Polyamide (PA), commonly known as nylon, has a density of about 1.15g/cm3. It is a general term for thermoplastic resins containing repeating amide groups — [NHCO] — on main chain of molecule, including aliphatic PA, aliphatic-aromatic PA and aromatic PA. Among them, aliphatic PA has many varieties, large output and wide application. Its name is determined by specific number of carbon atoms in synthetic monomer.
Because aliphatic polyamides contain amine and carbonyl groups, they are easy to form hydrogen bonds with water molecules. Therefore, various materials obtained are easy to absorb water when used, resulting in a plasticizing effect, which leads to volume expansion of material, a decrease in modulus, and obvious creep under stress.
Polycaprolactam and polyhexamethylene adipate (nylon 6 and nylon 66) are the most commonly used polyamide materials. As can be seen from table below, their water absorption is significantly higher than other materials. They can absorb up to 10% of water by mass from humid air, and can also absorb 2% to 4% of water by mass under normal humidity conditions, resulting in changes in various mechanical properties.

PA6 and PA66 molecules have inherent amide groups, hygroscopicity is inherent characteristic brought to material by amide groups. PA6 is formed by ring-opening and repolymerization of caprolactam, PA66 is formed by polymerization of hexamethylenediamine and adipic acid. Both molecular chains contain following amide group structure:

 

Figure 1: Amide molecular structure
When amide group meets water molecules, it is very easy to form hydrogen bonds, which is a binding force between van der Waals forces and chemical bonds. Therefore, nylon is very easy to absorb water and is not easy to dry.

 

Figure 2: Schematic diagram of hydrogen bond formation between PA 6 and water
Due to different structures of PA6 and PA66, equilibrium water absorption rates of two under same conditions are also different. In a standard environment of 23℃/50% relative humidity, equilibrium water absorption rate of pure PA6 products is about 3.0; equilibrium water absorption rate of pure PA66 products is about 2.8.

 

Figure 3: Molecular structure of PA 6 and PA 66

Nylon (PA) has a series of excellent properties such as high mechanical strength, chemical resistance, oil resistance, wear resistance, self-lubrication, easy processing and molding. It has become one of thermoplastic engineering plastics widely used at home and abroad.
However, in actual applications, performance requirements for nylon are different under different use conditions or environments. For example, components such as electric drill and motor housing, pump impeller, bearing, diesel engine and air conditioner all-plastic fan require nylon materials to have high strength, high rigidity and high dimensional stability; due to poor low-temperature toughness of nylon, it is necessary to toughen it at this time; in some outdoor application fields, nylon materials must be weather-resistant in long-term outdoor environments, etc.
Even so, in process of nylon modification (reinforcement, toughening, weather resistance, thermal conductivity, etc.), technical R&D personnel will still face various problems, such as poor fluidity and dissatisfaction in injection molding during glass fiber reinforcement modification; poor gloss and insufficient fluidity of materials during toughening modification; difficult feeding and insufficient gloss due to too high addition of thermal conductive powder during thermal conductivity modification; at this time, further functional modification is often required to solve above problems.

Reinforcing materials used for reinforced nylon are mainly fibrous substances such as glass fiber, carbon fiber, and whiskers, among which glass fiber reinforcement is the most widely used. Glass fiber reinforcement can significantly improve rigidity and hardness of material, significantly improve dimensional stability and heat resistance of material.
For glass fiber reinforced nylon, interface bonding between glass fiber and nylon resin, length of glass fiber in nylon material, dispersion of glass fiber in material, processing temperature, glass fiber diameter, glass fiber type, etc. will affect final performance of material. I believe that technicians engaged in modification of such materials should be able to think of it.
In modification of glass fiber reinforced nylon, it is also necessary to pay attention to protecting thermal oxidation degradation of nylon during processing. Taking glass fiber reinforced nylon 66 as an example, glass fiber is easily squeezed and rubbed with material, screw and inner wall of barrel in twin-screw extruder barrel, and a large amount of friction heat is generated, which often makes actual temperature of material in extruder barrel much higher than extruder display temperature. Such high temperature can easily cause thermal oxidation aging and degradation of nylon 66, and reduce mechanical properties of composite material.
Glass fiber reinforced nylon has poor fluidity. During injection molding process, problems such as high injection pressure, high injection temperature, unsatisfied injection, and poor surface quality are prone to occur, which seriously affect appearance of product and lead to a high rate of product defects.

