PP or PA66: Which Resin Fits Your Part?
PP and PA66 can both make a strong 40% long glass fiber part, but they do not fit the same part. PP LGF40 is the lighter, lower-moisture, lower-temperature option. PA66 LGF40 is the stiffer, higher-temperature option, with stricter drying and a higher mold temperature. Choosing the wrong resin does not just change one property. It changes heat resistance, moisture handling, processing, and part weight at the same time.
The short answer is therefore a boundary, not a ranking. Choose PP LGF40 when the part stays below the PP heat limit, does not need PA66 stiffness, and benefits from low moisture absorption and lower density. Choose PA66 LGF40 when the part needs higher heat deflection and higher stiffness, and the shop can dry it properly and run a hotter mold. If the application, current material datasheet, and target performance are not yet clear, send those three inputs first. A trial on the real part decides the rest.
Why Switching Resin Changes More Than Strength
fitted on a pump.
A buyer who asks PP versus PA66 usually already has a part in mind, and the part already works in one resin, in metal, or in short fiber material. The question is whether switching resin is safe. That question is harder than comparing two datasheets, because a resin change touches the whole part system. Wall thickness that worked in PP may not give the same stiffness in PA66, and a mold temperature that worked for PP will not fill and hold a PA66 part the same way.
Cost is part of the same system, but it is not pellet price alone. It includes drying equipment and time, mold heating, scrap from moisture or poor filling, assembly, and failure in service. A resin that looks cheaper per kilogram can still cost more if it needs longer drying, a hotter tool, or a redesign. A resin that looks stronger on paper can still fail if it absorbs moisture in service and loses dimensional stability.
The challenge, in one sentence, is this: the resin must hold stiffness and shape under the real load, temperature, and environment, and it must do so with a process the shop can repeat. If either side fails, the buyer should not switch on datasheet values alone.
Why PP and PA66 Behave Differently
PP and PA66 differ at the polymer level, and that difference explains most of the selection boundary. PP is a non-polar polyolefin. It absorbs very little moisture, so dimensional change from humidity is small and drying is simpler. Its heat deflection is lower, so it is not the right choice for parts that sit near sustained heat.
PA66 is a polar polyamide. Its amide groups give higher stiffness and higher heat deflection, but they also attract moisture. Moisture in PA66 pellets must be removed before molding, and moisture in service can change dimensions and properties. That is why PA66 drying is longer, hotter, and needs a dehumidifying dryer, while PP drying is shorter and simpler. Neither behavior is a defect. It is the reason the two resins have different processing windows and different application limits.
Long glass fiber does not erase the resin difference. In both grades, 40% long fibers form a load-bearing network that improves stiffness and impact resistance compared with short fiber material. The network raises both grades, but it raises them from different resin baselines. A foamed or metal-replacement claim that ignores the resin baseline will mislead the buyer.
PP LGF40 vs PA66 LGF40 on the Same Standards
The table below compares the two 40% grades only, on the same standards where the two factory datasheets share them. Values are solid, unfoamed reference values, not trial results for a specific part.
| Property | PP LGF40 | PA66 LGF40 | Standard |
|---|---|---|---|
| Density | 1.21 g/cm3 | 1.46 g/cm3 | ASTM D-792 |
| Tensile strength | 135 MPa | 215 MPa | ISO 527 |
| Tensile modulus | 8,893 MPa | 13,588 MPa | ISO 527 |
| Flexural strength | 186 MPa | 320 MPa | ASTM D-790 |
| Flexural modulus | 6,693 MPa | 10,115 MPa | ASTM D-790 |
| Tensile elongation | 1.8-2.6% | 1.7-2.5% | ISO 527 |
| Charpy impact strength | 35 kJ/m2 | 35 kJ/m2 | ISO 180 |
| Heat deflection temperature | 159 °C | 254 °C | ISO 75-2 |
| Molding shrinkage | 0.1-0.3% | 0.1-0.3% | ASTM D-955 |
| Flammability | HB at 1.5 mm | HB at 1.5 mm | UL-94 |
Reference: PP values are from the published PP LGF40 generic grade TDS, NG04-2, version A3, dated 2026-06-30. PA66 values are from the PA66 LGF40 TDS, NG04, version A3, dated 2026-06-30. Both cover black and natural color. GB/T values from either datasheet are not used on this page. The Charpy row uses ISO 180 for both grades and does not mix Izod results into the same row.
