Designing With Polycarbonate: Wall Thickness, Ribs and the Stress You Cannot See

Designing with polycarbonate wall thickness, ribs and the stress you cannot see begins with an amorphous polymer whose tensile modulus falls between 2350 MPa and 2450 MPa and whose notched Izod impact at 23°C is commonly reported between 600 J/m and 850 J/m at 3.2 mm thickness per ASTM D256-10e1. Molded components display useful transparency and dimensional stability, but the material retains process-induced internal stress that is not visible under ordinary factory lighting. Wall stock, rib geometry, gate location and mold temperature control the magnitude of that hidden stress, which later governs environmental stress crack resistance and fracture behaviour.

In production injection molding of general-purpose bisphenol-A polycarbonate, barrel temperatures are held between 280°C and 320°C, with mold surfaces between 80°C and 120°C. Pellets are dried in a desiccant dryer at 120°C for 4 h to a moisture content below 0.02% by weight, typically with a dew point of -40°C. Inadequate drying on a 3000 kN clamp machine produces splay, hydrolytic molecular weight loss, and reduced rib-to-wall ductility before any design rule is evaluated.

Does a 3.2 mm Wall Thickness Suppress Molded-In Stress in Transparent PC Housings?

For load-bearing transparent polycarbonate covers, nominal wall thickness is usually specified between 2.0 mm and 4.0 mm. At 3.2 mm, the frozen layer formed during filling reaches approximately 0.2 mm to 0.4 mm before packing begins. The still-molten core continues to carry flow-induced molecular orientation. If gate freeze-off occurs before cavity pressure compensation is complete, thermal contraction pulls the surface layer inward while the core is still solidifying. The resulting residual stress is stored in the wall as birefringence, observable only with polarized light. Molding at 70°C instead of 100°C increases the residual stress gradient in thin plaques, and layer-removal curvature measurements on unfilled PC have shown surface tensile stress increases of 10% to 30% in representative cases. Published data for this specific configuration is limited, but the direction is consistent across supplier technical literature.

Uniformity of wall thickness matters more than any single nominal value. An abrupt transition from 3.0 mm to 4.5 mm creates differential shrinkage at the shoulder. In a 120 mm by 80 mm housing side wall, the solidification-time difference between those two sections can be 8 s to 12 s, sufficient to produce a visible distortion of approximately 0.05 mm across the surface. Shrinkage plaques tested per ISO 294-3 quantify the same effect under controlled conditions before production tooling is committed.

Rib Base Thickness, Sink Marks, and the 50-to-60 Percent Rule

Ribs increase section modulus without increasing nominal wall stock. For unfilled polycarbonate, rib base thickness is held to 0.5 to 0.6 times the adjacent wall thickness. At 0.4 times wall, a rib is usually too weak to prevent warpage in a flat cover. At 0.7 times wall, the mass concentration at the root produces a visible sink mark opposite the rib. On a 2.5 mm wall, a rib base of 1.5 mm is therefore preferred to 1.75 mm. Rib height is limited to 2.5 times the rib base thickness for free-standing ribs, and rib spacing is set at no less than 2 times nominal wall to avoid excessive material concentration.

Rib root radius is specified at 0.25 to 0.5 times the rib base thickness, with a minimum of 0.5 mm. A sharp root acts as a notch. Because polycarbonate is notch sensitive, a 0.1 mm radius at the junction of a 1.0 mm rib can reduce energy absorbed at the root by more than 50% relative to a 0.5 mm radius in falling-dart tests on rectangular plaques. Draft of 0.5° to 1.0° on each rib side prevents ejection scuffing and reduces stress concentration at the base.

Sink marks are assessed under 45° incident light. A depth greater than 0.003 mm is often visible on high-gloss transparent polycarbonate. Sink depth is proportional to the local wall thickness difference and the packing pressure decay. On a 3200 kN direct-hydraulic clamping injection molding machine, increasing packing pressure to 140 MPa for 4 s reduced sink depth from 0.008 mm to 0.003 mm on a 2.5 mm wall with a 1.4 mm rib base. These are representative process observations, not unconditional values for every tool configuration.

