Integral Foam Structure Increases Strength
Investigations indicate that the mechanical properties of foamed moulded parts are determined largely by the compactness and thickness of the skin zone. Fine-celled foamed parts with a compact skin zone thus exhibit better mechanical characteristics than homogeneous microcellular foamed products.
Foaming of thermoplastic parts reduces part weight and saves material. The cooling time is shortened and parts can be produced faster. In addition, incorporation of gas reduces the melt viscosity – the cavity pressure is lower, so that the clamping force can be reduced. An additional benefit: foamed parts exhibit less distortion and better dimensional stability.
Unfortunately, however, foaming is always associated with a decrease in mechanical properties of the moulded parts. In a joint experimental program, BASF and Demag Plastics Group have investigated the correlation between foam structures and mechanical characteristics. Among other things, the relationship between the actual size and distribution of bubbles and the decrease in mechanical properties was established.
In the course of this work, it was found that the stiffness of a moulded part depends more strongly than previously assumed on the profile of the foam structure. The project group reports in the October issue of the magazine Kunststoffe that, above all, a dense and compact skin zone leads to high mechanical properties. According to the article this disproves the widely held belief that only a homogeneous foam cross-section with the smallest possible, microcellular bubbles produces good mechanical attributes.
The bubble size is not decisive
In the test series, the weights of two glass-filled ABS grades (ABS-GF20 and ABS-GF15) were reduced by 9% each through use of CO2 and N2 . With N2 a relatively homogeneous bubble structure resulted. In contrast, when foaming with CO2 a more inhomogeneous bubble structure resulted. The skin is relative homogeneous and compact, while coarse structures with considerably larger bubbles form in the interior. In spite of the coarser bubble structure, the parts that used CO2 as the blowing agent exhibited better mechanical properties.
It follows that the mechanical properties of the foamed parts are determined largely by the compactness and density of the skin zone. The investigations show that the formation of homogeneous microcellular foam structures is actually detrimental as far as mechanical properties are concerned.