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Metallographic Castable Mounting

Metallographic castable cold mounting


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SPECIMEN MOUNTING

The primary reasons for specimen mounting are to better hold the part to be ground and polished, and to provide protection to the edges of the specimen. Secondarily, mounted specimens are easier to fixture into automated machines or to hold manually. The orientation of the specimen can also be more easily controlled by fixturing it and then setting it in place via mounting. Metallographic mounting is accomplished by casting the specimen into a castable plastic material or by compression mounting the plastic under pressure and temperature.

CASTABLE RESIN PROPERTIES

Castable or cold mounting for metallographic mountsCastable or cold mounting for metallographic mounts

Castable resins are monomer resins which utilize a catalyst or hardener for polymerization. Polymerization results in cross-linking of the polymer to form a relatively hard mount. Castable resins also have the advantage of simultaneously mounting multiple samples at one time for increased throughput. A number of resin systems used for metallographic mounting and include:

-Epoxy resins
-Acrylic (castable) resins
-Polyester (clear) resins

EPOXY

ACRYLIC

POLYESTER Resin

Type

Epoxy resin and hardener

Acrylic resin and powder

Polyester resin and hardener

Peak Temperature

100-375° F

150 ° F

100 ° F

Shore D Hardness

82

80

76

Cure Time

30 minutes to 8 hours

5-8 minutes

6-8 hours

Comments

Moderate hardness, low shrinkage, transparent

Very fast cure, translucent, some shrinkage

Transparent, water clear


Epoxy Resins
The most common and best performing castable resins are epoxy based (Figure 4-2). Epoxy resins are typically two-part systems consisting of a resin and a catalyst (hardener). Mixing ratio's vary from ten parts resin with one part hardener to five parts resin with one part by weight of hardener. The advantages of mounting with epoxy resins include:

  • Low shrinkage
  • Relatively clear
  • Relatively low exotherms
  • Excellent adhesion
  • Excellent chemical resistance
  • Good hardness
  • Relatively inexpensive


Epoxy curing timesare dependent upon a number of variables including:


  1. Volume of mounting resin (larger mounts cure faster).
  2. Thermal mass of specimen (larger specimens absorb heat and therefore require longer curing time).
  3. Specimen material properties.
  4. Initial resin temperature (higher temperatures cure faster).
  5. Ambient temperature (higher temperatures cure faster).
  6. Relative humidity and shelf life (absorption of water degrades resin and shortens shelf life).
  7. Mounting mold (plastic, phenolic rings, rubber absorb heat differently).

Metallographic epoxy resinAs a general rule, curing times can vary from 30-45 minutes for fast curing epoxies up to 24 hours for slower curing epoxies. For metallographic epoxies to grind properly, the hardness needs to be at least a Shore D80. Note that epoxy resins typically will continue to harden over a longer period of time (maximum hardness, Shore D90).

In some cases, the curing time and temperature may need to be controlled to compensate for the above variables. For example, an 8-hour resin system can be cured in 30-45 minutes by preheating the resin to approximately 120°F (50°C) prior to mixing and then curing at room temperature. This procedure initiates the catalytic reaction sooner; however, this may also increase the maximum exotherm temperature.

TIP: Preheat the specimen to initiate the epoxy resin at the surface of the mount and thus have the epoxy shrink towards the sample for better edge retention.

Conversely, the resin curing cycle can be slowed or reduced by decreasing the curing temperature by forcing air over the curing mounts (fume hood or fan), placing the mounts into a water bath, or curing in a refrigerator. In these cases, care must be taken to not stop the reaction; however if this does occur or the resin is too soft after curing, heating it to 100-120°F for several hours should push the reaction to completion and the mount should be hard after cooling to room temperature.

Epoxy Standard Epoxy Quick Mounting Epoxy Low Viscosity Epoxy
Characteristic Lower cost Fast cure Lower viscosity, clearer, lower exotherm, low shrinkage
Cure Time 1-5 hours 30 minutes 2 hours
Mixing ratio (weight)
(resin : hardener)
5:1 10:1 10:1
Peak Exotherm
-20 grams
-30 grams

300° F
350° F

350° F
375° F

120° F
150° F
Color

Clear to slight
yellow tint

Slight yellow tint Clear
Viscosity Medium Medium Low
Shrinkage Moderate High Low

Metallographic acrylic resinACRYLICS (Castable) - Castable acrylics are easy to use and are very robust. The main advantage of mounting with castable acrylics is the fast curing time. Depending upon the mixing ratio, castable acrylic mounts are typically ready to use within 8-15 minutes. Also unlike epoxy resins, the ratio of the various acrylic parts (powder to liquid) can be altered by up to 25% with no adverse effect to the final properties of the mount. This is because both the liquid and powder are acrylics with various additives and curing agents. By varying the ratio of the liquid to powder, the curing time and viscosity can be altered.

