process temperature.
Cause: Over-cured surface.
Solution:
Decrease the process temperature.
Cause: Entrapped gas within mount.
Solution:
Preheat the resin.
Bulging
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Cause: Insufficient cooling.
Solution:
Increase the cooling time. 
Porosity
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Cause: Excessive temperature.  Solution:
Reduce the process temperature. 
Voiding within Large Mounts
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Cause: Insufficient heating time.  Solution:
Increase the heating time. 
Cause: Excessive temperature.  Solution:
Reduce the process temperature. 
Cause: Insufficient force/pressure.  Solution:
Increase the mounting force/pressure. 
Dull Surface Finish
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Cause: Insufficient heating time.  Solution:
Increase the heating time. 
Adhesion between Mount and Rams
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Cause: Insufficient application of mold release agent.  Solution:
Apply the mold release agent. This must always be applied to the mounting rams as a thin layer before the mounting process begins. This prevents the resins from sticking to the rams and makes it easier to remove the mounts afterwards. 
Cause: Insufficient heating time.  Solution:
Increase the heating time. 
Cause: Insufficient force/pressure.  Solution:
Increase the mounting force/pressure. 
Individual Grains Visible on Mount*
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Cause: The resin has cured without force/pressure.  Solution:
Increase the force/pressure during the heating cycle. 
Cause: Insufficient heating time.  Solution:
Increase the heating time and/or temperature. 

*Thermosetting resins only

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