Foam injection molding 2.0

• High-pressure foam injection molding
• Cellmould: the machine concept
• What potentials does high-pressure foam injection molding have to offer?
• High-gloss surfaces through dynamic mold tempering
• Elastomers are also suitable for foaming
• Technical paper of Wittmann Battenfeld

Lightweight design is a trend increasingly pervading all sectors of the manufacturing industry. In this domain, plastics play a vital part thanks to their favorable relation between performance data and low specific weight. But their lightweight potential can be increased even further by foaming, for example by foam injection molding. One of the pioneers in this field is the Austrian injection molding machine manufacturer Wittmann Battenfeld. Its Cellmould high-pressure process offers comparatively superior performance parameters together with a less complex and consequently more robust system technology by comparison with competitors. It is a 100% in-house development and serves as the basis for a number of new, innovative applications, such as solutions for improvement of the surface quality including high gloss, for partial combination of compact with foamed components in a single molded part and for foaming of thermoplastic elastomers. These solutions have been developed in cooperation with the Bavarian technology company Schaumform (Fig. 1).

Foam injection molding technology is not a new process. Applications in which chemical substances such as azodicarbonamide or phenyltetrazole are blended into the plastic granulate and plasticized with it, which release propellant gases following injection into the cavity of the mold, have been known and used in production for some 50 years. Since the expansion pressure of these chemically released gases is no more than about 15 to 40 bar, their use is limited to relatively thick-walled parts with short flow paths.
To further extend the application boundaries for foam injection molding, foaming by the addition of an inert gas, usually nitrogen, was developed about 40 years ago. The main advantage is that higher expansion pressures in the region of 100 to 200 bar can be reached with nitrogen. This enables exploitation of the lightweight design potential in foam injection molding for thin-walled components and components with long flow paths as well. The advantages in addition to weight reduction are a reduction of the specific injection pressure required to fill the cavities and consequently the clamping force, and compensation of shrinkage and warpage effects. Both processes are used in thermoplastic resin processing, all the way from PP to engineering plastics such as PC, PA or PBT. Most recent, promising developments aim to extend the fields of application to include thermoplastic elastomers as well.

Cellmould: the machine concept

The essential task of a foam injection molding line is to generate a one-phase polymer-gas solution dispersed as homogeneously as possible during the plasticizing process. The technology used by all suppliers for this purpose is very similar. Nevertheless there are some differences in the details of technical design. Dipl.-Ing.(FH) Wolfgang Roth, Head of Application Technology at Wittmann Battenfeld, puts it this way: “The more than 40 years of practical experience with the technology developed at our predecessor company Battenfeld, Meinerzhagen provided a solid foundation for us to build on. Our goal was to reduce the complexity of the system while simultaneously expanding the fields of application and thus make it more reliable. Therefore we have designed our Cellmould foam injection unit to come as close as possible to the standard injection unit. Accordingly, our machine operates with a 20 D standard screw, which has been extended at the front by adding a 5 D mixing section.”
The specific Battenfeld feature of the Cellmould technology is the separation between the plasticizing and gas injection sections of the screw, which is provided by a fixed, cylindrical barrier on the screw. It is the alternative to using an additional sleeve-type check valve. Wolfgang Roth adds: “The effort involved in adjusting two check valves to the operating conditions in every case to make them fail-safe, i.e. wear-resistant, motivated us to seek a simpler solution, which we have ultimately found in the barrier between the plasticizing and gas injection sections of the screw. This solution has been proven in production for all machine sizes. In this way, the wear problem could be eliminated without having to compromise to a significant extent on gas density in the direction of the screw’s plasticizing section.”
In the mixing section of the plasticizing unit, liquefied nitrogen (pressurized with up to 300 bar) is added into the plastic melt by an injector in during a metering stroke and subsequently diffuses into the melt. In the mixing section of the screw, the nitrogen distribution is intensified by dividing the melt flow into many separate currents.” (Fig.2) Since the barrel is kept closed by a needle shut-off valve in the direction of the mold during plasticizing and gas injection, the melt-and-gas mixture is kept under pressure inside the plasticizing unit. Consequently, a single-phase polymer/gas solution is achieved by the end of the mixing process. During injection into the cavity, it is subjected to pressure decrease, which reduces the solubility of the gas in the plastic melt. The finely distributed gas nucleates in the melt and thus provides the ingredient to form a foam structure with just as finely distributed cells.

The formation of this structure depends on the specific conditions of the injection molding process. These include the viscosity of the plastic melt, the injection speed (the higher the speed, the finer the foam) and finally the pre-set degree of foaming (material reduction). The latter is set either by injecting a corresponding underdosage into a fixed cavity, or by filling a cavity completely and subsequently opening it with a pre-set high-precision stroke. In order to reach the high injection speed which favors an even foam distribution, an injection accumulator is supplied as a part of the Cellmould equipment package (Fig. 3a+3b).

