Anionic ring-opening polymerisation of caprolactam in bulk is distinguished from hydrolytic PA6 production by the absence of water as initiator and by the application of a strong base catalyst in combination with an N-acylated activator. In monomer casting of semi-finished sheet and rod, molten caprolactam at approximately 120–180 °C is degassed under vacuum, treated with catalyst and activator in separate streams to prevent premature reaction, and metered into a mold cavity where polymerisation and crystallisation proceed in a single cooling profile. The resulting polymer is characterised by molecular weights that can exceed 250,000 g/mol, high isotropy in the cast cross-section, and crystalline fractions typically between 45% and 55% according to differential scanning calorimetry. Because the monomer melt has low viscosity at the filling stage, the process accommodates section thicknesses that are not accessible to melt extrusion, and the cast form is used primarily for machining stock in gears, rollers, wear pads, and structural components requiring high toughness and low moisture sensitivity relative to other nylons.

What Governs the Adiabatic Temperature Rise in Thick Cast PA6 Stock?

The principal process boundary in anionic caprolactam casting is the exothermic enthalpy release coupled with low thermal conductivity of the forming polymer. For a semi-finished block with thickness above 40 mm, the centreline temperature may exceed the boiling point of residual caprolactam or initiate thermo-oxidative discolouration if the mold wall temperature and fill temperature are not controlled. Heat removal during polymerisation is governed by the balance between reaction rate and conduction through the polymerising mass. In production-scale casting, heated mold walls are maintained between 150 °C and 170 °C, while the activated monomer feed temperature is typically 130–160 °C. Lower wall temperatures reduce cycle time but increase radial crystallisation gradients and internal stress; higher wall temperatures preserve optical clarity and minimise void formation but may prolong demold time and increase residual monomer concentration. The temperature profile across a cylindrical rod is parabolic during the reaction-front propagation, with centreline solidification occurring after the wall layers have already crystallised. This differential solidification produces a compressive skin and a core with lower density if volumetric shrinkage is not compensated by a riser or pressure plate. Casting of rods above 150 mm diameter often requires staged cooling, reduced catalyst levels, or insulating sleeves to prevent centreline porosity.

In vertical static mold casting of rod, the activated monomer is introduced through a bottom-fill port to avoid air entrapment, and the mold is sealed under a nitrogen blanket to exclude moisture and carbon dioxide. Sheet casting uses horizontal open molds with adjustable side dams and a heated top plate or infrared bank. Thickness tolerance in the as-cast sheet is generally controlled to ±0.5 mm for thicknesses up to 20 mm and proportionally larger for thicker sections; subsequent fly-cutting or surface grinding brings machinable blanks to final flatness. The absence of melt orientation in cast PA6 is a critical characteristic for machining stock, because cutting forces do not release anisotropic frozen-in stresses to the same extent as in high-speed extruded bar. However, volumetric shrinkage during polymerisation and crystallisation leaves a residual stress pattern that is dependent on section geometry, mold temperature, and post-curing annealing. Producers therefore stress-relieve cast shapes prior to precision machining by slow heating to 160–180 °C in oil or air, holding for 4–8 hours depending on section thickness, and cooling at rates not exceeding 10 °C/h until the part reaches 60 °C.

When Residual Caprolactam Monomer Exceeds Machining Tolerance Criteria

Residual unreacted caprolactam in anionically polymerised PA6 typically falls between 0.5 wt% and 3.0 wt% immediately after demolding, depending on catalyst ratio, mold temperature, and post-curing time. When residual monomer remains above approximately 1.5 wt% in the machined stock, subsequent service at temperatures above 60 °C can release monomer vapour, cause surface tack, and alter dimensions through migration and crystallinity changes. Dimensional drift in machined bearing components is less often caused by moisture absorption alone than by slow monomer migration from the core to the surface, particularly in thick sections where centreline conversion is lower. Annealing at 160–180 °C reduces residual monomer and completes conversion of active centres; the process also increases crystallinity and reduces the tendency of thin-walled machined parts to warp after machining. A production-scale stress-relief cycle for cast PA6 rod of 80 mm diameter may hold the part for 6 hours at 170 °C followed by controlled cooling; published data for specific residual monomer levels after such cycles is limited, but industrial practice is to specify maximum residual monomer below 1.0 wt% for precision gear and bearing applications. Vacuum post-treatment at 180 °C can further strip residual monomer, but this must be balanced against oxidative yellowing and the possible loss of impact modifier if a modified grade is used.

Because PA6 absorbs water from the surrounding environment, the machined dimensions of as-cast parts are not stable until the moisture content reaches equilibrium with the service atmosphere. According to ISO 62:2008, immersion at 23 °C for 24 hours produces water absorption values in unfilled cast PA6 of approximately 1.0–1.5 wt%, while saturation in water reaches 6–7 wt%. A dry-machined part will swell by approximately 0.3–0.6% in the linear dimension when conditioned to 50% relative humidity at 23 °C, and this change is sufficient to close running clearances in a rotating shaft bearing. Machining stock is therefore supplied either in a dry state with final machining allowances, or in a moisture-conditioned state to produce near-net dimensions. For parts requiring tight clearance control, the blank is often machined to within 0.5 mm of final dimension, conditioned in water or humid air to the target moisture content, and then finish-machined dry or with minimum coolant. This practice is documented in bearing manufacturer guidelines but the exact conditioning protocol depends on the end-use environment.

