Product literature identifies Mitsubishi PET-G 3D Printing Filament as a glycol-modified polyethylene terephthalate copolyester supplied in 1.75 mm and 2.85 mm nominal diameters. The smaller-diameter stock is controlled on manufacturer lot certificates to ±0.05 mm ovality using two-axis laser gauges; the larger-diameter stock typically carries a ±0.08 mm tolerance, though regional product codes may differ. The glycol modification substitutes a portion of the ethylene glycol repeat unit with 1,4-cyclohexanedimethanol, which suppresses crystallisation and produces an amorphous feedstock for fused filament fabrication. Intended processing includes direct-drive and Bowden-type extrusion platforms producing functional fixtures, machine guards, fluid-handling adapters, snap-fit housings, and transparent or translucent enclosures. Because Mitsubishi Chemical does not publish a single harmonised datasheet across every distributor, the user should verify lot-specific tensile behaviour under ISO 527-2 and melt-flow behaviour under ISO 1133-1:2022 before production release. The product is differentiated from unfilled PET by reduced spherulite growth, from PLA by higher elongation before fracture, and from ABS by lower styrene-related volatile load in enclosed print rooms.

Why does cyclohexanedimethanol substitution suppress crystallinity in PET-G?

In conventional PET, slow cooling from the melt produces spherulite growth, haze, and dimensional stress because the linear polymer chain can fold into ordered lamellae. The introduction of 1,4-cyclohexanedimethanol into the chain backbone disrupts that geometric regularity. Differential scanning calorimetry on representative PET-G filament at a heating rate of 10 K/min typically shows a glass transition near 78 °C and a weak or absent cold-crystallisation exotherm. The practical consequence is that printed walls remain amorphous unless deliberately annealed. The heat-distortion temperature at 0.45 MPa for unfilled product-family samples is approximately 68 °C under ISO 75-2 method B. Annealing at 90 °C for 2 h can raise that value by 10 °C to 15 °C, but annealing also introduces shrinkage of 0.2% to 0.6% depending on part geometry and infill density. Published data for this specific annealed configuration is limited; production parts should be measured after the proposed thermal cycle rather than relying on raw-filament heat-distortion values.

On enclosed production printers, the amorphous character reduces warping at the base when the build chamber is held between 40 °C and 50 °C. At chamber temperatures above 60 °C, the remaining glass-transition margin becomes too narrow for unsupported overhangs and thin vertical walls, and parts can distort during the build. This behaviour differs from PLA, which may soften earlier at comparable chamber temperatures, and from ABS, which benefits from chamber heating above 70 °C but releases a more noticeable styrenic odour. Field data from direct-drive systems with hardened steel 0.4 mm nozzles indicate that spool-to-spool viscosity variance becomes visible as skipped extruder steps when the melt mass-flow rate falls below 7 g/10 min at 250 °C under a 2.16 kg load. Conversely, flow above 14 g/10 min is associated with stringing and nozzle drool. These boundaries are generic PET-G process thresholds and should be revalidated with lot-specific melt volume-flow rate measurements under ISO 1133-1:2022. Pigment masterbatch can shift viscosity by 5% to 10% even when the unfilled pellets meet the nominal melt-flow specification.

Moisture absorption and pre-drying requirements for stable extrusion

Glycol-modified PET filaments are hygroscopic but less moisture-sensitive than polyamide. At 23 °C and 50% RH, equilibrium moisture uptake is approximately 0.2 wt%; at 85% RH, it can exceed 0.5 wt%. Because the filament draws moisture predominantly at the surface before diffusion into the core, a short open-air exposure during loading is not equivalent to an equilibrated spool. Wet filament hydrolyses in the hot zone above 230 °C, reducing molecular weight and producing brittle parts with poor interlayer adhesion. Pre-drying at 65 °C for 4 h to 6 h in a forced-air dryer with a dew point below -30 °C is typical for spools exposed to ambient humidity. Vacuum drying at -0.1 MPa and 65 °C can shorten the process, but the spool core must tolerate the temperature without creep; polycarbonate cores may deform above 70 °C. After drying, storage in sealed barrier film with a desiccant pouch maintains moisture below approximately 0.05 wt%. On an open production line, exposure beyond 4 h at RH above 60% justifies re-drying. Hydrolysis damage is not reversible by drying alone; molar-mass markers such as intrinsic viscosity should be verified if the spool has been stored for months in humid conditions.

When the filament exits a 0.4 mm nozzle, melt shear rates exceed typical capillary rheometry

A print speed of 50 mm/s through a 0.4 mm nozzle can generate an apparent wall shear rate above 2000 s⁻¹. That shear regime is far removed from the 100 s⁻¹ range commonly used in melt-flow testing. PET-G exhibits shear-thinning behaviour; product-family capillary data at 240 °C show apparent viscosity falling from roughly 800 Pa·s at 1 s⁻¹ to approximately 120 Pa·s at 1000 s⁻¹. These values are not lot-specific and should not replace capillary rheometry on the actual filament. The implication is that melt-flow index alone cannot predict printing behaviour. High melt elasticity can produce oscillatory flow in Bowden systems, especially when retraction distances exceed 5 mm, causing inconsistent extrusion and local diameter swell at restart points. Low-friction PTFE tubing with an internal diameter of 2.0 mm for 1.75 mm filament, with radial slack below 0.1 mm, reduces melt-pressure oscillation. For direct-drive platforms, retraction distances of 1 mm to 3 mm are commonly used, with retraction speeds below 40 mm/s to avoid melt fracture and air entrapment.