The nitrogen is either drawn from a battery of pressure cylinders or extracted from the ambient air by a nitrogen generator. In both cases, the gas is subsequently passed on to the gas injector via a pressure generator such as is also used in Airmould gas injection lines. A part of the Battenfeld line concept is that several machines can be supplied simultaneously by one gas supply system (Fig.4). A gas flow regulator is placed between the pressure generator and the gas injector on the plasticizing unit. Via its controllable valve system, the gas flow is controlled and coordinated with the process by the Cellmould software (Fig.5). The Cellmould equipment package is available for the entire portfolio of Wittmann Battenfeld machines.

What potentials does high-pressure foam injection molding have to offer?

Inside the mold cavity, the formation of foam in the outer shell of the melt is largely suppressed due to its contact with the cooled cavity wall and the resulting increase in viscosity, while the hotter core area favors the formation of the cell structure. In this way, “sandwich structures” are formed in major parts of the molded part, consisting of covering layers with a high density and core parts, whose bulk density is 5 to 20% lower (Fig. 6a+b).

The possible density reduction in the molded part shows a direct correlation with the flow path/wall thickness ratio for all commonly available types of plastic materials. In PP processing, for example, a density reduction of 15 per cent can be achieved at a ratio of 100 : 1, while at 150 : 1 a density reduction of only 10 per cent can be expected.
Quite apart from weight reduction, foam injection molding offers additional potential for improvement in the quality of molded parts, primarily with regard to shrinkage and warpage, thanks to the uniform effect of the expansion pressure inside the foam core. This effect is so strong that sink marks and warpage caused by shrinkage can be virtually eliminated to 100 per cent, thus increasing the overall dimensional accuracy. Processors will also benefit from several significant process technology advantages, such as a reduction in the required clamping force by up to 50 per cent due to a decrease in melt viscosity and consequently the injection pressure, as well as commercial advantages by a reduction in cycle time, in particular cooling time, thanks to the lower mass of the molded part which needs to be cooled.

High-gloss surfaces through dynamic mold tempering

In spite of exploiting the full range of parameter variations offered by the foam injection molding process, light-weight parts still show characteristic striations or grey fogging on the surface as a common attribute. This surface effect is due to gas bubbles penetrating into the flow front of the melt during the injection process. This structure then solidifies when coming into contact with the cooler cavity wall and subsequently remains unchanged. Polished surfaces, such as are required for visual parts of housing components, cannot be achieved with standard technology. However, a substantial improvement in surface quality can be achieved by a combination of foam injection molding with cyclical, dynamic mold tempering, as is offered, for example, by Wittmann Battenfeld in the form of BFMold and Variomould technology. These variants use a cooling system integrated in the mold to improve the visible surface of the molded part, following the part’s contour and operating cyclically with hot/cold temperature controllers. This system controls the temperature of limited mold areas close to the cavity. By heating the cavity wall, for example with pressurized water heated up to 180°C immediately before injection of the melt with gas content, the material does not come into contact with a cold cavity wall at first, so that a closed surface can form before it solidifies (Fig. 7). In this way, excellent surface quality can be achieved, which is on a par with that of compact plastic parts. The comparison between parts with and without dynamic cooling, as illustrated in Fig. 8, shows how strongly the effect of dynamic mold tempering can influence the quality of the surface.”

Elastomers are also suitable for foaming

Foam injection molding can also be extended to thermoplastic elastomers. While good foam structures can be achieved by chemical as well as physical foaming, for example, with polypropylene and polyamide, our test series have revealed that most types of TPE can only be foamed by physical foam injection molding. And only TPEs based on thermoplastic polyester show acceptable results in terms of foam structure, fineness of cells and evenness. Tests have shown that the softer a TPE formulation, the more strongly surface problems will show up in foaming, especially if foam injection molding is combined with high-precision mold opening. Especially when the cavity is draw polished or even high-gloss polished, the surface often shows numerous dents. Several different explanations have been proposed for this phenomenon. One is that air is already enclosed between the molded part and the cavity wall while the cavity is being filled, which cannot escape. An alternative assumption is that high-precision opening leads to a separation of the foam part from the cavity wall, and that the expanding foam part, when it comes into contact with the cavity wall again, encloses air or plastic gas in some places, which then causes the dents.”
Test series have shown that, in contrast to rigid and solid technical thermoplastic materials, the surface problems in TPE processing can be significantly reduced by using medium to low injection speeds. Equally positive effects can be obtained by structuring the cavity wall. A textured, bead blasted or grained surface allows any potential gas or air bubbles to escape via micro channels in the contact surface between the molded part and the cavity wall.
As to striations on the surface, the same principles generally apply as in foam injection molding with engineering plastics. Here, the solution is also to use dynamic tempering around the contours of the visible side. If high-precision opening is applied simultaneously, high-quality soft foam padding, for example for arm rests in vehicle construction, or shock absorbers for hand-held appliances which must be protected from damage dropped, can be produced at low cost. This will be further discussed in a separate report in one of the next issues.