Tool Edge Degradation in Reinforced and Unreinforced Monomer-Cast Nylon 6

Machining of cast PA6 generates continuous ribbon-like chips that can wrap around rotating tooling and workpieces, creating frictional heat and dimensional error. The material’s low thermal conductivity of approximately 0.23–0.29 W/m·K concentrates cutting heat at the tool-chip interface, and the uncrystallised or low-crystallinity skin layer may smear if cutting speeds exceed recommended values. Tools with positive rake angles between 5° and 15° and large clearance angles are preferred to reduce cutting pressure and heat generation; uncoated cemented carbide grades and polycrystalline diamond tooling are used for abrasive filled grades. The machining parameters for unfilled cast PA6 on a lathe typically fall within cutting speeds of 200–500 m/min for carbide tools and feed rates of 0.1–0.5 mm/rev, depending on depth of cut and surface finish requirements. At speeds above 500 m/min, melting and re-deposition of swarf become more frequent, and coolant is then required to control part temperature and chip adhesion. In milling operations, climb milling is preferred to conventional milling because the cutter exits the workpiece with a thinner chip, reducing heat and improving edge definition. Drilling of deep holes in cast PA6 requires peck cycles at intervals of 1–1.5 times diameter to clear swarf and prevent heat buildup that expands the hole wall and grabs the drill.

Oil-filled cast PA6 contains a uniformly dispersed hydrocarbon-based lubricant, typically at 1–4 wt%, which reduces friction in dry sliding without the abrasive nature of solid lubricants. MoS2-filled cast PA6 contains approximately 1.5–3.0 wt% molybdenum disulfide to provide nucleation and slip in wet and dry conditions. These fillers increase hardness and reduce wear rate but lower notched impact strength and tensile elongation. For bearing design, oil-filled cast PA6 is selected when lubrication is absent or intermittent, while MoS2-filled grades are specified in applications where external oil would be washed away. The solid lubricant grades are more sensitive to moisture cycling because the filler particles create microvoids that accelerate water ingress; property comparatives should be measured according to ASTM D638-14 and ASTM D256-10 after conditioning at 50% RH.

Measuring Moisture Uptake Before Final Machining of Precision Gears

Dimensional change due to moisture uptake is nonlinear with section thickness and is not uniform in anisotropic sections. A dry-machined PA6 gear with a tooth thickness of 5 mm at 0.2 wt% moisture may expand by as much as 0.15% when the tooth reaches 2.5 wt% moisture at equilibrium with 50% relative humidity, which is sufficient to change a DIN 8 gear backlash class to an interference condition. The relationship between linear expansion and moisture content is approximated by a coefficient of hygral expansion of 0.0020–0.0025 mm/mm per 1 wt% water content in the dry-to-conditioned range. To avoid dimensional rework, machinists condition cast PA6 blanks in water at 60–80 °C for 12–24 hours or in humid air at 70% RH until the target moisture level is reached; final finishing is then performed dry with allowances for the thermal expansion from cutting heat. This conditioning is not suitable for all dimensions because through-thickness moisture gradients remain after accelerated conditioning, and full equilibrium may take weeks in sections thicker than 50 mm. Published data for specific gear geometries is limited, so end-use trials with in-process CMM inspection are recommended for tolerance classes tighter than IT7.

A Post-Crystallisation Annealing Protocol Prevents Bore Collapse in Machined Components

Bore collapse in machined components made from cast PA6 arises when the residual compressive skin is removed unevenly, releasing internal stress and allowing the bore to close. This is particularly severe in tubular parts cut from solid rod, where the hoop stress gradient from cooling causes the inner diameter to shrink by 0.05–0.2 mm on a 50 mm bore after trepanning. A post-crystallisation annealing protocol at 170 °C for 4 hours per 25 mm of wall thickness, followed by slow cooling at no more than 5 °C/h to 80 °C, reduces the residual stress and stabilises the machined bore. The protocol is applied either before rough machining or after rough machining but before finish boring; both sequences are used in production, but the latter is preferred because stress release after roughing is more complete. Thermal blankets or circulating oil baths with temperature uniformity better than ±3 °C are required to avoid uneven stress relief and additional distortion.

Standard stock shapes are supplied as sheet in thicknesses from 10 mm to 200 mm and rods from 20 mm to 500 mm diameter. The as-cast tolerance is generally ±(0.5 mm + 0.002 × thickness) for sheet and ±(0.5% of diameter) for rod; machinists should specify oversize blanks for components requiring final tolerances below IT8. Cut surfaces from band sawing exhibit local heating and may have a melted or oxidized edge that must be removed by at least 1 mm in subsequent machining.

Compliance verification for cast PA6 machining stock is typically performed against a matrix of physical, mechanical, and regulatory standards. The table below summarises relevant test methods and typical verification conditions.

In dry sliding wear against hardened steel, unfilled cast PA6 typically exhibits pressure–velocity limits in the range of 0.05–0.10 MPa·m/s for continuous operation without external lubrication, while oil-filled grades may extend the limit to 0.15–0.25 MPa·m/s according to bearing manufacturer data. The values assume a hardened steel counterface with surface roughness Ra 0.2–0.4 µm, ambient temperature 23 °C, and no abrasive contamination. For applications involving food contact or chemically aggressive media, the grade must be validated against the specific migration or resistance standard; general-purpose cast PA6 is not suitable for immersion in strong acids, phenols, or chlorinated solvents. End-use tests with specified geometry and load cycles remain necessary because published data for wear rate and PV limits in machined cast PA6 components is limited to specific counterface and surface finish conditions.