CompositesWorld
Issue link: https://cw.epubxp.com/i/742140
NOVEMBER 2016 26 CompositesWorld PLANT TOUR "and we also did all new testing, starting with coupon testing to feed the simulations and then moving up through components to qualify each material for each part." At this point, BMW Group Plant Dingolfing's head of Press Shop and CFRP Production Peter Wolferseder takes over the tour. As he leads the way into the open production area, he explains that the CFRP Shop's 100 employees cover three shifts, five days per week. Inside, a bank of 10 automated CNC milling machines supplied by EIMA Maschinenbau (Frickenhausen, Germany) flanks the left side with a line of presses opposite — five for wet compres- sion molding and two for hybrid B-pillar pressing — all supplied by Dieffenbacher (Eppingen, Germany). Many of the tools in the presses, he says, are supplied by FRIMO (Lotte, Germany). "We have a good relationship with them, and we also build some tools in-house." Walking past the CNC milling cells, Wolferseder shows how a wet-compression-molded tunnel part is placed into the front of the machine while another is milled at the back, for maximum throughput and efficiency (Fig. 5, p. 27). "No dust escapes," he points outs. "It is a completely clean environment." Fig. 2 Hybrid CF/steel B-pillar Using carbon fiber prepreg reinforcement enabled a thinner, high-strength steel B-pillar, saving 2 kg while providing superior crash performance. Source | BMW AG Wet pressing of sills e sills that run along the lower sides of the BIW are assembled from two CFRP parts, also wet compression molded. A sill manu- facturing cell equipped with two KUKA (Augsburg, Germany) robotic arms applies epoxy resin simultaneously to two dry textile preforms. e non-crimp fabric (NCF) for the preforms is made at BMW's joint venture plant SGL Automotive Carbon Fibers, 60 minutes away in Wackersdorf, which also supplies the i3 and i8 lines. e resin is mixed and injected via a dual-head system supplied by KraussMaffei (Munich, Germany), with an integrated volume flowmeter that records the quantity applied to each stack. e resin forms a pool in the center of the stacks, leaving about an inch or so around the edges. "If the resin goes to the edges, then the needle grippers we use to pick up the preforms would get covered in resin and no longer work," explains Wolferseder (Figs. 6a and 6b, p. 28). "e resin application is programmed to fully impregnate to the edges during the pressing." Why two robots instead of one? "Because both preforms should be the same," Wolferseder responds, "with the resin sitting on them for the same amount of time. If you only had one robot, one preform would have resin sitting for some seconds more than the other. Also, the robots are not very costly, so it is no issue to have two." Designed with latency, so cure does not begin until the press cycle, the resin sits for some seconds, penetrating through the preforms vertically. e wet preforms are picked up and placed in the press by the robotically actuated needle grippers. e press then applies pressure according to specified ramp, with the final 10 mm of "daylight" closed very slowly. "Our special recipe to produce these parts is to coordinate the temperature, pressure and resin curve," says Wolferseder. Two molded stacks are produced with one stroke (Fig. 6c, p. 28). "We will cut these in half so that we end up with four parts from one cycle," says Wolferseder. "We mostly use tools with four cavities to increase the overall output capacity of our machines." He points out that this is a completely unattended process and totally reliable. "ere are two people at the end to take parts off the line and to do QA checks. If there is a defect, they can address this in the machines," says Wolferseder. "Wet pressing is not unique," he adds. "What is unique is this industrialization." Hybrid B-pillars e prepreg used to reinforce the B pillars is delivered from Hexcel Austria (Neumarkt) using epoxy resin from an undisclosed supplier and carbon fiber from SGL Carbon SE (Wiesbaden). It arrives precut to shape and on trays, which are stacked onto trolleys and rolled into the workcell's feed station. "e solid green layer on top is the epoxy adhesive which will bond the CFRP to the hot-formed steel B-pillar," says Wolferseder, "but also provide isolation against galvanic corrosion." A robot picks up two of the adhesive-coated prepreg preforms and places them on a light table to enable an automated stack orientation check, which is completed in a few seconds by a Vision Machine Technic (Mannheim, Germany) system. e robot then picks up the preforms again and after returning to the work- cell's periphery swivels to the other side of the robot transit aisle