Metallurgical encapsulation

Encapsulation of samples is a recommended technique in sample preparation to benefit from used for ease of manipulation and edge retention.

Encapsulation of micro samples is carried out in the metallurgical laboratory to improve the quality of metallurgical sample preparation.  When the correct type of enpasulating resin is used the metallographer will benefit from improved sample edge and ease of sample manipulation.

Hot mounting

The use of a metallurgical mounting press is used to produce hard mounts for encapsulating metallurgical samples in short cycle times. Two classifications of resin are used for hot mounting, these are thermosetting and thermoplastic resins.
Thermosetting resins require cure with heat and pressure. Once the baking time has been achieved samples can be ejected. It is recommended that samples are cooled for operator confort and easier ejection from the machine cylinder.
Thermoplastic resins remain fluid at maximum temperature, when the sample is cooled it will become more dence and transparent as cooling occurs.  Increase pressure on cooling.
Hot mounting parameters which require control are pressure, temperature and time. Advanced automatic mounting presses are capable of holding multiple programs to allow the metallurgist to select the correct parameters.

Thermosetting resin Defects.

Radial cracks can emanate from stress raisers such as sharp corners on the sample. These cracks are undesirable and can cause contamination problems and leaching of polishing fluids.  To overcome this problem it is advised to reduce the specimen size or increase the mould diameter. Care should be taken to remove sharp corners on samples to reduce the occurance of radial cracks.
The quality of edge retention is a consequence of shrinkage. If the moulding temperature is too high this can cause poor adhesion to sample surface.
When the cure time is not sufficient or the mould pressure is too low surface eruptions can occure on the base of the sample.
When samples are not fully cured this is a consequence of mounting parameters. A woody surface appearance is a consequence of the resin curing prior to the flow stage. We must prevent localised curing of the resin when preparing the mounting process.
When the mould temperature is too high moulds can become stuck to the lower ram. Dull mounts with excessive flash will be indications of too high mould temperature.

Thermoplastic Encapsulation defects.

Cottonball features can occur in thermosetting mounts as a consequence of the resin not reaching temperature. This typical issue can be overcome with increase of heating time to ensure full flow stage is achieved.
Crazing of the sample occurs when  stresses cause cracking of the mount after the sample is ejected from the mould. Increasing the cooling time will eleviate this problem and decreasing the pressure during the cooling stage will also help.  Mounts can be stress relieved in boiling water to overcome the problem of cracking.

Cold mount casting resin

When the conditions applied in hot mounting are unsuitable for delicate samples, cold mounting is a very good alternative option.  Micro samples produced by cold mounting will take longer than the hot mounting process.  Large batches of samples can be processed together to make the cold mounting process efficient.
The three main types of resin used in cold mounting are Epoxides, Acrylics and Polyesters.  All systems are supplied as two part systems with resin and hardener.  Curing of the resin involves an exothermic reaction and it is important to mix the correct proportions of constituents to enable high quality mounts.
Epoxy and polyester resins are transparent, acrylic resins are available in a range of colours but opaque is normal. If a clear acrylic mount is required we can mount under a pressure of 2 bar to achieve a high quality clear mount.

Important characteristics of resins

• Hardness  Epoxides, Polyester, Acrylic.
• Temperature Epoxide, Polyester, Acrylic.
• Shrinkage Epoxide, Acrylic, Polyester.
• Cure time Acrylic, Polyester, Epoxide.