Three readings matter more than the full table. First, PA66 is stiffer and far more heat resistant, but it is also denser: at the same volume, a PA66 part weighs more than a PP part, so stiffness per weight must be checked, not stiffness alone. Second, Charpy impact at ISO 180 is the same for both grades at 40%, so impact alone does not separate them; tensile, flexural, and heat deflection do. Third, elongation is low and similar for both grades, so neither grade should be chosen for a part that needs large ductility without a separate check.
When to Choose PP, and When to Choose PA66
Choose PP LGF40 when the part fits its boundary. That boundary is a structural part that needs long fiber stiffness and impact resistance, stays within the PP heat limit, benefits from low moisture absorption, and gains from lower density. Pump and valve housings, shift lever bases, and micro-mobility frames are the kind of parts where PP LGF40 already fits, provided the gate is 5-6 mm and pin-point gates are not used.
Choose PA66 LGF40 when the part fits its boundary. That boundary is a structural part that needs higher stiffness and higher heat deflection, such as engine covers, appliance frames, and fuel system components that face sustained heat and chemical exposure. The shop must be able to dry PA66 properly and run a hotter mold. If drying or mold heating cannot be guaranteed, PA66 stiffness on paper will not survive into the real part.
Neither resin is a general upgrade over the other. PP is not a cheap PA66, and PA66 is not a stronger PP. Each is the right answer only inside its own boundary, and a part outside that boundary should stay with the resin that fits, or be redesigned before switching.
Switching Resin Means Switching Process Windows
Switching resin means switching process windows. Do not run PA66 on a PP window, or PP on a PA66 window, and expect the same part.
| Item | PP LGF40 | PA66 LGF40 |
|---|---|---|
| Drying | 2-4 hours at 80-90 °C, moisture below 0.1% | 3-5 hours at 85-105 °C, moisture below 0.05%, dehumidifying dryer recommended |
| Melt temperature | 210-250 °C | 267-299 °C |
| Mold temperature | 38-69 °C | 75-90 °C |
| Injection pressure | 69-103 MPa | 70-125 MPa |
| Gate | 5-6 mm, no pin-point gates | 5-6 mm full-round, no pin-point gates |
Design checks follow the same boundary. For PP, check heat exposure first, then stiffness at the real wall thickness. For PA66, check moisture control first, then stiffness and heat at the real wall thickness. In both grades, thick walls, ribs, and bosses need a separate filling and warpage check, and a result from a different thickness or a different test cannot be used as proof for a new part.
How to Trial PP or PA66 LGF40 for Your Part
For a new part, the trial follows a fixed sequence:
- Send the part application and a product photo, so the load path, heat exposure, and environment can be seen.
- Send the current material and its datasheet, so the comparison starts from a known baseline.
- State the target performance, including load, temperature, environment, moisture exposure, and service life.
- Run a trial for the chosen resin, grade, and tool, and test it with the same standards as the baseline.
- Decide on production only if the trial meets stiffness, heat, dimensional stability, and repeatability for that part.
FAQ
Can I replace PP LGF40 with PA66 LGF40 without changing the mold?
No, not on datasheet values alone. Mold temperature, drying, and filling all change. Confirm gate size, mold heating, and drying capacity first, then trial the part.
Is PA66 always stronger than PP?
Not in every property. At 40%, PA66 has higher tensile, flexural, and heat deflection values, but Charpy impact at ISO 180 is the same for both grades, and PA66 is denser. Strength must be checked at the real wall thickness and load.
How does moisture affect the choice?
PP absorbs very little moisture, so humidity has a small effect. PA66 attracts moisture, so pellets must be dried properly before molding, and moisture in service can change dimensions. If drying cannot be controlled, PA66 is the riskier choice.
Which resin is lighter?
PP LGF40 is lighter at the same volume, with a density of 1.21 g/cm3 against 1.46 g/cm3 for PA66 LGF40. A lighter part still needs enough stiffness and heat resistance, so weight alone does not decide the resin.
What should I send for a resin trial?
Send the part application and a product photo, the current material and its datasheet, and the target performance, including load, temperature, environment, moisture exposure, and service life.
Trial PP or PA66 LGF40 for Your Part
Send your part application, current material datasheet, and target performance. LFT-G will review whether PP or PA66 LGF40 fits that part, and what the trial needs to prove.
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