Nylon Injection Molding: PA6, PA66, and Glass-Filled Grades
Nylon Injection Molding: PA6, PA66, and Glass-Filled Grades
Nylon injection molding fails more often from wet pellets than from bad tooling. Moisture above 0.2% in PA66 causes splay, voids, and a 15 to 30% drop in tensile strength in the finished part. Get the drying and processing parameters right before you cut steel, and you will avoid the majority of costly rework loops we see on offshore programs.
PA6 vs PA66: Choosing the Right Nylon Grade
PA6 and PA66 are both polyamides, but they are not interchangeable. PA66 runs hotter, absorbs less moisture at equilibrium, and carries a higher continuous-use temperature. PA6 is cheaper per pound, easier to process at lower barrel temperatures, and produces better surface finish on cosmetic parts. Choosing the wrong one adds cost without adding performance.
| Property | PA6 (Unfilled) | PA66 (Unfilled) |
|---|---|---|
| Melt temperature range | 230 to 260 °C | 260 to 290 °C |
| Mold temperature (typical) | 60 to 80 °C | 70 to 90 °C |
| Moisture absorption (23 °C, 50% RH) | 2.7 to 3.3% | 1.5 to 2.5% |
| Linear mold shrinkage (unfilled) | 1.0 to 1.5% | 1.5 to 2.0% |
| HDT at 1.8 MPa (unannealed) | 55 to 65 °C | 75 to 90 °C |
| Typical resin cost (US, 2024) | $1.40 to $1.70/lb | $1.80 to $2.20/lb |
For structural brackets, under-hood hardware, and bearing surfaces, PA66 is the correct call. For cable ties, textile components, and cosmetic housings, PA6 handles the job at lower cost. Confirm your continuous service temperature first. If the part sees sustained heat above 85 °C, unfilled PA6 is already out of the running.
Nylon Drying and Moisture Control
Inadequate nylon drying moisture control is the single most common root cause of scrap on first-shot PA programs. Both PA6 and PA66 are hygroscopic. They pull moisture from ambient air within hours of opening a bag. You cannot skip this step or shorten it.
The standard drying protocol for PA66 processing requires a desiccant dryer set to 80 °C for 4 to 6 hours with a dew point of negative 20 °C or lower at the dryer outlet. PA6 dries at 80 °C for 4 to 5 hours under the same dew point conditions. A hot-air dryer without desiccant is not adequate in humid environments. Resin that has been exposed to ambient conditions for more than 8 hours in a warm shop needs a full redry cycle, not a partial one.
Target moisture content before molding is 0.20% or less for both grades, measured by Karl Fischer titration or a calibrated moisture analyzer. We run incoming lot checks on every PA shipment at our partner shops before the material touches the hopper. Pellets that arrive above 0.25% get a mandatory 6-hour desiccant cycle before any trial shots.
- Desiccant dryer dew point: negative 20 °C or lower
- PA6 drying: 80 °C, 4 to 5 hours
- PA66 drying: 80 °C, 4 to 6 hours
- Maximum residence time in hopper after drying: 1 hour at room air exposure
- Reject threshold: above 0.25% moisture by Karl Fischer
Regrind changes this equation. Regrind absorbs moisture faster than virgin pellets because the surface area is higher. Hold regrind percentage to 15% or less on structural parts, and dry the blended mix as a fresh batch. Anything above 20% regrind in PA66 structural components is a quality risk we do not accept on critical programs.
Glass-Filled Nylon: Shrinkage, Warp, and Tool Design Implications
Glass-filled nylon changes the processing and tooling game significantly. A 33% glass-filled PA66 grade drops linear mold shrinkage to 0.4 to 0.7% in the flow direction, compared to 1.5 to 2.0% for unfilled PA66. That sounds like good news. The problem is anisotropic shrinkage: shrinkage perpendicular to flow is 0.8 to 1.2%, nearly double the flow-direction number. That differential is what causes warped flat panels and bowed ribs.
Gate location controls warp on glass-filled nylon more than any other single tool variable. A center gate on a flat plaque keeps fiber orientation symmetric and warp manageable. An edge gate on the same plaque creates a flow front that preferentially orients fibers along the fill path, maximizing the shrinkage differential. We have seen out-of-flat conditions of 0.8 mm on 200 mm panels with a single edge gate, corrected to under 0.15 mm by moving to a fan gate at the geometric center.
| Grade | Glass Content | Flow-Direction Shrinkage | Cross-Flow Shrinkage | Tensile Strength (typical) |
|---|---|---|---|---|
| PA66 Unfilled | 0% | 1.5 to 2.0% | 1.5 to 2.0% | 80 to 85 MPa |
| PA66 GF15 | 15% | 0.7 to 1.0% | 1.0 to 1.4% | 115 to 130 MPa |
| PA66 GF30 | 30% | 0.4 to 0.7% | 0.8 to 1.2% | 160 to 175 MPa |
| PA66 GF50 | 50% | 0.2 to 0.4% | 0.5 to 0.8% | 200 to 220 MPa |
Steel selection changes with glass content. Unfilled PA6 and PA66 run fine in P20 tooling for medium volumes (under 500,000 shots). At 30% glass fill or above, the abrasive fiber content attacks cavity surfaces. Core and cavity inserts for high-volume glass-filled nylon tools should be H13 at 48 to 52 HRC minimum, or 420SS for corrosion resistance in humid storage environments. Expect P20 cavity life to drop by 40 to 60% on a 30% GF program compared to an unfilled run of equal volume.
