Polyamide 6 (PA6)/polyolefin alloys are processed as hygroscopic two-phase thermoplastics in which the PA6 phase provides the continuous matrix for short-term mechanical strength and the polyolefin phase reduces moisture-conditioned stiffness loss and improves low-temperature impact. In injection molding of parts containing holes, ribs, or multi-gated filling, weld lines form at the confluence of separate melt fronts. The weld line is not a crack but a zone of frozen oriented material with limited chain interdiffusion across the impinging interfaces. The resulting local mechanical performance in PA6/polyolefin alloys is governed by melt rheology, local cooling rate, pressure history, moisture content, and the interfacial morphology between PA6 and the polyolefin domain. Because PA6 is hygroscopic and subject to hydrolytic chain scission above its melting point, drying before processing is a prerequisite. Moisture content should be below 0.10 % by weight as determined by ISO 15512 Method B. Processing conditions for these alloys are typically set between 230 °C and 280 °C for the melt, with mold temperatures between 40 °C and 120 °C depending on wall thickness and required weld-line toughness. A capillary rheometer operated according to ASTM D3835-16 and a melt index instrument operated according to ISO 1133-1:2022 provide conventional quality control data, but these values alone do not predict weld-line strength because weld-line strength is strongly influenced by the elongational flow at the melt front and by the dynamics of frozen-layer formation. Welded tensile specimens tested per ISO 527-2 or ASTM D638-14 generally show retention values relative to unwelded specimens between 0.40 and 0.80; the low end corresponds to uncompatibilized, moisture-contaminated, or cold-molded material, while the upper end corresponds to dried, compatibilized, hot-molded material. The impact strength at the weld line, measured by ISO 179-1/1eA Charpy or ISO 180/1A Izod, can be less than 50 % of the unwelded value, and this loss is more severe at sub-zero test temperatures. The exact numerical values depend on mold geometry, gate location, part thickness, and polyolefin type; published data for a specific commercial alloy vary sufficiently that production-scale validation is required.

What Limits Melt Front Interdiffusion in Weld Lines of PA6/Polyolefin Alloys?

The formation of a weld line begins with fountain flow at each advancing melt front. In a PA6/polyolefin alloy with continuous PA6 matrix, the fountain flow transports material from the midplane to the wall region, where it is deposited and quenched. The oriented skin layer at the front has a high degree of molecular extension. When two fronts impinge, the contacting surfaces are already partially frozen, and the available free volume for chain segment diffusion is low. For PA6 with a relative viscosity of 2.7 measured in 96 % sulfuric acid per ISO 307, the zero-shear viscosity at 250 °C is high, but the process-relevant viscosity at 1000 s⁻¹ is more informative; dry resin values between 200 Pa·s and 350 Pa·s are common, while moisture-containing resin shows lower values due to hydrolytic degradation. The polyolefin phase, which may be polypropylene, polyethylene, or an ethylene-octene copolymer, has a different crystallization temperature and a different viscosity. In an uncompatibilized alloy, the viscosity ratio between the dispersed polyolefin droplet and the PA6 matrix at the melt front determines whether the droplet stays in the core or is drawn into the weld line. If the polyolefin has a lower viscosity at the relevant shear rate, it can migrate to the wall and coat the weld line, creating a weak plane. The weld-line strength retention is reduced further when the melt front temperature at the weld line is below the PA6 crystallization temperature; under fast cooling, the PA6 crystallizes before the two fronts reach full molecular interdiffusion. Therefore the time available for interdiffusion is set by the temperature decay between the gate and the weld line, not by the total cycle time. Cavity pressure transducers at the weld line often show a pressure pulse that decays rapidly in thin sections. When the pressure at the weld line has dropped below approximately 100 bar, packing is insufficient to compensate for volumetric shrinkage, and the weld line opens into a surface notch.

Capillary rheometry according to ASTM D3835-16 using a die with L/D 20:1 at 250 °C and shear rates from 100 s⁻¹ to 5000 s⁻¹ provides the shear viscosity function required for mold filling simulation. The Cross-Williams-Landel-Ferry (Cross-WLF) or Carreau-Yasuda model is fitted to these data. For a dried PA6/polyolefin alloy with 30 wt% polypropylene, the zero-shear viscosity at 250 °C may be between 800 Pa·s and 1500 Pa·s, and the power-law index n is typically between 0.60 and 0.80 in the shear-thinning region. The polypropylene phase with a melt flow rate of 20 g/10 min at 230 °C/2.16 kg per ISO 1133-1:2022 has a shear viscosity at 1000 s⁻¹ around 120–180 Pa·s at 250 °C. The viscosity ratio at 1000 s⁻¹ is therefore in the range 0.4–0.9 depending on moisture and compatibilizer level. A viscosity ratio closer to 1.0 tends to preserve a dispersed or co-continuous morphology that maintains weld-line integrity. However, weld-line strength also depends on elongational viscosity. In the fountain flow region, the melt undergoes extensional deformation. Neat PA6 has weak strain hardening, while certain low-density polyethylene grades show pronounced strain hardening; the mismatch can cause local flow instabilities at the converging front and promote the formation of a V-notch. Melt strength measurement using a capillary rheometer with a haul-off device can identify differences, but there is no universal pass/fail criterion. A practical method is to measure tensile weld-line strength retention at controlled conditions per ISO 527-2 and to relate that value to cavity pressure at the weld line.

