Low-pressure reaction-injection molding is a way of making large nylon parts with low internal stress.
Quadrant Engineering Plastic Products
Reaction-injection molding (RIM) combines two lowviscosity liquid streams consisting of a reaction catalyst and initiators (containing mostly monomer) inside the mold. The resultant polymer grows in molecular weight as a chemical reaction completes. This transforms the monomer into a rigid, solid polymer part.
The handy thing about RIM is that it can produce high-strength components economically. The properties of RIM parts depend on the monomer system selected, any additives included such as reinforcements, stabilizers, or lubricants, and the process itself.
RIM is most commonly associated with polyurethane (PUR) chemistries but it is also widely applied in more demanding applications with the use of various nylon-based materials. Nylon, along with only a few thermoplastic resins (such as PUR) has the notable advantage: It can be processed from both pellets via melt-oriented processes such as extrusion, injection molding, and compression molding as well as cast where the monomer polymerizes in the aluminum mold cavity.
This monomer casting has been used since the 1960s to produce stock rod, plate, and tubular shapes for machined plastic parts. More recently, processors have adapted RIM to compensate for the greater challenge of casting nylon-based monomers. This innovation makes it more economical to RIM process parts rather than machine them from a cast stock shape.
RIM provides economies close to those made possible by injection molding. But it offers far more flexibility in part size, geometry, and tooling cost. Geometries with varying wall thickness in adjacent areas and length-to-wall thickness ratios approaching 100:1 are possible. The high molecular weight of monomer cast polymers plus the ease with which they can be formulated is a benefit seldom associated with conventional injection-molding grades.
RIM is a good candidate for parts ranging in size from a few ounces to 32 lb and in volumes of 1,000 to 100,000 parts/yr. The lower processing temperature of RIM nylon, coupled with low-injection pressures, lets RIM use aluminum tools rather than hardened tool steel as for conventional injection molding. Use of aluminum tools benefits parts made in smaller volumes and those that may need a redesign. It costs less to retool aluminum than hardened steel tools.
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THE RIM PROCESS
RIM is a closed-loop system in which the components constantly recirculate while temperature, pressure, and reaction chemistry are carefully controlled. Piston pumps can add reinforcements to the stream without compromising the aspect ratio (length:diameter) of the fiber.
Temperature and pressure management of the system yield low stress, highly repeatable parts. Tolerances are generally controlled to within the larger of ±0.5% of nominal dimensions or ±0.75 mm, although many parts are produced to far tighter tolerances especially on critical features. Batch-tobatch repeatability is generally quoted as 0.3%. The low residual stress in RIM cast parts lets finish-machining operations take place on standard machining equipment.
Tooling for RIM parts ranges from single-cavity molds for large, low-volume parts to 30-cavity molds that mold smaller components in larger quantities. Start-up costs for RIM tooling typically range from $5,000 to $15,000 depending on part complexity and expected tool life.
When RIM processing is a poor fit, atmospheric pressure casting is a possibility. This monomer casting process puts similar nylon chemistry in open molds under atmospheric pressure.
RIM NYLON MATERIALS
The formulation possibilities of RIM nylon are endless. Nylon's superior wear and abrasion resistance coupled with its high strength and stiffness provide a wide range of engineering properties. The relative ease in which nylon can be finish-machined also allows for easy finishing where parts are not complete out-of-the-mold.
Elastomer-modified formulations include an elastomer rubber molecule chemically bonded to the caprolactam monomer. These increase the impact properties as much as 10
3 that of unmodified nylon. And glass and/or aramid reinforcement boosts the flexural properties more than 50% over unreinforced nylons. Polymer-based wear additives reduce friction properties against mating substrates. This raises the wear life by a factor of eight while reducing the coefficient of friction 50% over standard nylon materials.
RIM nylon parts have served in heavy industrial equipment for years although predominately in Europe where much of the process equipment originates. The process lets designers of construction equipment, such as mobile cranes, use engineering plastics for machinery parts that are just too big for processing via injection molding. Parts such as sheaves, which go in the booms of mobile cranes and trackplates used in agriculture equipment, are notable examples of RIM nylon parts in service.
Highly custom parts that pose processing difficulties for injection molders, and those that make tooling too costly, may also be a good fit for RIM nylon. A RIM nylon fan blade, for example, replaced a wood laminate part that was fully machined. The RIM processed fan blade is molded around a steel insert providing a means for transmitting the input torque from the blower motor to the plastic part. Casting over metal inserts is made easier through the RIM process. There are no knit lines because the monomer polymerizes uniformly around the insert.
Likewise, a sprocket designed for conveying water treatment screens in a sludge collection system is another complex part successfully converted to RIM nylon. The sprocket weighs 13 lb and replaces what was once a 90-lb cast iron sprocket wheel. The gear segments are also RIM parts eliminating costly preassembly and hobbing operations.
Although RIM-processed nylon doesn't always beat injection molding, it can cost less and perform better than thermoforming. A thinwalled thermoformed UHMW part, for example, is being challenged by a RIM-processed nylon material for use as a thrust interface between the wheel bogey of a freight car and the car itself. The parts enable motion between the car body and the wheels. The performance advantage of RIM processing is that that the friction coefficient can be readily customized in the nylonbased material to improve the handling of the car under different track conditions. RIMprocessed parts have for years served in the demanding railroad environment for brake guides, pedestal liners, centerbowl/ centerpin liners, and hatch covers for tanker cars.
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