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Textured mold can cut costs on insert fabric
August 1, 2003
3 Min Read
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Figure 1. The tightly fissured cavity texture has raised the fibers on the right side of the small sample, while the smooth cavity surface on the left leaves them flattened. Kaufmann calls it the Velcro effect. |
Multilayer inmold film is expensive, but this toolmaker can reduce those costs by precisely reproducing mold textures in monolayer polypropylene.
A series of tests done in the new Technology Center of moldmaker Georg Kaufmann Formenbau of Busslingen, Switzerland (see August 2002 IMM, pp. 46-48) have shown that mold texturing can improve the results and lower the costs when backmolding with inmold fabric inserts. This technique is very popular for making automotive A-, B-, and C-pillars, door panels, and other interior components.
Kaufmann says that until now backmolding has required multilayer decorative fabrics. One layer, usually a nonwoven fabric, protects the decorative layer from receiving too much stress during molding. Using a mold with textured cavities permits the use of less complex, and therefore less expensive, fabrics. As a bonus, the tests showed that the texturing actually improves the textile structure of the fabric and leaves none of the shiny areas associated with backmolding against fabric inserts.
The Velcro Effect
For these tests, Kaufmann’s tech center used a well-proven mold for an interior auto door trim panel. Previously smooth, the cavity area was given a variety of grained and textured finishes to see what results each variation would produce.
The tests showed that a rounded, grainy texture like that of natural leather had only a very slight positive effect on the fabric. Best results came with a rough, tightly fissured mold surface that resembles mountainous terrain when magnified (see Figure 1).
Figure 2. When the backmolding process is done using frontal flow, the mold surface texture can be accurately reproduced in a smooth PP film, as in the two examples shown here. |
Kaufmann’s researchers say the decisive factor is that the fibers in the fabric intermesh with the fissures in the mold surface in what they describe as a “Velcro effect.†When the mold opens, the fibers straighten up as they cling to the mold surface. This is true even in areas where the fibers are pressed flat against the cavity—for example, near the gate or where the fabric is displaced by the injected melt.
The testing was done on one of Kaufmann’s Krauss-Maffei C Series machines, this one a 420-metric-ton model that has been specially equipped for backmolding with fabric inserts. Kaufmann has been a driving force in that technology, and has partnered with Delta Tooling Co. of Auburn Hills, MI (see April 2001 IMM).
Kaufmann’s Technology Center came up with several key determinations as a result of this testing:
 Surface improvement can be achieved with lower-cost decor fabrics that do not include protective linings.
 The texturing improved finished fabric surface quality both with injection-compression into partially open molds and with conventional backmolding into a closed mold.
 Improvement could also be gained when using foam-backed, or so-called soft-touch, fabrics.
 Fabric embossing can be achieved.
Further trials showed that an etched or spark-eroded texture can be reproduced with precision on smooth decor films of PP and PVC when the backmolding process takes place with frontal flow (see Figure 2).
Kaufmann says the next stage will involve trials with a C-pillar mold to determine the best processing parameters for achieving optimum surface properties of the inserted fabric. The test will also allow fabric manufacturers to run trials of low-cost decor fabrics that they can show to their auto industry customers.
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