MILAN—Laser marking liquid silicone rubber was demonstrated at the Silicone Elastomers Summit, sponsored by Smithers Rapra, in Milan.
The demonstration involved REA Jet FL fiber laser marking equipment from Mühltal, Germany-based REA Elektronik GmbH.
The Merck KgaA laser marketing pigments were dispersed in Alpa-LSR 550201 and 550202 laser marking grades from Tuebingen, Germany-based CHT Germany GmbH.
The project, which made luggage tags, was produced by Freiberg, Germany-based LSR and thermoplastics molder Antech Polymertechnik.
CHT's laser marking grades have compression set 12, Shore hardness 50A and viscosity 300 mPas at 10/s. But values differ for elongation to break at respectively 380 percent and 300 percent, and tensile strength at respectively 8.5 N/mm2 and 7.5 N/mm2.
Depending on laser beam intensity, markings range from light gray to black, on opaque colored or transparent LSR parts, including deeply engraved marking where required.
Also at the Silicone Elastomers Summit, Svenja Marl, a researcher at the UNIpace polymer application center at Kassel University in Germany, spoke about physical foaming LSR. Colleagues had shown water as a foaming agent at the 2017 summit.
Marl presented microscope images of foam structures from 2-4 percent water showing nonhomogenous cell distribution. Mold temperature has significant influence on structure: at 140° C, no foam structure close to the skin and central rough pores; completely foamed structure at 180° C, but still central rough pores.
She then moved onto the next development stage, mixing LSR with Expancel 031 DU 40 expandable thermoplastic microspheres from Stockviksverken, Sweden-based Akzo Nobel Pulp and Performance Chemicals AB.
Researchers, meanwhile, also used Unicell MS 140 DS expandable microspheres from Tramaco, Taiwan. As with Expancel microspheres, these spheres consist of a 2µm-thick 15-20 µm diameter shell filled with hydrocarbon. Marl revealed Unicell microspheres consist of an acrylonitrile/methacrylate copolymer, expanding at 120-125° C to a 50 µm sphere with a 0.1 µm thick shell.
UNIpace used 0.5-3 weight percent Unicell microspheres in LSR. Foaming was good at 1 and 2 wt percent, the latter giving higher Shore hardness due to filler network formation. The microspheres had initially reduced hardness with filler content up to 0.5 wt percent.
While 50 percent weight reduction was obtained with Expancel foaming earplug tips in Dow Silastic LTC 9400-50, weight reduction was lower at 30 percent when molding foam discs and spheres in KEG-2003H-40, a Shin-Etsu Silicone LSR grade from. In general, Marl spoke about 35 percent weight reduction being possible with Unicell MS 140 DS.
In a "low is the new high" presentation, Patrick Beyer of Dow Silicones showed advantages of new low-viscosity, low-compression set, low-temperature cure and low-volatility LSR grades.
Low viscosity Silastic RBL-9200-50 retains low viscosity prior to injection into mold cavities, enabling lower injection pressure and higher injection speeds, with benefits of a faster cycle time, a wider process window and improved consistency.
Dow reduced compression set in LSR by suppressing thermo-oxidative decomposition and reactive group presence in cured LSR that react with hydrosilation under compression, time and at elevated temperature, to form new cross-links and permanent deformation. The result is improved sealing performance under harsh conditions and elimination of post-cure treatment.
Beyer suggested these benefits become important as automotive under-the-hood temperatures continue to rise with engine downsizing, also to meet new technological requirements arising with shifts to electric drive vehicles and autonomous driving.
Silastic NPC 9300-xx series low volatile nonpost-cure grades typically have 0.25 percent certified volatile content, not influenced between 120° C press cure temperatures right up to in-mold curing of injection moldings at up to 200° C.
Dow Silastic LTC low-temperature cure grades address significantly longer cure times at reduced temperatures, which are required when overmolding LSR to lower-temperature thermoplastics and sensitive components.
While cure time hardly differs vs. standard LSR at 150° C, Beyer showed it being 60 percent faster at 100° C, yet with a long 72 hours pot life. He showed successful molding of a LTC grade to Dowlex 2035, a 97.8° C Vicat softening temperature polyethylene. As LTC grades have a low degree of temperature gradient sensitivity, they can be used at higher temperatures to obtain fast deep-section cure in thicker moldings.
Reviewing new self-bonding LSR grades, Clemens Trumm, global application development center manager at Momentive Performance Materials GmbH in Leverkusen, Germany, said peel strength against other substrates should be at a lower level and rising more slowly during part demolding. But this requirement works against the need for relatively high bond strength needed to demold highly complex parts.
Momentive proposes UV-curing LSR grades for bonding to polyolefins. They do not require post-cure treatment, but at least part of mold cavities need a transparent PMMA window.
Silopren LSR 2730/2740/2750 self-bonding grades have been available since at least 2014.
Trumm implied the potential to eliminate plasma treatment, used to raise surface tensile strength from 45 mN/m to 85 mN/m on a polycarbonate lens overmolded by polybutylene terephthalate and then LSR. The process limits between the LSR and polycarbonate in a multicomponent rain-and-light sensor molded since 2010 by Esslingen, Germany-based molder and mold maker Weber-Formenbau GmbH & Co. KG.
Momentive illustrated polycarbonate bonding by molding a cell phone holder at NPE2018 in Orlando, Fla.
he LSR 27x9/47x9 grades provide low temperature cure, do not need post-cure heat treatment and have "targeted adhesion," so that while bonding well to PC and copolyester, the materials do not adhere during molding to metal mold and cold runner metal surfaces.
Trumm said work now focuses on soft/hard combinations with new BPA-free resins, adhesion to UL94 V-0 flame retardancy resins and low temperature cure with longer pot life.
Stefan Roenisch, global manufacturing engineering LSR at Aptiv Services Deutschland GmbH in Wuppertal, Germany, described how Industry 4.0 principles improve silicone rubber molding.
As a large molding plant with 160 LSR machines running with 16 colors, great emphasis is placed in Wuppertal on using data to optimize molding processes and displaying it on large display screens throughout the plant.
These inform management, production, maintenance and toolshop staff about molding machines' live status, planned vs. achieved cycle times, downtime reasons, planned maintenance schedules and tool servicing, and material changeovers.
A quality assurance measurement trolley connects to molding machines via each machine's interface, calling up data on the actual situation live, overviewing data on the last five boxes filled, recording last check time stamping and scanning quality check results.
Roenisch illustrated how data generated since monitoring was first introduced in January 2016 improved control of mold fouling, which depends on many factors, such as weather, storage conditions, material properties, machine settings, color masterbatch and mold cleaning methods.
Discussions started with LSR material suppliers in June 2016 established further factors that eliminate mold fouling "when used in the right combination" — for example, platinum catalyst, cross-link agent, water and silane contents, filler treatment, water/silane, stripping time.
Roenisch concluded, "If we share our big data with supplier data, we get real big data, as has been used to overcome mold fouling."