Coil Durability

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Premise

"The coil is a toy. Stretch and pull as needed. Embrace true function."

As perhaps the most tedious step in the hobby's typical cable assembly process, coiling may sometimes find itself oversimplified as a straightforward procedure that only needs proper industry temperatures and timing. Conventional cable assembly industry wisdom then provides the "best" method to achieve long-lasting coil memory in two key points:

  1. The coil must be reversed after the heat cycle, forcing the cable jacket to permanently fight against its own heat-set memory.
  2. The heating process is most effective using an oven chamber for uniform heat saturation across the entire coil.

Such conventional wisdom applies well in the context of industrial manufacturing, and point one holds true universally. Point two in particular, however, fails to consider a key difference—the presence of custom sleeving—that separates the artisan cable process from industrial practices. The addition of braided outer PET (polyethylene terephthalate) sleeving, most commonly Techflex® and MDPC-X®, introduces new fundamental material properties that must be considered when determining coil heat cycle temperatures.

In-Depth

The principle issue can be summed up in several points of detail:

  • Standard practice (i.e. conventional wisdom) is an oven heat cycle of approximately 105°C for 20 minutes, applying the proper level of heat to sufficiently soften the cable's outer jacket and create new shape memory once cooled. This method is well-proven when working with bare, unsleeved cables.
  • The Techflex® and MDPC-X® datasheets list a maximum continuous operating temperature of 125°C and a melting point of 250°C. These properties prevent sufficient softening of the PET sleeving during an oven heat cycle of only 105°C, leading to an inadequate shape memory reset and a considerably weakened coil result.
  • A higher oven temperature at a value necessary to effectively soften and reset PET shape memory is not possible as the chamber's uniform heat saturation will fuse and destroy the internal wires.

As a result:

  • Manual, more labor-intensive directed heat is required at significantly higher temperatures (nearing the 250°C PET melting point) to force a selectively molten outer surface (PET sleeving plus cable jacket) without causing damage to the internal wires. When compounded with the necessary coil reversal step, we refer to this coiling method as the HELIX method.

Examples

Videos may better illustrate the resulting differences between methods; see below:

  1. Oven Chamber (Single Coil) - standard oven heat cycle at 105°C, coil not reversed
  2. Oven Chamber (Reverse Coil) - standard oven heat cycle at 105°C, coil reversed after cooled
  3. HELIX / Directed Heat (Reverse Coil) - manual directed heat at greater than 200°C, coil reversed after cooled

1. Oven Chamber (Single Coil)

2. Oven Chamber (Reverse Coil)

3. HELIX / Directed Heat (Reverse Coil)

Final Comparison

With each coil having undergone repeated stretching to 250% (20 inches) of its retracted length at rest (8 inches), the results are immediately noticeable and described below, followed by final video footage comparing all three coils side-by-side:

  1. Oven Chamber (Single Coil) - Results show a permanently loosened coil. There are now significant gaps, and the overall outer diameter has increased by more than 10% due to unravelling of the insufficiently heated and poorly reshaped PET outer sleeving.
  2. Oven Chamber (Reverse Coil) - Results show a permanently deformed coil. The coil shows no gaps and retains its retraction strength due to the reversal process but is now misshapen with unsightly bulges. The overall outer diameter has increased by more than 10% due to unravelling of the insufficiently heated and poorly reshaped PET outer sleeving.
  3. HELIX / Directed Heat (Reverse Coil) - Results show no discernable damage. Retraction strength remains intact, and the shape at resting state is straight and uniform. There are no significant gaps, no bulges, and the overall outer diameter shows no change.

Final Comparison