Kubota Research’s documented heritage in infrared processing began with a specific challenge: preserving high-tenacity fibers during composite consolidation. Early P-Wave™ systems demonstrated that precise, pulsed IR radiation could consolidate pseudo-prepreg layers—such as ImpacGard™—onto carbon composites without the thermal degradation caused by conventional epoxy-based methods. That work, validated with impact resistance testing, established a core principle: controlled infrared energy can bond advanced materials while maintaining their structural integrity.
That same principle of controlled, non-contact radiant heat now informs a more focused question in modern manufacturing: through transmission infrared welding. Where earlier efforts addressed impact absorption and shatter containment, current engineering interest centers on joining thermoplastic components by passing IR energy through an upper, transmissive layer to melt a lower, absorptive interface. The legacy of managing heat input to avoid fiber damage translates directly to managing wavelength and exposure to prevent bulk heating in the transmissive layer.
This site’s documented history with P-Wave™ radiation units provides the technical foundation for exploring how through transmission infrared welding adapts those earlier consolidation concepts to precise, repeatable assembly joints. The following discussion examines the parameters that distinguish this process from other IR welding methods.
Process Fundamentals and Energy Delivery
Through transmission infrared (TTIR) welding is a fusion joining method for thermoplastic composites in which infrared radiation passes through a transparent or semi-transparent upper layer and is absorbed at the weld interface, typically by a carbon-filled or otherwise absorptive interlayer. The process differs from laser-based through transmission welding in that the energy source is a broad-spectrum infrared emitter rather than a coherent beam, which changes how energy is delivered, how the heat-affected zone develops, and how process windows are managed.
The governing principle is selective absorption: the upper laminate must transmit sufficient infrared energy, while the lower substrate or an interlayer must absorb it efficiently to generate heat at the bond line. In practice, this means the upper ply is often unreinforced polymer or a thin film, and the absorbing layer is a carbon-fiber-reinforced composite or a dedicated susceptor film. The heat generated at the interface conducts upward and downward, melting a controlled volume of polymer on both faying surfaces. Pressure is applied to consolidate the joint as the polymer cools.
Quantitative Process Parameters from Evidence
Reported process conditions for related thermoplastic fusion methods provide a useful reference frame for TTIR development, even where the specific infrared parameters are not yet standardized. For resistance welding of PPS, one NASA study cites a post-weld consolidation step at 625°F (329°C) for 30 minutes to reassemble the joint at strength similar to the original [3]. This temperature is well above the melting point of PPS and indicates the thermal budget needed to fully remelt and recrystallize the semicrystalline polymer at the interface. For TTIR, the equivalent thermal exposure must be achieved without overheating the transparent upper layer, which constrains the maximum emitter intensity and dwell time.
Mechanical test data from ultrasonic welding of PEEK adherends with an LM-PAEK interlayer show a maximum stress of 6.56 ± 1.07 MPa at room temperature (~23°C) and 8.47 ± 1.19 MPa at elevated temperature (121°C/250°F) [6]. The displacement at failure was 1.09 ± 0.21 mm at room temperature [6]. These values are not directly transferable to TTIR because the heating mechanism and thermal history differ, but they establish the order of magnitude for lap-shear strength that aerospace-grade thermoplastic joints must achieve. A TTIR process that produces joints below this range would likely be considered under-consolidated, while values above it may indicate excessive degradation or void formation.
For joint preparation, metal shims approximately 0.02 inches thick, wrapped in high-temperature Kapton tape, have been used to control bond-line thickness in resistance welding of PEKK and other thermoplastics [5]. The shim approach is directly applicable to TTIR, where maintaining a consistent gap and pressure distribution is critical for uniform heat generation. The 0.02-inch figure provides a starting point for bond-line control, though the optimal value will depend on the polymer viscosity, fiber architecture, and applied pressure.
Applying These Numbers in Process Development
The 625°F/30-minute consolidation condition [3] should be interpreted as a ceiling for thermal exposure in TTIR, not a target. In TTIR, the heat is generated at the interface and conducted outward, so the upper layer experiences a lower peak temperature than the bond line. If the upper layer is a low-melting-point film, the emitter power must be limited so that the upper surface does not exceed its degradation temperature while the interface reaches the melting range. A practical approach is to run a thermal model or instrumented trial with thermocouples at the upper surface and interface, then adjust emitter distance, power, and dwell time until the interface reaches the target temperature while the upper surface stays below its limit.
The lap-shear values from ultrasonic welding [6] serve as acceptance criteria for TTIR joint qualification. If a TTIR joint with similar materials and geometry does not reach at least the lower bound of the reported range (approximately 6.5 MPa at room temperature), the likely causes are insufficient interface temperature, inadequate pressure, or too short a consolidation time. Conversely, if the joint exceeds the elevated-temperature value of 8.47 MPa [6], the process may be over-consolidating, which could lead to polymer degradation or fiber displacement in the weld zone. Testing at both room temperature and 121°C is recommended because semicrystalline thermoplastics lose modulus with temperature, and a joint that passes at room temperature may fail at elevated service conditions.
The 0.02-inch shim thickness [5] provides a baseline for controlling the melt layer thickness in TTIR. If the shims are too thin, the molten polymer may be squeezed out under pressure, leaving a starved joint. If too thick, the bond line may be resin-rich and weak in shear. The shim approach also helps maintain uniform pressure across the joint, which is essential because TTIR heating is sensitive to contact pressure—higher pressure improves thermal contact but can also displace molten polymer before it cools.
Process Control and Material Considerations
TTIR requires careful matching of the infrared source spectrum to the absorption characteristics of the upper layer and the susceptor. Most thermoplastics have transmission windows in the near-infrared region, but their absorption increases in the mid-infrared. A broad-spectrum emitter will deposit some energy in the upper layer, which can be beneficial for preheating but must be accounted for in the thermal model. Carbon-fiber-reinforced upper layers are generally not suitable for TTIR because the fibers absorb and scatter infrared radiation, preventing through-transmission. This limits TTIR to applications where the upper layer is unreinforced or lightly reinforced polymer.
The process window is narrower than for resistance or induction welding because there is no embedded heating element to provide a secondary heat source. The infrared energy must be delivered uniformly across the joint area, which becomes challenging for large or curved parts. Reflectors and focusing optics can improve uniformity, but they add complexity and cost. For aerospace applications, where joint quality is critical, the trade-off between process simplicity and joint reliability must be evaluated on a part-by-part basis.
Comparison with Adjacent Fusion Methods
TTIR sits between laser through-transmission welding and resistance welding in terms of energy density and process control. Laser welding offers higher energy density and smaller heat-affected zones but requires precise beam positioning and is sensitive to part fit-up. Resistance welding uses an embedded heating element that provides uniform heating but leaves the element in the joint unless it is removed. TTIR offers a middle ground: no embedded element, broader energy distribution than a laser, and the ability to heat large areas simultaneously. However, the lack of an embedded element means the process is more sensitive to variations in part thickness, surface roughness, and infrared transmission.
Comparative studies of ultrasonic, induction, and through-transmission laser welding of CF/PEEK have been reported for aerospace assembly [2], and the same comparative logic applies to TTIR. Each method has distinct advantages and drawbacks related to energy delivery, tooling complexity, and joint quality. TTIR is most competitive when the part geometry allows direct line-of-sight access to the weld area and when the upper layer is thin enough to transmit sufficient infrared energy without excessive absorption.
This independent educational reference summarizes general technical concepts. Verify current standards, dimensions, and manufacturer specifications before making a procurement or engineering decision.
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Every figure above traces to the reports below. Check the original document before using a number in a live design.