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Business flows to liquid silicone rubber

November 2, 2004

8 Min Read
Business flows to liquid silicone rubber

Gaining acceptance in the health care industry for use in catheters, Tufel high-consistency silicone and LIM liquid silicone rubber from GE Advanced Materials (GEAM; Pittsfield, MA) are used for extruded tubes and injection molded catheter terminations. Reported benefits are design flexibility, radio-opaqueness, and biological compatibility.

As patients heal from surgeries, bulbs and drains like these are used to accumulate fluids. Molded from GEAM?s Tufel high-consistency silicone and LIM liquid silicone rubber, the materials offer elastic response and clarity so fluid levels can be monitored.

After displaying an Eltec 2 machine at last year?s NPE in Chicago, Krauss-Maffei sold the press to LSR material supplier Dow Corning (Midland, MI) for use in that company?s lab. Dow Corning had supplied a mold for the machine to run during NPE, and now uses the Eltec for application development and testing.

POLYAMIDE AND LSR IN THE SAME TOOLMolding two materials with temperature profiles as disparate as LSR and polyamide in the same tool, a multimaterial cell was displayed at K by Arburg (Lossburg, Germany), highlighting that company?s silicone processing expertise. Running a thermally isolated two-component mold from Austrian toolmaker Rico (Thalheim, Austria), a 220-ton Allrounder created a sealed diaphragm for use in disposable bottles that included a PA substrate and LSR overmolding. The injection units were arranged in an L, with the main screw and barrel feeding PA into a two-cavity mold via an eight-drop hot runner. Situated in a L position, the second injection unit injected silicone into a 32-drop cold runner via the machine?s rear. The rotary tool transported the PA component to the LSR side for final overmolding, and an Arburg Multilift H robot system removed the parts, and the nylon sprue, simultaneously.

Toshiba is exploring the micromolding LSR market, running a 5-ton all-electric EC5 at Plastics USA that molded four .0125g umbrella valves from a .05g shot.

Engel offers four types of injection units to process LSR that are built to service specific market niches, featuring dynamic or static mixers as well as screw or plunger injection. For molders who want to process LSR and thermoplastics on the same machine, the company provides a standard reciprocating screw. A plunger system is designed specifically for LSR and installed on Engel's electric Emotion Series. Engel also has a double-plunger or piston-style unit (shown) that is designed for very large shot sizes to serve the high-voltage electric market.

Traditional thermoplastic molders and machinery suppliers who have watched their markets dry up or relocate have been increasingly seeking out new business opportunities, and many have settled on or at least flirted with LSR.

A stone?s throw from silicone pioneer GE Advanced Materials in Pittsfield, MA, Sinicon Plastics always had liquid silicone rubber (LSR) in its backyard, but it didn?t have the technology on its shop floor until three years ago. ?We?ve always dealt with the high-end engineering resins, and we?re very used to high mold heats,? explains David Allen, Sinicon president. ?You have to run silicone at 350°C or so in the tool, but we run [polyetherimide] and PEEK everyday, so we?re used to having hot molds, that doesn?t bother us.? Sounds simple, right? Sinicon, which runs 11 machines (one for LSR), has 14 employees, and generates $2 million in sales, got an assist from Krauss-Maffei (K-M; Florence, KY) when it started the ?LIM thing?, buying a LSR press from K-M and receiving training and setup for its first silicone project: a baby bottle nipple.

?The way I describe silicone molding,? says Sinicon?s Allen, ?is you?ve got to take your molding hat, turn it around, and put it on backwards, because in regular thermoplastic molding, heat is your enemy. You take the plastic, melt it, get it into the mold, and pull the heat out to resolidify it.? But in LSR, faster cycles and less flash require more heat. ?Heat is your friend,? explains Allen.

Allen?s situation is certainly not unique. ?The last few years have been pretty hard,? says George Kipe, founder of Kipe Mold (Placentia, CA) and the widely anointed ?grandfather? of LSR in the States with 35 years of experience, going back to the material?s germinal days in the ?70s. ?People are looking around and saying, ?Well this plastic thing is pretty flat right now; let?s explore some other areas; let?s do the LIM thing.??

Kipe?who worked at a traditional compression, transfer-molding rubber plant in his teens, performing mold maintenance and construction before starting his own rubber tooling shop at the age of 24?offers his own analogy. ?The one thing that I think is important for everybody to realize is the compressibility of silicone,? Kipe says. ?It?s like a marshmallow?if you ever tried to mold an exact amount of marshmallows into a shoebox, it becomes tricky.?

