SAMPE Journal - March/April 2012 - (Page 24)

Feature Article Advanced Diaphragm Forming Process for High Temperature Thermoplastic Composite Applications C. Hopmann, W. Michaeli and R. Schöldgen Institute of Plastics Processing (IKV), RWTH Aachen University Aachen, Germany Email: schoeldgen@ikv.rwth-aachen.de Abstract Continuous fiber reinforced plastics with a thermoplastic matrix are recyclable, weldable and suitable for many structural applications. Due to the short cycle times of about 1 minute, forming processes for thermoplastic composite parts are already capable of series production. Two of the most important forming techniques are Matched Die Forming and Diaphragm Forming. The Matched Die Forming provides short cycle times and high processing temperatures but requires complex and expensive molds. The Diaphragm Forming Process enables the use of low cost tooling and machinery and provides a high degree of flexibility. On the other hand its processing temperature range is limited to 250°C due to the silicone diaphragms, which are used for cost-efficient series production. The paper presents a new processing technique that was developed at IKV Aachen and combines the advantages of both processes. These particularly are a big variety of processable matrices and low cost equipment with a high degree of flexibility. The developed machinery like the new handling system, the innovative IR-heating station and the modified forming technique are described in detail. The development focuses on a temporal reduction of the thermal load of the diaphragms by using them only for the forming process. As a result, the processing temperature range has been extended from 250°C to 320°C so that it is now possible to additionally process PA6, PA66 and PPS based prepregs. Introduction Light weight design is getting more and more important for many fields of application. A lot of structural parts for example in the automotive industry are still made of steel or aluminum. Continuous fiber reinforced plastics can, in many cases, lead to a significant weight reduction compared to metal1,2. When recycling and short cycle times also play an important role, continuous fiber reinforced thermoplastics (FRTP) can be the adequate material for many applications. Fiber reinforced thermoplastics provide very short cycle times, because no polymerization has to take place during the forming process. Furthermore they offer an inherent weldability, excellent mechanical properties (especially impact performance), a low degree of emission and a complete recyclability1,2,3,4. Initial Situation In recent years various forming processes for thermoplastic composites have been developed. Among them is the matched die forming process and the Double-Diaphragm-Forming process. Matched die forming (often also called thermoforming) for thermoplastic composites is widely used. A scheme of the process is shown in Figure 1. The thermoplastic prepreg, which in this case has to be completely consolidated, is fixed within a transport frame. The transport frame has to be equipped with grippers or holding-down clamps, which allow a guided feed of the prepreg and provide a counter force during the forming process. After fixing the prepreg in the transport frame the setup is transferred into the heating station. The thermoplastic prepreg is heated up to temperatures at least 10°K above the melting point of the matrix. This is usually realized by means of infrared radiation. When the required processing temperature is reached, the prepreg is transferred into the forming station and then formed between the two mold halves5,6,7. As the mold is made of steel and the fabric reinforced prepreg changes its local thickness during the forming process, the gap between the two mold halves has to be calculated and manufactured elaborately. It may not simply be set to a constant value. The required molds therefore are comparatively expensive. Furthermore the forming pressure is applied by a linear motion of the mold halves. This leads to a loss of forming pressure in steep sections of the mold. Therefore parts with perpendicular walls cannot be manufactured using the matched die forming process. A rubber die is sometimes used and can lead to an improvement of the consolidation in steep sections but also has to be designed for each single part8. As a high forming pressure is required to enable a good consolidation, hydraulic driven presses are used. This causes high costs also for the machinery. Advantages of the process, on the other hand, are the very short cycle time and the processability of all thermoplastic matrices. A second forming process for thermoplastic composites is the Double-Diaphragm-Forming (DDF) process that is undergoing continuous development at the Institute of Plastics Processing9. This process makes it possible to manufacture a wide variety of molded parts economically (Figure 2). The thermoplastic prepreg SAMPE Journal, Volume 48, No. 2, March/April 2012 Figure 1. The matched die forming process. 24

Table of Contents for the Digital Edition of SAMPE Journal - March/April 2012

SAMPE Journal - March/April 2012
Contents
President’s Message
Technical Director’s Corner
Butt-joined, Thermoplastic Stiffenedskin Concept Development
Perspectives
Europe News & Views
SAMPE Europe’s SEICO 2012, Paris
SAMPE Europe’s SEMAT 2012, Munich
The SAMPE Foundation
SAMPE Proceedings
Advanced Diaphragm Forming Process for High Temperature Thermoplastic Composite Applications
Perspectives
Materials & Products
SAMPE Books & CD’s Order Form
SAMPE LinkedIn Communities
SAMPE Journal Editorial Calender
Welcome SAMPE’s Newest Members
SAMPE Tech 2012 Call for Papers
Industry News
Corporate Partners
SAMPE 2012|Baltimore
SAMPE 2012 Exhibition
SAMPE 2013|Long Beach Call for Papers
Advertiser’s Index
Resource Center
Membership Application
Industry Events Calendar
Chapter Meetings Dates and Contacts

SAMPE Journal - March/April 2012