PLA Melt Ovality Limits in Multipass Water Calibration Troughs

Ovality in PLA profiles exiting multipass water calibration troughs is defined as the difference between the maximum and minimum measured diameter in the same transverse plane, expressed as a percentage of nominal diameter, and it is controlled by the interaction of quench asymmetry, melt drawdown, vacuum calibration force, residual stress, and the thermal history imposed by repeated water immersion. In extrusion grades of PLA, melt flow index measured according to ISO 1133-1:2022 at 210 °C under 2.16 kg generally falls between 2.0 and 6.0 g/10 min; grades outside this band tend to exhibit either insufficient melt strength for open-trough sag control or excessive shear heating that promotes hydrolysis and diameter drift. The melt density of PLA in the die range of 190 °C to 210 °C is approximately 1.08 to 1.12 g/cm³, while the solid density is typically 1.24 to 1.26 g/cm³, and this density change is directly linked to diametric contraction during solidification. Solid PLA has a thermal conductivity of only 0.13 to 0.16 W/m·K, which is roughly one order of magnitude lower than brass or aluminium calibration sleeves, and a specific heat capacity near 1.8 to 2.0 J/g·K. These properties mean that the surface temperature of the extrudate drops rapidly upon water contact while the core remains above the glass transition for a significant axial length; the resulting radial temperature gradient generates differential shrinkage that can lock in ovality whenever the water film or calibration sleeve contact is non-uniform around the circumference. A multipass water trough with a single-pass length of 6 m and 4 passes provides 24 m of effective immersion length in a footprint of approximately 2.5 m, but each turnaround introduces a local bending force, a change in water velocity vector, and a short unsupported span in which the solidifying profile can depart from circularity. The rollers that redirect the profile are usually fixed in pairs, and any deviation in roller plane alignment greater than 0.2 mm per metre of trough width is known from industrial setup practice to impose a repeating ovality signature. Published data for this specific configuration is limited in peer-reviewed literature, but equipment manufacturers and line auditors generally recommend laser alignment of turnaround rollers with the calibrator exit centreline to within 0.05 mm absolute and a water immersion depth of at least 30 mm above the top surface of the profile to prevent buoyancy-induced asymmetry.

How Does Multipass Water Trough Hydrodynamics Alter PLA Quench Symmetry?

The rate of heat transfer from the surface of a PLA profile to water in a multipass trough is not uniform around the circumference unless the relative water velocity and incident flow angle are deliberately balanced at every pass. The water-side heat transfer coefficient for forced convection over a submerged cylindrical surface can be estimated from correlations of Nusselt number as a function of Reynolds number and Prandtl number. For water at 30 °C, the density is approximately 995 kg/m³, dynamic viscosity is 0.0008 Pa·s, thermal conductivity is 0.618 W/m·K, and Prandtl number is near 5.4. A typical trough circulation velocity of 0.3 m/s to 0.8 m/s across a hydraulic diameter of 40 mm to 80 mm yields Reynolds numbers between 1.5 × 10⁴ and 8.0 × 10⁴, which reflect transitional-to-turbulent conditions. The highest surface heat transfer occurs at the upstream stagnation point of the profile relative to the water flow; the downstream separation zone has lower heat transfer and can leave a hot stripe along the profile. In a multipass arrangement the orientation of the profile relative to the flow can be changed at each turnaround by rotating the roller axis or by alternating water jets, but if the flow remains consistently transverse to the profile axis in the same direction across all passes, the asymmetry accumulates rather than cancels. The resulting circumferential solidification front becomes non-circular: the quenched side reaches the crystallization growth window earlier and develops a skin layer with different residual stress than the opposite side. Under such conditions, even a precision-ground calibrator sleeve cannot correct the internal stress asymmetry, and the profile may exit in specification but relax within 24 h to an ovality that exceeds the original tolerance. Water bath temperature rise across the trough is commonly limited to 2 °C to 3 °C in high-rate production, because a larger rise changes local viscosity and heat transfer coefficient; the inlet manifold temperature is typically controlled to 30 °C to 40 °C for PLA profiles of 2 mm to 12 mm diameter. At line speeds from 8 m/min to 25 m/min, exit surface temperature should remain below 45 °C to avoid deformation at the final turnaround roller and above 25 °C to prevent excessive residual stress from overly rapid skin quench.

