Beyond price/pound

With prices of some engineering resins on the rise, buyers need to closely examine their selection criteria to optimize performance at the lowest cost. That doesn’t necessarily mean choosing the lowest price per pound. There are a number of strategies that can lower overall program cost even if the material cost per pound is not minimized. Choosing a highly engineered, special purpose grade, for example, may optimize flow, reduce scrap, or minimize cycle time, thereby increasing productivity.

Another strategy is to compare the resins from various manufacturers within a generic resin type. If one manufacturer’s nylon doesn’t meet your needs, another nylon might.

Evaluate all phases of production

One selection criteria should be ease of use. Think beyond cycle time and physical properties to every phase of production, upstream or downstream of the machine. Handling requirements, scrap rate and post-molding operations can all affect total per-part costs.

Eastman’s Thermx CG933 PCT (polycyclohexylenedimethylene terephthalate) polyester offers performance advantages, cost savings and process efficiencies for small, thin-walled parts such as connectors. The material is a glass-fibre reinforced, flame-retardant polyester for electronic applications. The company reports that Thermx CG933 is more competitively priced than high-temperature nylons, and cycles 20 to 30 percent faster than comparably priced PPS, at mold temperatures that permit water cooling. “There is no cheaper surface-mount compatible material that molds as clean or is as fast cycling,” says Paul Flynn, Eastman business market manager for electrical/electronic plastics.

The bottom line result, says Randy Hendrich, vice president of the injection molding firm Maloney Plastics, is improved productivity. “We have had fill and outgassing problems with other materials. Since we switched to CG933, we have been able to shorten cycle times and run production for much longer periods between mold cleanings. This has helped us maximize our productivity.”

In addition, because of their low rate of moisture absorption, Thermx polyesters need less drying than competitive materials, such as polyamide and poly(phthalamide). Those resins are also more prone to blistering during soldering, says Eastman.

Scrap and regind issues are another factor in total resin cost. In automotive lighting systems, some manufacturers have been switching from bulk molding compound (BMC) to polyetherimide (PEI) for complex reflectors, a metallized component of the forward lighting systems. The PEI resin delivers significant weight savings, substantial cost savings and the ability to recycle molding scrap, according to GE Plastics. GE Plastics has now stated that manufacturers using its Ultem PEI resin can recycle even the metallized part scrap, up to a level of 25 percent.

“With direct metallized Ultem resin, both molding scrap and metallized scrap can be re-introduced into the process feed stream,” reports Jim Wilson, GE Plastics commercial technology manager. “The net result is that Ultem resin can be a 100 percent yield product.”

Another thermoplastic that can replace a thermoset is Chevron-Phillips Chemical’s new grade of Ryton polyphenylene sulfide (PPS). In applications that require precise injection molded parts, Ryton R10-110BL, a glass- and mineral-filled grade, achieves thinner walls, thus using less material, Chevron-Phillips reports.

“Particularly in the appliance industry, this compound can result in lower total part costs versus phenolic resins because Ryton R10-110BL provides faster cycle times, very low scrap rates and can eliminate secondary operations, such as deflashing,” explains Mark Goydich, Ryton PPS marketing and sales manager.

Seek out the best in class

The case of a polybutylene terephthalate (PBT) automotive part illustrates that all resins in a given class do not perform equally. Tyco International chose DuPont’s Crastin HR5015F PBT to produce the cover for a power network box for DaimlerChrysler 2001 minivans. High flow is essential to consistent filling of the part’s mold, which has a relatively long flow path. The 15-percent glass-filled Crastin grade consistently fills the 10 in. x 5 in. part without resorting to excessive melt temperature or pressure. In contrast, a competitive PBT evaluated for a smaller cover filled the mold only with great difficulty and produced noticeable weld lines, says DuPont development specialist Paul Kane. Crastin HR5015F also achieves 76 percent higher notched Izod impact strength than a typical, standard PBT with 15 percent glass reinforcement, reports DuPont.

