Centrifugal Casting Centrifugal Casting is used for making cylindrical, hollow shapes such as tanks, pipes and poles. Chopped strand mat is placed into a hollow, cylindrical mold, or continuous roving is chopped and directed onto the inside walls of the mold. Resin is applied to the inside of the rotating mold.
Tape / Fiber Placement Tape placement is an advanced composites process that orients resin preimpregnated high-strength reinforcement fibers in specific directions. Part weight is minimized by concentrating high strength in only the directions needed. It is widely used in the fabrication of high strength/weight ratio parts in the aerospace industry such as aircraft wing and body skins,control surfaces, spacecraft and missiles. Automated equipment is predominately used for large structures while hand tape placement remains widely used but usually for smaller parts. Most automated machines are large, sophisticat sophisticated ed and represent represent a major capitol capitol investment. investment. Resin choices for tapes tapes can be grouped grouped by processing characteristics. These are, Tacky, B-staged resins later cured to a thermoset stage, e.g., epoxies. Nontacky solid resins that melt and flow prior to curing to a thermoset polymer, e.g., bismaleimides and non-tacky thermoplastic resins processed entirely by melting and freezing, e.g., polyetherketone, (PEEK), and polyphenylene sulfide, (PPS).
Compression Molding Compression molding is done with matched metal molds utilizing sheet-molding compound (SMC), bulk-molding compound (BMC), or preform mat. A weighed charge of SMC or BMC, or a preform of glass reinforcement shaped to the mold is placed on a press ranging in size from 300 to 4,000 tons. Resin is added with the preform while SMC and BMC contain all components including fiber, resin, fillers, catalyst etc. Heat and pressure is applied, with temperature ranges of 225 to 325 oF. and 150 to 1,000 psi pressure required to cure parts. Cycles can range from less than one to five minutes. Typical thermoset resins used in compression molded parts are polyesters, vinyl esters, epoxies, and phenolics. Compression molded products vary from dinnerware, trays, buttons, appliance housings, large containers, electrical, electrical, to recreational vehicle body panels such as snow mobiles, and jetskis.
Pressure Bag Molding Pressure bag molding is similar to the vacuum bag moldimg method except that air pressure, usually 30 to 50 psi, is applied to a rubber bag, or sheet that covers the laid up composite to force out entrapped air and excess resin. Pressurized steam may be used instead, to accelerate the cure. Cores and inserts can be used with the process, and undercuts are practical, but only female and split molds can be used to make items such as tanks, containers, and wind turbine blades.
Vacuum Bag Molding Vacuum bag molding, a refinement of hand lay-up, uses a vacuum to eliminate entrapped air and excess resin. After the lay-up is fabricated on either a male or female mold from precut plies of glass mat or fabric and resin, a nonadhering film of polyvinyl alcohol or nylon is placed over the lay-up and sealed at the mold flange. A vacuum is drawn on the bag formed by the film while the composite is cured at room or elevated temperatures. Compared to hand lay-up, the vacuum method provides higher reinforcement concentrations, better adhesion between layers, and more control over resin/glass ratios. Advanced composite parts utilize this method with preimpregnated fabrics rather than wet lay-up materials and require oven or autoclave cures.
Autoclave Molding Autoclave molding is a modification of pressure-bag and vacuum-bag molding. This advanced composite process produces denser, void free moldings because higher heat and pressure are used for curing. It is widely used in the aerospace industry to fabricate high strength/weight ratio parts from preimpregnated high strength fibers for
aircraft, spacecraft and missiles. Autoclaves are essentially heated pressure vessels usually equipped with vacuum systems into which the bagged lay-up on the mold is taken for the cure cycle. Curing pressures are generally in the range of 50 to 100 psi and cure cycles normally involve many hours. The method accommodates higher temperature matrix resins such as epoxies, having higher properties than conventional resins. Autoclave size limits part size.
