Thermoformed plastics are being asked to take on more challenging and higher-value tasks in large, thin-walled parts. Thermoforming has begun to allow manufacturers to dispense with metal for such components and avoid more expensive processing methods such as injection molding and machining stock shapes.
As thermoforming has advanced in these more demanding applications, formers have sought out higher-performing resins. Traditionally, they have worked with polymers in the lower tier of the price-performance pyramid, such as polypropylene, ABS, PVC, and HDPE. The search for high-performance thermoformable plastics has led to the adoption of polycarbonate, nylon 6, polyethersulfone, polyetherimide, PVDF, and now polyphenylene sulfide (PPS). Thermoformable
PPS, which was introduced in sheet and roll form in early 2004, has opened new possibilities in aerospace, chemical, electronics, food, and transportation applications.
PPS is a good replacement for fluoropolymers in chemical tanks because it is practically inert to so many compounds. Development is under way on PPS liners for large steel storage vessels. Fiber backing on the PPS allows for bonding to the interior surface of the steel outer layer. The entire shell of smaller tanks can be thermoformed of PPS.
In a prototype aircraft application, PPS was thermoformed to create an interior window reveal. Forming allows the reveal to be larger and thinner than a corresponding injection molded part because the knit lines that form in molding are often a source of weakness and rejects. Another aircraft application involves thermoforming 30% glass-filled PPS sheet to create an overhead storage bin. The component measures 20 x 30 in. and is 6 in. deep.
A thermoformed control panel uses 15% glass-reinforced PPS to resist the heat that builds up within the assembled unit. The 18 x 6 in. part is CNC machined to cut out windows and holes for various devices.
Train and bus manufacturers are considering thermoformed PPS panels for walls and doors because they must withstand high heat and have low smoke generation. Such large, thin-walled parts can’t be made easily or economically by injection molding.
Thermoformed PPS is also finding use in oven trays that tolerate high temperatures for extended periods and replace those of aluminum and stainless steel. A developmental 1.5-in.-deep PPS tray measures 20 x 30 in. and has a rolled-over lip. It will replace a stainless steel tray that must be refinished regularly to remove burrs and food baked onto it. PPS trays need no refinishing after use.
PPS was also chosen for a 6-in.-deep thermoformed lighting canister and panel because it tolerates the heat generated by high-intensity halogen lights. The 24 x 36 in. canister was CNC machined to provide a fit for the lamp. Other applications under evaluation include plenum liners for cleanroom exhaust ducts and polishing carriers used in producing hard drives.
Thermoformable grades of PPS provide exceptional heat and chemical resistance, low creep under load, high strength-to-weight ratio, excellent resistance to wear and friction, and low water-vapor transmission.
These grades are made of linear PPS, which offers far better melt elasticity at high molecular weight. Linear PPS thus extrudes easier and more consistently into sheets and rolls and can be drawn to significant depths, unlike the more brittle branched PPS. Linear PPS also offers better colorability, strength, toughness, and purity.
From a thermal standpoint, linear PPS outperforms nearly all other engineering resins. It melts at 285 C (558 F) and withstands short-term excursions to 270 C (518 F). As for longer-term performance, unfilled and glass-filled PPS grades have UL thermal indices of 180 C (356 F) and 220 C (428 F), respectively.
PPS also has strong resistance to solvents and performs well under prolonged exposure to acids, bases, strong bleaches, and automotive coolant, transmission, and brake fluids. Immersion tests lasting 5000 hr show PPS is highly stable in aggressive alternate fuels. It also has excellent resistance to hydrolysis and deionized water.
Standard grades are inherently flame retardant and achieve the UL 94V-0 rating. PPS generates little smoke when burned.
PPS has excellent dimensional stability in use because it absorbs no moisture, resists creep, has a low coefficient of thermal expansion, and resists most chemicals.
PPS can be used with common sterilization methods. It withstands up to 1000 steam sterilization cycles and gamma sterilization at doses to 1500 kGy, which is more than most materials can tolerate.
