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Metallographic Compression Mounting

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COMPRESSION MOUNTING


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Metallographic Compression Mounting

Compression mounting is a very useful mounting technique which can provide better specimen edge retention compared to castable mounting resins. Compression mounting resins are available in different colors and with various fillers to improve hardness or conductivity.

Several compression mounting characteristics include:

  • Convenient means to hold the specimen
  • Provides a standard format to mount multiple specimens
  • Protects edges
  • Provides proper specimen orientation
  • Provides the ability to label and store the specimensTERAPRESS metallographic compression mounting press

Compression mounts are quick and easy to produce, requiring several minutes to cure at the appropriate mounting temperature. Most of the time required in compression mounting occurs during the heating and cooling cycles. When choosing a compression mounting machine, the most important features include its maximum heating temperature and how intimately the heater and water cooler are connected to the mold assembly. The better compression mounting machines have heaters which can reach temperatures of at least 200°C ( 450°F). For faster turn around time, water cooling is essential.

 

Compression mounting resins include:

    The primary compression mounting resins include:

    • Phenolic Resins (standard colors are black, red and green)
    • Acrylic Resins (clear)
    • Diallyl Phthalate Resins (blue and black)
    • Epoxy Resins (glass-filled)
    • Conductive Resins (phenolics with copper or graphite filler)
Phenolic (bakelite) compression mounting resin   Diallyl phthalate metallographic compression mounting resin
     
Glass-filled metallographic epoxy compression mounting resin   Metallographic conductive compression mounting resins
     

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Compression Mounting Resin Properties

There are a variety of compression mounting materials. The two main classes of compression mounting materials are thermoset and thermoplastics. Thermoset resins require heat and pressure to cross-link the polymer and the reaction is irreversible. Thermoplastic, on the other hand, can theoretically be remelted.

See the following tables for more specific information for the various compression mounting resins.

Phenolics

Acrylics

Epoxy (glass filled)

Diallyl Phthalates

Cost

Low

Moderate

Moderate

Moderate

Ease of use

Excellent

Moderate

Good

Good

Availability of colors

Yes

No

No

Yes

Cycle times

Excellent

Moderate

Good

Good

Edge retention

Fair

Good

Excellent

Excellent

Clarity

None

Excellent

None

None

Hardness

Low

Medium

High

High

TIP: Compression mounting at higher then the recommended minimum temperature generally improves the properties of the mount.

TIP: A usefuMetallographic acrylic compression mountingl tip for marking or identifying a specimen is to mold the label inside of the mount. If the entire mount is an acrylic, just place the label on top the mount and cover it with a little acrylic powder. To label other compression mounting resins, add a thin layer of acrylic over the other mounting material and then position the label on this layer. Finish off the mount with another layer of acrylic.

 

Resin

Phenolic

Acrylic

Epoxy

Diallyl Phthalate

Form

Granular

Powder

Granular

Granular

Specific gravity
(gm/cm3)

1.4

0.95

1.75-2.05

1.7-1.9

Colors

Black, Red, Green, Orange, White

Clear

Black

Blue

Shrinkage (compression)(in/in)

0.006

 

0.001-0.003

0.001-0.003

Coefficient of Linear Thermal Expansion
(in/in/C 10-6)

50

 

28

19

Chemical resistance

Glycol, petrochemicals, solvents, some acids and bases

Alcohol, dilute acids & alkalies, and oxidizers

Solvents, acids, alkalies

Solvents, acids, alkalies

Molding temperature

200C (392F)

220°C (428°F)

200°C (392°F)

200°C (392°F)

Hardness

 

Rockwell M63

Barcol 72

 

Curing time
(1/2" mount @ temp.
and pressure)

8-10 minutes

12-15 minutes

8-10 minutes

8-10 minutes

Phenolics - In general, phenolics are used because of their relatively low cost. In addition, phenolics are available in a variety of colors.

Undercured phenolic compression mounting resin mounts

TIP: Use different color phenolics to color code jobs, specimens types, or for different testing dates. For example, changing the phenolic color each month will show which samples or jobs are getting old.