Note: the powder contains a catalyst that reacts with the liquid hardener to start the curing process. Fillers are added to increase hardness and to reduce shrinkage.

Characteristics of Acrylics

  • Rapid mounting
  • Very repeatable and consistent
  • Moderate shrinkage
  • Good hardness
  • Semi-transparent
  • High odor

TIP: Acrylics can be submerged into a water bath during curing. This reduces the exotherm heat and thus reduces the shrinkage of the mount at the specimen interface. A secondary advantage is that the water absorbs the odor.

Acrylic CASTAMOUNT ACRYLIC PLUS SUPERMOUNT
Characteristic Fast, clear, low shrinkage Fast, clear, low shrinkage Fast, hard
Cure time 10 minutes 8 minutes 10 minutes
Mixing ratio (volume)
Powder : Liquid
3:2 2:1 3:2
Color Semi-clear Semi-clear Gray to black


POLYESTERS are typically used when a very clear mount is required. Polyester resins are also useful for mounting parts for display. In this case, the part appears suspended in the plastic. The procedure for molding samples for display is to first determine the mixing ratio of the resin to hardener (catalyst). This ratio is variable depending upon the mass of the casting.

Single Layer Casting (metallographic) Multiple Layer Casting (display)
Layer Thickness Drops of catalyst per ounce of resin Layer Drops fo catalyst per ounce of resin
1/8" 15 1st layer 5
1/4" 5 2 nd layer 4
1/2" 6 3rd layer 3
3/4" 5 4th layer 2
1" - 1-1/2" 4 5th layer or more 2

Polyester clear metallographic resinsFor larger volumes, the amount of hardener needs to be reduced significantly. The procedure for suspending the sample in the mount is to pour an initial layer and allow it to pot or gel (do not let it fully cure). The object or specimen is then placed on the initial rubbery polyester layer and another layer of the liquid polyester is poured. Multiple layers can be poured in this fashion if required.

Characteristics of Polyester

  • Very clear (water clear)
  • High odor
  • Best resin system for large castings

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CASTABLE MOUNTING PROCEDURES
  • Clean and thoroughly dry specimens to remove cutting and handling residues.
  • Remove debris from molding cups.
  • Apply thin coat of mold release compound to molding cup.
  • Center specimen in molding cup.
  • Accurately measure resin and hardener.
  • Mix thoroughly (gentle mixing to avoid producing excessive air bubbles).
  • To reduce air bubbles, pull a vacuum on the specimen before pouring the resin. After pouring the resin over the specimen, cure at room pressure or apply pressure in an autoclave chamber.
  • TIP: Before mixing, preheat resin, hardener and specimen to 30°C (85°F) to expedite curing cycle Note: this will also increase maximum exotherm temperature.

    Vacuum/Pressure Mounting
    Vacuum impregnation is a very useful technique used to fill in pores or voids prior to specimen preparation. It is highly useful for thermal spray coatings and other porous samples.

    Metallographic vacuum impregnationThe most effective technique is to pour the resin under vacuum and/or apply pressure during the curing cycle (advantages - better infiltration of pores and cracks, more transparent mounts, and fewer air bubbles).

    For porous or cracked specimens, the resin can aid in supporting these features. Filling these voids can be difficult depending upon their size, with the smaller voids being much more difficult to impregnate than larger voids. This arises mainly because of the compressibility and volume of air within the void. By applying a vacuum to the specimen and pouring while under vacuum the total pressure of this air can be reduced significantly. Subsequent curing at increased pressures will force (or push) the resin into the voids. Note that the vacuum time on both the resin and specimen should be kept to a minimum in order to minimize degassing of the resin.

    PV = nRT (gas law)
    P - Pressure
    V - Volume
    T - Temperature

    V(bubble size) = nRT/ P

    Thus in order to decrease the air bubble size, impregnate at low pressures and cure at higher pressures.