At wall intersections where a 4.0 mm outer flange meets a 2.5 mm lens window, the cooling rate differential produces a visible sink line and a birefringence band extending approximately 0.6 mm into the wall. The band is not observed under normal factory lighting. The hidden stress lowers critical strain for environmental stress cracking in the presence of isopropanol or surgical disinfectants. For medical enclosures wiped with 70% isopropanol, a corner radius of 0.5 mm to 1.0 mm is specified at the flange-to-window junction. Without that radius, the same part can develop microcracks after 50 to 100 cycles of chemical exposure. ISO 22088-3:2002 bent-strip fixtures with strain levels between 0.2% and 1.0% provide the comparative test platform.

Hot-runner manifolds for polycarbonate are balanced to within ±2°C melt temperature across all drops. A temperature difference of 6°C between two cavities changes fill speed, packing pressure and residual stress. On an eight-cavity tool for transparent vials, the cavity with the lower manifold temperature showed more birefringence near the gate and a 7% lower notched Charpy value when tested per ISO 179-1/eA:2010. The runner system is therefore not neutral in hidden-stress control.

When Weld Lines and Chemical Exposure Combine Below the Surface

Weld lines form when two melt fronts meet. In polycarbonate, weld-line tensile strength is often 40% to 60% of the bulk value. The notch effect is more severe when the weld line is located in a ribbed section. A weld line combined with a rib root radius below 0.4 mm creates a fracture initiator that cannot be detected by dimensional inspection. For a housing with a hole in a 2.5 mm wall, changing gate placement to create a flow-front angle of 30° improved weld-line strength from 28 MPa to 41 MPa in tensile plaques molded per ISO 527-2:2012. Published data for this specific configuration is limited but illustrates the sensitivity to flow-front angle.

Chemical exposure magnifies the stress field at weld lines. Aromatic hydrocarbons, esters, ketones, chlorofluorocarbons and amine-containing products are known stress-cracking agents for polycarbonate. Critical strain for cracking in a given fluid is not a single material constant; it depends on residual stress, molecular weight, temperature and wall thickness. In a 2.0 mm wall, critical strain in 70% isopropanol at 23°C may fall from 1.0% to 0.2% when a weld line is present. Design verification therefore requires testing actual molded components, not generic test bars.

On a coextrusion sheet line producing 3.0 mm polycarbonate sheet at 900 kg/h, the roll-stack temperature profile is set to 120°C, 130°C, and 140°C on the first three rolls. Center roll pressure controls sheet thickness to ±0.05 mm. A thicker edge bead alters internal stress; it is removed by in-line edge trim. In flat-die extrusion, nonuniform output from a 1.5 m flexible-lip die produced a center-to-edge deviation of 0.08 mm in a 3.0 mm sheet. When cooled below 85°C, the edge cools faster and induces bowing. The locked-in stress is invisible in the flat sheet but appears after CNC routing because stress release distorts the part. For such operations, stress relief at 120°C for 2 h before cutting reduces camber after routing by approximately 60% in a 400 mm square plaque.

Process validation for transparent polycarbonate includes polarized light inspection of at least 5 consecutive shots and chemical resistance trials on stressed specimens per ISO 22088-3:2002 before production release. The design stress for a continuously loaded transparent cover should not exceed 20 MPa at 23°C unless the application is short-term. At 60°C, allowable tensile stress is lower by approximately 30%. The combination of 0.5% tensile strain and a high-pH cleaning agent can generate microcracks after 48 h in 60°C hot water. Because the stress is internal, failure appears unrelated to the applied load. Wall thickness balance, rib root geometry, mold temperature, gate location and chemical exposure must therefore be evaluated as a single design system.