Sponsored By

Technical article: Soft-touch for the price of two-component injection molding

One of the key differentiating features of modern cars is the interior. In addition to the overall design, special attention is being paid to the quality of the surfaces. A variety of manufacturing processes have been created to give interior grained surfaces a soft feel. The Dolphin process, which is unrivalled in cost effectiveness, is set to join the ranks.

ENGEL

September 19, 2014

11 Min Read
Technical article: Soft-touch for the price of two-component injection molding

One of the key differentiating features of modern cars is the interior. In addition to the overall design, special attention is being paid to the quality of the surfaces. A variety of manufacturing processes have been created to give interior grained surfaces a soft feel. The Dolphin process, which is unrivalled in cost effectiveness, is set to join the ranks.

Methods for manufacturing parts with a soft-touch surface must strike a balance between high quality requirements, technical feasibility and cost. Conventional processes used to generate a pleasant tactile feel - such as the laminating of compact injection molded thermoplastic parts or in-mold foaming with polyurethane or slush processes -have one thing in common; they are multistage processes, which inevitably involve trimming, folding or bending after the last shaping stage. The linking of the individual process steps and the secondary finishing processes are time consuming, increase the investment costs for the production line, require a logistical effort for conveying the semi-finished products that is considerable in many cases, and - like the production of PVC slush skins - are often very energy intensive.

For those reasons, the aim of developing the Dolphin process1 was to use thermoplastic injection molding to achieve soft-touch surfaces with a good level of softness, acceptable density and foam heights, with no secondary process required apart from sprue cutting. That is to say the soft surface should be applied (created) directly in the injection molding machine to (on) the hard support material. Also, the trend toward greater flexibility of manufacturing technology was taken into account. An injection molding machine that is suitable for this complex process can be used for different injection molds and processes and adapted to future product generations.

Balance between quality and efficiency

According to a study on the market positioning of various surface enhancement techniques (Fig. 2), the Dolphin process closes the gap between mold skins for extreme quality requirements in high-end automotive engineering and classic film-decorated injection moldings for cost-effective car production. The first applications have confirmed the high efficiency of the process (Fig. 3).

The high saving potential of the Dolphin process can be illustrated in reference to a car instrument panel. The material costs for the part are calculated at $15.56 for the part, $5.75 for the backing and $9.81 for the foam layer. These values are based on a backing volume of 1,300 cm3. The backing has a surface area of 4,500 cm2. With a starting value of approx. 2 mm for the thickness of the TPE foam layer, this results in a TPE volume of 1,050 cm3. For a final wall thickness of 8 mm, the cycle time is about 150 s. The cycle time of the entire process is determined by the machine-specific movement times and the cooling times. The cooling time for the foam components depends on the final layer thickness of the foam. For a 3 mm-thick foam layer, the cycle time is assumed to be about 90 s.

From a commercial point of view, the high flexibility of the production line and the associated high plant utilization has a positive effect on the ROI. In addition, less investment is required in the infrastructure and technology-specific plant components. In many cases, production cells of this size have been in use for 15 years.

The Dolphin process was developed together by Engel Austria, Georg Kaufmann Formenbau, BASF and the P-Group (now SO.F.TER), and was presented at K2007 - together with the auto supplier Johnson Controls. In the fall of 2011, the first series application began. The Italian Daimler supplier SOLE S.p.A. used this method to produce cockpit covers for the new Mercedes Benz Actros (Fig. 4). The Dolphin consortium now includes Engel, Georg Kaufmann Formenbau and SO.F.TER.

Close tolerances in moldmaking

The Dolphin process combines two-component injection molding (Engel combimelt) with physical foaming (MuCell) and compression technology. First, on an injection molding machine equipped with a rotary-platen (Fig. 5), a thermoplastic backing part is conventionally manufactured in the first parting plane - e.g. from PC+ABS - with the aid of a needle-valve hot-runner system. After cooling, the backing part is transferred to the second parting plane position by swiveling the center rotary mold platen. A thermoplastic elastomer (TPE) is charged with an inert gas in supercritical state after plastication in the melt cylinder, and the single-phase plastic/blowing gas solution is injected into the cavity - distributed via multiple cold runners. During the process, the backing structure is flooded to a thickness of about 2mm. The MuCell process unit is mounted on the moving platen of the injection molding machine for this process step.

During a short cooling phase, the melt solidifies on the mold wall and forms a skin layer which reproduces the grained surface of the cavity. Immediately afterwards, the clamping unit is opened by a defined compression stroke to allow the blowing gas to expand. Since the pressure within the cavity now decreases with a gradient that is equal across the cavity, particularly fine and uniform foam cells form - the prerequisite for a high-quality surface. The cavity is held in the extended position until the TPE has cooled completely and crystallized. Besides the adjustable delay time, the opening velocity and platen parallelism can also be controlled.

Since, simultaneous with foaming, the next backing part is being produced on the other side of the mold, the clamping force must not decrease during the opening stroke. This is ensured by a blocking system on the second mold half. Another special feature of the mold engineering is a surrounding spring strip that is incorporated into the plunger, which reliably seals the two mold halves during the opening stroke so that neither melt nor foam can escape. The Dolphin process therefore requires particularly stable and high-precision molds. The tolerance in the temperature-controlled and movable elements is 0.03 mm. The cooling is just as big a challenge. To generate parts of the best quality, the two mold halves must be held at different temperatures. The backing part requires a temperature that is 35 °C higher than the overmolding of the soft component.