For nylon users, Rhodia has introduced a new nylon family, Technylstar, based on proprietary polymerization and compounding technology. It has outstanding flow characteristics and can carry up to 65 percent reinforcement. The company states the heavily reinforced compounds will compete with polyarylamide, PPS, light alloys and polyphthalamide (PPA) in terms of stiffness, but will be less expensive and easier to mold.

The higher-flow grades of Technylstar present processing advantages such as faster cycle times, fewer injection points on large components, smaller runners and less scrap. In addition, the material can be processed at lower clamp forces, and can be molded at lower temperatures than conventional nylons.

New products require new thinking

In their search for a resin to mold an exterior dryer vent, Deflect-o Corp. evaluated an array of polymers including ABS, PC/ABS, ASA, PVC and engineered polyolefins. The company was seeking a material that was weather-resistant, paintable, easy-to-process and environmentally sound. Using the resources of a local university, Deflect-o measured heat deflection, specific gravity, Izod impact and processing parameters in side-by-side testing, and then reviewed the results in a matrix format. Hivalloy W engineering resin alloy from Basell (formerly Montell) emerged as the best choice for this application.

The resulting dryer vents have a protective grid to keep rodents out, and can be painted to match the color of the house.

For Compaq Computer Corp., the goal for its new iPAQ desktop PC was to develop a low-cost, durable and aesthetically pleasing personal computer for the corporate market. After evaluating ABS resins and PC/ABS blends for the outer housing, the company chose Lustran ABS 650 grade from Bayer (650 is available in Asia, and is equivalent to the 648 grade sold in North America).

“Compaq had specific performance requirements in terms of impact resistance, shock and vibration resistance, and these were met by ABS,” explains Roger Rumer, director of information technology markets for Bayer. He adds that PC/ABS is often specified for this type of product, but that it was not necessary to move up to a blend for this application. Compaq produced prototypes in various materials and subjected them to testing before making its final material selection.

Should you choose a specialist or generalist?

While the point is sometimes made that switching some applications to a versatile resin that meets the needs of many applications can reduce costs by simplifying purchasing, there is an opposite argument that choosing a highly-engineered, targeted resin may save money by doing the job better.

GE Plastics, for example, has introduced a new grade of Noryl GTX that can reduce the noise, vibration and harshness (NVH) generated by many automotive underhood applications. This may allow designers to avoid using foams and tapes for damping, thereby eliminating both the cost of foams and tapes, and the cost of secondary operations to apply them.

GE’s vibration damping technology changes the mechanical response of Noryl GTX, and permits the material to be tailored to meet certain temperature and frequency requirements. The technology will be applied to general-purpose Noryl GTX, glass-filled, alloy and fire-retardant grades. Noryl GTX is a polyamide product reinforced with polyphenylene ether (PPE, or PPO in GE Plastics’ lexicon).

Another specialized resin is Honeywel
l’s Capron SEGM35, which is designed for gas-assist molding. It aims to achieve improved surface appearance despite the slower fill speeds and melt front hesitation commonly associated with gas-assist injection molding. The resin is a 40 percent mineral- and glass-reinforced nylon 6 suitable for vehicle mirror housings, roof racks, door handles and bezels.

On the generalist side of the debate, Electrolux do Brasil S.A., a unit of global appliance manufacturer Electrolux, has converted a number of its refrigerator and freezer applications to Dow Plastics’ Styron A-Tech 1110 advanced polystyrene resin. The property balance of Styron A-Tech 1110 high-impact polystyrene (HIPS) allowed Electrolux to optimize inventory costs by replacing both ABS and high-gloss HIPS with a single resin.

“We make refrigerators all over the world for indoor applications. The fewer different resins we can use, the more profitable our operations. Styron A-Tech polystyrene is the closest we’ve come to a resin that works for all our applications,” says Adilson J. Nogueira, purchasing manager for Electrolux do Brasil.

Material cost generally accounts for the largest portion of total part cost, so resin selection can make or break the profitability of a contract. For best results, look beyond price/pound and consider the material’s impact on the entire production process before making a decision.