Against the hydrodynamic backdrop, the crystallization kinetics of PLA impose a second boundary on ovality control because PLA does not crystallize rapidly from the melt in the same manner as polyethylene or polypropylene. The glass transition temperature of PLA is approximately 55 °C to 60 °C, and its maximum cold crystallization rate in the solid state occurs near 95 °C to 110 °C; when the surface of a strand is quenched too rapidly to a temperature below 50 °C, the outer skin may be largely amorphous while the core continues to cool more slowly. As the core passes through the cold crystallization range, it experiences additional densification and shrinkage, but the already frozen amorphous skin resists that contraction. The crystalline density of PLA can be taken as approximately 1.36 g/cm³, compared with an amorphous solid density near 1.25 g/cm³, which means that even a modest difference in crystalline fraction between the skin and core creates a measurable radial stress. If the water trough is operated at 25 °C or below, the skin quench is severe, the temperature gradient across the diameter can exceed 20 °C for a 4 mm profile, and ovality often increases by 0.5% to 2.0% compared with the same line speed at 35 °C. Conversely, water temperatures above 45 °C slow the skin formation sufficiently that the extrudate remains deformable at the contact rollers, allowing gravitational sag and roller contact flattening to imprint ovality. The practical quench window for many PLA extrusion lines is therefore between 30 °C and 40 °C, but the precise setpoint depends on line speed, profile diameter, and whether the trough is operated open or under vacuum. Thermal analysis of pellets and extrudate according to ASTM D3418-15 should be used to establish the glass transition and cold crystallization peaks for the specific grade; a formulation with a cold crystallization peak below 90 °C may require a lower water temperature setpoint than one with a peak at 105 °C because the solidification front moves through the critical temperature window at a different rate.

Molten PLA Drawdown, Vacuum Sizing, and Roller Contact Loads Interact to Set Ovality Limits

Melt drawdown ratio, defined as the cross-sectional area of the annular die divided by the cross-sectional area of the final product, is a primary determinant of molecular orientation, residual stress, and ovality in PLA round profiles. For PLA tube with a nominal outside diameter of 6.0 mm and wall thickness of 1.0 mm, typical die gaps are selected to give drawdown ratios between 1.5 and 3.0; ratios above 3.0 orient the PLA molecules strongly in the axial direction and can produce a profile that has anisotropic shrinkage after reheating. PLA melt has relatively low melt strength compared with polyolefins, and instrumented melt tension tests at 190 °C often record melt strength values of 2 cN to 6 cN for extrusion grades. Excessive drawdown causes draw resonance and periodic diameter variation that appears as ovality because the amplitude is not always equal in the vertical and horizontal axes. Vacuum calibration sleeves operate by pulling the hot profile against the internal wall of a cooled sleeve through a series of annular slots or holes while a water film lubricates the contact interface. The vacuum level in a PLA tube line is normally set between 0.2 bar and 0.5 bar below atmospheric pressure, depending on profile diameter and calibrator length. Vacuum levels below 0.15 bar may not produce sufficient contact force to round the profile before the skin freezes, while vacuum levels above 0.6 bar increase friction and can induce stick-slip marking or local overheating at the calibrator wall. The calibrator itself is usually fabricated from brass or stainless steel with internal land lengths of 20 mm to 80 mm and has a diametral clearance that is typically 0.1 mm to 0.3 mm larger than the target outside diameter to allow for thermal contraction. If the clearance is too large, the profile may rotate or float before solidifying; if too small, the contact pressure concentrates on the upper and lower surfaces and creates a two-point contact condition that leads to ovality. Roller contact loads after the calibration sleeve also contribute to ovality. The pinch force on the profile at each turnaround should be limited to the minimum required to maintain path tension; a peak contact stress above 0.3 MPa on a PLA surface at 40 °C is sufficient to produce measurable flattening, particularly if the surface is still above its Vicat softening temperature. Because PLA has a low surface hardness and a high coefficient of friction on polished steel, water lubrication at every roller is critical; dry or intermittently wet rollers are a common source of repeating ovality defects at intervals corresponding to the roller circumference.

Prior to extrusion, the moisture content of PLA pellets must be reduced below the threshold at which hydrolytic chain scission changes melt viscosity and melt strength during residence in the extruder and multipass trough. PLA is a polyester, and the ester linkage is susceptible to hydrolysis when moisture is present above approximately 0.025% (250 ppm) by weight at processing temperatures. Predrying in a desiccant bed or vacuum dryer at 80 °C for 4 h to 6 h typically brings moisture below 100 ppm to 200 ppm, and hopper dryers with a dew point of -40 °C or lower are required where ambient relative humidity exceeds 60%. In-line melt viscosity monitoring or MFR spot checks according to ISO 1133-1:2022 at the dryer outlet provide a practical verification of lot-to-lot stability. If moisture is not controlled, the MFR of an extrusion-grade PLA can drift by 2 g/10 min to 5 g/10 min during a single production shift, reducing melt strength and altering the drawdown response. In a multipass water trough, the hydrolyzed lower-viscosity melt is less able to resist the bending forces at the turnaround rollers, and the ovality signature changes progressively from the start to the end of the batch. The water in the trough itself is not a major source of hydrolysis within the residence time of the profile because hydrolysis is kinetically slow at water temperatures below 50 °C, but a water film adhering to the hot extrudate before the first roller can remain in contact at temperatures above 100 °C for several seconds; this is sufficient to produce a surface hydrolysis layer of reduced molecular weight and increased tack, contributing to roller stick and surface marking. A single-screw extruder with an L/D ratio of 30:1 and a compression ratio of 2.5:1 to 3.0:1 is typical for PLA; high compression ratios above 3.5:1 can generate excessive shear heating and localized temperature spikes above 220 °C, at which point thermal degradation and lactide formation accelerate. Processing melt temperature at the die should be measured with an immersion thermocouple and held within a 10 °C band, usually 190 °C to 210 °C, because deviations above 220 °C produce yellowing, molecular weight loss, and unstable drawdown.