Resin Transfer Molding Resin transfer molding (RTM) is a low pressure closed molding process for moderate volume production quantities, filling the gap between the slow, contact molding processes and the faster, compression molding process, which requires higher tooling costs. Continuous strand mats and woven reinforcement is laid up dry in the bottom mold half. Preformed glass reinforcements are often used for complex mold shapes. The mold is closed and clamped, and a low viscosity, catalyzed resin is pumped in, displacing the air through strategically located vents. Metered mixing equipment is used to control resin/catalyst ratios that are mixed through a motionless/static mixer and injected into the mold port. Common matrix resins include polyester, vinyl ester, epoxy, and phenolics. Advantages over contact molding methods are a uniform thickness, two finished sides and low emissions. For optimum surface finish, a gel coat would be applied to the mold surface prior to molding. High quality parts produced by this method include automotive body parts, bathtubs, and containers.
Reaction Injection Molding (RIM) Reaction injection molding (RIM) is also known as liquid reaction molding or high- pressure impingement mixing. The low pressure method involves mechanical mixing often with plural component dispensing equipment rather than by impingement and immediately injects the mixture into the closed mold cavity. These reactive polymer resins such as polyols, isocyanates, polyurethanes, and polyamides provide fast molding cycles well suited for automotive and furniture applications. Common RIM parts include automotive bumpers, fender and panel components, appliance housings, and furniture components. When short fibers or flakes are used to produce a more isotropic product, the process is called reinforced reaction injection molding (RRIM). Many hybrids such as polyurethane/urea, polyurethane/polyester hybrid systems are currently being used as well as those for RIM already mentioned.
Structural Reaction Injection Molding (SRIM) This process uses two resin components which are combined and mixed together, then injected into a mold cavity containing reinforcement. In the mold cavity, the resin rapidly reacts and cures to form the composite part.
Pultrusion
Strongwell brings unmatched capacity, technology, engineering, developmental and overall general pultrusion firepower to its customers by producing hundreds of thousands of feet of pultruded fiber reinforced polymer (FRP) shapes every day! The unique advantages possible with FRP have enabled pultruded profiles to penetrate markets where other materials could not meet the design or end use requirements efficiently. The Pultrusion Process Pultrusion is a manufacturing process for producing continuous lengths of reinforced polymer structural shapes with constant cross-sections. Raw materials are a liquid resin mixture (containing resin, fillers and specialized additives) and flexible textile reinforcing fibers. The process involves pulling these raw materials (rather than pushing, as is the case in extrusion) through a heated steel forming die using a continuous pulling device.
The reinforcement materials are in continuous forms such as rolls of fiberglass mat and doffs of fiberglass roving. As the reinforcements are saturated with the resin mixture ("wet-out") in the resin bath and pulled through the die, the gelation, or hardening, of the resin is initiated by the heat from the die and a rigid, cured profile is formed that corresponds to the shape of the die. While pultrusion machine design varies with part geometry, the basic p ultrusion process concept is described in the schematic shown below.
Watch the Pultrusion Process The creels position the reinforcements for subsequent feeding into the guide plate. The reinforcement must be located properly within the composite and this is the function of the reinforcement guides. The resin bath saturates (wets out) the reinforcement with a solution containing the resin, fillers, pigment, and catalyst plus any other additives required. The interior of the resin impregnator is carefully designed to optimize the wet-out of the reinforcement. On exiting the resin bath, the composite is in a flat sheet form. The preformer is an array of tooling which squeezes away excess resin as the product is moving forward and gently shapes the materials prior to entering the forming and curing die. In the forming and curing die, the thermosetting reaction is heat activated (energy is primarily supplied electrically) and the composite is cured (hardened). On exiting the die, it is necessary to cool the hot part before it is gripped by the pull blocks (made of durable urethane foam) to prevent cracking and/or deformation by the pull blocks.
Strongwell uses two distinct pulling systems, one that is a caterpillar counter-rotating type and the other a hand-over-hand reciprocating type to pull the cured profile to the saw for cutting to length. In certain applications an RF (radio frequency wave generator) unit is used to preheat the composite before entering the die. When in use, the RF heater is positioned between the preformer and the die.
