(Click link to print this page)

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 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 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 times are 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).

As 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.

ACRYLICS (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.

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

For 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

Return to top

---

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.

The 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.

Return to top

---

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

Return to top

---

CASTABLE RESIN CONSUMABLES

CASTAMOUNT ACRYLICS (Castable)
Description	Quantity	Catalog Number
CASTAMOUNT Acrylic resin kit (1 lb)	1 lb resin with 12 oz hardener	AK-5000
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

PREMIUM ACRYLICS (Castable)
Description	Quantity	Catalog Number
PREMIUM Acrylic resin kit (1 lb)	1 lb resin with 12 oz hardener	PAK-5000
PREMIUM Acrylic resin powder	1 lb (454 gms) 5 lbs (2.2 kg) 25 lbs 100 lbs	PAR-5000-P1 PAR-5000-P5 PAR-5000-P25 PAR-5000-P100
PREMIUM Acrylic hardener (clear liquid)	12 oz (0.36 l) 32 oz 1/2 gallon 1 gallon 5 gallons	PAH-5000-H12 PAH-5000-H32 PAH-5000-H64 PAH-5000H128 PAH-5000-H640

SUPERMOUNT Acrylic kits (with filler)
Description	Quantity	Catalog Number
Glass Filled Acrylic kit (black)	1 lb powder with hardener	GKAR-3000
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
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
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
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
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" molds 1.50" molds	12/pkg 12/pkg	METPREP-0125 METPREP-0150
Disposable plastic mounting molds 1" molds 1.25" molds 1.50" molds	50/pkg	MOUNT-0100 MOUNT-0125 MOUNT-0150
Reuseable Silicon rubber 1-inch diameter 1.25-inch diameter 1.5-inch diameter 2-inch diameter 3"x1.5"x7/8" rectangular	3/pkg 3/pkg 3/pkg 3/pkg each	RMOUNT-0100 RMOUNT-0125 RMOUNT-0150 RMOUNT-0200 MOUNT-3050R
Phenolic Ring forms 1.0" (25 mm) 1.25" (32 mm) 1.5" (38 mm)	100/pkg	RF-0100 RF-0125 RF-0150

MISCELLANEOUS MOUNTING ACCESSORIES
Description	Quantity	Catalog Number
Mold release	8 oz (0.24l) 16 oz (0.47l)	MR-1000-08 MR-1000-16
Stirring sticks (6" tongue depressors)	100/pkg	SS-1000-100
Mixing cups (3 oz graduated cups)	100/pkg	MCUPS-1000-50
Plastic mixing cups ( 9 oz)	50/pkg	PCUPS-050
Plastic specimen mounting clips	100/pkg	KLIP-0100
Metal specimen mounting clips	100/pkg	MKLIP-0100
1.25" specimen storage containers	25/pkg	STORE-0125

Return to top

---