When Melt Drawdown Ratio Exceeds 2.5 in Small-Diameter PLA Profiles

When the melt drawdown ratio exceeds 2.5 for PLA profiles with outside diameters below 3.0 mm, the processing window for ovality control narrows substantially because the draw resonance and the solidification front move closer to the die face. At a die diameter of 5.0 mm and a final diameter of 1.75 mm, the drawdown ratio is approximately 8.2 by area, which is far above the stable range for many PLA grades and requires a specially formulated high-melt-strength PLA or a melt pump and closed-loop diameter control to prevent periodic ovality. Published data for this specific configuration is limited because most PLA filament lines use larger die gaps and moderate drawdown ratios near 3.0 to 5.0 with water troughs operated at 35 °C to 45 °C. The high drawdown condition also increases molecular orientation in the outer layer, which elevates residual stress and magnifies ovality if the water flow is not perfectly axisymmetric. In multipass water troughs, the first pass is the most critical for small-diameter profiles because the surface is still soft and the water drag force can displace the filament laterally by more than 0.1 mm if the water velocity is excessive. Reducing water velocity to 0.2 m/s to 0.4 m/s in the first pass and increasing it in subsequent passes is a standard line setup procedure that balances heat transfer with drag. Mechanical testing according to ASTM D638-14 of samples with high drawdown often shows transverse tensile strength less than 35 MPa while axial tensile strength exceeds 60 MPa; the anisotropy is an indicator of orientation and a predictor of poor ovality after storage.

Ovality Measurement Protocol and In-Line Laser Micrometer Data Acquisition

Ovality measurement for PLA round profiles should be performed in-line with a dual-axis or multi-axis laser micrometer or optical gauging system that samples diameter at 100 Hz to 1000 Hz and records maximum, minimum, and average diameter values for each transverse plane. Off-line verification is carried out with a calibrated bench micrometer or a coordinate measuring machine at 20 °C to 23 °C after conditioning the specimen for 4 h to 24 h at 23 °C and 50% relative humidity in accordance with ISO 291:2008. Ovality is calculated as (D_max − D_min)/D_nominal × 100; for a 6.00 mm nominal tube, an ovality of 1.0% corresponds to a diameter difference of 0.06 mm. The in-line device should be placed after the final dewatering pass, but before any haul-off or winder that might apply contact pressure, so that the measurement reflects the free, solidified profile. Data acquisition should include minimum, maximum, mean, ovality, and standard deviation over a moving window of 500 mm to 2000 mm of product length. A sampling interval of 10 ms is generally sufficient to capture periodic defects caused by out-of-round rollers or vacuum slot spacing; defects associated with roller circumference typically appear at frequencies between 0.5 Hz and 5 Hz at line speeds of 10 m/min to 25 m/min.

Table 1 provides a compliance matrix for dimensional and material verification of PLA round profiles subject to ovality control.

In production practice, the ovality signature of a PLA profile through a multipass water trough is rarely the result of a single variable but rather the interaction of several control parameters that exhibit non-linear thresholds. The most sensitive control band is the water bath temperature at the inlet manifold; the practical window for PLA is commonly 30 °C to 40 °C, and a shift of more than ±4 °C around the setpoint can increase ovality by 0.5% to 1.5% within 5 min to 10 min because the solidification front moves axially along the trough. The second sensitive parameter is vacuum level in the calibration sleeve: the normal negative pressure band is 0.2 bar to 0.5 bar, and pressures below 0.15 bar may permit the profile to collapse or float, whereas pressures above 0.6 bar increase contact friction and can generate a visible axial stick-slip marking on the upper and lower quadrants. Puller force or haul-off tension is usually maintained below 10 N for profiles of 4 mm to 6 mm outside diameter; higher tensions stretch the hot skin and increase the orientation along the profile axis, which leads to ovality after residual stress relaxation. Melt temperature at the die is controlled within 190 °C to 210 °C for processes without a melt pump; with a melt pump installed, a 5 °C temperature increase at the die can be compensated by slightly retarding the pump speed, but the resulting shear history change may alter the die swell and require a new calibration sleeve clearance.