S-LEC Wedge-shaped Film

On automotive OEM laminated-glass lines, S-LEC Wedge-shaped Film is integrated as the interlayer in windscreens that carry a head-up display projector. The film is received in rolls with a machine-direction thickness gradient; the wedge axis must be aligned to the vertical or mixed orientation defined by the projector coordinate system. Layup follows a glass-inboard / wedge film / glass-outboard sequence after curved glass bending. The glass pair is washed and dried; the interlayer is conditioned at 18–20 °C and 20–30 % RH for 2–4 h. If conditioning is skipped at high ambient moisture, the PVB surface absorbs water and autoclave delamination risk increases. Incoming film moisture is held below 0.5 wt% by Karl Fischer coulometric measurement ISO 15512. Vacuum de-airing is performed on a flat-bed or ring-bag system at a residual pressure of 20–25 kPa absolute for 20–40 min. The assembly then enters an autoclave at 135–145 °C and 1.2–1.5 MPa for 90–120 min. After lamination, the windscreen is inspected for optical distortion, bubble content, and wedge-angle profile across the HUD eye box. The double-image separation is evaluated with a HUD optical bench, not solely by visual inspection of ghost image. The residual ghost image specification is defined by the HUD system integrator; no ISO standard currently covers HUD ghost image limits in laminated glass. This lack of public standard makes OEM-specific wedge-angle data the controlling dimensional input. The wedge-angle profile is measured by laser profilometry with 0.001 mm thickness resolution before layup, and the same profile is rechecked after autoclave to confirm that melt flow has not shifted the local wedge angle.

When AR-HUD Virtual Image Distance Exceeds 10 m, What Changes in Lamination Stack Design?

Published HUD optical models show that when the virtual image distance is extended beyond 10 m for augmented reality navigation, the allowable wedge-angle error contracts substantially. The wedge film’s thickness gradient must be maintained not only as a global average but also as a local profile matching the locally varying glass curvature. OEM specifications commonly require local wedge-angle repeatability rather than a single global thickness difference; the exact numerical tolerance is controlled by the HUD system integrator because published data for specific AR-HUD limits is limited. The glass bending master and the wedge film profile are matched to the reflecting surface coordinate system. Autoclave flow of the wedge film tends to redistribute thickness along the wedge axis; therefore the initial wedge profile is cut with a higher edge thickness differential than the final target, compensating for melt redistribution. This compensation factor is determined on each line by flat-glass lamination trials, because the magnitude depends on autoclave pressure ramp and glass bending stiffness. Pressure ramp rates above 0.10–0.15 MPa/min before the glass surface reaches 100 °C can cause channelized flow along the wedge gradient, producing localized wedge-angle dropout. The process window for AR-HUD windshields is therefore narrower than for conventional PVB interlayers. Lamination cycle validation is performed with a glass pair thickness of 2.1 mm inboard and 2.1 mm outboard or with asymmetric 2.1 mm/1.6 mm glass combinations where weight reduction is specified; asymmetric stacks require the thicker glass on the inboard side to control impactor performance. Optical distortion is measured per ISO 12543-6. The windshield must still satisfy the fragmentation and mechanical strength requirements of UN/ECE R43 for the applicable vehicle category. Where the HUD system uses a large mirror and a long optical path, the local wedge-angle requirement is not uniform across the windscreen. The film supplier’s wedge profile is usually defined by a wedge table that lists thickness differential at graduated positions along the machine direction. The glass bender and the laminator use the same coordinate table for post-autoclave verification. The verification scan uses a non-contact laser displacement sensor on a linear stage; the scan line is offset from the windshield installed datum by a known distance. The scan data is compared with the wedge table, not with a single average value. This method detects local profile washout, which a single-point measurement cannot detect. In AR-HUD laminations, a post-autoclave hot-air knife edge-sealing step is sometimes used to reflow edge serrations; this step must be qualified because local heating can alter the wedge profile within the HUD eye box if the eye box is located close to the edge. Published data for this specific configuration is limited, so the hot-air knife parameter set is validated by cross-sectioning prototype windshields and measuring wedge angle by optical microscopy. The terminal product is a laminated AR-HUD windshield in which the wedge correction is maintained across the projector eye box after autoclave.