TIP: If the color dye in the mount bleeds out when rinsing with an alcohol, this is an indication that the mount was not cured either at a high enough temperature or for the proper length of time.

 

Acrylics -the main application for compression mounting acrylics is based on their excellent clarity. This is particularly important for locating a specific feature within the specimen mount.

Cotton ball effect in compression mounted acrylic resins

TIP: A common problem, know as the “cotton ball” effect, can occur with thermoplastic resins if they are not heated and held for a sufficiently long enough time to completely melt the plastic. For acrylic resins, the unmelted resin takes the appearance of a cotton ball in the middle of the mount. To correct this problem, simply put the mount back into the mounting press and either increase the time or temperature of the press. Eventually this will eliminate the “cotton ball.”

Epoxies / Diallyl Phthalates - Glass-filled epoxies and diallyl phthalates are compression mounting resins used to provide a harder mounting support edge next to the specimen. These resins are commonly used to support the edges of coatings, heat treated samples and other specimens requiring better flatness. The following figure shows the polished interface between a glass-filled epoxy and tungsten carbide specimen. Note that there is no noticeable gap between the specimen and the mounting material, therefore showing that glass-filled epoxies provide excellent support to the specimen edge even for extremely hard specimens.

Edge retention for glass-filled metallographic mounts
Polished edge for tungsten carbide mounted in glass-filled epoxy.

TIP: Epoxies (glass-filled) and diallyl phthalates are significantly more expensive then phenolic and acrylics. In order to rPhenolic and glass-filled epoxy for cost reduction of metallographic mountseduce the cost of these mounts, they can be laminated with a lower cost mounting compound such as a phenolic. The technique is to place a sufficiently thick enough layer of the glass-filled epoxy or diallyl phthalate around the specimen in order to compensate for any grinding loss. The rest of the mount can then be supported with a lower cost compression mounting compound such as a phenolic. Red phenolics are used frequently for this technique.

 

Specialized Compression Mounting Resins - With the addition of fillers such as graphite or copper, the compression mounting compounds can be made conductive. Conductive mounts are used in scanning electron microscopes (SEM) to prevent the specimen from building up a charge. Conductive mounts are also used for specimens requiring electrolytic etching or polishing.

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RECOMMENDED COMPRESSION MOUNTING PROCEDURES

  • Clean specimens to remove cutting and handling residues
  • Remove debris from mold assembly
  • Apply thin coat of mold release compound to mold assembly
  • Raise mold ram to up position
  • Center specimen on ram
  • Lower ram assembly
  • Pour predetermined amount of resin into mold
  • Clean and remove any excess resin from around the mold assembly threads
  • Lock mold assembly cover
  • Slowly raise ram into up position
  • Apply recommended heat and maintain pressure for specified period of time
  • Cool to near room temperature
  • Remove mounted specimen
  • Clean mold and ram assembly

TIP: Preheat resin and sample to 35°C (95°F) to expedite the initial heating process and for increasing throughput.

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COMPRESSION MOUNTING TROUBLE SHOOTING

Symptom

Cause

Action

Large bubbles in acrylic resins

Insufficient mounting pressure

Increase mounting pressure or reduce temperature

Soft surfaces on mount

Mount did not completely polymerize because of polymer incompatibility with mold release or oil on the specimen surface

Clean specimen and mounting machine to remove incompatible coatings. Use a compatible mold release

Voids or cracks

High internal stress due to overly rapid cooling

Allow mounts to cool more slowly and longer

Haze around specimen (acrylic mounts)

Specimen contains moisture


Specimens contain copper or some other polymerization retarding alloy

Use a desiccator or low
temperature oven to dry specimen

Coat specimens with an appropriate lacquer before mounting

Phenolic dye leaching out with alcohol rinsing

Insufficient mounting temperature

Increase mounting temperature or service check heater element

Distortion or cracking of specimen

Autoclave pressure is too great for the specimen

Reduce mounting pressure or use a castable epoxy resin


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PRODUCT DESCRIPTIONS

PHENOLIC RESINS

Description

Quantity

Catalog Number

 

