Multi-material solutions, particularly those that join metals with plastics, are viewed as a promising means of automobile lightweighting by many OEMs globally. However Japanese automakers at least are treading cautiously when it comes to actual deployment of such solutions due to a number of reasons.
Notes Kiyoshiba Mase, General Manager, Organic Materials Engineering Division at Toyota Motor Corp., “When you are joining materials with rivets you can analyze the design with CAE but that’s no longer possible if you are using adhesives.” Mase does concede, however, that Europe is well ahead of Japan when it comes to data analysis regarding adhesion.
Mase also notes that the anisotropic nature of carbon fiber reinforced plastics (CFRP) makes bonding to steel challenging. “You need to design around this anisotropy and there are not many options for bonding. Those that do exist take time and effort,” he says. As such, extensive use of CFRP is only an option for supercars such as Toyota’s Lexus LFA.
Mase also voiced concerns about techniques that etch the metal parts in a pretreatment step in order that resin can be over-molded, generating a so-called anchor effect. He questioned whether there are really effective, especially over the long term. Toyota’s view is that adhesive bonding contributes 80% of the joining effect whereas the uneven metal surface created by etching only contributes 20%.
Japan’s Daicel Polymer thinks it has come up with a solution for joining metals to plastics with its DLAMP continuous wave metal melting process. As opposed to a pulsed laser that would ablate the metal, a continuous wave laser melts the metal and forms 200-micron deep holes and hooked features on the metal surface. These interact with molten resin in an over-molding process to form much stronger physical bonds than with so-called nano-etching processes. Ideally, the metal parts should be at least 1-mm thick.
In bonding tests using a 60% glass fiber-reinforced grade of polyamide (PA) 66 (Plastron PA66-GF60-01) supplied by Daicel Polymer, the bond strength with A5052 aluminum was 45 MPa, while with SUS304 stainless steel it was also 45 MPa. DLAMP can be used to join steel, copper, titanium, zinc and magnesium with PA 66, with bond strengths varying from 20 MPa for SPCC steel to 50 MPa for ZDC2 zinc.
DLAMP process realizes strong metal-plastic bonding through molten resin flowing into the occluded spaces that are formed via melting of metal using a continuous wavelength laser.