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1 20 SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding [VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTICS] A Little Extra? Process Combinations Open up New Horizons for Foam Injection Molding The trend towards lightweight construction and the desire to further exploit the savings potential in plastics processing are ensuring the increasing adoption of thermoplastic foam injection molding. By selectively combining this with other processes, it is possible to positively influence the mechanical and tactile properties, and surface appearance of foamed parts. In thermoplastic foam injection molding, plastic melt charged with a blowing agent is injected into the cavity. The foaming process occurring there replaces the holding pressure required in nonfoam injection molding and leads to a homogeneous pressure level. Since no pressure gradient forms between the gate and flow path end, the warpage tendency is significantly reduced. In addition, the choice of sprue position and wall thickness need not be chosen so as to allow the holding pressure to be transferred even to areas remote from the sprue. The wall thicknesses can thus be reduced in many applications, in addition injection from thin to thick is possible [1]. Modeled on Nature: a Similar Structure to Bones and Tree Trunks The cross-section of a foamed part shows a sandwich-like structure: the nonfoamed outer layers and the fine-celled foam structure on the inside, which are responsible for the reduced part weight, among other things. With most materials, however, streaking occurs on the part surface, since small bubbles occur at the melt front during injection (Fig. 1). The sandwich-like structure often improves the weight-specific properties of the part, i. e. the property data that are quoted per kilogram of material. Examples of this can also be found in Nature. For example, human bones and a tree trunk each have structure similar to that of a foamed part, in which a non-foamed outer layer surrounds a cellular core structure. In both cases, the core transmits the A combination of variotherm mold temperature control with a thermoplastic foam injection molding process such as MuCell allows foamed parts to be manufactured with high surface quality. Engel demonstrated together with Roctool the potential of this technology combination at the Fakuma 2014 with the production of middle console components. shear forces or the compressive loads, while the flexural properties are for the most part derived from the non-foamed outer layer. Based on calculations [2, 3], it can be shown how the tensile and flexural stiffnesses behave with decreasing density in relation to non-foamed parts (Fig. 2). The wall thicknesses of the parts are not changed with these considerations. It can be seen that foamed parts initially have a very steep decline of tensile strength

2 Injection Molding RENAISSANCE OF FOAM PROCESSES 21 Fig. 1. The surface quality is usually streaky with unfilled amorphous plastics (left). Depending on the material type used, filled semicrystalline plastics have a higher surface quality (right) with decreasing density. Because of the sandwich-like structure, the flexural stiffness behaves differently and decreases less sharply in the range relevant to the applications. A density reduction of 10 % reduces, for example, the tensile strength by about 20 %. In this example, however, the flexural strength decreases by less than 10 %. Besides the density, the thickness of the non-foamed outer layer is also suitable for changing the part properties, as regards the process technology, within certain limits. For example, a lower gas content usually involves the formation of a thicker non-foamed outer layer. The mold temperature, or melt temperature, and the injection velocity influence the sandwich structure and therefore the properties of the part. Detailed investigations on the influence of the processing parameters are described, for example, in [4]. Different Process Goals in Combination with Negative Coining controlled parallelism to a preset end position. The effect of greater wall thicknesses with constant part weight on the mechanical properties can be accurately estimated. Graphs determined from calculations [2, 3] make it clear that the flexural stiffness increases with increasing wall thickness (Fig. 4). A doubling of the wall thickness, e. g. from 2 to 4 mm, is not rare in the production of dashboard backings in the automotive industry. It is possible to make use of the possibility of improving the flexural strength without increasing the weight of the non-foamed part. If the focus is not on greater flexural stiffness but on lower part weight, the initial wall thickness h 0 of the unfoamed part is reduced and the mold is opened after the injection of the blowing-agent-charged melt to the extent necessary to achieve the required flexural stiffness of the foamed part. In this manner, the part weight can be reduced more strongly than with conventional foaming methods. In implementing this process combination, users should pay particular attention to two points: the plastic used and the part design. The process sequence illustrates the high requirements on the material (Fig. 5). In step 1, the cavity, with a basic wall thickness of 2 mm, is filled. In step 2, a non-foamed outer layer of 0.3 mm in each case is set. In step 3, the mold is opened so far that the finished part has an end wall thickness of 6 mm. When the mold is opened, the foaming process starts in