Lightweight Injection Molding

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 lightweight 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.

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-lightweight injection molding. 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.” 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.

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 which provides perfect gas assisted injection molding process. A part of the Battenfeld line concept is that several machines can be supplied simultaneously by one gas supply system. 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. The Cellmould equipment package is available for the entire portfolio of Wittmann Battenfeld machines.

Foam injection molding technology has received a new innovative boost due to the progressively increasing trend towards lightweight injection molding 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.

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