Performance

Plastic impregnation generally has little or no effect on tensile strength and ductility, and often resin sealants for impregnated parts have very similar physical properties. Therefore, sealants are often considered roughly equivalent in terms of component strength and machinability.

However, there are important performance differences that have arisen with further developments and evaluation. For example, developments in improved sealing performance have coincided with adhesive strength improvement from 1 to 50 N/cm2 (Ref 1). Tests of the radial crushing strengths for unimpregnated and impregnated components using different impregnants also show changes in both strength and ductility (Table 1, Fig. 6). Resin 1 has a 10% improvement in yield strength, while resin 2 shows improved ductility. The resin that improves the strength has good adhesive properties, whereas the resin improving ductility forms the harder polymer. The harder polymer is more brittle, yet the components impregnated with resin 1 exhibit more pronounced brittle failure (Ref 1).

Table 1 Radial crushing strength (K) comparison

Sample

Maximum

Strength

Extension to

Minimum

load,^

(K),

maximum load,

deformation,

MPa

mm

mm

Unimpregnated

6S04

2S8

0.382

0.432

Resin 1

7l78

28S

0.376

0.4l4

Resin 2

6768

269

0.3S2

0.472

Source: Ref l

ffl

300

CL

S

250

£ O)

200

iZ

JJ

—■ tfi

150

a>

Ù.

(f>

100

n

O

"râ

50

T3

(fi

ÙL

0

Unimpregnated

Resin 1

V

\ Resin 2

Fig. 6 Radial crushing strength of impregnated and unimpregnated P/M part. Source: Ref 1

Corrosion Resistance. Corrosion and surface blemishes are a chronic problem in P/M parts. Pits, blisters, stains, and other imperfections break out because corrosives and industrial solvents are absorbed into pores. Even after surfaces have been treated with protective coatings and platings, the effects of internal corrosion may not appear until well after other such treatments as tumbling, spraying, painting, polishing, cleaning, and anodizing have been completed (Fig. 7).

Fig. 7 Surface condition of P/M part after plating

Without a protective thermoset plastic impregnant, corrosive solvents soak into pores, eventually seeping upward and damaging the most hardened and lustrous surfaces. The fluids involved in P/M applications are numerous, including:

• Ethylene glycol

• Lubrication oil

• Carbon removal compounds

• Hydrocarbon fluids

• Hydraulic fluids

• Machining fluids

• Alkaline cleaners

Machining Benefits. One of the more remarkable developments is the demonstrable improvements in machinability with impregnated metal parts versus those that remain untreated. The precise reason for this result is unclear, but the consensus points to a natural lubricity present in thermoset plastic resins used in most P/M impregnation processes. These cured polymers minimize tool chatter or vibration, heat buildup, and the interrupted cut associated with the machining of P/M parts. They also reduce chip thickness and adhesion, improve surface finishes, help achieve consistent finish dimensions, and improve dimensional control of parts.

By filling voids in the porous metal, the impregnating material promotes better chip formation and separation by the cutting tool. The plastic fill cushions the tool as it passes through metal, giving the tool edge an uninterrupted feel, thus extending its service life through a reduction in cutting force.

Many manufacturers impregnate parts solely to derive machining benefits. For example, a major equipment manufacturer verified benefits from impregnating parts with thermoset resins including higher-quality electroplating, pressure-tight sealing of hydraulic and pneumatic parts, and major improvements and cost savings in machinability. Tool life doubled in tapping and turning operations and tripled on parts with high hardness. Overall, perishable tooling costs dropped about 50% and, in some cases, the company is machining almost ten times as many pieces per tool.

Drilling is the most common machining operation used on P/M parts, and several studies (Ref 2, 3, 4, 5, and 6) confirm lower drilling forces and longer tool life with impregnated parts. Some deterioration in surface roughness occurs (Ref 6), presumably due to melting of the impregnated resin. Nonetheless, several research programs definitively support resin impregnation as a method to significantly improve machinability. In one major program, exhaustive tests performed on various P/M alloys found that resins reduced drilling forces up to 75% in some cases (Fig. 8, 9, 10, and 11).

Fig. 8 Machining chart of P/M iron. UI, unimpregnated; OI, oil impregnated; RIR, resin-impregnated resinol
Milien bntHn Eulmiig nililma viiciillag VHd

Fig. 9 Machining chart of P/M 304 stainless steel. UI, unimpregnated; RIR, resin-impregnated resinol e 2000 E

E 1500

1000

e 2000 E

E 1500

1000

IR^"-**'^

S = 0.09 P _i------ 1 . ,

100 200 300 400 Drill load, N

500 600

100 200 300 400 Drill load, N

500 600

Fig. 10 Effect of resin impregnation on drilling. UI, unimpregnated; IR, impregnated resin. Efficiency of improved machinability: 370% = 3.28 + 0.89 x 100

Fig. 10 Effect of resin impregnation on drilling. UI, unimpregnated; IR, impregnated resin. Efficiency of improved machinability: 370% = 3.28 + 0.89 x 100

Test

Value

condition

Density

7.0 g/cm3

Cutting tool

Carbide, P20

Depth of cut, mm

1.0

Feed rate, mm/rev

0.1

Cutting speed, m/min

100

Fig. 11 Effect of resin impregnation on turning. Atomized iron powder SMF1015 impregnated with resinol. UI, unimpregnated; IR, impregnated resinol

0 0

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