When Knurling Becomes the Bottleneck: Who Exactly Is Being Held Back by Traditional Thermal Break Aluminum Composite Processes?

In insulated aluminum alloy door, window and curtain wall systems, there is a nearly undisputed consensus: thermal insulation profiles are the foundation of all performance metrics. Wind pressure resistance, sound insulation, air tightness, water tightness, energy efficiency indicators... Whether you focus on structural or thermal performance, everything ultimately boils down to one question: how firmly and stably are the thermal barrier strips connected to the aluminum profiles? For this very reason, "composite processes" have always been a critical matter in the insulated aluminum door, window and curtain wall industry.

 

For decades, the industry has widely adopted a seemingly mature process

flow: Knurling → Inserting → Rolling.

Knurling

Inserting

Rolling

This process has indeed supported the entire development of thermal break aluminum doors, windows and curtain walls from scratch to scale. However, as the market pursues larger sizes, higher safety levels, greater stability and higher consistency, the "shortcomings" of this traditional process are becoming increasingly prominent. And the most glaring issue lies in the first step that many take for granted: Knurling.

The image above shows a failure case caused by inadequate knurling of thermal insulation profiles. When the assembled window frame arrived at the site, severe bending deformation occurred in the lower horizontal frame.

 

I.Success or Failure Often Hinges on the Most Overlooked Details

From a mechanical perspective, thermal insulation profiles do not rely solely on "friction" or "compression" to maintain connection; instead, they require a definite mechanical interlock. The essence of knurling is to pre-form a row of "grappling hooks" on the aluminum profile notches. Details such as the sharpness of the tooth profile and the depth of the teeth are rarely discussed repeatedly in daily production, but they are ruthlessly exposed during shear tests. The practical problem is that the knurling step is highly dependent on "operational conditions".

 

II.Why Is Knurling the Most Difficult Process to Stabilize Long-Term?

If you visit the composite workshop of any profile factory, you will notice an interesting phenomenon: inserting and rolling usually pose few problems, while knurling is always the real concern in practice. The quality of knurling is affected by three factors:
1.Cutting tools: Knurling discs inevitably wear out, and the wear occurs silently;
2.Equipment: Equipment concentricity and vibration status directly affect the consistency of the tooth profile;
3.Workers: The operator's experience, feel, judgment and working condition are all indispensable.

 

Very Good Knurling  

Good Knurling

Not Good Knurling

No Knurling

 

More realistically, in a high-tempo, high-load production environment, it is very common to "keep using" worn knurling discs for a little longer. As a result, you may find that the shear strength just meets the minimum standard today, but the data may drop sharply tomorrow. The problem does not surface immediately but accumulates gradually. The final outcome is often obvious fluctuations in shear test results for the same batch of profiles. Hidden dangers are laid in engineering projects in advance, followed by rework, scrap and claims.
 

III.The Efficiency Calculation Makes Knurling's Flaws Even More Obvious

If knurling is already troublesome enough in terms of mechanical strength, its problems become even more prominent from the perspective of production tempo. In the traditional three-step composite process, the inner aluminum profile of a composite profile needs knurling once, and the outer aluminum profile requires another knurling. In contrast, inserting and rolling can both be completed in one step at the same tempo. This means knurling is inherently a "time amplifier" on the composite production line. Against the backdrop of rising labor costs, energy consumption and equipment footprint, this structural inefficiency has become increasingly hard to ignore.

 

IV.As the Industry Upgrades, How Long Can Traditional Processes "Make Do"?

As doors and windows evolve towards super-large sizes, extra-large glass and overweight sashes, the loads borne by thermal insulation profiles are far beyond the levels of a decade ago. The industry's expectation for shear strength has also shifted from "meeting standards" to long-term stability, batch consistency and predictable results. Thus, a question emerges: if there is a method that no longer relies on knurling conditions for quality control, yet can achieve higher and more stable shear strength, is this process worthy of being redefined?