The recommended nozzle temperature window for unfilled Mitsubishi PET-G is 230 °C to 250 °C. Below 220 °C, interlayer peel strength can decline sharply because insufficient polymer chain diffusion occurs at the interface. Above 260 °C, visible yellowing can occur, and low-molecular-weight volatiles can deposit on the nozzle and heat block. The bed temperature is generally maintained at 70 °C to 80 °C on textured PEI or polycarbonate build surfaces. First-layer height of 0.2 mm with a print speed of 20 mm/s improves adhesion on polished glass with a polyvinylpyrrolidone-based adhesive. Cooling fans are typically disabled for the first two layers and then run at 20% to 40% duty cycle to balance overhang quality against interlayer cracking. These settings are processing boundaries, not universal rules; chamber temperature, part cross-section, and colour masterbatch can shift the practical window by several degrees.

Dimensional drift and winding tension effects on lot-to-lot feed consistency

Filament diameter is not the only feed variable. Winding tension, spool core release, and ovality create transient diameter perturbations at the extruder gear. On a standard dual-gear extruder, an ovality of 0.03 mm can change cross-sectional area by approximately 4%, altering volumetric feed rate at constant linear speed. Manufacturer inspection typically reports average diameter drift below 0.02 mm over a 500 m spool length, but this figure is insufficient without a point-by-point deviation record. A conservative incoming acceptance test logs two-axis laser-micrometer data at 10 Hz during an unwind of the first 50 m. Any excursion beyond ±0.08 mm can create over-extrusion in low-back-pressure nozzle paths and visible surface banding. Winding tension above 200 g can embed stress in the filament, particularly with rigid grades at low humidity, and contribute to coil spring-back when the spool is partially unwound. Lot-to-lot colour changes should be treated as new material qualifications because the masterbatch particle size and rheology modifier load can alter die swell and filament roundness.

Comparative mechanical response against PLA, ABS, and PET feedstock

The table below compares product-family typical values for unfilled Mitsubishi PET-G filament with representative unfilled PLA, ABS, and PET filament data. The values are drawn from published technical bulletins and test methods; they do not replace lot-specific certificates. Mitsubishi PET-G occupies an intermediate position between rigid PLA and impact-responsive ABS. Its elongation at break is higher than PLA, but its notched Izod impact remains below typical styrenic filament.

The key performance distinction of the glycol-modified grade is not tensile strength but the ratio of toughness to stiffness. At 6 kJ/m² notched Izod and 2020 MPa tensile modulus, the material resists cracking in snap-fit and vibration-loaded fixtures better than PLA while retaining a higher modulus than many flexible polyester-based filaments. Compared with PET, the glycol modification reduces modulus and heat-distortion temperature but improves printed-layer fusion because crystallisation during cooling is suppressed. Compared with ABS, the material is less tolerant of high-temperature service above 80 °C, but it can be processed on open-frame machines without the same degree of heated-chamber control and with a lower styrene-related odour.

Chemical resistance thresholds and compliance testing

Mitsubishi PET-G filament is unsuitable for continuous immersion in ketones, chlorinated solvents, and strong aromatic hydrocarbons. Printed coupons exposed to acetone under a flexural strain of 0.5% for 24 h can exhibit whitening and stress crazing when evaluated under ASTM D543. The grade tolerates intermittent contact with dilute acids, many alkaline cleaning solutions, aliphatic hydrocarbons, and isopropyl alcohol wipe-down service. However, continuous contact with 70% isopropanol at 23 °C for 72 h has been reported in generic PET-G literature to reduce notched Izod impact by up to 15% because of solvent plasticisation and localised stress cracking. A specific compatibility test for the printed part is required before using the material in solvent-carrying adapters. Food-contact and medical applications must be revalidated on the final printed article under the applicable national or regional regulation, such as EU Regulation 10/2011 or FDA 21 CFR 177.1315. Printing with non-food-grade coloured masterbatch may alter migration behaviour and cannot be assumed compliant from the natural resin data alone. Raw-material REACH Regulation 1907/2006 and RoHS 2011/65/EU compliance are typically documented by the resin supplier; they do not automatically cover additives introduced during compounding or pigments used to produce coloured spools.

Fused filament parts in this material are not suitable for autoclave sterilisation above 121 °C because load-bearing dimensional stability is lost near the glass-transition range. Low-temperature hydrogen peroxide plasma or ethylene oxide cycles may be compatible if the fixture is dried before treatment and the geometry avoids entrapped pores. Residual stress from printing magnifies solvent ingress and environmental stress cracking; annealing at 60 °C to 70 °C for 1 h can reduce internal stress but may invalidate tight dimensional tolerances. The recommended storage range for unopened spools is 5 °C to 30 °C in sealed barrier film with desiccant. If intrinsic viscosity is reported near 0.75 dL/g on the certificate of analysis, a drop below 0.65 dL/g after prolonged storage or repeated drying indicates molecular weight loss and a corresponding reduction in melt strength. Dilute solution viscometry in 60/40 phenol/tetrachloroethane at 30 °C can be carried out under ASTM D4603 or ISO 1628-5 to verify lot integrity. The material is compatible with standard PLA purge sequences and can be removed with polypropylene purge; it should not be blended with polyamide remnants, which can build high-pressure blockages in the hot zone because of incompatible melt rheology and thermal degradation behaviour.