With innovative mold and machine technology for broad application

It has already been mentioned in connection with surface improvement that innovative mold technology plays a vital part in foam injection molding. Another area of mold and machine technology specially geared to foam injection molding is the system of partial mold opening via the injection molding machine, which enables the combination of compact with foamed components in a single injection-molded part. This is necessary whenever functional elements made of rather compact material, such as hooks, springs or bolts, must be combined with panel components made of foamed material. To realize this, the part of the cavity to be foamed around the foaming stroke is made movable. In a first step, the entire cavity for the molded part is filled as is done for a compact molded part. Subsequently, only the part to be foamed is opened by a high-precision stroke. In this way, housing components with complex mechanical interfaces to partner components can also be realized in light-weight design.

Mechanical key values can be predicted reliably

High-pressure foamed injection molded parts have a characteristic sandwich structure with compact covering layers and a foamed core layer. The borderline between the covering layer and the core is relatively abrupt. In low-thickness components, the core layer has a virtually constant density throughout the entire width of the core, while in the case of large total thickness a characteristic density profile is present. The process implementation has just as little influence on the density of the compact covering layer as the type of gas injection chosen. Consequently, the most important design parameters are the reduction of density designed for the core part and the wall thickness. These can be clearly defined by measurement results and serve as key figures for a calculation model developed by Dr. Norbert Müller, the founder of Schaumform, as part of his dissertation to predict the mechanical attributes of components.

Process design based on a model calculation

The starting point for the model calculation is a symmetrical sandwich structure, in which, in slightly simplified terms, the specific material values of the compact material are assumed for the covering layers. For the foamed core, key values close to reality are assumed for the E module and fracture strain (yield strain for ductile materials). The behavior of the foamed core is derived from the behavior of the entire sandwich component, which functions well if the thickness of the covering layers is known. Tests in which the foamed core is extracted from a component and subsequently tested mechanically are possible, but lead to strongly scattered measurement results, which consequently have only a very limited significance.

Theory and practice are consistent

The optimal method to test rigidity and strength is by using standard test rods produced from injection-molded structured foam sheets. Alternatively, if this option is not available, standard test rods with a 4 x 10 mm cross-section (e.g. campus tension rods) can be used. However, when analyzing the measurement values, it must be taken into account that not only the 10 mm-wide covering layers of the standard rods are compact, but also the 4 mm-deep lateral surfaces. Consequently, a foamed standard tension rod is comparable to a small rectangular tube (10 x 4 mm) with approx. 0.4 to 1.0 mm wall thickness and a foamed core.
As can be expected, the evaluation of tensile strain tests shows that as the proportion of foam increases, the tensile elastic modulus and tensile strength decrease accordingly. This is due to the fact that only the quantity of material still contained in the component can withstand mechanical stress or contribute to load bearing. So foamed injection-molded products show a higher expansion rate when exposed to the same load and break under a lower maximum load. Added to this are notching effects caused by foam cells close to the covering layer. The measurement results regularly show that the decline in tensile strength invariably at least equals the reduction in part weight. (Fig.9)

tensile strength (N/mm²) –  charpy impact resistance (kJ/m²) – fracture strain (%) – compact PP

Under bending load, the absolute values of flexural resistance and flexural strength are also reduced. However, since sandwich structures are much more resistant to this type of load, the loss of strength here is significantly less than for tensile strain. The flexural resistance falls by a lower percentage than the part weight. In figures 10 and 11 it is documented that, for instance, with a foaming degree of 15% the rigidity in relation to weight has increased by 4.8 per cent compared to the compact part without foam or, in other words, it is possible to realize rigid components with a lower weight.

Summary

Foam injection molding technology has received a new innovative boost due to the progressively increasing trend towards lightweight applications. The most recent innovations concern methods to improve the surface quality in the direction of high gloss, as well as the combination of compact with foamed segments in a single molded part. The most important contributions have been made by further developments in process and mold technology, ranging from dynamic mold tempering to high-precision opening of entire molds or cavity segments in one or several steps. The proven model calculations, which are now generally available, offer additional potential for assistance in parts design and layout. All in all, the foam injection molding process has thus reached a similarly high degree of maturity as conventional processing by injection molding. It delivers precise, repeatable density reductions and sandwich structures for a constantly growing range of plastic materials, including thermoplastic elastomers.