Metin Atmaca 030080007 7th week definitions

1. Reaction injection molding (RIM) (Manufacturing Process)

Previous Definition:

Reaction injection molding (RIM) involves the mixing of two highly reactive liquid ingredients and immediately injecting the mixture into a mold cavity, where chemical reactions leading to solidification occur. The two ingredients form the components used in catalyst-activated or mixing-activated thermoset systems.Urethanes, epoxies, and urea-formaldehyde are examples of these systems. RIM was developed with polyurethane to produce large automotive components such as bumpers, spoilers, and fenders. These kinds of parts still constitute the major appli­cation of the process. RIM-molded polyurethane parts typically possess a foam internal structure surrounded by a dense outer skin.

As shown in Figure 13.27, liquid ingredients are pumped in precisely measured amounts from separate holding tanks into a mixing head. The ingredients are rapidly mixed and then injected into the mold cavity at relatively low pressure where polymerization and curing occur. A typical cycle time is around 2 min. For relatively large cavities the molds for RIM are much less costly than corresponding molds for conventional injec­tion molding. This is due to the low clamping forces required in RIM and the opportunity to use lightweight components in the molds.

Advantages of RIM include (1) low energy is required in the process; (2) equip­ment and mold costs are less than injection molding; (3) a variety of chemical systems are available that enable specific properties to be obtained in the molded product; and (4) the production equipment is reliable, and the chemical systems and machine relation­ships are well understood.

 (Mikell P. Groover,Fundamentals of Modern Manufacturing,4th Edition,pg.285-286)

New Definition (Better):

Process description

. Two components of a thermosetting resin are injected into a mixing chamber and then injected into the mold at high speed where polymerization and subsequent solidification takes place

Materials

. Mostly thermosets.

. Foamed materials possessing a solid skin can be created by setting up a pressure differential between mixing chamber and mold.

. Can add chopped fiber material (glass, carbon) for added stiffness to mixing to produce composites.

Process variations

. Mold material is usually aluminum. Can also use resin for low production runs or hardened tool steel for very high volumes.

. Further heating of resin components before mixing is dependent on material used.

Economic considerations

. Production rates from 1 to 10/h.

. Lead times can be several weeks.

. Material utilization good. Less than 1 per cent lost in scrap.

. Scrap cannot be recycled.

. Flexibility limited by dedicated dies, die changeover and machine setup times.

. Economical for low to medium production volumes (10–10 000).

. Can be used for one-offs, e.g. prototyping.

. Tooling costs low.

. Equipment costs high.

. Direct labor costs moderate to high.

. Finishing costs low. A little trimming required.

Typical applications

. Car bumpers

. Cups

. Containers

. Panels

. Housings

. Footwear

. Garden furniture

(Swift, K. G., Booker, J.D., Process Selection From Design To Manufacture, p.67)

2. Thermoset Sheet Molding Compound (SMC) (Manufacturing Technique):

Previous Definition:

Sheet molding compound is a combination of TS polymer resin, fillers and other additives, and chopped glass fibers (randomly oriented) all rolled into a sheet of typical thickness = 6.5 mm (0.250 in). The most common resin is unsaturated polyester; fillers are usually mineral powders such as talc, silica, limestone; and the glass fibers are typically 12-75 mm long and account for about %30 of the SMC by volume. SMC’s are very convenient for handling and cutting to proper size as molding charges. Sheet molding compounds are generally produced between thin layers of polyethylene to limit evaporation of volatiles from the thermosetting resin. The protective coating also improves surface finish on subsequent molded parts.

(Mikell P. Groover; Fundamentals of Modern Manufacturing Materials, Processes, and Systems 3rd Edition; pg.323)

New Definition (Better):

Polyester or vinyl ester resins can be used to make SMC. Proper thickening with the usual thickening agents is an essential resin characteristic if the SMC is to be ready to mold, i.e., mature, in a reasonable time and to remain moldable long enough that it can be all used up under normal production schedules. Long moldability is especially important in SMC that is intended for use in remotely located satellite plants or SMC intended for outside sales. As previously mentioned, the most widely used thickening systems depend on the reaction of Group II metal oxides and hydroxides with carboxylic functionality on the resin. Carboxylic functionality on the shrink control or “low profile” resin used with the polyester or vinyl ester can also contribute to the thickening reaction. The thickening reaction results in the formation of a certain amount of chain extended high molecular weight fractions which can then greatly increase the viscosity of the SMC by entanglement and hydrogen bonding.

For resins to be satisfactory for SMC, they must also develop sufficient incompatibility with the shrink control additive resin on gelation and cure. This is necessary so that the shrink control additive properly precipitates as a second, monomer solvated, phase. The resultant expansion during cure can offset the curing shrinkage of the polyester or vinyl ester resin.

A good SMC resin should also have good hot strength to minimize damage to the molded part on ejection and removal from the die. Molding temperatures for SMC generally range from 275” to 300°F (135” to 149°C). SMC is commonly made on special continuous machines designed for this purpose. Usually an SMC paste made from the molding resin, low profile resin, filler, mold release, catalyst, thickener, and pigment is fed onto a continuous web of carrier film. Chopped fiber glass roving, often one inch in length, is dropped into the paste layer as it passes the glass chopping station on the machine. The film carrying the paste layer and the chopped glass is then continuously combined with a top carrier film also having a layer of SMC paste.

The SMC, now encased between two layers of film, on passing between a number of rolls on the SMC machine is kneaded and squeezed to complete the impregnation of the chopped glass by the SMC paste. A take-off stand at the end of the machine rolls up the SMC which is then taken off the machine in rolls of convenient size for transfer to a “maturing” area where the rolls are stored until moldable. SMC made on equipment of this type is generally made in weights of 12 to 24 ounces per square foot. Heavier weight SMC is often more readily made on “TMC” or thick molding compound machines which are of different design than ordinary SMC machines.

Highly unsaturated isophthalic and propylene glycol maleate polyester resins are probably the most often used resins for SMC molding.

The SMC paste is then combined on an SMC machine with 1 inch length chopped roving to a 25 to 30% glass content in the finished SMC.

SMC is widely used now for molding automotive body parts such as grille opening panels, truck lids and hoods. Business machine housings are another growing application as are sections from which satellite reception TV antennas are assembled. There are a diversity of other applications such as for swimming pool filter tanks, seating and snowmobile body parts.

(Goodman, S.H., Handbook of Thermoset Plastics (2nd Edition), p.158)