Phenolic powder - Black

5 lbs (2.3 kg)
25 lbs (11.4 kg)

CM-2001B-5
CM-2001B-25

Metallographic phenolic compression mounting

Phenolic powder - Red

5 lbs (2.3 kg)
25 lbs (11.4 kg)

CM-2001R-5
CM-2001R-25

Metallographic phenolic compression mounting

Phenolic powder - Green

5 lbs (2.3 kg)
25 lbs (11.4 kg)

CM-2001G-5
CM-2001G-25

Metallographic phenolic compression mounting

Phenolic powder - Orange

5 lbs (2.3 kg)
25 lbs (11.4 kg)

CM-2001OR-5
CM-2001OR-25

Metallographic phenolic compression mounting

Phenolic powder - White

5 lbs (2.3 kg)
25 lbs (11.4 kg)

CM-2001W-5
CM-2001W-25

Metallographic phenolic compression mounting

1.25-inch PHENOLIC PREFORMS

Description

Quantity

Catalog Number

 

1.25" Phenolic PREFORMS - Black

105/pkg
500/pkg

PCM-2125B-105
PCM-2125B-500

Metallographic phenolic compression mounting

1.25" Phenolic PREFORMS - Red

105/pkg
500/pkg

PCM-2125R-105
PCM-2125R-500

1.25" Phenolic PREFORMS - Green

105/pkg
500/pkg

PCM-2125G-105
PCM-2125G-500

1.50-inch PHENOLIC PREFORMS

Description

Quantity

Catalog Number

 

1.50" Phenolic PREFORMS - Black

100/pkg
500/pkg

PCM-2150B-100
PCM-2150B-500

Metallographic phenolic compression mounting

1.50" Phenolic PREFORMS - Red

100/pkg
500/pkg

PCM-2150R-100
PCM-2150R-500

1.50" Phenolic PREFORMS - Green

100/pkg
500/pkg

PCM-2150G-100
PCM-2150G-500

TRANSACRYLIC COMPRESSION MOUNTING RESIN

Description

Quantity

Catalog Number

 
TRANSACRYLIC Resin powder (transparent) 5 lbs (2.3 kg) CM-2101A-5 Metallographic compression mounting acrylic

TRANSACRYLIC Resin powder (transparent)

25 lbs (11.4 kg)

CM-2101A-25

EPOXY (Glass Reinforced) COMPRESSION MOUNTING RESIN

Description

Quantity

Catalog Number

 

EPOCOMP Epoxy Compression Resin powder

5 lbs (2.3 kg)
25 lbs (11.4 kg)

EP-2201A-5
EP-2201A-25

Metallographic compression mounting glass filled epoxy

EPOCOMP Epoxy FINE Compression Resin powder

5 lbs (2.3 kg)
25 lbs (11.4 kg)

EPF-2201A-5
EPF-2201A-25

DIALLYL PHTHALATE COMPRESSION MOUNTING RESIN

Description

Quantity

Catalog Number

 

Diallyl Phthalate Compression Resin powder (short-glass filled)

5 lbs (2.3 kg)

DY-2301A-5

Metallographic compression mounting diallyl phthalate
Diallyl Phthalate Compression Resin powder (short-glass filled) 25 lbs (11.4 kg) DY-2301A-25


CONDUCTIVE MOUNTING RESIN

Description

Quantity

Catalog Number

 

Conductive (graphite) Compression Resin powder

2.5 lbs (1.1 kg)
5 lbs (2.8 kg)

CONDUCTO-1
CONDUCTO-5

Metallographic compression  conductive mounting

Conductive (copper) Compression Resin powder

2.5 lbs (1.1 kg)
5 lbs (2.8 kg)

CONDUCTO-CU1
CONDUCTO-CU5


MISCELLANEOUS MOUNTING ACCESSORIES

Description

Quantity

Catalog Number

 

30 ml measuring scoop

30 ml

MS-1000-30

Metallographic compression mounting

Mold release

8 oz (0.24l)
16 oz (0.47l)

MR-1000-08
MR-1000-16

Metallographic compression mounting

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