Cast Nylons Nycast 12 is a cast polyamide 12 stock-shape material produced by activated anionic polymerization of ω-laurolactam in low-pressure moulds. The cast polymerization route yields semi-crystalline PA12 with lower equilibrium moisture uptake than cast PA6 and PA66, a density in the range of 1.01–1.02 g/cm³ under ISO 1183-1, and Shore D hardness of 70–75 under ISO 868. Standard forms supplied are natural and black rod, plate, and tubular bar; custom castings are machined from blanks that have been stress-relieved to control skin-to-core crystallinity gradients. The grade is distinct from extruded PA12 because the cast anionic route produces higher molar mass and permits very thick sections without melt orientation.

Typical ISO 527-2 tensile yield stress for dry-as-cast material is 40–50 MPa, with elongation at break above 200%; flexural modulus under ISO 178 lies near 1200–1400 MPa. The water absorption at saturation under ISO 62 is approximately 0.8–1.0 wt%, compared with 8.0–9.0 wt% for cast PA6. That difference controls dimensional change in humid service. The property table gives representative values for unfilled natural Nycast 12 stock; release values for a particular lot must be taken from the supplier certificate.

The anionic polymerization of laurolactam in the mould proceeds at 160–190 °C, with the ring-opening exotherm removed through the mould wall. Because cooling is directional, the outer skin solidifies first and develops finer spherulites than the slowly cooled core; the resulting hardness gradient across a 100 mm thick block can be 2–3 Shore D points if the casting is not post-annealed. Stress-relief annealing at 80–100 °C in circulating air is commonly specified before finish machining, with holding time of 1 h per 25 mm of thickness. This treatment reduces warpage after material removal and improves batch-to-batch reproducibility on multi-part CNC runs.

Cast PA12 has a melting peak near 175–180 °C determined by differential scanning calorimetry under ISO 11357-3, and a crystallinity typically between 40% and 50%. The combination of high molar mass and moderate crystallinity provides the balance of toughness and creep resistance; higher crystallinity would raise modulus and heat deflection temperature but reduce elongation and impact. For thick-section components, the centre may have lower crystallinity than the skin because cooling is slower; this is not a defect but must be considered when measuring hardness on exposed cut faces. Published data for the centre-core condition of very thick billets is limited; design values for sections above 150 mm should be verified by coupon testing because cooling-rate effects reduce tensile elongation and impact relative to the surface.

Why Does Saturation Moisture Uptake of 0.8–1.0 wt% Govern Tolerance Retention in Humid Process Environments?

At saturation, unfilled Nycast 12 absorbs 0.8–1.0 wt% water under ISO 62, whereas cast PA6 absorbs 8.0–9.0 wt% and cast PA66 roughly 7.0–8.0 wt%. Because absorbed water occupies free volume and hydrogen-bonds to amide groups, the linear swelling of PA12 at saturation is substantially lower than that of the higher-amide-density nylons; this is the central reason tight-clearance components machined from Nycast 12 do not seize after washdown.

The moisture-driven dimensional change is not linear with relative humidity. At 50% RH, PA12 reaches approximately 0.5–0.6 wt% moisture, so most machined parts tested in room-conditioned metrology laboratories are closer to saturated behaviour than is often assumed. For parts that must hold ±0.05 mm on a diameter, the shop should document equilibrium time; on 80 mm thick plate, moisture equilibration from centre to air can require several days. For accelerated preparation, drying may be performed at 60–80 °C for 4–6 h when ambient relative humidity exceeds 60%; welding or adhesive bonding requires the surface to be dried because water vapour creates porosity at the joint.

Before metrology, parts should be conditioned at 23 °C and 50% RH for at least 48 h after machining because internal stresses relax and ambient moisture equilibrates. The use of solvent wipes immediately before gauging can extract surface water and change dimensions less than 0.01 mm on a 50 mm diameter part; this is not a substitute for proper conditioning. For parts shipped to high-humidity climates, sealed polyethylene packaging with desiccant prevents in-transit growth.

Where components operate in cold storage at −30 °C or are cleaned with dilute caustic, Nycast 12 retains ductile behaviour more reliably than POM homopolymer and PA6. Low-temperature Charpy notched impact under ISO 179-1/1eA is typically above 5 kJ/m²; the glass transition of PA12 occurs near 45 °C, but because water-related plasticisation is small, the low-temperature impact transition is not shifted as strongly as PA6 after moisture ageing. Chemical resistance under immersion in aliphatic hydrocarbons, mineral oils, and dilute aqueous alkalis is acceptable; strong mineral acids and polar solvents at elevated temperature attack the amide group. High-pressure steam above 110 °C is not recommended for continuous service because hydrolysis reduces molar mass.

Bearing and Wear Service Against Stainless Steel Counterfaces

Sliding wear in cast PA12 is governed by frictional heating and counterface topography. On 316L stainless steel with surface finish Ra ≤ 0.2 µm, unfilled Nycast 12 exhibits a wear rate lower than cast PA6 in dry conditions because the lower moisture content reduces softening and because the longer aliphatic segment between amide groups lowers melt viscosity and improves transfer-film stability. The recommended counterface hardness is above 40 HRC for long life; hardened stainless steel, nitrided steel, or ceramic-coated shafts are preferred. A water-lubricated bearing in 0.5–1.5 m/s sliding velocity may run with low stick-slip, but the specific PV limit must be derived from manufacturer bearing curves because the wear constant depends on pressure-velocity cycles, cooling, and clearance.

