Injection-Molding Liquid-Crystal Polymers

With the miniaturization of many products, manufacturers are streamlining devices to pack more features into smaller packages. For instance, the pitch (spacing) between contacts in electrical connectors is narrower than ever before — as small as 0.2 mm compared to older-style power connectors with pitches exceeding 12.0 mm. Nominal wall stocks have also thinned considerably as the overall size of connectors has shrunk.

These thin-wall designs pose a challenge during the injection molding of microconnectors for devices such as iPads, robots, computers, cell phones, and satellites.

A suitable material must be a tough insulator with high fluidity during molding, excellent electrical properties, high heat tolerance, chemical/oil/gas resistance, dimensional stability, flatness, and high stiffness. The material must also withstand reflow soldering without distorting, melting, or blistering. Liquid-crystal polymers (LCPs) prove to be just the ticket when compared to other polymers.

For the same reasons, LCPs are also injection molded into IC sockets, HF network switches, power modules for wind and solar inverters and converters, custom high-power electrical connectors, and many other precision devices.

Several grades of LCPs have even been optimized for electrical high-frequency properties that outperform ceramics, thermosets, and metal, while significantly cutting costs. LCP is particularly suitable for microwave-frequency electronics due to its low relative dielectric constants, low-dissipation factors, and the commercial availability of laminates. Packaging microelectromechanical systems (MEMS) is yet another suitable application for LCP.

Because LCPs resist gamma radiation, steam autoclaving, and most chemical-sterilization methods, they are now replacing stainless-steel medical devices, such as dental tools and sterilizable trays, and being used in drug-delivery systems and high-tech instrumentation. Several grades of LCPs comply with ISO 10993-1 and USP Class VI medical standards.

LCPs come in many standard grades with glass-fiber or mineral reinforcements. There are also specialty grades for applications that require platability, HF shielding, improved lubricity, low-specific gravity, static dissipation, or thermal conductivity.

What are liquid-crystal polymers?
Liquid crystallinity in polymers comes about either by dissolving a polymer in a solvent (lyotropic liquid-crystal polymers), or by heating a polymer above its glass or melting transition point (thermotropic liquid-crystal polymers). Liquid-crystal polymers come in melted or solid forms. The main example of a solid lyotropic LCP is the commercial aramid known as Kevlar. Its chemical structure consists of linearly substituted aromatic rings linked by amide groups. Additionally, several companies including Toray, Sumitomo Chemical, Ticona, and Polyplastics produce commercially available grades of thermotropic LCPs.

Liquid-crystal polymers are unique in that they can form regions of highly ordered structure while in the liquid phase. However, the degree of order is somewhat less than that of a regular solid crystal. LCPs comprise a class of partially crystalline aromatic polyesters based on p-hydroxybenzoic acid and related monomers.

Typically, LCP has high mechanical strength at high temperatures, exceptional chemical resistance, flame retardancy, and good weatherability. The polymers come in a variety of forms from sinterable high-temperature materials to injection-moldable compounds. LCP also has a high Z-axis coefficient of thermal expansion.

Additionally, LCPs are exceptionally inert. They resist stress cracking when subject to most chemicals at elevated temperatures. These chemicals include aromatic or halogenated hydrocarbons, strong acids, bases, ketones, and other aggressive industrial substances. LCPs feature an excellent hydrolytic stability in boiling water. However, high-temperature steam, concentrated sulfuric acid, and boiling caustic materials cause the polymers to deteriorate.

Selecting an injection-molding machine
LCP can be molded using either a standard in-line-type or a plunger-type (ram-fed) injection-molding machine. Materials such as Sumika Electronic Material Inc.’s Sumikasuper LCP E4000 and E6000 Series require molding temperatures of less than 400°C. Thus, there are no problems with heater capacity even when a standard injection-molding machine is used. But other materials, such as the E5000 Series, require molding temperatures up to 420°C, making high-temperature machine necessary.

