I. Definition and Engineering Significance of Composite Compatibility

The composite compatibility between thermal insulation strips and aluminum alloy profiles refers to the comprehensive matching capability: after strip feeding and rolling compounding, the thermal insulation strip forms a stable engagement structure with the aluminum profile groove. Throughout the full cycle of processing, warehousing and service, the assembly jointly bears mechanical loads, temperature alternation and environmental aging without defects such as slippage, separation, cracking or arching. Strip-fed broken-bridge thermal insulation profiles rely on knurling and rolling engagement of aluminum grooves to form a mechanical lock, with no adhesive layer. The matching relationship between the two materials is maintained entirely by geometric structure, material properties and engagement stress. This is the core feature distinguishing composite compatibility from ordinary adhesive composite systems.

In door, window and curtain wall engineering, composite compatibility directly determines the shear strength, transverse tensile strength and high-low temperature cycle stability of thermal insulation profiles. It serves as a core failing indicator for type inspection. Profiles with insufficient composite compatibility may suffer composite failure in subsequent processing or outdoor service even if all individual raw material indicators of the thermal insulation strip meet standards. This leads to deterioration of airtightness and watertightness, decline in thermal insulation performance. Under extreme working conditions, structural separation of profiles may occur, creating safety hazards. For ultra-low-energy buildings, high-rise curtain walls, coastal and alpine cold-region projects, composite compatibility is a key item for material selection and profile matching verification, as well as a critical control point for system door and window supply chain matching.

II. Matching Linear Expansion Coefficient: Foundation for Long-Term Coordinated Deformation

The linear expansion coefficient of 6063 series aluminum alloy profiles is approximately 2.35×10⁻⁵/K. Qualified PA66GF25 thermal insulation strips are controlled within the range of 2.0×10⁻⁵/K ~ 3.0×10⁻⁵/K. Their close values enable synchronous deformation during temperature rise and fall, lowering interfacial shear stress. Expansion difference caused by temperature gap is the main source of alternating loads sustained continuously by the composite interface. Profiles heat up under solar radiation in summer and shrink at low temperatures in winter; the temperature difference between inner and outer aluminum profiles can exceed 60℃. If the thermal insulation strip matrix lacks sufficient glass fiber or adopts PVC or heavily modified regrind materials, its expansion coefficient will deviate greatly from that of aluminum profiles. Each temperature cycle generates extra shear and pulling force at the knurling teeth.

This accumulative repeated shear stress does not trigger failure immediately; it causes gradual fatigue damage. After long-term cycles, microcracks emerge where aluminum teeth press into the thermal insulation strip, locking capacity weakens gradually, and tiny slip gaps form between the strip and aluminum groove. Some inferior thermal insulation strips have a linear expansion coefficient more than twice that of aluminum profiles. They suffer profile bending and outward extrusion of strips even in short-term high-temperature tests, with direct damage to the composite structure. Therefore, matching of linear expansion coefficient is not a simple comparison of material parameters, but a fundamental prerequisite for stable service of composite structures under years of temperature cycling.

III. Cross-Sectional Dimensional Tolerance and Groove Structure Matching Logic

Strip feeding compounding belongs to interference-fit engagement assembly. The width, thickness, side chamfer and tooth root contour of the thermal insulation strip must form a matched tolerance system with the dovetail groove and knurling dimensions of aluminum profiles. The groove width, groove depth, knurling height and tooth pitch of aluminum profiles carry precise tolerances formed during extrusion and knurling processes. The extruded thermal insulation strip also has tolerance ranges for cross-sectional size, edge flatness and straightness. Superposition of these two sets of tolerances directly decides the interference after rolling.

If the thermal insulation strip cross-section is too thin, aluminum teeth achieve insufficient penetration after rolling, the locking area is small, and composite shear strength and transverse tensile strength fail to meet requirements, making slip and strip pulling easy under external force. If the cross-section is excessively thick and the interference exceeds the reasonable range, excessive compressive stress forms during rolling and persists inside the strip. As stress releases slowly after unloading, the profile bends laterally and the thermal insulation strip arches. Mismatched side chamfers, edge burrs or wavy bending of strips cause uneven feeding resistance, resulting in jamming and offset on high-speed strip feeding lines, inconsistent engagement depth along one single profile, and high dispersion of composite strength.

The angle and opening width of dovetail grooves also govern compatibility. Standard conventional grooves match national standard cross-section strips. Non-standard special-shaped grooves and shallow-tooth grooves hardly reach standard composite strength even with qualified strip materials. Profile factories cannot directly mix different thermal insulation strip suppliers. Trial compounding of cross-section matching must be conducted first, with shear and tensile indicators tested. Mass production can start only after tolerance matching is confirmed.

