The family of nylons consists of several different types.
Nylon 6/6, nylon 6, nylon 6/10, nylon 6/12, nylon 11, nylon 12, and nylon 6-6/6
copolymer are the most common. Of these, nylon 6/6 and nylon 6 dominate the
market. The numbers refer to how many methyl units (-CH2-) occur on each side of
the nitrogen atoms (amide groups). The difference in number of methyl units
influences the property profiles of the various nylons. Moisture absorbance is
decreased due to reduced polarity with further separation and less regular
location of the very polar amide roups. Resistance to thermal deformation is
lowered due to more flexiblity and mobility in these methyl unit sections of the
main chain. As these units increase in length, making the molecules appear more
like polyethylene, the properties of the nylon shift slightly toward those of
polyethylene. Not considering the effects of moisture, Nylon 6/12 has lower
modulus, higher elongation, lower strength, lower thermal distortion
temperature, lower hardness and lower melting point than nylon 6/6. One
relationship which does not conform is price. Nylon 6/12 is more expensive than
nylon 6/6. The property which gives nylon 6/12 its utility is moisture
absorption which is approximately half of that of nylon 6/6. This means the
properties are much more consistent and experience less fluctuation due to
ambient humidity levels in the end application.
Moisture absorption by nylon has been a source of great
study for many years. Although all polymers absorb some amount of moisture, on
none does it have such a significant effect as on nylons. Table 6.1 illustrates
the moisture absorption levels of various types of nylons. (Ref 16)
Table 6.1 Absorption of Moisture by Nylons by Weight %
at 50% R.H. and Saturation @ 23�C (Ref 16)
Water molecules produce polar bonds with the amide groups
in the nylon molecules. Although small, water molecules take up space and
displace the nylon molecules. This results in the nylon molecular matrix
swelling. Dimensional changes of 0.7% can result in nylon parts from the
"as-molded" state to equilibrium at 50% R.H. environments. This change occurs in
approximately 150 days for a 0.0 60 inch (1.5 mm) thick part. (Ref 17) Molecular
mobility is increased through the absorption of water. The increase in spacing
between nylon molecules lowers the secondary forces allowing easier
translational motion. This is the major reasons for the change in physical
properties discussed above. There is less resistence to applied stress from the
decrease in intermolecular friction. The change in molecular mobility is
significant enough that molded nylon parts can relieve molded in stresses as
they absorb moisture. Pretty neat 'eh?
The absorption of moisture by nylon is a completely
reversible physical reaction. Drying in an oven will drive off all but a small
percentage of the water molecules which can only be removed through dissolution
of the nylon molecular matrix. The rate of absorption/desorption varies with
type of nylon as well as temperature and relative humidity. Addition of fillers
reduces the effect of moisture both due to volume reduction of the ammount of
nylon polymer in the mixture, and by sharing the attraction of the molecules
somewhat reducing polarity and the available space for moisture molecules.
Reinforcements reduce the effects more than fillers due to nylons strong
affinity for reinforcement. In addition to the mechanisms which take place with
fillers, the adhesion of the nylon molecular matrix to dimensionally stable
reinforcements is stronger than than polar bonding of the water molecules and it
dominates. Kinda like my ex...
Another area where moisture has significant effects on
nylons is in processing. Heated to molding temperatures while wet (ie., >0.2 %
water) will result is hydrolytic degradation and a significant loss of physical
properties. (Hydrolytic degradation is a chemical reaction which occurs at high
temperature with some polymers in the presence of water. It causes primary bonds
in the molecular chains to be severed thus reducing molecular weight.) Over
drying (ie., <0.08% water) will remove the plasticizing effect of the water
molecules and make the resin very viscous and hard to flow. The plasticizing
effect in processing has to do with mobility and relative spacing of the nylon
molecules, the same influence as on physical properties. This low level of
moisture does not cause significant degradation during processing. The
absorption of moisture by nylon must be considered in mold making. The shrinkage
factor used in designing the mold must take the the potential for change in post
molded dimensional into account. Although moisture causes problems in working
with nylons, it does contribute to: better dyeability, toughness, softness and
greater flexibility in nylon parts.
Another dominant feature of nylons is crystallinity. As
with most crystalline polymers, the molecular chains are uncluttered by large
substituent groups. They are flexible and regular in group spacing and
crystallize readily. As with acetals, this crystallinity is responsible for
properties of wear resistance, chemical resistance, thermal resistance, and
unfortunately, higher mold shrinkage. The overall excellent property profile of
nylons results in their probably having the most diverse range of applications
of all thermoplastic polymers. Now let's talk about cutting nylon.
TIPS FOR MACHINING NYLON
STORAGE
Nylon has a high coefficient of thermal expansion (about three times that of
aluminum) and low heat conductivity. Make sure that it has been exposed to
normal room temperature for several hours before it is machined into finished
parts.
SAWING
Nylon can be easily sawed on standard metal working equipment. Wood working
equipment may be suitable but the high cutting speeds may cause excessive heat
build-up. A blade that has been used for cutting metal is usually not sharp
enough for nylon. Use a new coarse tooth blade with good set. Coolant may be
used to control heat buildup and to prevent melting the nylon.