Screws and cylinders in the molding machine should be fabricated from materials with good abrasion resistance because LCPs can have high filler content. It is preferable to use standard full-flight screws. Subflight screws and high-mixing screws are not recommended because they extend metering time. In addition, it is a good idea to use screw heads with backflow prevention (check valve). Also, because the moldability of LCPs is temperature sensitive, we recommend PID-type cylinder-temperature controllers.

Again, open nozzles are optimal because shut-off nozzles have excessive dead space that can trap and retain resin. Nozzle heaters must have independent PID temperature controls. When using extension nozzles, ensuring consistent and uniform temperature distribution is an important design consideration.

In injection-molding Sumikasuper LCPs, both standard open and closed-loop control machines work. But for molding thin-walled components, use a molding machine that features a quick response to the initial injection. This is necessary because the material sets rapidly and its shear rate is highly dependent on melt viscosity. In general, better results come from molding machines that are capable of continuously metering one-third to three-quarters of the total injection capacity. Insufficient metering results in excess resin, which causes molding defects.

Sumikasuper LCPs is an example of a material that does not produce a lot of flash during production because it has a low-melt viscosity and sets rapidly. However, during the molding of ultrathin-walled products (<0.2 mm), resin can set in the walls, hindering flow. An accumulator on the machine remedies this situation.

Mold design
Injection molding LCPs causes the chained molecules to orient in the flow direction near the mold wall. It is, therefore, important to carefully consider flow patterns and anisotropy in the cavities. The sprue should have an angle of 1 to 2°/side. To easily remove the cold material, include a “sucker pin” or cold slug well (~4 to 5 mm in diameter × 5 mm or more in depth) at the opposite end of the sprue. The addition of a sprue lock ensures positive sprue removal.

It is feasible to use standard runners having either a circular, semicircular, or trapezoidal cross-sectional shape. But circular and trapezoidal shapes are most efficient in terms of pressure loss and processibility. The general rule of thumb for runner diameters is two-third to one-half of that used for PPS and PBT, with the smallest being a 3-mm diameter. Multicavity tools should be designed to have balanced runners so individual cavities fill simultaneously with resin. There should be provisions for cold slug wells at the ends of main and secondary runners.

The weld strength of LCPs is lower than most other engineering plastics, so it is necessary to limit gate locations to only one or two places to avoid excess weld lines. Side-edge gates should have land lengths of 0.6 mm or less, with a width of no more than 5.0 mm. Land depths should be 0.7× the thickness of the typical wall stock, with a minimum depth of 0.2 mm. The appropriate diameter of pinpoint gates ranges from 0.3 to 1.5 mm in diameter, with a maximum land length of 0.5 mm. Increasing the gate diameter can cause stringing and gate warpage, but can significantly reduce fill pressures.

The optimum draft angle are typically 0.5 to 1°/side for thin-walled moldings and 1 to 2° for thick-walled moldings. However, it is necessary to enlarge the extraction taper for components having greater depths. Mold-release (MR) grades let parts release from the mold more easily than general grades.

LCP components are often manufactured by high-speed injection machines, so it is critical to design adequate air-venting in the cavity to ensure all gas remaining in the mold discharges quickly and effectively. Proper air venting eliminates the problem of short shots resulting from welds in thin walls or at the ends of flow.

Because of the major differences between molding-direction (MD) and transverse-direction (TD) properties (anisotropy), which have a substantial effect on shrinkage, mold shrinkage for LCP must be specified for the correctable direction, based on the mean of the MD and TD values. For compact size or thin-walled components, design shrinkage should be specified as 0% in the MD. Specialized blends of mineral and mineral-glass are available to minimize the anisotropy properties of LCP.

Standard Sumikasuper LCP grades contain glass fiber as filler so tools should be made of a steel alloy with rigidity equivalent to SKD11 and a hardness of HRC55 to 62 (for example, HPM31, PD613, S7, and Rigor). The LCPs emit almost no corrosive gases so general-purpose tool materials work well. They do not require corrosion-resistant surface coatings or specialized stainless steel for cavities and cores.

© 2011 Penton Media, Inc.