Machinery suppliers like K-M face the same problem as molders, with the market for new thermoplastic injection machines in the U.S. diminishing over the last few years, they are also trying to develop new markets. Most of these companies have at least retrofitted machines to run LSR since its broader market introduction in the late 1970s, and a few?like Engel (Guelph, ON), Arburg (Newington, CT), K-M, and Battenfeld (S.Elgin, IL)?offer dedicated LSR machines off the shelf.

John Timmerman, LSR specialist at Engel, says ?traditional plastics machine suppliers have been looking for other markets, so they?ve come up with conversion kits to take their plastic machine and allow it to process LSR.?

?Three or four years ago, mainstream machinery makers started to look at niches to gain market share,? explains Chris Kightlinger, an engineering manager with Toshiba (Elk Grove Village, IL). His company used the Plastics USA exhibition in Chicago to showcase a LSR umbrella valve that was fed by cold runner and molded on a 5-ton all-electric. A response to the North American market, the technology for the Japanese company was actually developed in the U.S. and then sent back to Asia.

Demag Plastics Group (Strongsville, OH) planned a presence for LSR equipment at K 2004, and Larry Doyle, business development manager, notes that DPG ran LSR on a 50-ton hydraulic at the Düsseldorf show. It featured an antirotation circuit to keep material from leaking back into the barrel. He says LSR gives customers a technology option to specialize their shops. ?LSR allows our customers to differentiate themselves,? Doyle says. ?They?re also looking at growth markets, and medical is one of those markets.?

?Heat is Your Friend?

In spite of machinery suppliers? willingness to promote the technology, adding LSR to your repertoire is not as simple as installing a new screen on your machine control. A thermoset material with viscosities ranging from liquid to paste, LSRs are equally mixed parts of polysiloxane polymers that are vulcanized by polyaddition. The two components come in tubs or pails that are pumped into a customized barrel.

Because of the material?s viscosity, the screw?s flights and barrel?s inner lining must be sealed to prevent LSR from seeping behind the screw during injection. Also, the thermoset material processes in reverse from its thermoplastic counterpart, meaning the barrel is cooled to keep the material from crosslinking once mixed, and the mold is heated to 350°C ±25 deg C to cure the part in the tool, requiring cartridge heaters in the mold.

The material doesn?t shrink in the mold as a thermoplastic would upon cooling, expanding instead and requiring precise control of the screw position and a short shot (approximately 98% to 99% fill). The material must cure at the gate to seal off the cavity and prevent LSR from retreating into the barrel, but this can also be assisted with a high quality shutoff valve.

In addition to machine requirements, most other precautions required with LSR have to do with the mold. Extremely tight shutoffs are needed since the material can flash in .002 inch, but such a tight seal also traps air. Conventional vents invite LSR to fill them and become stuck, so the few moldmakers that serve this market resort to smaller vents (1 to 3 mm wide and .004 to .005 mm deep) on the parting line. Vacuums are also used to remove trapped air, which can scorch part surfaces or leave surface voids. The material?s durometer also requires care in part removal (stripper plates vs. ejector pins) and measurement (noncontact vs. calipers).

Also, as a thermoset once it crosslinks, LSR cannot be reused. Scrap is scrap. Cold runner systems that deliver cooled silicone to the cavity to eliminate runners help, but with the pastier high-consistency silicone running from $2 to $4/lb and LSR costing $4 to $7/lb, lost material of any sort is a costly proposition.

?You can?t regrind it and reuse it,? Kipe says, ?so if you make a scrap part, not only do you lose the time that it took you to make it, you have to use more material and more time to get back to where you were.?

Room to Grow

In spite of its challenges, LSR presents molders with growth opportunities due to its relative inertness and unique temperature profile. As an application example, GE Silicones displayed recently an oven mitt made of the material, driving home its temperature resistance. Medical OEMs who once sourced a silicone seal and thermoplastic component from two shops, for the sake of FDA tracking, are looking to one-stop-shop for whole assemblies, giving traditional molders an entry into the market.

Likewise, seals and gaskets for automotive uses are increasingly made from LSR, replacing rubber counterparts which break down over time or deteriorate under compression and heat. The gasket and component can also be made in one piece, with the softer silicone overmolded onto a harder, thermoplastic substrate.

In terms of growth, Engel?s Timmerman has heard anything from 6% to 10%/yr, but that can be deceiving given its relatively minute market. ?Silicone is still very small,? says Mike Werner, technical sales manager at Toshiba. ?It?s still in its infancy but it is growing, and a surge is taking place.?

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