Thermoforming, Plug Design and the Crush Test

By Kathleen Boivin, Sr. Materials Engineer

How do you ensure that your thermoformed container is going to mechanically withstand the rigors of the end use application?  One common method of evaluating structural performance is the crush test.  Crush testing, which is otherwise known as top load compression, measures the amount of force it takes to crush the container to failure.  The test entails placing the container upright between two platens, as shown in Figure 1.  The bottom platen remains stationary while the top platen moves downward, applying force to the container until it fails.  A load cell measures the force at failure. The most common units are pound force (lbf) and Newtons (N).  The top load test can be performed on a universal testing machine, such as an Instron machine, or a unit specifically designed for the test such as a TMI Top Load Compression Tester.1

Crush_test_fig_1

Figure 1.  Top load compression test.

Why is crush resistance important for thermoforming?  The container needs to survive its intended use without any signs of failure.  For example, a clear drink cup needs to be rigid enough to avoid buckling and spillage when the lid is applied.  A berry basket needs to be strong enough to survive stacking of the filled containers to prevent damaging the fragile contents.

During the crush test, the weakest point of the container is the first to yield to the force.  The weakest point is typically the thinnest point of the container.  In order to achieve the highest crush resistance possible, consistent material distribution is required.  Container material distribution is evaluated by measuring the thickness of the container in numerous areas.  Thickness is measured in mils or millimeters (mm) using a micrometer or thickness gauge, such as a Magna-Mike.  Usually, the thinnest parts of a container are the bottom sidewall and corner areas.

The relationship between material distribution and crush resistance was apparent during a polypropylene cup forming study.2  In addition to other factors, the effect of plug material and plug geometry on material distribution and crush resistance were studied.  As both the lower sidewall and corner thickness increased, the crush resistance increased proportionally as evidenced by the greater top load force required to crush the cups (see Figures 2 and 3).

Crush_test_fig_2

Figure 2.  Effect of lower sidewall thickness on crush resistance.

Crush_test_fig_3

Figure 3.  Effect of corner thickness on crush resistance.

The parameters that resulted in greater lower sidewall / corner thickness improved material distribution and therefore increased crush resistance.  Plug material impacted material distribution through coefficient of friction at the plug-sheet interface.  Compared to HYTAC-WFT (syntactic plug material containing Teflon for low friction), HYTAC-FLX (a copolymer material with higher friction) allowed the plug to pull more material into the cup.  The improved material distribution with HYTAC-FLX, resulted in greater crush resistance.

Crush resistance is also affected by plug geometry.  An increase in plug diameter improves the ability of the plug to pull material into the cup, thereby increasing crush resistance.  A reduction in plug top radius (more blunt design) also improves the ability of the plug to bring more material to the lower sidewall and corner, again improving crush resistance.

  1. M. Strachan, uVu Technologies, LLC.
  2. N. Tessier & T. Gallagher, “Measuring Plug to Sheet Interactions During the Thermoforming Process”, SPE Thermoforming Conference (2006).
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