IV. Support of Material Mechanical Properties for Engagement Bearing Capacity

During rolling compounding, hard tooth tips of aluminum profiles are pressed into the side of PA66GF25 thermal insulation strips under external force. Shear force and tensile force are transferred via mechanical interlock between teeth and the matrix. This process requires the strip matrix to possess suitable hardness, tensile strength and compressive yield performance: it should not be too hard for aluminum teeth to penetrate, nor too soft to suffer plastic creep under compression and tooth loosening over long service.

Standard PA66GF25 thermal insulation strips have tensile strength ≥80MPa, balancing rigidity and toughness. Stable interlock structures form after aluminum tooth penetration. Materials around teeth disperse local stress through slight plastic deformation and resist brittle chipping. Insufficient glass fiber content or unbalanced toughening systems make strips brittle, which crack directly at tooth indentation positions during rolling. Low glass fiber content and excessive regrind soften the material; creep occurs under sustained extrusion of tooth tips. Creep rate rises further at high temperatures, engagement gaps expand gradually, and composite strength decays continuously.

Meanwhile, transverse stiffness and bending resistance of thermal insulation strips affect the straightness of compounded profiles. Two aluminum profiles are connected as a whole by the strip after feeding. Insufficient flexural modulus of the strip easily causes lateral warpage of compounded profiles, and deformation is amplified in subsequent spraying and aging processes, impairing assembly precision of doors and windows.

V. Surface Characteristics and Stress Distribution at Engagement Interfaces

The surface condition of extruded PA66GF25 thermal insulation strips also influences composite compatibility. A smooth, dense and uniform surface free of local porosity, blowholes or surface delamination ensures even force distribution after aluminum tooth penetration. If the strip surface contains foaming, pores or peeling, tooth tips land on defective spots, triggering local locking failure and forming weak points of the composite structure.

Adhesion is unnecessary for strip-fed systems relying purely on mechanical engagement. Excessive surface lubrication and migration of precipitated additives to the surface do not reduce static composite strength directly, yet micro-slip is more likely to occur at the surface interface after long-term aging. Uncontrolled addition of lubricants and release agents in raw materials leads to precipitation and enrichment at side engagement zones, accelerating fatigue failure of engagement interfaces under high-low temperature cycles and humid environments and weakening long-term service stability.

VI. Basic Compatibility Verification Items and Judgment Logic

For compatibility verification on material and cross-section levels, static mechanical tests are prioritized, including longitudinal shear strength and transverse tensile strength. These two indicators are fundamental for evaluating composite compatibility. Test specimens must use thermal insulation strips from the same batch and matched aluminum profiles, compounded by normal factory rolling processes; separate strip specimens cannot replace them. Besides static strength, high-low temperature cycle tests and high-temperature long-duration creep tests verify compatibility stability under temperature alternation.

For qualified specimens, failure occurs as tearing of the thermal insulation strip matrix while aluminum teeth remain embedded inside the material. If failure takes the form of direct pull-out of aluminum teeth or smooth interfacial separation, geometric or material matching is inadequate and composite compatibility fails. This is the most intuitive method to distinguish “qualified raw material” from “qualified matched profile assembly”.

VII. Engineering Consequences Caused by Material Compatibility Defects

Broken-bridge profiles with poor composite compatibility show defects at downstream processing stages: loose strips at saw-cut ends, local separation under stress during corner bracket assembly. High-temperature baking in spraying triggers profile bending and strip arching as stress releases. During engineering service, symptoms include stiff door and window operation, continuous extrusion failure of sealing gaskets, and degraded airtightness and watertightness. After long-term temperature cycles, water penetrates composite interface gaps, causing electrochemical corrosion of aluminum grooves. Expanded corrosion products further pry open engagement zones, forming a vicious cycle. For high-rise curtain wall conditions with repeated alternating wind pressure, fatigue of composite interfaces accelerates, bringing safety risks of profile separation.

VIII. Selection and Control Strategy for Material and Structural Matching

Supply chain matching control for profiles shall establish a three-in-one verification mechanism covering “material indicators + cross-sectional tolerances + groove structure”. When introducing new materials, besides testing individual indicators of thermal insulation strips such as ash content, glass fiber content, low-temperature impact and linear expansion coefficient, trial compounding with aluminum profiles must be performed to test composite mechanical performance and cycle stability. Production lines with fixed groove specifications should preferably fix matched cross-sectional tolerances of strips to reduce compatibility fluctuations caused by mixing materials from different manufacturers. For multi-cavity and wide-width thermal insulation strips with higher cross-sectional rigidity, knurling parameters and rolling pressure of grooves need synchronous adjustment; process standards for solid narrow strips cannot be directly adopted.