Thermoplastic microcellular foams (such as those produced by the MuCell process of Trexel Inc.) provide cost saving of the products and thus provide economy. One of the key problems is to correlate cell structure with foam mechanical properties. The researchers at IKV in Germany evaluated three variables: length of core movement, speed of core movement and hold time before core movement. One variable was changed at a time while the others were held constant.

After molding, samples were sliced and polished for viewing under a microscope. The following effects were noted:
Effect Of Density
Density reduction in these experiments ranged from 10-25%. For parts with the same skin thickness and different core density, lower density resulted in fewer and bigger cells. This correlated with lower tensile strength and modulus, though the correlation with flexural properties was less clear and less pronounced. An interesting result was higher tensile elongation and much higher instrumented multi-axial impact toughness at lower densities. Both the force required to break the part and the energy absorbed were greater with increased cell formation, even at very low levels of density reduction.
Effect Of Skin Thickness
Longer hold time produced finer and more uniform cells with the same degree of density reduction (mold cavity expansion) in the part's core. However, the target variable, skin thickness was only slightly affected. Finer cells correlated with higher tensile strength, but there was no significant change in tensile modulus. Flexural properties showed no clear trend. But tensile elongation and impact energy absorption increased dramatically at the highest hold time, which produced a proportionally thicker skin and extremely fine cells.
Effect Of Cell Size
Keeping the nominal skin thickness and core density reduction the same and varying only the speed of cavity expansion, produced more and finer cells at faster expansion rates While the tensile modulus was unaffected, tensile strength increased 50% with a finer cell structure. Effects on flexural properties were smaller and the trend was less clear. But impact energy absorption increased almost three-fold with a finer cell structure.
The researchers noted that these experiments varied the skin thickness by only 8%. To achieve a greater thickness variation, the IKV team is now combining a reduction in mold temperature with longer hold time. This appears not to affect cell size. Confirming the previous studies, early results with this technique show higher tensile strength and modulus with larger increases in skin thickness. The study concluded that not just foam density, but also foam cell morphology significantly affects part properties. However, the effect of foam morphology also depends on the type of loading of the part�tensile, flexural, or impact. Density reduction substantially diminishes tensile properties but is less critical if the part is exposed primarily to impact loads.