the molten interior of the part in this example over a thickness of 1.4 mm. Here, the typical cell structure develops, which is increasingly stretched as the opening stroke progresses. The requirement on the material used consists in stabilizing the fine cell structure during the opening movement and not tearing it open. For the part design, it should be noted that surfaces that are not at a right angle to the opening direction undergo» According to Steiner s law, the flexural strength is the higher, the more distant the outer layers are from the neutral phase. In practice, a greater distance of the outer layers can be achieved by combining thermoplastic foam injection molding with negative coining. In thermoplastic foam injection molding, plastic melt charged with a blowing agent is injected into the cavity with the basic wall thickness h 0. After cavity filling and the subsequent formation of the non-foamed outer layer, the cavity or partial regions of the cavity are increased to the wall thickness h 1 (Fig. 3) either by means of a movable core or by selective opening of the mold. With the use of Engel duo injection molding machines, the mold is opened with Relative tensile/flexural strength Tensile strength Flexural strength 0.2 Practical field of application % 50 Weight reduction Fig. 2. Change of mechanical properties with part weight for constant wall thickness: at first, the relative tensile strength drops steeply with increasing density reduction. For example, with a density reduction of 10 %, the tensile strength falls by about 20 %. The flexural stiffness falls less steeply because of the sandwich structure Kunststoffe international 12/2014

3 22 SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding Foam Injection Molding Thermoplastic foam injection molding can be subdivided into two parts depending on the type of blowing agent addition: For example, in chemical foam injection molding, the plastic melt is charged by the decomposition of a chemical blowing agent added to the molding compound. In physical foaming, a physical blowing agent, usually nitrogen, in rare cases carbon dioxide, is introduced directly into the melt via an injector. In the mixing sector of the screw, blowing agent and melt are homogenized until a single-phase mixture is present in the front region of the plastication unit. In physical foaming, there are a number of process variations, with MuCell being the most widespread. The MuCell method is based on developments made at the Massachusetts Institute of Technology (MIT) in Boston, and is marketed by Trexel, headquartered in Wilmington, both in Massachusetts/USA. The machine manufacturer Engel offers completely integrated injection molding systems for the MuCell foam injection molding process from a single source under the name Engel foammelt. The Authors Dipl.-Ing. Robert Endlweber is technology manager in process technology development at Engel Austria GmbH, Schwertberg, Austria, robert.endlweber@engel.at Dipl.-Ing. Josef Giessauf heads the process technology development department at Engel; josef.giessauf@engel.at Service References & Digital Version BB You can find the list of references and a PDF file of the article at German Version BB Read the German version of the article in our magazine Kunststoffe or at h 0 h 1 Fig. 3. In negative coining, the cavity is volumetrically filled in the first step (left). Due to the delay time, the thickness of the compact outer layer can be adjusted (center). The subsequent opening stroke initiates the foaming process (right) a lower expansion of the wall thickness during mold opening (Fig. 6). Consequently, the degree of foaming is not equal everywhere and the mechanical properties can vary. To estimate the mechanical properties that can be expected, the above-mentioned graph (Fig. 4) can be used. Controlling the Haptics: The Negative Coining of TPE The above-described combination of foaming and subsequent negative coining permits the tailored design of mechanical properties and supports lightweight construction and reduced consumption of raw material. It is often used for manufacturing sheet-like hard-foam beams. If, instead of a thermoplastic, a soft material such as TPE is used, the softness or haptics of the part can be adjusted by means of the opening stroke and other process parameters. This principle is used, for example, in the Dolphin process [5], in which, on an Engel duo combi M injection molding machine with rotary table, a thermoplastic beam can be produced, and overmolded with blowing-agent-charged TPE in a second process step. In this case, the soft-touch effect is selectively adjusted by means of the opening stroke. For Visible Applications: Overmolding of Films and Textiles with Foam The advantages offered by the foaming of thermoplastics lead to the ever increasing use of foam processes such as Engel foammelt (see Info box) for visible parts, e. g. panels or various covers. Several combinations of foam injection molding with other processes make this possible now. Decorative materials such as film or textiles are often overmolded with blowing-agent-charged melt. The characteristic streaky structure of the foamed part is thereby easily concealed behind a non-transparent insert part and is not further relevant for the surface quality. In many cases, the film also forms a heat barrier to the inner wall of the cavity [1], so that the polymer melt does not solidify immediately after the filling process and streaking cannot occur on the part surface. Especially when amorphous plastics are used, transparent films can thus be produced with high surface quality. The process combination using the blowing-agent-charged melt hardly differs from the process without blowing agent as regards the requirements on product design, material selection, etc. It is only necessary to consider the following additional points:

4 Injection Molding RENAISSANCE OF FOAM PROCESSES 23 Relative tensile strength (EA/(EA 0 ) Relative flexural stiffness (EI/(EI 0 ) Final wall thickness/initial wall thickness Final wall thickness/initial wall thickness Fig. 4. Change of mechanical properties with wall thickness at constant part weight: with negative coining, only the tensile strength values are reduced (left). In the case of the flexural stiffness, on the other hand, the greater wall thickness leads to a significant increase (right) The films inserted in the mold must permit outgassing of the blowing agent. If the film forms a gastight barrier, bubble formation or delamination of the film from the carrier may occur. Because of the low cavity pressure during foam injection molding, the thickness of the film or of the textile can be frequently reduced in many cases. Due to the lower cavity pressure effect, the soft decorative material is compressed less strongly. The elimination of sink marks due to the foam pressure can be used to achieve new design possibilities for ribs and bosses. Consequently, parts can be produced, which have advantages in their surface quality and mold complexity, compared to conventionally produced parts. Variotherm Process Control: High Gloss without Secondary Finishing The combination of the TSG process with variotherm mold temperature control is already used for the series production of optically high quality panels in automotive engineering. In the first step, the cavity surface is heated. There are different possibilities for this: the entire mold, a mold insert or ideally only the surface of the cavity can be heated. Known heat sources are induction, infra-red or laser radiation, ceramic resistance heaters or conformal fluid temperature control with water or steam [6]. Once the cavity surface has been heated, the blowing-agent-charged polymer is injected. While the occurring streaks solidify immediately on a cold mold wall, they are eliminated again on the preheated surface. Cooling takes place parallel to the foaming process. Parts manufactured with this process combination have a high surface quality. Due to the foaming, rib/wall thickness ratios of 1 : 1 are implemented without sink marks becoming apparent on the surface of the final part. With laser texturing, matte and high-gloss surfaces can be combined (Title figure).» Kunststoffe international 12/2014

5 24 SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding Effective foam stroke [% of opening stroke] h 0 = 2mm 0.3 mm The prerequisite for a high surface quality is often good venting, since, in the thermoplastic foam injection molding process, injection is generally very fast to prevent foaming occurring already during injection. In addition, the gas cushion that forms on injection due to outgassing blowing agent requires good venting. 1.4 mm Direction of the opening stroke 20 B Angle to opening stroke Fig. 6. All part surfaces, as in detail A, are not always oriented normal to the opening direction. The region of the part shown in detail B is inclined by, for example, 60. The wall thickness here is enlarged by about 50 % of the actual opening stroke (figures: Engel) Foaming with Gas Counter Pressure only for Simple Geometries A h 1 = 6mm Fig. 5. In this example, in the first step, the cavity is volumetrically filled with a basic wall thickness of 2 mm (left). Then a 0.3 mm outer layer forms (center). The 1.4 mm-thick plastic core is strongly stretched in step 3 (right) making high demands on the plastic During foaming with gas counter pressure, the pressure in the cavity is first increased with nitrogen. The level of this varies depending on the blowing agent and blowing agent charge that are used. For chemical foaming, about 10 bar is required, and 50 to 100 bar when physical blowing agent is used. Subsequently, the blowing-agentcharged melt is injected and the nitrogen previously introduced can escape from the cavity again via a relief valve. Due to the injection against the high pressure, foaming is prevented directly at the melt front and streaking is thus prevented [7]. The pressure level required for injection, however, limits the possible applications to very simple geometries, since it is very difficult to seal movable elements, such as slides. Elongated parts, in which the gating point and gas inlet and outlet are opposite one another are suitable for this process. Only with this arrangement the gas cushion can be through the gas outlet without uncontrolled gas inclusions occurring. Summary In thermoplastic foam injection molding, users profit from holding-pressure-free process control, efficient machine dimensioning, high freedom of design and advantageous part properties. In addition, the process is often used to minimize the part weight. If the foam process is combined with negative coining, the part weight can be reduced even more significantly than with conventional thermoplastic foam injection molding process. In addition, this is accomplished without loss of the flexural stiffness. In combination with the overmolding of films and textiles, variotherm process control, or the gas counter pressure process, even the challenges of manufacturing high-quality visible parts can be mastered. W