In sleeve bearing service, the coefficient of friction of cast PA12 against steel at 0.5 m/s and 1 MPa is typically 0.25–0.35 dry and 0.08–0.12 water-lubricated. These values should be confirmed on the actual counterface because transfer-film formation, roughness, and temperature alter them. Cast PA12 exhibits lower stick-slip than PA66 in slow linear guides; for this reason it is used in bottling star wheels and pharmaceutical packaging cams where intermittent motion causes chatter with higher-friction materials.

The wear equation used for design is w = K(P · v) / H, where K is the dimensionless wear factor. For unfilled cast PA12, supplier design guides commonly use a K range of 10⁻⁷–10⁻⁶ mm³/Nm for clean steel counterfaces at moderate speed. At surface temperatures above 80 °C, the effective wear factor increases by an order of magnitude because the polymer modulus falls. The PV limit is therefore not a material constant but a temperature-management limit; a bearing with a thinner wall will have a higher PV capability than a thick wall because heat is conducted away faster.

Unfilled cast PA12 is normally selected for unit loads below 10 MPa in continuous dry sliding at 20 °C, with the allowable bearing pressure reduced linearly as the interface temperature rises toward the 120–140 °C heat deflection temperature under ISO 75-2/B. Water-lubricated service can raise the allowable load because the thin water film carries part of the contact pressure; dry service against Ra ≤ 0.2 µm stainless steel is preferred for low wear. If a component is run without lubrication above 80 °C surface temperature, surface melt fracture can occur before bulk failure.

For wet running, the cooling effect of water dominates the apparent PV limit. A sleeve bearing of 25 mm inside diameter running at 1 m/s with 0.5 MPa load in 20 °C water typically operates 30–40 °C below the dry-running interface temperature. If water flow is interrupted, the same bearing reaches surface temperature above 80 °C within minutes; therefore dual-condition failure analysis is required. The clearance allowance for water-lubricated cast PA12 is often set at 0.5% of shaft diameter plus 0.1 mm; thermal expansion and humidity swell are both included. For a 25 mm shaft, this yields roughly 0.22 mm total clearance. Insufficient clearance causes seizure not from bulk melting but from loss of clearance as the heat-affected surface expands.

Production machining of thick cast PA12 plate requires stress-relief annealing before roughing because skin crystallization occurs faster than core solidification in low-pressure casting. A CNC lathe or machining centre with carbide inserts having 8°–10° positive rake, 5°–7° clearance, and coolant directed at the cut point is used to prevent galling and dimensional drift. Thin ribs machined to 3 mm width from 80 mm plate must be rough-machined, released from the fixture, and allowed to stabilise for 24 h before finishing; otherwise the release of asymmetric casting stress produces bowing that cannot be corrected by a final pass.

When PA12 Replaces PA6, PA66, POM, or UHMWPE in Machine Components

Compared with cast PA6 and PA66, the primary selection difference is moisture-dependent dimensional movement. At equilibrium in 50% RH, PA6 absorbs roughly 2.7 wt% water, while PA12 absorbs below 1.0 wt%; this yields less tightening and clearance loss in humid food-processing lines. In structural blocks and jigs, PA12 is chosen over acetal when impact loading is high and when water or dilute acid washdown would attack acetal via acid hydrolysis of the acetal linkage. The flexural modulus of PA12 at 1200–1400 MPa is approximately half that of acetal, so section thickness must be increased for equivalent rigidity. Under 30 MPa compressive stress at 23 °C, PA12 shows measurable creep; design guides limit continuous compressive design stress to 5–10 MPa for structural PA12 parts. UHMWPE is limited to 65–85 °C under load, so PA12 replaces it in pasteurization tunnel guides and hot-water bearing applications.

The effect of moisture on mechanical properties is non-linear. At 50% RH, the tensile modulus of cast PA12 drops by approximately 10–20% relative to dry values, whereas cast PA6 loses more than 25% of modulus after conditioning. This distinction matters in gears and cams because lubricant-free components in ambient air do not see dry properties. Design calculations should therefore use conditioned properties, not dry-as-cast values, for parts operating in ventilated factory air at 50% RH.

Batch-to-batch variation in cast PA12 is most visible in residual monomer content and colour. Natural PA12 may shift from water-white to pale yellow with thermal degradation during casting; this does not necessarily affect short-term tensile properties but can increase brittleness if the melt has been overheated. The supplier should report intrinsic viscosity or molar mass data for critical rotating components; if not, a 48 h accelerated hot-water ageing test at 80 °C is used to expose hydrolytically unstable lots before machining.

In elevated-humidity food-contact zones, the lower water uptake of Nycast 12 reduces the frequency of clearance re-boring after washdown. When food-contact certification is required, the grade must be specified as compliant with FDA 21 CFR 177.1500 or an equivalent positive-list regulation before machining, and the supplier must provide lot-level migration documentation because not all cast polyamide 12 formulations are automatically compliant. The use of MoS₂-filled or oil-filled variants in direct product-contact zones is generally prohibited unless the additive package is listed in the relevant positive list.

Nycast 12 is incompatible with strong mineral acids at elevated temperature, halogenated solvents under stress, and high-pressure steam above 110 °C. Prolonged outdoor exposure causes surface embrittlement due to photo-oxidation of the polyamide chain; UV-stabilised black grades are preferred for sunlight-adjacent service. For machined parts exposed to intermittent hot water at 80 °C, dimensional stability remains acceptable provided the part is conditioned to the service environment before final inspection.