A new plastics-processing technology enables efficient manufacturing of electronic connector, socket, and lead-frame components in computer, communications, and automotive applications.
BY Lee J. Hyde and Levi A. Kishbaugh
The MuCell injection-molding process is being applied by leading electronic connector manufacturers for operating and capital cost savings, productivity improvements, and the ability to commercialize products faster. MuCell is a thermoplastic injection-molding process technology that results in electronic connector, socket, and insert-molded lead-frame components that have less residual stress, superior conformance to mold dimensions, and improved dimensional stability at extreme operating and soldering temperatures compared to conventionally molded parts.
The MuCell process uses supercritical fluids (SCF) of inert gases (nitrogen or carbon dioxide), to create a uniform, microscopic closed-cell structure throughout a component with a solid skin. The MuCell process enhances thin-wall connector design, improves processing efficiency, and reduces product costs by an average of 12% to 20%.
MuCell development and implementation is occurring within an extensive range of connection systems, including wiring-harness connectors, electronic-control-module headers, insert-molded lead frames, IC sockets (such as DIMM, RIMM, backplane connectors), and LGA and BGA sockets. The MuCell process has been deployed globally across an extensive range of automotive, business machinery, industrial, and electrical/electronic markets as a strategic cost savings technology.
 Figure 1. Cycle-time reductions from the MuCell process average 20% to 30% and are realized by eliminating pack-and-hold time and reducing cooling time. |
Using glass-fiber-reinforced engineering thermoplastics such as PBT, Nylon 66, and high-temperature Nylon (HTN), MuCell electronic connector components are being implemented for three strategic benefits: cycle time reduction, faster product release, and low-cost material substitution. The MuCell process reduces cycle time by an average of 20% to 30% compared to conventional molding by eliminating the pack-and-hold phase and reducing cooling time (see Fig. 1). For connector manufacturers, the economic significance of MuCell cycle time reductions includes reduced operating costs, freed production capacity, and capital cost avoidance. The cycle time of an automotive electronic-control-module header insulator molded in a two-cavity mold using glass-reinforced PBT was reduced from 32 seconds in conventional molding to 24 seconds using the MuCell process.
Acceptable part dimensions are achieved in less time using the MuCell process through better dimensional conformance to the mold and the elimination of residual stress. More consistent and predictable part dimensions lead to faster part-to-steel, fewer tooling iterations, and thus the ability to launch new connector programs in less time. Flatness variance along the length of an insert-molded automotive connector housing molded in glass-filled nylon was reduced from 0.06" in conventional molding to 0.008" using the MuCell process and the elimination of residual stress.
With the trend toward smaller electronic connection designs, reduced melt viscosity and pressures in MuCell processing result in a wider processing window to achieve complete filling of thin-wall sections and flexible members without pressure-induced stress. MuCell extends the selection window of the least expensive material that is functionally suited for a given application. In some cases, conventionally molded liquid-crystal polymers (LCP) can be replaced with MuCell-molded HTNs, with HTNs benefiting from improved flow and greater dimensional stability using the MuCell process. Liquid-crystal polymer is the only thermoplastic known to be incompatible with MuCell molding (although blends of LCP with other high-temperature thermoplastics are being successfully processed in MuCell).
Process fundamentals
The process begins with the precisely controlled injection of the SCF of nitrogen or carbon dioxide into the barrel during screw recovery. A specially designed MuCell screw then disperses the SCF into the polymer melt creating a single-phase solution. The SCF acts as a temporary polymer plasticizer, reducing the viscosity of the resin by up to 35%, providing improved filling of thin-wall sections and flexible members.
During filling and cooling, microscopic cell growth takes place within the cavity, creating the internal pressure needed to completely fill the part. Thus, the pack-and-hold phase, which contributes to cycle time and is a common source of molded-in part stress in conventional molding, is eliminated. Peak cavity pressure is reduced by up to 65% and cooling time is often shortened due to reduced mass and the use of lower mold temperatures. The result is a more dimensionally stable part with reduced warpage and the elimination of sink marks without any chemical change to the polymer.
The elimination of the hold phase results in a significantly reduced peak clamp-force requirement. That allows machine size reduction by up to 50% using MuCell processing and reduced operating costs through the use of the lower corresponding hourly machine rate.
Replacing the pack-and-hold phase of the conventional molding process with cell growth of the MuCell process provides improved filling of thick-wall sections located at the end of thin flow paths. In conventional molding, optimum part design includes gating, such that thinner wall sections are at the end of fill, and this gating location makes it difficult to completely pack out common electronic connector part geometries such as thin flexible members. The cell growth that occurs with the MuCell process allows complete filling of these thicker features, providing a part with much sharper detail without residual stress.
 Figure 2. MuCell molding capability can be added to nearly any injection-molding machine through the modular MuCell Modular Upgrade. |
The MuCell Modular Upgrade (MMU) represents a significant advancement for MuCell technology since MuCell capability can be added in a straightforward modular way to nearly any existing electric or hydraulic injection-molding machine (see Fig. 2). The MMU allows replacement of current screws and barrels without complex and costly mechanical adaptations.
Materials manufacturers have established that for most semi-crystalline thermoplastics, including PBT, SPS/PA, PA 66, and HTNs, significantly lower mold temperatures can be used with the MuCell process without a loss in crystallinity (see Fig. 3). That creates opportunities for substantial cooling-time reductions using the MuCell process (see Fig. 4).
MuCell automotive connectors, including those for sealed under-the-hood systems, have been validated according to the SAE/USCAR performance standards for automotive electrical connector systems. For computer, consumer, and communication electronics, all UL-recognized component plastics are also recognized by UL using the MuCell process at up to 5% density reductions.
A comparison of terminal retention force measured after wave soldering at 260°C for a MuCell and conventionally molded electronic-control module shows the MuCell header achieving higher average and minimum terminal retention force values. The minimum and average terminal retention force values were 28.3 N and 41.4 N for the MuCell headers and 25.5 N and 31.6 N for the conventionally molded header. In this case, MuCell's higher terminal retention force is attributed to lower residual stress levels, resulting in superior dimensional stability of the thin-wall sections surrounding the pin holes.
Terminal retention force values without TPA for a MuCell automotive wiring harness connector with 0.64-mm and 1.50-mm terminals exceeded USCAR/SAE-2 performance standards by a substantial margin. The minimum MuCell terminal retention force of 76 N compared favorably to the USCAR/SAE-2 specification of 30 N. For 1.5-mm terminals, the minimum MuCell retention force of 134 N compares favorably to a specification of 50 N. These functional results are typical of MuCell automotive connectors molded in glass-reinforced Nylon, HTN, SPS/PA, and PBT.
Deployment of the MuCell process lowers the cost structure of electronic connection systems and reduces the time and expense to develop and launch new products. Through reductions in operating costs, material costs, and capital costs, an investment in MuCell technology for electronic connection systems generates an average investment payback period of less than 18 months.
Across a wide spectrum of electronic connectors ranging from board level to automotive wiring-harness connectors, the MuCell process is enabling manufacturers to mold stress-free parts with repeatable and predictable dimensions while generating significant operating and capital cost savings.
LEE J. HYDE is business director and LEVI A. KISHBAUGH is vice president, engineering, at Trexel, 45 Sixth Road, Woburn, MA 01810. Tel: (781) 932-0202; email: l.hyde@trexel.com.