For toughened nylon, PE, POE, EPDM and their grafts are often used as toughening agents. Especially in ultra-tough modification of nylon, amount of toughening agent added can sometimes be as high as 20 parts. Due to in-situ reaction of functional groups on this type of graft with nylon, improvement in toughness is quite obvious, but it also brings problems such as poor fluidity and rough material surface. Toughening agent cannot be squeezed out when it exceeds 20 parts. After adding 20 parts of toughening agent, notched impact strength of toughened nylon reaches 69KJ/m2. At this time, fluidity of material is greatly reduced, particles are rough and not smooth. In fact, with a slight adjustment in formula, its notched impact strength can be further increased to nearly 80KJ/m2, and surface gloss is significantly improved.

When nylon materials are used in outdoor environments for a long time, they will be affected by external conditions such as sunlight, temperature changes, wind and rain, and will experience a series of aging phenomena such as fading, discoloration, cracking, powdering and strength reduction. By adding carbon black to nylon, its weather resistance can be significantly improved, which is why weather-resistant nylon is currently mainly black.
But even for black products, carbon black alone cannot meet weathering requirements for many years. At the same time, there is also problem of yellowing of natural or light-colored nylon parts when used outdoors.

Thermal conductive plastics have been heated for a period of time before, especially white insulating thermal conductive nylon. There are relatively large problems, mainly because amount of insulating thermal conductive powder that needs to be added is very high, and it is difficult to discharge material during pelletizing process. Dispersion of thermal conductive powder in nylon resin is uneven, and thermal conductivity perpendicular to flow direction is low. How to improve fluidity of thermal conductive nylon materials, dispersion of thermal conductive powders and processing and preparation technology are crucial.
Thermal conductivity of nylon PA66 and PA6 pure resin is generally around 0.3 (W/m-K). After adding some thermal conductive additives to resin, thermal conductivity of nylon materials can reach 0.5-1.0, which greatly improves thermal conductivity of material and has been widely used in emerging field of LED lighting.
IV. PA66 injection molding process (reference)
Cooling temperature of barrel discharge port 60～90℃ (80℃)
Plasticization utilization rate is best at 35%-65%, and flow length ratio of plastic parts is 50-100
Melt temperature: 270～290℃
Mold temperature: 60～100℃
Injection pressure: 100～160MPa (1000～1600bar), if it is processing thin-section long flow channel products (such as wire ties), it needs to reach 180MPa (1800bar)
Holding pressure: 50-65% of injection pressure; since material solidifies relatively quickly, a short holding time is sufficient.
Lowering holding pressure can reduce stress inside product.
Back pressure: 10-20 bar), which needs to be adjusted accurately.
Injection speed: It is recommended to use a medium-high injection speed; mold has good exhaust properties, otherwise product is prone to coking.
Screw speed: It is best to set screw speed to a medium-low speed, as long as plasticization process can be completed before end of cooling time.
Residual material: 2-6mm depends on metering stroke and screw diameter.
Material drying: Dry at 80℃ for 4h. Nylon is hygroscopic to prevent moisture in hopper; water content exceeding 0.25% will cause molding defects.
Recycled material addition rate: 10% recycled material can be added
Shrinkage rate: 0.7%~2.0%, or 0.4%~0.7% if 30% glass fiber is added;
If mold temperature exceeds 60℃, product should be cooled gradually; gradual cooling can reduce shrinkage after molding, that is, product shows better dimensional stability and small internal stress;
Gate system: point type, latent type, sheet type and straight gate are all OK, and hot runner can also be used; due to narrow processable temperature range of molten material, hot runner should provide closed-loop temperature control.
Injection molding shutdown: no need to clean with other materials; molten material should not remain in barrel for more than 20 minutes to prevent thermal degradation of material in barrel.
Injection molding machine barrel: In view of characteristics of PA low viscosity, difficult coloring, fast melting speed, good self-lubricating property, screw has good color mixing effect, stable feed amount and good exhaust effect. For reinforced materials with glass fiber added, a highly wear-resistant bimetallic barrel is required.