Nylon is an engineering thermoplastic and commonly used injection molding material that is a polyamide (PA). It exhibits high crystallinity, an important attribute that improves a plastic’s mechanical strength and thermal performance. Nylon’s material properties support its use as a replacement for metal and make it a good choice for pump parts, fan blades, cable ties, screws, and gears.
Nylon comes in many different forms, but Nylon 6, Nylon 66, and PA 12 are the most common types. Adding fillers such as glass fibers can enhance nylon’s mechanical and thermal properties, but at the expense of flexibility. During material selection, it’s also important to consider that nylon tends to absorb moisture and must be dried before injection molding occurs. During end-use applications, nylon can swell in high-humidity environments.
Advantages of Nylon Injection Molding
Nylon is a commonly used injection molding material because of its advantageous physical properties, mechanical properties, and moldability. These advantages include:
Low melt viscosity
Chemical and abrasive resistance
High-temperature resistance
Fatigue resistance
Tensile and mechanical strength
Low Melt Viscosity
Nylon’s low melt viscosity improves its moldability for parts with thin side walls or other thin features. During the injection phase, the melted nylon flows easily through tight spaces.
Chemical and Abrasive Resistance
Nylon resists dilute acids, inorganic chemicals such as ammonia and alkaline solutions, and organic solvents like halogenated hydrocarbons. Nylon is also highly resistant to fuel and oils, and has excellent abrasion resistance due in part to its low coefficient of friction.
High-Temperature Resistance
Nylon has a higher maximum operating temperature than other engineering plastics. Although nylon injection molding requires higher energy inputs, parts can be used in applications with long-term exposure to temperatures up to 150°C. Glass-reinforced nylon has a higher melt temperature and even greater high-temperature resistance.
Fatigue Resistance
Nylon’s excellent fatigue resistance makes it ideal for components, such as gears, that undergo cyclic loading. Designing injection molded nylon parts with the proper corner radii further strengthens fatigue resistance.
Tensile and Mechanical Strength
Nylon has a tensile strength comparable to some metals; however, nylon is not as rigid. Nylon’s mechanical strength can be further increased by adding fillers such as glass fibers, or increasing the mold temperature to improve the crystalline structure of the material.
There are also four nylon-specific elements to consider:
Wall thickness
Radii
Draft angle
Part tolerances
Wall Thickness
When designing parts for nylon injection molding, use a wall thickness between 0.030 and 0.115 in (0.76 – 2.92 mm). Note that this is slightly less than the recommended wall thickness for most other injection molding materials.
Also, leep wall thicknesses uniform. If that’s not possible, apply a gradual change of no more than 10% to 15% in thickness between adjacent walls. A wall thickness of greater than 6 mm is not recommended.
Without compromising moldability, nylon parts can be lightweight with relatively thin walls. The relative thinness of nylon’s minimal wall thickness is due in part to its low melt viscosity.
Radii
Nylon is sensitive to stress concentrations caused by sharp corners. Therefore, it’s critical to design parts with corner radii larger than 0.5 mm. Optimal part performance is achieved when radii are equivalent to 75% of the nominal wall thickness.
Draft Angle
Nylon parts can be molded without draft angles because nylon’s low coefficient of friction promotes part ejection. This characteristic also supports the use of injection molded nylon for gears and other components that require flat surfaces. Applying a draft angle to nylon parts can shorten cycle times, however, and typically, draft angles beween 0.5° and 1° per side are used.
Part Tolerances
Because nylon has a higher shrinkage rate than most thermoplastics, it can be challenging to maintain dimensional tolerances with nylon parts. When shrinkage rates are controlled, however, injection molded nylon has consistent part tolerances. This makes nylon a popular choice for precision components like gears and bearings.
Nylon Material Properties
Table 1 below lists the physical, mechanical, and molding properties of various grades of nylon.
| Nylon 11 | Nylon 12 | Nylon 46 | Nylon 66 | Nylon 6630% GF | ||
| Physical | Density (g/cm3) | 1.04 | 1.31 | 1.2 | 1.17 | 1.38 |
| Linear Mold Shrinkage Rate (cm/cm) | 0.0083 | 0.00694 | 0.019 | 0.0139 | 0.0044 | |
| Rockwell Hardness (R) | 107 | 98 | 95 | 114 | 117 | |
| Mechanical | Tensile Strength at Yield (MPa) | 37.1 | 46.1 | 73.9 | 72.5 | 155 |
| Elongation at break (%) | 119 | 67.4 | 43.4 | 47.1 | 4.03 | |
| Flexural Modulus (GPa) | 0.948 | 5.66 | 2.64 | 3.09 | 7.96 | |
| Flexural Yield Strength (MPa) | 119 | 136 | 108 | 229 | ||
| Nylon Injection Molding | Drying Temperature (°C) | 90 | 92.6 | 93.7 | 81 | 82.2 |
| Melt Temperature (°C) | 261 | 224 | 303 | 279 | 285 | |
| Mold Temperature (°C) | 48.9 | 70.7 | 103 | 74.9 | 86.1 |
As this data shows, nylon has excellent tensile and flexural strength. Nylon is also inherently hygroscopic and, as such, must be dried thoroughly before injection molding.
Nylon Material Processing
Plastic injection molding with nylon involves choosing the right processing parameters — below are some of the most important to consider:
Viscosity
Moisture
Temperature control
Injection pressure
Injection speed
Gassing
Shrinkage
Viscosity
Nylon has a low melt viscosity and can flow quickly through tight channels before setting. This supports its use with injection molded parts that have thin walls and long injection paths. Nylon’s low melt viscosity also helps to minmize cycle times since nylon will fill a mold cavity more quickly than other plastics.
Moisture
Nylon absorbs moisture over time if it isn’t stored correctly. In injection molded parts, excessive moisture can cause voids and surface defects. The rate of water absorption depends on factors such as the specific nylon grade and the relative humidity. A moisture content of between 0.15 and 0.20% is optimal, and higher humidity results in faster water absorption.
Temperature Control
Nylon injection molding requires proper temperature control of both the mold and the material. Higher mold temperatures result in less shrinkage and enable the production of thin-walled parts due to the decrease in viscosity. Increased temperatures also improve the material’s crystallinity, which increases strength and hardness. Note, however, that temperatures above 330°C can result in thermal degradation.
Injection Pressure
To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. Inadequate injection pressure can result in knit lines and a surface that appears corrugated. Excessive injection pressure can result in part defects such as overflow or flash.
Injection Speed
Nylon supports faster injection speeds because of its low viscosity, which results in shorter cycle times for reduced per-part costs.
Gassing
Gassing refers to the generation of excess gasses during injection molding. If these gasses are not venting properly, underfilling and surface defects can result. The causes of gassing include poor mold venting, and improper barrel temperature and throughput.
Shrinkage
Nylon is prone to shrinkage, so it’s important for injection molders to limit and control shrinkage rates — otherwise, shrinkage can cause dimensional inconsistencies and warping. To reduce shrinkage, increase the mold and barrel temperature.
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