Commercial Vehicle Glazing Lines and R43 Marking Compliance

Commercial vehicle HUD windshields use larger glass formats and often higher installed rake angles; the wedge film must withstand longer vacuum bag cycles and uneven autoclave loading. The lamination process on large flat-bed lines uses segmented vacuum channels because the wedge film’s thickness differential can prevent uniform air evacuation if a single perimeter vacuum port is used. Cooling after autoclave is staged to avoid thermal shock in the thick-to-thin transition zone. The windshield must pass UN/ECE R43 headform impact, fragmentation, and light transmittance requirements. Visible light transmittance must remain not less than 70 % where the applicable passenger-car windscreen requirement applies; for commercial vehicle categories, the specific limit is confirmed against the type-approval text. The wedge film plus tinted glass builds are pre-screened with a spectrophotometer. The lamination glass is tested for fragmentation number and particle size per the national type approval test. On a typical commercial vehicle line, the autoclave basket may hold multiple windshield sets with different curvature; the wedge film blanks are loaded so that the thick edge does not hang over the support rails. Because the wedge film is thicker on one edge, the assembled stack can shift during initial vacuum bag closure if the support table is not level. Flat-bed lines with membrane presses use an inflatable membrane pressure of 0.03–0.06 MPa before autoclave to secure the stack. The membrane pressure must be applied after the vacuum is drawn, not before, to avoid trapping air. R43 fragmentation testing is performed on the final assembly; the wedge film does not change the fragmentation mechanism of the glass, but the interlayer adhesion level influences the pummel rating after impact. The component approval file includes a matrix of glass thickness, PVB grade, and wedge angle; any change to one of these parameters requires a new type test. Replacement glass manufacturers replicate the original lamination stack but must re-validate because a change in glass thickness or PVB grade alters the ghost image compensation. The terminal product is an R43-marked commercial vehicle windshield with HUD wedge correction.

Does Inverted Wedge Orientation Create a Reversed Ghost Image?

Orientation of the wedge film is a critical control point. An inverted wedge axis reverses the sign of the thickness gradient and can double the ghost image separation instead of canceling it. On production lines the film is marked with a machine-direction wedge arrow; the arrow is checked against the glass-bending orientation and the HUD projector coordinates before layup. The wedge film is placed with the inboard glass surface, because the primary HUD reflection originates at the inboard glass-air interface. In multi-layer stacks where the wedge film is combined with an acoustic PVB core, the wedge layer is typically the inboard-side PVB layer. The acoustic core is a trilayer with lower glass transition temperature; the wedge film remains the stiffer outer layer to maintain the wedge profile during autoclave. Laminating a wedge film on the outboard side can still produce a laminated safety glass, but the ghost image correction vector may be reversed relative to the HUD optical model. An inverted wedge is not detectable by thickness profile alone because the roll may have a symmetric edge build; the wedge direction marker is the only reliable field check. Some lines install an automatic camera system that reads the arrow and compares it with the cutting program before the film enters the layup cell. The wedge film has a thin edge and a thick edge; during layup, the thin edge is placed on the lower section of the windshield when the HUD projector is mounted low in the instrument panel. If the projector is mounted high, the orientation is reversed. This relationship is defined by the optical path geometry, not by the glass-shape drawing alone. In multi-layer stacks, the acoustic PVB core is placed outboard of the wedge layer when the inboard glass is used as the primary reflecting surface. The acoustic trilayer is not used as the HUD wedge carrier because its softer core can deform during autoclave and reduce the local wedge-angle stability. The stiffer wedge film remains dimensionally stable under the same autoclave pressure. The incoming film is cut with a negative or positive wedge direction depending on the mounting orientation; batch records include the roll number, wedge angle class, and cut orientation. The terminal product is a multi-layer windshield in which the wedge function remains on the correct optical path.

