The word "nylon" for a person who is not
familiar with the science behind the plastics instantly reminds: socks, velcro,
reeds for guitar, fishing line and different types of clothing. But the
application of polyamides is not limited to simple parts and textile yarn, far
from it, polyamides are the main engineering plastics in the market. To give you
an idea, in 2002 (sorry for delayed data) the total world consumption of
polyamide amounted to 464,000 tonnes!
Polyamides are not polymerized from the same substances, but they all have the
amide functional group (CONH). Some of these polymers are obtained from two
basic materials, each with a number of carbon atoms corresponding to the carbon
number of the polyamide. PA 6.6 for example, is so called because of hexamethylenediamine and adipic acid, its raw materials, having 6 carbon atoms each. Since the PA 6 is polymerized from caprolactam
has 6 carbon atoms.
structure of polyamides
The polar amide groups (CONH) directly influence the properties of polyamides,
as much shorter the distance between these groups as better its mechanical and
thermal properties. However, resistance to water absorption decreases due to
increased hydrogen bonding. The best examples are the PA 6 and PA 6.6 which are
most used in the injection molding of engineering parts and have high values of
water absorption when compared to PA 6.10, 11 and 12.
The influence of moisture on the injection molding of polyamides
In some cases, drying of the PA is required prior to injection molding to
prevent the appearance of stains on parts, differences in polymer melt viscosity
and other typical problems presented by wet plastics, particularly in cases of
recycled materials. Drying may be performed through tray dryers, circulating air
dryers or dehumidifiers hoppers over a period of 4 hours at 176°F (80°C) (PA 6 and 6.6).
Leave this much beyond the recommended time can cause yellowing of polymer of
natural color. Preheating also facilitates the processing, making part of the
cylinder work, preventing the occurrence of burn points. Drying can be
eliminated from the process depending on factors such as geometry and
application of part; type and capacity of the machine; cost due to the electric
power expense, time and drying capacity. Injection molding tests and costing may
be necessary to verify whether the removal of this step of the process really
In most cases, after the molding injection is done the opposite way: the
immersion of parts in a container with water to make this acts as a plasticizer
in the polymer, separating molecular chains and decreasing crystallinity and
glass transition temperature (Tg) of a value of about 122°F (50°C) to 32°F
(0°C), i.e., the part that might need to overcome 122°F not to stand so hard and
brittle as glass will need only 32°F. This makes a rigid and fragile piece
becomes tough and resistant to impact after hydration. The time required for
this process step, or its elimination, varies with the following factors:
mechanical properties required by the application part, the presence of metal
inserts that can become loose after hydration and available time between the
production of the part and its packaging. It is recommended that the moisture
content in parts made from polyamide is between 1 and 2%.
Polyamides have a very good acceptance of the incorporation of glass fiber,
mineral fillers, pigments, stabilizers, lubricants and impact modifiers.
- High resistance to fatigue
- Good impact resistance
- High melting temperature
- Low coefficient of friction
- Resistance to weathering
- Great mechanical properties
- High flow
- Impervious to gases
- Low resistance to inorganic acids (nitric, hydrochloric, sulfuric etc.)
- Low resistance to aromatic alcohols (benzyl alcohol, phenols, cresols etc.)
Gears, automotive parts, bushings, seals and clothes.
Glass transition (Tg)