Moisture, Hydrolytic Degradation, and Viscosity Collapse at the PA6 Phase

PA6 is hygroscopic; moisture level determines both processing viscosity and weld-line fusion. PA6 containing 0.20 % moisture at 270 °C undergoes hydrolysis, reducing molecular weight by chain scission, which lowers melt viscosity and weld-line strength. The hydrolytic chain scission reaction follows pseudo-first-order behavior with respect to water concentration at melt temperature, and its rate increases sharply above the melting point. Drying to less than 0.08 % moisture by weight with desiccant dryers operating at a dew point below −30 °C and air temperature of 80 °C for 4–6 h is required. In production-scale operations, PA6/polyolefin blends that are not dried have shown viscosity drops of 15–25 % as measured by capillary rheometry at 1000 s⁻¹, and weld-line tensile strength retention can fall below 0.40 under ASTM D638-14. The polyolefin phase reduces overall moisture uptake but does not protect PA6 from hydrolysis at the weld line because the weld line is enriched in PA6 at the surface. Moisture also generates volatiles that produce surface splay and microscopic voids at the weld-line contact. When ambient relative humidity exceeds 60 %, open hopper residence must be minimized and sealed hopper/dryer circulation used to prevent reabsorption. Over-drying above 90 °C can cause yellowing and thermal oxidation of the polyolefin phase, particularly when the blend contains unstabilized polypropylene. The lower viscosity caused by moisture may reduce injection pressure, but the loss of molecular weight reduces chain interdiffusion at the weld line. A lot with moisture content above 0.15 % shows surface splay, foaming at the weld line, and inconsistent cavity pressure transfer. Moisture analysis per ISO 15512 and drying control are therefore mandatory boundary conditions for any weld-line critical application.

When the Dispersed Polyolefin Phase Carries Maleic Anhydride Graft Functionality

Maleic anhydride grafted polypropylene and maleic anhydride grafted ethylene-octene copolymers react with the terminal amine groups of PA6 during melt compounding. The acid number of the maleated polyolefin, typically 3–8 mg KOH/g as stated on the manufacturer's certificate of analysis, determines the extent of interfacial reaction. During compounding in a co-rotating twin-screw extruder with L/D 40:1, two kneading blocks, and atmospheric venting at barrel 8, the reaction produces a PA6-g-polyolefin copolymer at the phase boundary. This copolymer reduces interfacial tension from approximate values of 5–10 mN/m to 1–3 mN/m at 250 °C; published data for this specific configuration is limited and values depend on molecular weight and graft level. The morphological consequence is a reduction in dispersed phase droplet size from more than 10 µm to below 2 µm. At the weld line, the finer domain structure prevents the formation of a continuous weak polyolefin layer at the impingement plane. The weld-line tensile strength retention of a PA6/PP 70/30 alloy can improve from about 0.45 to 0.70–0.78 with 3–5 wt% of an appropriate maleated polyolefin when tested according to ISO 527-2. Impact properties at the weld line, measured by ISO 179-1/1eA at −30 °C, also improve but remain below the unwelded values. However, excess maleic anhydride can generate additional amide-imide branching, increasing shear viscosity and heat dissipation, leading to thermal degradation of the PA6 phase when barrel residence time exceeds 5 min at 280 °C. The processing stabilization package must therefore include a hindered phenol antioxidant and a phosphite secondary antioxidant, as specified for PA6 at melt temperatures above 260 °C. Amine-based heat stabilizers or chain extenders containing free primary amine groups should be avoided in maleated polyolefin alloys because they react with maleic anhydride grafts and can cause premature crosslinking, increasing viscosity and reducing weld-line fusion.

For systematic comparison, a PA6/PP 70/30 alloy can be evaluated with controlled moisture content below 0.08 %, a melt temperature of 260 °C, a mold temperature of 80 °C, and a double-edge-gated plaque of 2 mm thickness. The table below lists representative values from laboratory molding; these are not specification limits and have been compiled from typical published data.

