Processing of intermingled polyamide partially oriented yarn (POY) for fine gauge hosiery and medical knits begins with selection of linear density and filament geometry against the needle gauge and loop length of the intended circular knitting machine. A 15 dtex f7 polyamide 6 POY with elongation at break of 62–70% and tenacity of 3.5–4.0 cN/dtex under ISO 2062:2009 permits loop formation on a 32–34 gauge single-cylinder hosiery machine with 400 needles; the same yarn fed from an overhead creel at a distance of 1.2–1.8 m through ceramic thread guides with a coefficient of friction below 0.20 maintains a yarn path tension of 2–4 cN before the positive yarn feeder. If intermingling node density is below 20 nodes/m, filaments separate between the creel and the yarn feeder, producing periodic slubs and an increase in laddering propensity. If node density exceeds 110 nodes/m, the nodes themselves act as temporary anchoring points that raise short-duration tension peaks above 6 cN during stitch cam passage, generating loop length variability greater than 0.15 mm in a 3.0 mm stitch cam. Intermingling in POY is therefore neither a twist equivalent nor a texturing step; it imposes discrete compressed nodes at intervals of 9–50 mm that constrain filament lateral displacement but leave the untwisted filament bundle between nodes free to flatten in the knitting zone. Linear density is verified by skein method according to ISO 2060:1994, and the coefficient of variation of yarn linear density should remain below 1.5% within a package and below 2.0% between packages for acceptable stitch uniformity. Published data for the exact interaction between intermingling node geometry and loop distortion on 34 gauge hosiery machines is limited; however, process logs from single-cylinder circular knitting lines indicate that node density is adjusted before the draw point on the POY machine and is not increased after drawing because of filament damage risk.

What Limits Intermingling Node Density in Polyamide 66 POY for Medical Compression Fabrics?

In medical compression knits manufactured from 44 dtex f34 polyamide 66 POY, intermingling node density is constrained by both the knitting mechanics of the body-size circular machine and the subsequent dyeing and finishing requirements. Polyamide 66 exhibits a higher glass transition temperature and a higher tenacity than polyamide 6, but its acid dye uptake is slower under standard conditions; a tightly compressed intermingling node functions as a localised barrier to dye penetration because the free volume within the node is reduced and the filament surfaces are brought into close contact. On a 4-feed single-jersey circular knitting machine with positive storage feed and take-down tension of 0.20–0.35 cN/dtex, node densities above 100 nodes/m generate measurable drag at each ceramic eyelet, raising the average yarn tension before the knitting point from 3.5 cN to 5.5–7.0 cN. The consequence is not immediate yarn breakage but a progressive reduction in loop length, which changes the compression level of the finished stocking after heat-setting; compression class boundaries under RAL-GZ 387 are separated by as little as 5 mmHg, so a loop length shift of 0.10 mm across the circumference is sufficient to move a class II stocking into class I or class III performance. Intermingling nodes below 20 nodes/m, by contrast, allow filament splaying at the knitting elements, and loose surface filaments entangle with latch needles on 34 gauge machines, increasing needle latch failures and causing vertical ladder defects. The practical working range for 44 dtex f34 polyamide 66 POY in medical compression fabrics is therefore 30–80 nodes/m, with an upper limit of 80 nodes/m for dark shades and a lower limit of 30 nodes/m for continuous high-speed knitting above 900 rpm. Node stability can be assessed on a production-scale warping or knitting creel by measuring yarn tension variance over 10,000 m using a 100 Hz electronic tension transducer; peak-to-peak tension variation should remain below 2.5 cN for uniform loop formation.

