经济实用的高强度尼龙制品

1、经济实用的高强度尼龙制品No stress nylon Low-pressure reaction-injection molding is a way of making large nylon parts with low internal stress. Reaction-injection molding (RIM) combines two lowviscosity liquid streams consisting of a reaction catalyst and initiators (containing mostly monomer) inside the mold. T

2、he 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, an

3、y 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 fe

4、w 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. The part depicted on the right is

5、 UHMW centerbowl. The part on the left is a RIM-processed centerbowl and the one in the center is a RIM-processed centerplate liner. All parts fit into the 17-in.-diameter cavity in the steel casting that is part of every freight car bogey.This monomer casting has been used since the 1960s to produc

6、e 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 p

7、rovides 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

8、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

9、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

10、 tools.THE RIM PROCESSRIM 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. Tempe

11、rature 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 repeata

12、bility is generally quoted as 0.3%. The low residual stress in RIM cast parts lets finish-machining operations take place on standard machining equipment. RIM nylon parts provide big part capability as well as high strength and durability for the demanding environments encountered by construction an

13、d agricultural 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.

14、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 MATERIALSThe formulation possibilities of RIM nylon are endless. Nylons superior wear and abrasion resistanc

15、e 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 m