Alternative plastic-molding processes can produce hollow, multicolored or multilayered components which are often bigger, stronger, and less costly than their injectionmolded counterparts.
Tough enough for the Hummer, thermoformed ABS interior components withstand the riggers of off-road driving. But they are stylish enough to give the vehicle a celebrity following. The civilian version of the high-mobility, multipurpose, wheeled vehicle, or HMMWV, sports many interior parts made from the high-impact, UV stabilized plastic. ESA Manufacturing & Trim Specialties Inc., Okemos, Mich., makes parts for only 1,200 to 1,500 Hummers annually. They chose thermoforming because the process makes economical sense for the small production runs, when compared to other molding techniques such as injection molding.
Injection molding is the most wellknown plastic-molding process and is used to produce nearly one-third of all plastic components. However, other molding processes such as thermoformin, rotational molding, or blow molding, to name a few, offer significant cost advantages, provide better structural capabilities, and can make larger parts compared to injection molding.
Gas-assist molding — An extension of injection molding, gas-assist partially fills the mold with material in what is commonly referred to as a “short-shot.” Near the end of polymer fill, nitrogen is injected into the system. The gas bubble forces the polymer deep into mold cavities, hollowing out channels as it travels through the material. Nitrogen is either injected via the same nozzle as the polymer, into the mold cavity by way of runners, or directly into the part or rib sections. Multinozzle gas fill is called web molding. Molders must design both the part and nozzle’s location carefully to optimize the flow patterns of the gas. Otherwise gas bubbles will follow the path of least resistance and may bleed into thin areas.
The biggest advantage of gas-assist is that the resulting hollow channels boost stiffness without adding weight. Gas-assist can produce large parts with large cross sections that once were considered impractical for molding. Other benefits of gas-assist include reduced cycle times, more uniform packing with reduced stress, less part warpage, and less sink marks.
Structural-foam molding — Constructed of foam core encased in a solid skin, structural-foam-molded parts have higher strengthto- weight ratios than those made by conventional injection molding. Skin-core construction improves load bearing performance by up to 70%. Structural foam is made either by introducing inert gas into the melt or preblending a chemical blowing agent into the resin. The resin/gas mixture is injected into the mold, in a short-shot. The pressure from the gas expands the resin into the rest of the mold. Structural-foam parts have low molded-in stress because short-shots do not require high-pressure packing of the mold. Tooling costs may be lowered by 10 to 20%. The process additionally uses a low clamp tonnage. Which makes larger parts or multicavity molds practical.
Structural foam provides good dimensional control. Inserts such as brackets, threaded fasteners, or structural supports are easily molded-in and eliminate costly secondary assembly operations. The process also reduces sink marks on part surfaces even when designs incorporate large ribs, bosses, standoffs, or mounting pads. Sink marks and warpage are also not as prevalent even when parts with relatively thicker walls of 0.25 to 0.125 in. are molded.
The primary disadvantage of the process is a characteristic swirl-patterned surface. When surface cosmetics are a priority, structural- foam parts need secondary sanding and painting operations to get the same quality as injection molding. The process also requires longer cycle times because of thicker walls and foam’s insulating properties.
Coinjection — Also called sandwich molding, co-injection uses a specially designed nozzle to inject two materials into a mold so that one completely encapsulates the other. Several constructions are possible. The most common are composites with either a solid-skin/solid-core or solid-skin/foam-core.
Coinjection molding provides an aesthetic surface in either case, even when recycled or EMI shielding material serves as the core. Solidskin/ solid-core gives a rigid part while maintaining a nice looking or flexible surface. Advantages of solid-skin/foam-core include thick walls with injection quality surfaces, no sink marks, and good rigidity at less cost.
Material selection for coinjection, however, is critical. Core and skin material must shrink and expand at the same rate and be compatible. Viscosity of the materials must also match up well. Typically, the skin is a lower viscosity material and lets higher viscosity core materials flow through its center. Use of recycled core material and a quality surface make this process attractive. However, there are some economic hurdles. For one thing, the capital equipment required costs 50 to 100% more than that of injection molding.
Reaction-injection molding (RIM) — In-mold polymerization lets RIM form large parts with 10 to 15% less injection pressure and less than 5% of the clamping force used in conventional injection molding.
RIM starts with two low viscosity components, that when mixed and injected into the closed mold, quickly react and polymerize. The low pressures needed to mold the low-viscosity components produce complex geometries with no molded-in stresses and little shrinkage. It accepts inserts or stiffeners and readily duplicates mold surfaces. The low-pressure, slow-cavity fill also handles walls whose thickness varies. Ribs and bosses can be molded without the threat of sink marks, and the process can accommodate in-mold coating which reduces finishing costs.
Polyurethane remains the dominant RIM material. Designers can tailor part structural properties through judicious selection of its different families. Elastomeric urethanes exhibit good impact resistance, while solid urethane RIMs can be made to resist static buildup or can be flame retardant. Other varieties of urethane include RIM foams which range from flexible to rigid. Like other engineered resins the RIM polyurethanes accept fillers or other reinforcements. The resulting processes are commonly referred to as RRIM (reinforced RIM) or SRIM (structural RIM).
RRIM combines short-fiber or flake reinforcement directly into the reaction process, while SRIM uses molds containing structural preforms. Preforms are three-dimensional precursors of the part and can be plastic or metallic inserts, fibrous reinforcements, or core materials. The most common preforms are fiberglass and can be mats of either thermoformed continuous strands or chopped fibers sprayed onto part-shaped screens.