Conditioning of polyamide POY packages at 21±1°C and 65±2% relative humidity according to ISO 139:2005 is a basic but non-negotiable stage because the equilibrium moisture content of polyamide 6 and polyamide 66 is 4.0–4.5% and 3.5–4.0% respectively at those conditions. When packages are transferred from a storage environment below 35% relative humidity to the knitting floor without conditioning, static charge accumulation on intermingled POY exceeds 2.5 kV on the package surface, causing adjacent ends to repel one another and increasing yarn breakage at the creel. In contrast, storage at a relative humidity above 70% causes the paper tube and the outer yarn layers to swell unevenly, producing package hardness gradients from 35 to 52 Shore A across the bobbin radius. Such a gradient generates cyclical tension fluctuations during unwinding because the yarn release angle changes as the package diameter decreases from 170 mm to 140 mm. Moisture absorption also modifies the frictional behaviour of the intermingled yarn against ceramic guides; the coefficient of friction of conditioned polyamide 6 POY at 0.25 can rise to 0.32 after 4 h at 85% relative humidity, which is enough to alter loop length on positive-feed machines. The conditioning time required for stable knitting is typically 24–48 h in the knitter’s room, with packages removed from polyethylene wrapping at least 12 h before creeling.

Draw Ratio and Residual Elongation Mismatches on Single-Feed Circular Hosiery Machines

Residual elongation after the draw point is the primary predictor of knitting performance on single-cylinder fine gauge hosiery machines, because the yarn is not fully drawn before it enters the first needle. A polyamide 6 POY with a draw ratio of 1.25–1.35 and a residual elongation of 25–35% is suitable for 15 dtex f7 yarns used in 32 gauge stockings, whereas a residual elongation above 80% produces excessive stretch and loop distortion during the take-down stage. If residual elongation falls below 20%, the yarn is over-drawn and brittle, and breaks occur preferentially at the latch needle contact point where the yarn is bent around a radius of 0.15–0.25 mm. Polyamide 66 POY requires a lower draw ratio, typically 1.20–1.30, because its molecular chain orientation is already high and its elongation at break under ISO 2062:2009 is 58–67%. The draw ratio on a draw-texturing or draw-warping line must be matched to the intrinsic viscosity of the POY; for polyamide 6 chips with a relative viscosity of 2.45–2.60 measured in 96% sulfuric acid at 25°C, the corresponding POY elongation at break ranges from 62% to 70%. A batch-to-batch shift in relative viscosity from 2.50 to 2.58 can lower elongation at break by 8% under a fixed draw ratio, a change that is visible on the knitting line as an increase in needle breaks and loop length variation but is not easily detected by package hardness or visual inspection. The correct response is not to increase intermingling node density to mask filament separation, but to adjust the draw ratio in increments of 0.02 until the residual elongation returns to the target band.

Under ISO 10993-1:2018, a medical compression stocking made from intermingled polyamide POY is a surface-contacting medical device with prolonged contact duration, so the finished textile must be evaluated for cytotoxicity, sensitisation, and irritation potential according to ISO 10993-5:2009, ISO 10993-10:2010, and ISO 10993-23:2021 where applicable. The spin finish applied to POY is a process aid that can contribute extractables; therefore, scouring and finishing of medical knits typically require a non-ionic detergent wash followed by rinsing with deionised water at 60–80°C to reduce non-volatile residue below 0.1% by mass before final heat-setting. Formaldehyde content of the finished knit should be below 16 mg/kg according to ISO 14184-1:2011, and the product should conform to the Oeko-Tex Standard 100 Annex 4 limits for products with direct skin contact, including extractable heavy metals below 1.0 mg/kg for lead and below 0.02 mg/kg for mercury. The selected polyamide POY must also be free of spin finishes containing alkylphenol ethoxylates above the REACH Annex XVII restrictions, because these can be released during wear and are specifically regulated. Medical knits made from polyamide 66 POY generally have a higher dimensional stability during repeated sterilisation up to 134°C, whereas polyamide 6 POY is more sensitive to hydrolytic degradation and should not be selected for reusable medical textiles that are autoclaved more than 50 cycles at 121°C unless a stabiliser package is specified.