    Recommended Procedure:
    1. Place mold and sample into impregnation chamber
    2. Mix castable mounting resin
    3. Place cover on chamber and pull vacuum
    4. Pour resin into mount
    5. Slowly increase the pressure
    6. Allow the mount to cure at room pressure or apply an external pressure.

    TIP: Do not pull vacuum for more than 60 seconds. Extended vacuum causes the dissolved gases in the liquid resin to degass and bubble (similar to opening up a carbonated beverage bottle).

    TIP: To reduce the curing time, preheat resin, hardener and specimen to 30°C (85°F). Note: this will also increase maximum exotherm.

    TIP: Slight preheating of the epoxy will also reduce the viscosity of the resin and allow it to flow better.

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    TROUBLE SHOOTING (CASATABLE RESINS)

    Symptom

    Cause

    Action

    Lack of or partial curing of resin

    Improper or insufficient mixing

  • Remount taking care to sufficiently mix epoxy resin and catalyst
  • Check expiration date on catalyst (typically 1-year)
  • Soft or gummy resins (Grinding produces a matted finish)

    Insufficient curing of resin primarily by too low an exotherm

    Heat mount in an oven at 90-100°F (30-40°C) for 1-2 hours and let cool. Resin should harden upon cooling


    Bubbling, cracking, or yellowing of resin


    Too hot a curing exotherm

  • Mount at room temperatures below 85°F (30°C)
  • Decrease volume or volume percentage of hardener
  • Too long a curing time

    Improper resin mixture, old hardener, or too cold a mounting temperature

  • Replace old hardener
  • Mount at room temperatures 70-80°F (30°C)
  • Preheat resin but cure at room temperature
  • Bubbles in resin

    Improper mixing or degassing of specimen

  • Mix with a slow folding motion
  • Pour resin under vacuum and/or cure under higher pressures
  • Clean specimen prior to mounting

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    CASTABLE RESIN CONSUMABLES

    CASTAMOUNT ACRYLICS (Castable)

    Description

    Quantity

    Catalog Number

     

    CASTAMOUNT Acrylic resin kits
    (1 lb and 5 lbs)

    1 lb resin with 12 oz hardener
    5 lbs resin with 1/2 gallon hardener

    AK-5000

    AK-5000-5

    Metallographic castable CASTAMOUNT acrylic

    CASTAMOUNT Acrylic
    resin powder

    1 lb (454 gms)
    5 lbs (2.2 kg)
    25 lbs
    100 lbs

    AR-5000-P1
    AR-5000-P5
    AR-5000-P25
    AR-5000-P100

    CASTAMOUNT Acrylic hardener (clear liquid)

    12 oz (0.36 l)
    32 oz
    1/2 gallon
    1 gallon
    5 gallons

    AH-5000-H12
    AH-5000-H32
    AH-5000-H64
    AH-5000H128
    AH-5000-H640

    ACRYLIC PLUS (Castable)

    Description

    Quantity

    Catalog Number

    ACRYLIC PLUS resin kits
    (1 lb and 5 lbs)

    1 lb resin with 12 oz hardener
    5 lbs resin with 1/2 gallon hardener

    ACYP-5000-K1

    ACYP-5000-K5

    Metallographic Premium Acrylic

    ACRYLIC PLUS
    resin powder

    1 lb (454 gms)
    5 lbs (2.2 kg)
    25 lbs
    100 lbs

    ACYP-5000-P1
    ACYP-5000-P5
    ACYP-5000-P25
    ACYP-5000-P100

    ACRYLIC PLUS hardener (clear liquid)

    12 oz (0.36 l)
    32 oz
    1/2 gallon
    1 gallon
    5 gallons

    ACYH-5000-H12
    ACYH-5000-H32
    ACYH-5000-H64
    ACYH-5000H128
    ACYH-5000-H640

    SUPERMOUNT Acrylic kits (with filler)

    Description

    Quantity

    Catalog Number

     

    Glass Filled Acrylic kit resin kits
    (1 lb and 5 lbs)

    1 lb powder with hardener
    5 lbs powder with 1/2 gallon hardener

    GKAR-3000

    GKAR-3000-5P

    Metallographic Supermount Acrylic

    Glass Filled Acrylic powder (black)

    1 lb (454 gms)
    5 lbs (2.2 kg)