Various grades are FDA compliant for repeat-use food contact, as per FCN No. 40; are certified by Underwriters Laboratories as complying with NSF/ANSI Std. 61 for potable water contact; conform to USP Class VI requirements; and have established FDA Drug & Device Master Files.
|TYPICAL PROPERTIES OF THERMOFORMABLE PPS|
1115L0 (15% Glass)
|Water absorption (23 C), %|
|Elongation at Break, %|
|Tensile Strength at Break, psi|
|Tensile Modulus, psi|
|Flexural Strength, psi|
|Flexural Modulus, psi|
|Notched Izod Impact, kJ/m2|
|HDT, F @264 psi|
PPS is less dense than many similarly priced engineering resins, which means it offers greater economy on a volumetric cost ($/in.3) basis. Specific gravity for neat and impact-modified grades is 1.35 and 1.24, respectively. By contrast, fluoropolymers tend to have a specific gravity range of 1.8 to 1.9.
Unlike most other polymers, PPS can go directly from storage to thermoforming without pre-drying because it absorbs little moisture.
Various material option
Thermoformers have many PPS options to choose from. Sheet and roll grades are sold in neat, glass-filled, and impact-modified versions in thicknesses from 0.01 to 0.25 in. (0.25 to 6.35 mm) and in widths up to 50 in. (1250 mm). Sheet can be ordered with polyester and fiberglass fabric backing to foster adhesion when bonded to other materials. In addition, spools of PPS welding rods are available for joining adjacent PPS thermoformed elements. A welding gun melts the rod and edges of a seam to form coherent structures on tanks and other components.
Processors have vacuum formed and pressure formed PPS in many tooling geometries and product designs. Since PPS has high-flow, it gives more detail in forming than most polymers. It allows for relatively sharp corners in vacuum forming, so parts can have corners with small internal radii without resorting to the more expensive pressure-forming process.
PPS draws much the same as polycarbonate. Glass-filled, 1/8-in. PPS sheet, for example, has been formed to a depth of 6 in. with good consistency and repeatability. Greater depth is possible with neat grades or when parts have wide radii. PPS panels as large as 5 x 9 ft have been formed to date.
Competitive cycle times
Although process parameters vary with the tool and part, PPS sheet should be heated uniformly in a high-temperature oven at 320 to 329 C (610 to 625 F). Infrared ovens are best for this task. A 1/8-in. PPS sheet processed in a tool without a cooling system typically cycles from initial heating to forming to final cooling in 60 to 90 sec. This is equivalent to polycarbonate and is shorter than for ABS or PVC. PPS generally has a more consistent and predictable look, thickness, and sag than ABS or HDPE when it emerges from the oven just before forming.
Thermoformed PPS has tensile, flexural, and impact properties that hold up well during processing, according to testing by the Polymer Science and Plastics Engineering Program at Pennsylvania College of Technology in Williamsport, Pa. These tests evaluated 1/8-in. PPS sheet with 30% glass reinforcement (made with Fortron PPS 1130L0) formed on a male box mold. Comparison of elongation at yield of the formed samples and the flat sheet showed that little was compromised in processing. The flexural modulus of the formed material was just 8% lower than that of the unformed sheet, while the tensile modulus was about 25% lower.
PPS is thermoformed with aluminum tools, which cost much less than the specialty steel tools usually required for injection molding. Thermoforming tools for PPS are generally preheated to about 200 C (390 F) and come to operating temperature and their full dimensions within several cycles.
After processing, PPS parts can be trimmed on conventional CNC routers with carbide bits and machined with the same routers, sanders, saws, lathes, and other tools used for metal or wood. They also can be assembled by a variety of thermal, mechanical, and adhesive methods—e.g., ultrasonic welding, bolts, and bonding with epoxy, methacrylate, polyurethane, or other common contact adhesives. PPS can be painted with conventional topcoat systems and printed by gravure, flexographic, pad, offset, screen, digital, and other printing methods. It can also be laser marked and is readily metalized.
Mike Gehrig is general manager of Ensinger/Penn Fibre, which extrudes PPS sheet.
Sheridan Kelly is national sales manager for Magee Plastics Co. in Warrendale, Pa., a thermoformer that uses PPS. Tel. (724) 776-2220 or e-mail firstname.lastname@example.org.
Marshall Carr is account manager for Fortron PPS resin supplier Ticona. Tel. (610) 873-7383, e-mail email@example.com.