When High Intermingling Density Creates Dye Barrier Formation in Polyamide 6 Medical Knits

When intermingling node density on 22 dtex f7 polyamide 6 POY exceeds 130 nodes/m, the compressed regions act as diffusion barriers during acid dyeing, and the resulting medical knit can show alternating light and dark bands even though the yarn itself is uniformly spin-finished. Acid dyes for polyamide 6 are typically applied at 95–98°C in a dyebath buffered to pH 5.0–5.5; under these conditions dye uptake at a normal intermingling node is only slightly reduced, but at node densities above 130 nodes/m the total amount of compressed polymer mass per metre increases and the time required for dye to penetrate the node exceeds the 30–60 min hold period. The visual difference between the node and the open yarn segment after dyeing can be quantified by colour measurement according to ISO 105-J03, with a ΔE CMC(2:1) greater than 1.5 considered unacceptable for uniform skin-tone hosiery. Dye barrier effects are more severe on polyamide 66 than on polyamide 6 because polyamide 66 has a more compact hydrogen-bonded structure and higher thermal transitions. The corrective action on the POY line is to reduce the intermingling air pressure from 3.5 bar to 2.5 bar or to increase the yarn speed through the intermingling jet from 600 m/min to 800 m/min, both of which reduce node density while maintaining adequate filament cohesion. A laboratory-scale knot retention test with a 50 g weight suspended from a single node for 60 s is used by some mills, but no ISO standard exists for this test, and published data for the relationship between node density and dye penetration in medical compression knits is limited.

Comparative Tensile and Shrinkage Compliance Matrix for 15 dtex f7 and 44 dtex f34 Polyamide POY

Package build defects on 3.5 kg polyamide 6 POY bobbins are a recurring source of knitting faults that cannot be corrected by adjusting intermingling node density or machine tension. A package with a shore A hardness below 38 permits inner layer migration during high-speed unwinding, causing the yarn to slip and create tangles, while a package with a hardness above 42 compresses the intermingled nodes and can increase the apparent node density at the unwinding point by 10–15% because the yarn is flattened. Cross-wound packages with a traverse ratio of 8.5–9.5 and a cone angle of 5°57′ for hosiery creels provide stable unwinding; the unwinding tension measured at 600 m/min should remain between 2.5 cN and 5.0 cN for 15 dtex f7 yarns. At the knitting machine, ceramic tension discs must be maintained with a surface roughness Ra below 0.4 µm; higher roughness values abrade the intermingled nodes and release filaments, producing fly waste on the cylinder. The combination of a stable package hardness of 38–42 Shore A, a controlled package diameter of 170–180 mm, and a creel distance of 1.2–1.5 m from the yarn feeder is more effective than increasing intermingling pressure for reducing knitting faults in fine gauge hosiery.

The compliance matrix below maps each required property to a test method and numerical acceptance limit

On-body frictional shear stress for class III medical knits made from 44 dtex f34 polyamide 66 POY is influenced by the interaction between intermingling nodes and the loop surface, but the primary selection variable remains the residual elongation and tenacity of the yarn after drawing. A knit prepared with 22 dtex f7 polyamide 6 POY at 80 nodes/m and residual elongation of 28% exhibits lower loop distortion during wear than the same yarn at 120 nodes/m, because the excessive nodes create stiff localised hinges that reduce the uniform elastic recovery of the knitted fabric. Repeated tensile testing of the finished fabric according to ISO 13934-1:2013 at 20 N load shows a loss of elastic recovery of 4–6% after 10,000 cycles when node density is above 100 nodes/m, whereas the reference fabric at 60 nodes/m retains elastic recovery within 2% of the original. These differences are not captured by standard yarn tensile tests alone and require fabric-level fatigue testing on a cyclic tensile tester with a 50 mm gauge length and a frequency of 1 Hz. The acceptable node density range for a 34 gauge single-cylinder machine with take-down tension of 0.30 cN/dtex is 40–70 nodes/m, provided the residual elongation remains between 25–35%.