Cavity pressure transducers placed at the gate and at the weld line on a 1500 kN servo-electric injection molding machine with a 60 mm three-zone screw provide local pressure histories during filling and packing. The weld-line pressure at the moment of front contact is lower than the inlet pressure by 30–50 %; restoring pressure quickly is required. Melt temperature, mold temperature, injection speed, and holding pressure are relevant. Increasing mold temperature from 40 °C to 120 °C increases time for interdiffusion and improves weld-line tensile strength by 10–20 %, but increases cycle time. Reducing melt temperature below 250 °C to avoid PA6 thermal degradation may reduce interfacial temperature and degrade weld-line strength. A processing window of ±5 °C is sometimes required for thin-wall parts, because temperature variation at the weld line alters frozen layer thickness and weld-line strength. For parts with wall thickness less than 1.5 mm, the injection time may be below 0.3 s; the weld line is fully frozen before packing pressure can act. Published production-scale data for this specific configuration is limited, but cavity pressure sensors indicate a pressure drop below 100 bar at the weld line during fill for such thin sections, below the threshold needed for chain interdiffusion.

Weld-line tensile strength retention is governed by cavity pressure decay rate.

The cavity pressure decay rate at the weld line after filling determines whether the material remains in intimate contact during crystallization shrinkage. If the pressure decay is too fast, the two fronts separate, producing a surface notch and a low molecular orientation region. The decay rate can be estimated from cavity pressure transducer data sampled at 1 kHz. In a 2 mm plaque with a weld line, a pressure decay from 500 bar to 100 bar in less than 0.5 s is associated with poor weld-line tensile retention; holding pressure must be increased or gate freeze time extended. The melt temperature must be high enough to allow the pressure wave to reach the weld line before the gate freezes. In hot runner systems with valve gates, the gate freeze time can be very short, and the pressure at the weld line is controlled by the valve pin closing time rather than by the screw holding phase. The use of sequential valve gating can position the weld line away from highly stressed areas, but the last melt fronts still create a weld line. A pressure-sensor-guided packing strategy may be required for parts with wall thickness below 2 mm. The processing limit is often mold temperature: if mold temperature is below the PA6 glass transition temperature of approximately 55–65 °C, the frozen skin thickness increases and the cavity pressure necessary for weld-line healing rises. When mold temperature is raised to 120 °C, the weld line remains above the PA6 glass transition for a longer period, allowing polymer chain segment diffusion across the interface.

Fracture surfaces of welded specimens tested per ASTM D638-14 show a smooth region at the weld line, indicating brittle failure, while the surrounding matrix may show ductile tearing. Scanning electron microscopy at 250× and 1000× reveals the dispersed polyolefin phase and the degree of interfacial adhesion. In an uncompatibilized alloy, large polyolefin droplets are visible at the weld line and are pulled from the PA6 matrix. In a compatibilized alloy, the droplets are finer and remain embedded in the fracture surface. The weld line region can have a skin layer of PA6 of 50–150 µm thickness, depending on mold temperature and injection rate. This skin layer is depleted of polyolefin and limits ductility. Differential scanning calorimetry per ISO 11357-3 at 10 K/min shows PA6 crystallinity at the weld line may be higher than in the bulk because of orientation-induced crystallization, while the polyolefin phase crystallizes later and creates shrinkage differences. Polarized optical microscopy on thin sections after etching reveals the V-shaped weld line and the frozen skin. The degree of dispersion of polyolefin in the core is evaluated based on phase size; a phase size below 2 µm is generally associated with better weld-line impact strength. Environmental conditioning of welded specimens according to ISO 1110 can raise weld-line impact strength due to plasticization of PA6, but tensile strength decreases; therefore data must be reported with moisture condition.

Injection molding simulation packages use the Cross-WLF viscosity model parameters determined from capillary rheometry and PVT data obtained under standard conditions. Mold filling simulation predicts the weld line location and the temperature and pressure at the weld line. Validation is performed by short-shot studies and cavity pressure transducer data. The simulated weld-line temperature in a PA6/PP alloy for a 2 mm plaque should remain above the PA6 crystallization temperature for a sufficient time before packing. If the simulated temperature at the weld line at the end of fill is below 190 °C, tensile strength retention under ISO 527-2 is likely below 0.50. If the temperature is above 220 °C and the cavity pressure exceeds 500 bar, retention above 0.70 is possible. These values are specific to mold geometry and alloy formulation; published data for this specific configuration is limited. Production-scale validation requires at least three consecutive lots with moisture content below 0.08 %, injection speed, holding pressure, and mold temperature logged. Parts should be tested dry as molded and after conditioning per ISO 1110 because PA6 properties shift with absorbed moisture. The weld-line position, surface appearance, and mechanical property retention must be documented for process capability. For any PA6/polyolefin alloy, the operational boundary is set by the combination of melt temperature, moisture control, compatibilizer level, and cavity pressure at the weld line; outside this window the weld-line tensile strength retention drops below the part specification even when bulk material properties remain acceptable.