Contoured in-mold cores, some times referred to as the male, hollow out three-dimensional SRIM parts giving thicker profiles and reduced weight. Metallic inserts provide localized stiffening, attachment points for high stress areas, and weldable studs. SRIM uses urethanes similar to those of RIM. However, the SRIM resins must have lower viscosities in order to better penetrate the preforms.
BLOWN UP OR SPUN AROUND
Technical blow molding — Blow molding produces lightweight parts with the highest stiffness-to-weight ratio of any thermoplastic process. It starts with a hollow tube of material (parison) extruded between open mold halves. As the mold closes, the parison is grasped and pinched off at each end. Air injects through a blow pin, expanding the parison into the mold cavity, where it solidifies and takes shape.
Blow-molded parts have lower molded-in stresses than injectionmolded components. Consequently, there is less warpage and fewer failures from stress. This single-operation process easily molds complex parts or flat panels. The technique employs foam filling, molded-in stiffeners or compression welds — tackoffs — between opposite sides of the parison to boost stiffness and strength in flat panels.
It is also possible to form parts with varying compositions or multiple layers through sequential and coextrusion blow molding. Sequential extrusion mixes various materials as the parison forms so that selected locations have different properties. Coextrusion builds multilayer parts and can use recycled material for inner layers. Coextrusion also gives inner and outer surfaces different performance qualities. For example, blowmolded fuel tanks have internal surfaces which resist corrosion and provide a vapor barrier. But their outer layers are tough and impact resistant.
Blow molding cannot produce the same surface finish or complex surface detail as injection molding. And because the parison fills the cavity by stretching, thinner walls result in areas with radii or deep-draw. Sharp corners are not practical and parts will have “witness” lines where cavity sections meet. Blow-molded parts also need secondary operations of trimming and deburring to remove pinchline flash. Holes and cutouts must be machined as well.
Rotational molding — Large, thick walled hollow or open-side parts are good candidates for rotational or rotomolding. Using mainly thermoplastic liquids or powders, rotomolding forms parts by simultaneously rotating molds around two right-angle axes. As the molds heat up, the resins fuse together forming uniform layers on mold surfaces. The amount of resin placed in the mold controls wall thickness.
Rotomolding tools cost less and are easier to make than those for injection molding. The process can handle a large array of part sizes from doll heads to boat hulls with no mold lines, sprue, or ejection marks. As a low-pressure process, it makes parts having no molded-in stress. Structurally, rotomolded parts have good load-bearing properties and stiffeners can be molded in if additional stiffness is needed.
SHAPED Thermoforming — Thermoplastic sheets can be formed in a number of ways using vacuum, low pressure, or a combination of both. The basic idea is to lay a heated sheet across a mold cavity. A pressure box makes contact with the sheet surface, forming two sealed areas above and below the sheet. Vacuum draws the sheet toward the mold, evacuating the cavity at the bottom, while compressed air pushes down on the sheet’s top surface.
Similarly, hollow parts are made using a twin-sheet forming process. Two opposing molds come together with the heated sheets between them. Vacuum draws each sheet to the corresponding mold half, while compressed air is introduced between them. Ballooning out, the sheets take the shape of the mold and fuse together where the molds meet. Vacuum can also be used alone to draw thermoplastic sheets into female molds or over male ones.
Materials for thermoforming are more expensive because they have already been processed into sheet form. Finished parts also require secondary trimming or routing for holes, louvers, or grills. Robotic trimmers, however, have reduced the cost of these operations so thermoforming prices, especially at low volumes, are comparable to injection molding.
Compression molding — Here matched male and female dies close around a “charge” of material filing the cavity. The pressure and heat inside the mold cures the resin. One of the most common thermoset materials is sheet-molding compound (SMC). It consists of 20 to 25% glass fibers, uncured thermoset resin, and thickeners sandwiched between carrier films. Before molding, the carrier films are removed and the SMC is placed in the bottom mold where it is compressed as the molds come together. Molded openings are possible by the use of shear edges or “mash-offs.” As the molds come together, the mash-off technique squashes the material in selected locations to a membranelike thickness. A secondary process cuts the membrane from the part. Holes created by mash-off give the parts less stress in the surrounding material.
SMC is a low pressure process and can use less robust tools made from materials such as reinforced thermosets for low-volume components that do not require shear edges or slides. Reinforced SMCs have good strength-to-weight ratios and tight tolerances, but low-impact resistance. The biggest disadvantage is the refrigeration storage requirements and shelf life of the SMC.
Glass Mat Thermoplastic (GMT) — Molded composites made of reinforced thermoplastic sheets offer advantages for large parts when compared to SMCs, wood, or steel. The preheated, reinforced thermoplastic sheets are laminated together in a single mold. Often called stamping, it differs from conventional steel stamping because it forms parts in just one operation instead of the progressive steps required by steel. The ability to laminate layers of varying reinforcement, continuous, unidirectional, and chopped fibers, make it extremely flexible. When compared to SMCs, thermoplastic molding produces less scrap and has shorter cycle times. The thermoplastic material also has an indefinite shelf life.
Threaded fasteners, steel-edge stiffeners, and sheet metal plates are easily molded-in. Dissimilar, decorative materials such as fabrics or carpet can also be laminated-in with proper temperature and mold pressures. The technique also easily produces textured mold surfaces.