Understanding the True Cause of Failure in Plugs

This week we received a package with a damaged plug and its mounting system. After reviewing the plug material and the failure modes, we suspect the cracking was due to a mismatch of thermal conductivity rates in a tightly confined area.   The plug design was efficient and neatly put together – at room temperature.   The chosen plug material, HYTAC-WF, is a thermoset epoxy-based material formulated for low thermal conductivity and minimal thermal expansion to provide consistent performance when contacting a heated sheet.  In this particular plug design, those same properties provide a challenge for the mounting system and result in significant stress on the most brittle element – the syntactic foam.

WF_damaged_plug_1

Looking at the material properties for the base plate, mounting bolts and syntactic foam shows the following:

Material    Thermal Conductivity Thermal Expansion
HYTAC-WF 0.19 W/moK (0.11 BTU/hr-ft-OF) 32 x 10-6 m/m/oC (18 x 10-6 in/in/oF)
Al 6061 170 W/moK (98.3 BTU/hr-ft-OF) 23 x 10-6 m/m/oC (12.7 x 10-6 in/in/oF)
Steel Bolt 10.5 W/moK (6.07 BTU/hr-ft-OF) 49.8 x 10-6 m/m/oC (27.6 x 10-6 in/in/oF)

While the aluminum base plate has the lowest coefficient of thermal expansion, its rate of thermal conductivity is nearly 900 times that of the syntactic foam and over 15 times that of the steel bolt and insert.  The rate of thermal conductivity of the syntactic foam is so low as to be virtually non-existent in comparison.  (Ultra-low thermal conductivity is one of the primary reasons syntactic foam is so successful as a plug assist material.)

So how does a designer make use the physics when designing a plug?  If we take the numbers above and our broken plug, let’s assign dimensions to the base plate for an example.  If the plate is 8” long x 5.5” wide x 0.5” thick, for every increase of 50 degrees F in temperature, the base plate will expand in length by .005”, in width by .0035” and by .0007” in thickness.  Because the syntactic foam has not absorbed virtually any heat in this same time period, it will not change at all.

If we assume a typical temperature change from room temperature (when the plug was assembled) at 70 degrees F, and an ambient temperature inside a thermoformer mold area of 300 degrees F, we have a temperature change of 230 degrees F.  The base plate used in the example above would grow by 0.023” while the syntactic foam did not change at all.

If there is no room allowed for the plate to grow, the result would likely first be seen in a cracking of the syntactic foam plug wall at the weakest point – the transitional corner between the plate and the foam.  The secondary result is the pull force against the insert mounted inside the foam.

Suggested options to prevent recurrence:

  •          Significantly reduce the length and width of the mounting plate.  This would reduce the growth of the plate and provide more syntactic foam to surround the mounting plate to delay the impact of the conductivity and expansion impact.  This is the lowest cost option, but it will only delay the potential problem rather than eliminate it.
  •          Utilize a flat base interface between the plug and the mounting plate.  This would eliminate the constrained condition of the plate inside the plug.  Again in the long run, the mismatch will occur because there would be nothing to shelter the mounting plate from ambient heat in the process.
  •          Switching to a thermoplastic-based syntactic foam (HYTAC-B1X or HYTAC-XTL) offers the most secure route for success.  These materials both have significantly higher flexural toughness than an epoxy-based syntactic foam. Flexural toughness actually increases as process temperatures rise giving even more security as opposed to a flat line toughness value for thermoset epoxy-based options.

We hope this information is helpful.  Please feel free to give us a call with questions or fill out our online form for specific application problems.