HOLDING
Keep in mind that nylon is not as strong as metal and can be deformed by
improper chucking methods. On small accurately sized rod, use standard spring
collets. On larger parts, use a 6-jaw universal chuck instead of a conventional
3-jaw chuck to distribute the holding force more uniformly. For thin walled
tubular shapes, machine soft jaws so that the part is almost entirely confined.
TURNING
Satisfactory finishes can be easily obtained on nylon over a wide range of
surface speeds. Use tools that are honed sharp and have high rake and clearance
angles, to minimize cutting force and reduce heat build-up. Chips will be
continuous and stringy. They should be directed away from the cut and prevented
from winding around the workpiece. Coolants are generally not necessary for
lathe work unless there is excessive heat build-up.
MILLING
Milling cutters should be honed sharp and should have high positive cutting
angles. Care should be used in clamping the part to prevent distortion.
Double-faced pressure sensitive tape can be used to hold down flat parts.
Cutting speeds and speeds will be determined by the finish required and will be
limited by heat build-up.
DRILLING
Use conventional twist drill or flat type drills. Polished flutes will aid in
the removal of chips. Do not use metal cutting reamers with nylon. They do not
cut freely enough. Drill small holes to size in one operation. Rough drill large
holes and finish by single point boring.
THREADING
Use only sharp taps and dies on nylon parts. Don't use tools that have been used
to cut metal. H5 or even larger oversized taps may be required because a
threaded hole in nylon closes in when the tap is removed. Threads to close
tolerances can be easily single point chased.
GRINDING
The large amounts of heat generated by grinding, together with the low heat
conductance of nylon, usually dictate that liberal amounts of coolant he used in
most grinding operations. Thru-feed centerless grinding of long, flexible parts
of nylon can be easily accomplished, and tolerances as close as .0005" are
possible. Cylindrical grinding on nylon is usually not required because it is
easy to get good finishes and close tolerances on a lathe. Suface grinding of
nylon is usually not necessary. If a flat suriace with close tolerances and good
finish are required, the best approach is fly cutting in a milling machine. No,
not cutting a fly on your milling machine, FLY cutting.
STAMPING
Thin pieces may be stamped with standard equipment. Thick sections will require
high shear angles if good edges are needed. Steel rule dies may be used for some
parts.
MEASURING
Use ordinary measuring equipment. However, use a light touch because the
material is not as hard as metal. A micrometer anvil can deform a nylon surface
as much as several thousandths. Homemade, soft plug and ring gauges are useful
on thin walled parts. If extremely close tolerances are involved, make SURE any
temperature changes that the part will see are taken into account.
PROPERTIES |
A.S.T.M
Test Method |
NYLON |
NYLON |
NYLON |
NYLON |
|
|
TYPE 6 |
TYPE 66 |
TYPE 612 |
CAST TYPE
6 |
Specific Gravity |
D792 |
1.12 - 1.14 |
1.14 - 1.1 |
1.06 |
1.15 |
Water Absorption
Method A |
D570 |
2.9 |
1.24 |
0.25 |
-=- |
Tensile strength
at yield, 1000 psi |
D638 |
9.4 |
12 |
8.8 |
11 - 14 |
Elongation at
yield, % |
D638 |
25 |
>150 |
7 |
10 |
Elastic Modulus
in Tension, 10~5 psi |
D638 |
-=- |
4.4 |
-=- |
3.5 - 4.5 |
Flexural Strength
at yield, 1000 psi |
D790 |
NO YIELD |
16 |
NO YIELD |
16 - 17.5 |
Elastic modulus
in flexure, 10~5 psi |
D790 |
1.50 |
4.1 |
2.95 |
-=- |
Rockwell Hardness
(Method A) |
D785 |
R104 |
88 |
R114 |
R112 |
Izod impact
strength, ft-lb/in. notch 1/8 in. speciman |
D256 |
2.2 |
1.2 |
1.5 |
-=- |
Deform. under
load(2000 psi; 122f), % |
D621 |
-=- |
0.8 |
1.6 |
0.5 - 1.0 |
Deflection
temperature, F at 66 psi fiber stress |
D648 |
340 |
450 |
356 |
400 |
Max recommended
service Temp., F continuous use |
-=- |
175 |
270 |
290 |
200 - 225 |
Coeff. of Linear
Thermal Expansion, F |
D696 |
4 x 10~5 |
4.5 x 10~5 |
5 x 10~5 |
5.0 x 10~5 |
Underwriters' Lab
Rating (Subj. 94) |
-=- |
HB |
V - 2 |
V - 2 |
-=- |
Dielectric
strength, v/mil, short time |
D149 |
-=- |
555 |
650 |
500 |
Dielectric
constant at 60 Hertz |
D150 |
7.2 |
4.0 |
4.0 |
3.7 |
Dielectric
constant at 1 MegaHertz |
D150 |
3.7 |
3.5 |
3.5 |
3.7 |
Dissipation
factor, at 60 Hertz |
D150 |
-=- |
0.02 |
.02 |
-=- |
Dissipation
factor, at 1 MegaHertz |
D150 |
0.12 |
0.03 |
0.2 |
-=- |
Volume
resistivity, ohm-cm |
D257 |
10~12 |
10~15 |
10~15 |
-=- |
Arc resistance
(SS Electrode), sec. |
D495 |
-=- |
123 |
-=- |
-=- |