Walk into any filament shop and you will see PETG everywhere. It is the default “tough PLA” upgrade, the material people reach for when they want a part that bends instead of snaps. Although sometimes PLA is "stronger materials". Then you see PET, often with carbon or glass fibre, sitting in a darker corner of the catalogue with a higher price tag and a shorter name. Plenty of people assume PET is just a cheaper, stripped-down PETG. That assumption is wrong in almost every way that matters for a functional print.
TLDR: PET CF/GF is stiff, creep-resistant, and far stronger under constant load than PETG CF/GF, but it costs more and demands an enclosed printer(for complex parts). PETG CF is more ductile, easier to print, and better for impact-prone parts. If your part must hold its shape under stress, pick PET; if it needs to survive drops, pick PETG.
At DREMC, we have printed and supplied hundreds of kilograms of PET CF and GF for clients in mining, defence, and manufacturing. We run it alongside PETG on our farm, and we have learned exactly where each material belongs. Here is the honest, hands-on comparison, including the mechanical differences, the printing gotchas, and the single question that tells you which spool to buy.
The Chemistry That Changes Everything
PETG is PET that has been modified with glycol. That addition does two things: it makes the filament dramatically easier to print by reducing warping and cracking, and it makes the finished part more ductile, able to flex and absorb impact without shattering. The trade-off is that you lose stiffness and long-term dimensional stability under load. PETG creeps. Put a constant force on a PETG bracket, and over days or weeks it will slowly bend out of shape. For anything structural, that is a dealbreaker.
Straight PET, and its filled variants PET CF and PET GF, is the opposite. It prints stiffer, it stays stiff under load, and it does not creep. The penalty is that it is more brittle, more demanding to print, and more expensive. But for a jig that must hold a dimension for a thousand cycles, or a mount that cannot relax over time, PET is the material we load first.
How They Print: Bed Adhesion, Drying, and the Sticky Truth
Both PET and PETG stick aggressively to build plates. In fact, they can stick too well. We print both on textured or satin PEI, and neither requires a glue stick or adhesion helper. If anything, the risk is over-adhesion. PETG on smooth PEI can tear the coating right off the steel if you remove it hot. PET is no different. A textured or satin sheet and a little patience while the bed cools will save you a lot of grief.
Drying is non-negotiable for both. PET and PETG are moisture-sensitive, and wet filament will produce brittle, bubbly, dimensionally inaccurate parts. PET CF absorbs slightly less moisture than PETG CF, so it stays dry a little longer in humid environments, but we still dry both before every print. Do not trust the vacuum bag. A dedicated filament dryer set to the manufacturer's recommended temperature, run for several hours before printing, is the single biggest factor in getting consistent mechanical properties. Skipping this step is why so many people think PET is "unprintable."
Mechanical Properties: PET CF vs PETG CF
We have put together a comparison table for the key mechanical differences between PET CF and PETG CF. We will fill in the numbers, but the qualitative behaviour is what drives most decisions.
| Property | PET CF | PETG CF |
|---|---|---|
| Tensile Strength (MPa) | 82 MPa | 43 MPa |
| Flexural Modulus (MPa) | 145 MPa | 84 MPa |
| Elongation at Break (%) | 2 % | 11 % |
| HDT at 0.45 MPa (°C) | 80°C ( Up to 218°C once annealed) | 74 °C |
| Impact Strength (kJ/m²) | 28 | 5 |
| Density (g/cm³) | 1.3 | 1.28 |
| Printing Difficulty | Chamber/Enclosure Required | No Chamber/Enclosure Required |
| Dimensional Stability | Excellent, low thermal expansion, and low creep | Okay, low thermal expansion, and medium creep (medium - long term) |
Under constant load, PET CF holds its shape indefinitely. PETG CF will slowly deform. This is the single most important distinction. If your part is structural — a bracket, a jig, a mount that will see sustained force — PET CF is the correct material. The stiffness advantage is significant, and if you anneal the part after printing, the heat deflection temperature can nearly double, pushing it well past ABS and ASA into territory usually reserved for engineering thermoplastics.
PETG CF fights back on ductility and ease of printing. It survives impacts that would crack PET. It prints on an open printer without an enclosure. It is cheaper and lighter per cubic centimetre. For a drone frame or a cosplay prop, those matter more than creep resistance.
Where We Use Each Material
Our PET CF and GF work has gone out the door for mining jigs, military prototyping, and factory floor fixtures. These are parts that cannot fail slowly over time. They need to hold a dimension from the first use to the thousandth. PET CF delivers that straight off the printer, and annealing pushes it further. We print it in enclosed machines with hardened nozzles, and we dry it religiously.
PETG CF we recommend for parts that need to flex before they break, for users who do not have an enclosed printer, and for projects where weight and cost are the primary constraints. It is a superb material for impact-resistant housings, snap-fit enclosures, and anything that will be dropped repeatedly.
The One Question That Settles It
When a customer stands in front of the shelf torn between PET and PETG, we ask a single question: Does this part need to hold a constant load over time?
If the answer is yes, we hand them PET CF or GF. The stiffness, the creep resistance, and the higher thermal performance are exactly what the application demands. If the answer is no, and the part needs to bend without breaking or they are printing on an open machine, PETG CF is the practical, affordable choice or honesty if they don't need temperature resistant PLA can perform better under load (just not suitable in environment that gets warm).
Most people who hear that question leave with PET. Once you understand that PETG slowly moves under load, the decision becomes obvious for anything functional.