On flat-bed lines, the wedge film produces a thickness differential across the glass blank that can trap air at the thin-edge side if the vacuum bag is closed too quickly. Perimeter vacuum ports are not sufficient; a center-line vacuum channel or zone-controlled vacuum bed is used. Some laminators use a pre-pressing nip roller set before vacuum bagging. The nip roller pressure is limited to 0.02–0.04 MPa because higher pressure forces the molten PVB wedge profile to smear before autoclave. The rollers are adjusted so that the first contact occurs at the thin edge, then the roller traverses toward the thick edge; this direction prevents air entrapment at the wedge transition. Incoming film rolls are stored horizontally in a dark area at 10–25 °C. A roll stored on its edge can develop a flat spot that appears as a thickness anomaly in the HUD eye box. The roll core end-face is marked with the wedge direction; if the end-face label is damaged, the roll is quarantined until the wedge direction is confirmed by laser thickness mapping. The pre-autoclave de-airing step is maintained for 20–40 min at 20–25 kPa absolute until the edge transparency appears uniform. Residual air pockets at the wedge transition tend to nucleate bubbles during autoclave. Batch-to-batch variance in wedge angle is controlled by incoming laser profilometry on the film roll. The wedge profile is measured at machine direction intervals of 100 mm; if the measured edge thickness differential deviates from the drawing by more than the specified tolerance, the roll is quarantined. After lamination, adhesive performance is checked by pummel adhesion or compressive shear on test laminates; the method and acceptance limit are defined in the interlayer supplier’s technical bulletin. No public ISO standard replaces the supplier’s adhesion method for PVB. After autoclave, the finished windshield is placed on a CMM with a laser displacement probe; the wedge angle is calculated from the measured inner and outer surface profiles. The measured profile is subtracted from the nominal HUD wedge table; if the residual exceeds the supplier or OEM limit, the windshield is rejected and the autoclave cycle record is reviewed. This closed-loop measurement prevents batch-to-batch drift in the lamination process. The finished HUD windshield is then evaluated for optical distortion per ISO 12543-6 and for residual double image on an OEM HUD bench. Published data for specific double-image pass/fail limits is limited; the pass/fail threshold is controlled by the HUD system integrator.

Thermal Ramp Limits for Non-Uniform PVB Melt Flow in Autoclave Cycles

Because the wedge film has a thickness gradient, autoclave heating creates non-uniform melt flow along the wedge axis. The thicker end reaches the viscoelastic flow regime earlier than the thinner end. Rapid heating can produce waviness in the HUD section, local wedge-angle dropout, or glass edge displacement. The rheology of the wedge film is shear-thinning. As the film temperature passes the glass transition region, the zero-shear viscosity drops from a high solid-like value to a viscous melt. The wedge film’s thicker region has a higher heat capacity per unit area; the thinner region heats faster. If the autoclave heat-up is uncontrolled, the thin side reaches the flow regime first, and the pressure drives material toward the thick side, steepening the wedge angle. This phenomenon is opposite to the intended compensation direction in some glass shapes. The autoclave cycle is therefore staged: a heating ramp of 2–5 °C/min is used up to 100 °C, then a slower ramp to 135–145 °C. Pressure is held at 0.5–0.8 MPa during the early stage before the final pressure is raised to 1.2–1.5 MPa. This staged pressure profile prevents the molten PVB from being forced from the thick end toward the thin end. Multi-zone autoclaves with side-wall and floor heaters are set to reduce the temperature spread between the thick edge and thin edge. A maximum spread of 5 °C is maintained between the two edge zones until the load reaches 100 °C. After the final pressure is reached, the spread is allowed to relax to the normal autoclave control band. The glass bending fixture must support the windshield so that the wedge axis is horizontal during autoclave; if the wedge axis is vertical, gravitational flow can shift the wedge profile. Autoclave loading orientation is recorded in the batch sheet. Strain patterns in the laminated glass are inspected with a white-light stress viewer. Wedge-induced strain is concentrated at the thickness transition and must be distinguished from bending-tempered strain in the glass. If the strain pattern shows a sharply bounded band, the laminate is flagged for wedge-angle re-measurement. The window for the wedge angle after autoclave is narrower than the incoming film tolerance due to flow-induced profile shift; therefore the incoming film may be selected in a narrower supplier tolerance band than standard PVB. The terminal product is a laminated HUD windshield with wedge-angle stability after autoclave and no optical waviness in the projector eye box.