BMW, Ford, Porsche and VW all presented papers at the recent VDI plastics in automotive engineering conference in Mannheim about how foam molding can save weight and cost.
Ford started using structural foam in 2003 to avoid warping and sink marks, as well as the weight and cost benefits, said Ford research engineer Carsten Starke.
Ford has produced a climate control panel made with 17 percent glass fiber reinforced ABS by using Trexel’s MuCell microcellular physical foam process. More recently, the company applied MuCell to Ecoboost engine covers on various cars and the instrument panel on the 2012/2013 model year Ford Kuga CUV.
Starke said Ford is looking at online pressure, viscosity and temperature measurement to more reliably predict processing conditions and part properties.
Wheel surround body panels, made in structural foam with a Class A painted surface, were described by Fred Wölfle, Porsche materials specialist, and Ralf Müller, product development manager at molder Scherer+Trier.
The 20 percent talc-filled PP material exhibited flow front bubbles, surface streaking and creases with high physical foam pressure (100-250bar). The company selected low pressure (less than 30bar) chemical foaming with Clariant’s Hydrocerol blowing agent masterbatch to mold a complex ribbed part without sink marks.
Closer temperature control reduced variations in weight and eliminated occasional “hammered” surface effects which were due to an inhomogeneous foam structure.
Surface appearance was still matt, requiring four-layer painting instead of conventional three-layer painting. Wölfle said the single part solution is much less expensive than two-part designs in hand-painted low series cars.
He added that three-layer painting may be possible using gas counter pressure, variotherm tool temperature control, thermal insulation coating of tools and/or two-component sandwich molding. This would favour large series production, he concluded.
Ignacio Bersch, based in the component development department at Volkswagen’s Wolfsburg plant, described a material concept for large-scale production of finished color horizontal body panels. This process involves in-mold coated structural foam molding, and is intended to compete with resin transfer molding, sheet molding compounds and steel/aluminum.
Bersch presented “same bending strength” comparisons to benchmark 0.75mm steel, for which the cost was represented as 100 percent. To achieve the same bending strength would require: 1.1mm aluminum (at 192 percent cost); 4.2mm compact mineral filled EPDM and modified PP (at 109 percent); and 2.7mm mineral filled PC-ABS (at 196 percent). Foaming reduced the latter to 140 percent, and its weight fell by 40 percent from 3.5kg/m3 to 2.5kg/m3. Comparative kg/m3 figures are 5.9 for steel, 3.0 for aluminum and 4.6 for compact mineral filled EPDM/PP.
Bersch said the in-mold PUR coating reflowed to self-heal scratches after one hour at 60°C or one week at room temperature.
VW’s new Golf 7’s IP will involve MuCell foam when launched at the end of 2012. R&D chief Dr Ulrich Hackenberg showed 20 percent IP stiffness improvement with structural foam over compact thermoplastic. It also provides a 20 percent weight saving and 15 percent cost reduction.
“Structural foam looks like a coming trend,” said Hackenburg.
Borealis said BMW is using its Nepol GB215HP material in its SGI (SpritzgussIntegralschaum) chemical integral foaming injection process. The material is 60 percent long glass fiber reinforced PP masterbatch, and this is dosed 33 percent with unfilled virgin and recycled PP for instrument carrier panels.
In a paper on a cockpit concept of the future, based on the BMW 1 and 3 series cars, Thomas Reisch, interiors project manager at BMW, said masterbatch saves 20 percent material cost over ready LGF PP granulate, and recyclate saves another 8 percent.
Reisch said further lightweighting should come from using injection-compression structural foam molding to reduce component wall thickness from 2.0mm to 1.8mm.