Physical Foam Injection Molding (eBook)

Thermoplastic, foamed molded parts are produced by injection molding by adding a blowing agent to the polymer. A distinction is made between chemical and physical blowing agents. The chemical blowing agents are offered to the processing industry as masterbatch. During the decomposition process of the masterbatch, in addition to the proportion of blowing gas, other residual materials are produced during injection molding that remain in the molded part. Physical blowing agents are mainly the inert gases nitrogen (N2) and carbon dioxide (CO2), which are present in various aggregate states from gaseous to supercritical during processing.

Independent of both the TSG process and the blowing agent used, the molded body is in the form of a structural foam, with a core of closed bubbles, also known as cells, and a compact boundary layer (skin) on both sides.

The properties of structural foams are derived from this characteristic structure, and thus ultimately also determine the material parameters for the component designer. By varying the boundary layer thickness as well as the cell size, cell density and cell orientation (in the case of elliptical cells), the property can also be partially optimized in line with the load. If one considers the two phases of the production of a single-phase mixture of matrix polymer and blowing gas in the injection molding machine, and the production of the molded part in the cavity of the mold, it is clear that not only the process parameters of the injection molding machine are critical, but that the mold also makes a significant contribution to the structural properties. Last but not least, in addition to the polymer to be processed, the proportion or type of nucleating agent also plays a role. At first glance, this apparent complexity in foam production appears difficult, but in the end, for the expert who masters his/her profession, there are possible solutions in lightweight design that cannot be achieved in any other comparable cost-effective way. Let us therefore go into the details.

In Figure 3.1, we see a typical example of a structural foam. Such a structure appears independent of the blowing agent used and independent of the manufacturing technology. A compact boundary layer and a foamed core can be seen, with the density usually increasing slightly from the center axis of the core to the boundary layer. The mechanical properties of such a structure – also called integral foam – which are important for the component designer can be used to advantage in component design.

However, until we go into this in more detail later, a quote from Cramer’s dissertation from 2008 [1] should explain the status at that time:

“Another challenge is found in the mechanical properties of thermoplastic foams. Basically, their mechanics are reduced by the absence of load-transmitting material compared to compact components with the same dimensions. This applies to components loaded in tension, in bending, as well as in impact. Only if the properties are normalized to the weight can better mechanical properties be achieved, depending on the load case. The sandwich-like structure of injection-molded thermoplastic foams results in a high area moment of inertia at low weight, which can mainly improve the load absorption in relation to weight under bending stress compared to compact injection molding. However, this potential often remains unused because in many cases the molded parts are designed according to conventional injection molding criteria. One reason is the often-cited lack of knowledge of the relationships between the foam structures and the resulting mechanical properties. A prediction of the molding process, let alone a structural simulation of the manufactured parts, is not yet possible today, despite many years of work in this field. However, this prediction must be a long-term goal in order to enable and expand the use of foamed components as technical injection molded articles.”

This quotation reflects the correct state of the art at that time! However, all the unsolved problems mentioned at that time will be taken up and answered in this book, especially in Chapters 4 (Design Guidelines) and 5 (Simulation). So there are effectively no more open questions today!

Typical weight reductions of finished parts are between 5% and 15%, depending on the material used (polymer and amount of blowing agent) and the process conditions during the production of the molded part. However, since the material parameters required for the design of the part depend on the thickness of the surface layer and the cell structure, any desired high weight reduction always competes with the necessary and sufficient material properties. For design reasons, this always leads to compromises.

Sink marks can be almost completely eliminated on the molded parts – especially in the case of wall thickness variations – because the necessary pressure in the mold has an almost uniform effect on the polymer melt from the inside due to the blowing agent. This provides the part designer with a great deal of additional potential: In contrast to the familiar plastic-specific design, in which there is a considerable pressure difference between the sprue and the end of the flow line inside the cavity in the holding pressure phase, the mechanical requirements of rib structures, for example, can be made more suitable for lightweight design with a view to notch effects or force transmission. See also Figure 3.2 for an explanation.

The process also dramatically reduces the warpage of the molded parts. As already mentioned in the previous chapter, the pressure required to form the single-phase mixture in the cavity is generated “from within” by the blowing gas. No holding pressure is required. The residual stresses and frozen orientations of the molecular chains typical of injection molding are minimized. As a result, components can then be manufactured with much tighter tolerances for the subsequent assembly process with a better fit.

In general, TSG foams are produced with lower shrinkage behavior than the classic injection molded parts. However, this must be divided into different shrinkage behavior in the thickness and longitudinal directions [2]. The observed shrinkage in the thickness direction is significantly lower than in compact injection molding, whereas a look at longitudinal shrinkage shows little to no difference from the known shrinkage behavior of classic injection molding.

This different behavior can be well understood based on the integral foam shown in Figure 3.1: In the thickness direction, the bubble growth inside the integral foam directly counteracts shrinkage. In the longitudinal direction, on the other hand, the compact boundary layers of the structural foam act similarly to a compact component, while the bubble growth has no major influence. This means that the shrinkage behavior is also similar.

The mechanical properties of structural foams can be varied within a certain range by the process conditions. For the bending stress or the bending modulus, the main influence is the boundary layer thickness. The main influence on stresses and elongations at break, on the other hand, is determined by the density of the component [3], as Figure 3.4 shows well. More detailed information on this and the different influences on amorphous or semi-crystalline thermoplastic materials can be found in Chapter 6 “Polymers for Foam Injection Molding”.

It is well known that foams insulate against heat/cold, which of course also applies to structural foams. In particular, the cell size, but also the cell uniformity, plays the main role. The heat transfer decreases,...