The Dolphin technique permits a high degree of design freedom since even complex geometries can be easily achieved. For example, undercuts can be generated (Figs. 6 and 7) with the aid of slides; and the foaming process allows very narrow radii to be produced. For the grained surface structure, Georg Kaufmann Formenbau has developed a special method. In its pilot plant, the mold maker has built a trial mold with interchangeable grain inserts to present a broad range of options to product developers (Fig. 8).

Decompression stroke determines the softness

During foaming, the initial wall thickness of the foam cavity determines the shot weight of the part and the wall thickness to be obtained after the decompression stroke determines the foam height (Fig. 9 and 10). The density of the foam can be adjusted accordingly, via the decompression stroke, and is indirectly proportional to the ratio between the initial and final wall thickness. Besides other parameters, the adjustable stroke also determines the softness of the product.

For a test series, with a starting wall thickness of 2 mm and a density of 1.12 g/cm3, the decompression stroke was varied between 2, 3 and 4 mm. The calculated average densities are thus at 0.56, 0.45 and 0.37 g/cm3, already taking into account the compact top layer. This develops both at the mold wall as well as at the contact surface to the carrier part. The high filling pressure supports the development of a long-life, adhesive connection between the foam layer and backing structure.

Based on its Pibiflex product range, the raw material manufacturer SO.F.TER has developed a TPE grade especially for the Dolphin process. Pibiflex 3567 S is characterized by a high UV stability and scratch resistance, and particularly pleasant tactile properties. The material is currently processed in the colors beige, gray, charcoal and black. And, since it keeps the gas used for foaming in solution, it permits uniform, controlled expansion. In addition, the TPE-E bonds strongly to the thermoplastic backing part. Because of their favorable mechanical properties across a wide temperature range from -45 to 150 °C, the products of the Pibiflex series - block copolymers of crystalline PBT and amorphous polyether glycol - are generally predestined for applications in auto engineering. The softness of the material is retained even at very low temperatures, and the hardness and tactile properties can be individually adjusted.

Foamed backing parts for optimum areal weights

In the current series application from Sole and other applications that are in trial, the soft component is foamed on the part surface. However, the backing structure will also have to play its part in future lightweight construction requirements. Therefore, the aim of current R&D is to physically foam the hard component while modifying the part design and mold construction appropriately. This would probably reduce the average density of the backing part by 7 to 10%.

Foaming of the backing by decompression technology instead of classic foaming opens up an even greater savings potential since the filling volume and weight are determined just as much by the initial cavity and the final backing dimensions -defined by the decompression stroke - as during the foaming of the TPE. During physical foaming of PC+ABS as backing material, area weights of less than 5.1 kg/m2 appear realistic, particularly when the current backing wall thicknesses are optimized.

The favorable material properties of the Pibiflex material across a broad temperature range make other applications, for example the manufacture of airbag covers, conceivable. The predetermined rupture behavior of this material would be independent of temperature, and the bursting of the cover under both cold and warm conditions would be repeatable.

References

1 Anon. High Soft-touch Quality in One Single Process. Kunststoffe international 96 (2006) 7, pp. 76-77, PE103626

The Authors

Dipl.-Ing. Michael Fischer, sales manager for technologies, Engel Austria GmbH, Schwertberg/Austria, www.engelglobal.com

Roger Kaufmann, head of development and sales, Georg Kaufmann Group, Busslingen/Switzerland, www.gktool.ch

Alessandro Giordani, technical service, SO.F.TER SPA, Forlì/Italy, www.softerspa.com

Editor's note: These authors are PlasticsToday contributors. The opinions expressed are those of the writers.  

Dolphin_Fig_1.jpeg

Fig. 1. More comfort in the driver's cabin: blinds and covers of the new Mercedes-Benz Actros are produced with softtouch-surfaces utilizing the Dolphin-method. (Figure: Daimler)

DolphinFig2highres.jpeg

Fig. 2. Cost versus quality: market positioning of the Dolphin process compared to conventional techniques. The results are based on a study performed in 2008 on cockpit and door lining parts, which has not yet been published. (Figure: Engel)

Dolphin_Fig_3a_4.jpeg

Dolphin_Fig_3b_3.jpeg

Dolphin_Fig_3c_1.jpeg

Fig. 3. Instrument panels, armrests and folding interior elements are among the first products manufactured as prototypes or in small series. (Figures: Engel, Daimler)

Dolphin_Fig_4_1.jpeg

Fig. 4. The production line manufactures cockpit covers with softtouch-surface in just a single process step. (Figure: SOLE)

Dolphin_Fig_5_4.jpeg

Fig. 5. The Dolphin process is based on multicomponent injection molding with two horizontal injection units and swivel platens, combined with physical foaming. (Figure: Engel) 

Dolphin_Fig_6_1.jpeg

Fig. 6. The Dolphin process opens up a high degree of freedom of design. Undercuts and narrow radii are possible. (Figure: SO.F.TER)

Dolphin_Fig_7a_0.jpeg

Dolphin_Fig_7b_0.jpeg

Fig. 7. Modified mold design: For invisible parting lines, unobtrusive slide edges are placed. (Figures: Engel)

Dolphin_Fig_8a_0.jpeg

Dolphin_Fig_8b_0.jpeg

Fig. 8. The desired foam quality can be adjusted by varying the process parameters (Figures: Engel)

Dolphin_Fig_9_0.jpeg

Fig. 9. The MuCell technique permits the foam structure in the final product to be selectively controlled (Figure: Engel)

Dolphin_Fig_1.jpeg

Fig. 10. The density of the foam can be adjusted via the decompression stroke and is indirectly proportional to the ratio between the initial and final wall thickness. Density of Pibiflex acc. To datasheet: 1.12 g/cm³ (Source: Engel)

About the Author(s)

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like