    GRAR-3000-P1
    GRAR-3000-P5

    Lliquid hardener (64 oz)

    12 oz (0.36 l)
    32 oz
    1/2 gallon
    1 gallon

    GRAH-3000-H12
    GRAH-3000-H32
    GRAH-3000-H64
    GRAH-3000-H128

    EPOXY Resins

    Description

    Quantity

    Catalog Number

     

    Epoxy Resin

    32 oz (0.95 l)
    1 gallon (3.8 l)

    EP-3000-32
    EP-3000-128

    Metallographic castable Epocast Epoxy

    Epoxy Hardener

    8 oz (0.24 l)
    32 oz (0.95 l)

    EH-3000-08
    EH-3000-32

    1-gallon pump dispenser

    each

    ED-3000-01

    QUICKMOUNT 2 Fast EPOXY Resin

    Description

    Quantity

    Catalog Number

     

    QUICKMOUNT Fast Epoxy Resin

    32 oz (0.95 l)
    1 gallon (3.8 l)

    EPF-3000-32
    EPF-3000-128

    Metallographic Quickmount2 epoxy

    QUICKMOUNT Fast Hardener

    8 oz (0.24 l)
    32 oz (0.95 l)

    EHF-3000-08
    EHF-3000-32

    ULTRATHIN 2 Low Viscosity EPOXY Resin

    Description

    Quantity

    Catalog Number

     

    ULTRATHIN 2 Low Viscosity Casting Epoxy Resin

    32 oz (0.95 l)
    1 gallon (3.8 l)

    ULTRA-3000-32
    ULTRA-3000-128

    Metallographic Ultrathin2 epoxy
    ULTRATHIN 2 Viscosity Hardener

    8 oz (0.24 l)
    32 oz (0.95 l)

    ULTRA-3000H-08
    ULTRA-3000H-32

    POLYCAST Clear Polyester Resin

    Description

    Quantity

    Catalog Number

     

    Polyester Resin

    1 gallon (3.8 l)

    POLYCAST-128

    Metallographic polyester Polycast resin

    Polyester Hardener

    2 oz

    POLYHARD-02

    Polyester dye kit 3 colors/pkg DYE-1000

    Castable Mounting Molds

    Description

    Quantity

    Catalog Number

     

    Reuseable two piece
    plastic molds
    1.25-inch molds
    1.50-inch molds
    2-inch molds


    12/pkg
    12/pkg
    12/pkg


    METPREP-0125
    METPREP-0150
    METPREP-0200

    Metallographic 2-piece reusable mounts

    Disposable plastic mounting molds
    1-inch molds
    1.25-inch molds
    1.50-inch molds

    50/pkg


    MOUNT-0100
    MOUNT-0125
    MOUNT-0150

    Metallographic castable disposable mounts

    Reuseable Silicon rubber
    1-inch diameter
    1.25-inch diameter
    1.5-inch diameter
    2-inch diameter
    3x2x7/8-inch rectangular
    2.75x1.625x7/8-inch rectangular


    3/pkg
    3/pkg
    3/pkg
    3/pkg
    each
    each


    RMOUNT-0100
    RMOUNT-0125
    RMOUNT-0150
    RMOUNT-0200
    MOUNT-3150R
    MOUNT-302R

    Metallographic castaable rubber mounts


    MISCELLANEOUS MOUNTING ACCESSORIES

    Description

    Quantity

    Catalog Number

     

    Mold release

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

    MR-1000-08
    MR-1000-16

    Metallographic castable mold release

    Stirring sticks
    (6-inch tongue depressors)

    100/pkg

    SS-1000-100

    Metallographic stirring sticks

    Mixing cups
    (3 oz graduated cups)

    100/pkg

    MCUPS-1000-50

    Metallographic castable mixing cups
    Plastic mixing cups ( 9 oz)

    50/pkg

    PCUPS-050

    Metallographic castable plastic mixing cups

    Plastic specimen mounting clips

    100/pkg

    KLIP-0100

    Metallographic castable metal mounting clips

    Plastic specimen mounting clips

    Metal specimen mounting clips

    100/pkg
    1000/pkg
    100/pkg

    SKLIP-0100
    SKLIP-01000
    MKLIP-0100

    Metallographic castable metal mounting clips

    1.25-inch specimen storage containers

    25/pkg

    STORE-0125

    Metallographic specimen storage holders

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