PA66 Processing Parameters and Common Defects
PA66 processing is less forgiving than ABS or polypropylene. The processing window is narrower, and the melt viscosity drops sharply with temperature. A barrel temperature 10 °C above the upper recommended limit on a thin-wall part produces significant flash and potential material degradation. Set barrel zones front to rear at 270, 280, 285, and 285 °C for most unfilled PA66 grades, with a nozzle at 275 °C. Adjust 5 to 10 °C for glass-filled grades.
Injection speed matters. PA66 freezes fast. Slow fill on thin walls (under 1.5 mm) causes short shots even when pack pressure is adequate. A fill time of 0.5 to 1.5 seconds is the working range for most medium-sized PA66 parts. Use a velocity-to-pressure transfer at 95 to 98% volumetric fill, not at a fixed position.
- Back pressure: 50 to 100 bar (keep it low to minimize fiber breakage in glass-filled grades)
- Screw speed: 60 to 100 RPM for unfilled; 40 to 70 RPM for glass-filled
- Cushion: 3 to 6 mm
- Pack pressure: 50 to 70% of peak injection pressure
- Pack time: 5 to 15 seconds depending on wall section
- Cooling time: 10 to 25 seconds for wall sections of 2 to 3 mm
Draft angle minimums for PA66 are 0.5 degrees per side on polished cores, 1.0 degree per side on textured surfaces. Glass-filled grades are stiffer and eject with more force. Add 0.25 to 0.5 degrees of draft over the unfilled minimums on deep cores with GF30 or above. Insufficient draft on glass-filled nylon is a leading cause of ejection scoring and part cracking at the ejector pin contact points.
Offshore Tooling Considerations for Nylon Programs
Offshore tools built for nylon require careful specification of steel grade, cooling circuit density, and gate type. We have audited Chinese tools built to SPI Class 103 specifications that ran unfilled PA66 acceptably but destroyed cavity finish in under 200,000 shots once the customer switched to a 33% glass-filled grade. The customer paid $24,000 for a recut and re-heat-treatment on inserts that should have been H13 from the start.
Cooling circuit density directly affects cycle time and warp on PA66. A minimum of two circuits per core and two per cavity is our baseline on any part larger than 75 mm in its longest dimension. Baffles or bubblers in long cores reduce hot spots that cause differential shrinkage. Mold temperature uniformity within plus or minus 3 °C across the cavity surface is achievable with proper circuit design. Beyond that tolerance, warp becomes inconsistent across cavities in a family tool.
Venting is critical for PA66. The high melt temperature and fast injection speed generate significant gas pressure at the end of fill. Vent depth should be 0.015 to 0.020 mm for unfilled grades and 0.020 to 0.025 mm for glass-filled grades. Inadequate venting causes burn marks, short shots at the end of fill, and elevated injection pressure that accelerates wear on the parting line surface.
Frequently Asked Questions
What is the difference between nylon 6 6 molding and PA6 molding in practice?
The chemistry differs, but the practical gap comes down to melt temperature and stiffness. Nylon 6 6 molding runs 30 to 40 °C hotter than PA6, requires tighter drying control, and produces a harder, stiffer part. PA6 gives you better surface cosmetics and lower processing cost. For parts requiring dimensional stability above 80 °C, PA66 is the correct choice.
How do I prevent warp on glass-filled nylon parts?
Gate at the geometric center of the part whenever possible to create symmetric fiber orientation. Keep mold temperature consistent within plus or minus 3 °C using separate cooling circuits on core and cavity. If warp persists after gating and cooling corrections, review wall thickness uniformity. Differential wall sections are a primary driver of post-mold warp on glass-filled nylon.
What steel grade should I specify for a 30% glass-filled nylon tool running 1 million cycles?
Specify H13 at 48 to 52 HRC for all core and cavity inserts. P20 will not survive 1 million cycles of GF30 nylon without significant surface erosion in the gate area and high-velocity fill zones. If your tool will also be stored in a humid environment between production runs, consider 420SS for corrosion resistance alongside the hardness requirement.
What moisture level is acceptable for PA66 before molding?
The accepted processing limit is 0.20% moisture by weight, measured by Karl Fischer titration or an equivalent calibrated moisture analyzer. Above 0.25%, expect splay, surface bubbling, and reduced impact strength in the molded part. According to Dupont PA66 processing guidelines, exceeding 0.3% moisture at the barrel inlet produces measurable molecular weight degradation in the shot.
Can I use the same tool for both PA6 and PA66?
Yes, with reservations. The tool steel holds up equally well for both grades at moderate volumes. The issue is shrinkage. PA6 shrinks at 1.0 to 1.5% while PA66 shrinks at 1.5 to 2.0%, so a tool optimized for one grade will produce parts at different dimensions when running the other. Confirm whether your dimensional tolerances allow for that shift before running both grades in the same cavity.
Run your clamp force and shot size estimates before you finalize tool design. Use our clamp force calculator at /tools/clamp-force-calculator to confirm press requirements for your PA66 part geometry, or contact our injection molding consulting team at /services/injection-molding-consulting to review your offshore nylon tooling specification before you issue an RFQ.