Matched metal molding

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

Injection Molding. Matched metal molds are used in the fabrication of plastic closures, specialty packages, and botde preforms. In conventional injection mol ding the plastic resin is melted in an extmder which forces a measured quantity or shot into a precision-machined chilled mold after which the nozzle of the extmder is withdrawn. 

Class A tolerances apply to parts compression molded with precision matched-metal molds. BMC, SMC, and preform are included. 

FIGURE 2.45 Matched metal molding (a) before closing of die (b) after closing of die. 

As reviewed in conventional RTM, the preform is placed into a closed, matched tool and resin is injected under pressure about 100 to 200 psi (0.69 to 1.38 MPa). Early RTM processes lacked the consistency needed for aerospace components, in both dimensional tolerances and mechanical properties. Fiber volume fractions were significantly lower than the 60 to 65 wt% typical of prepregs. Problems with predicting flow fironts as well as flaws that were introduced into the preform when closing the matched metal molds often led to high void contents and dry spots. 

Uses Colorant for polyester resins, match metal molding compds. (SMC, BMC, TMC, preform, premix), flat panels, gel coals, resin transfer molding... 

This refers to an open mold technique used for the production of many articles where the resin and reinforcement systems can be tailored to meet specific end use requirements. Typical products produced by this method are skis, boat hulls, tubs, and shower stalls. The molded cavity is sprayed with a gel coat to a desired thickness to produce a desired surface finish or color. The reinforcement, which is usually glass fibers, mats, or sheets, is then hand rolled to secure proper wetting of the fibers as well as the required wall thickness. The final curing takes place through the use of a catalyst or hardener reaction. Matched metal molds are often used with hand lay-up formulations in order to achieve high quality surface finishes on both sides of the molded article. 

A matched metal mold must be carefully ground from hardened steel and the finished dimensions carefully checked. The mold, when new, should be coated with silicone grease and baked for 24 h at 200°C, to acquire a patina with an improved release. All assembly set screws and their sockets should be smeared with silicone grease to prevent resin penetration... 

A machined matched metal mold (Figure 21.47) treated with a release agent is used to contain the resin/reinforcement and pressure applied to the stops to give the requisite laminate thickness by using a hydraulic press (10-50 MPa) to consolidate the reinforcement and remove any entrained air along with excess resin. Subsequently, the resin is cured by the application of heat. This method is used to prepare laboratory composite samples for routine quality control purposes (Section 17.8.2). DMC, BMC and SMC can be processed by this method. 

Figure 21.47 Matched metal mold for laboratory test composites.

Low material cost Minimal material storage problem Close control on part thickness and fiber content Thick complex parts can be molded in one shot High material cost (approximately 2x cost of RTM) Prepreg has short life at room temperature and must be stored in a freezer To achieve accurate thickness control, must use matched metal molds and computer controlled presses Difficult to mold complex parts in one shot... 

The advantages of matched metal molding are rapid production rates along with good quality and reproducible parts. The disadvantages are high mold and equipment costs. The parts often require extensive surface finishing. 

The RTM process begins with a dry fiber preform. The preform is placed into a matched metal mold and the mold is closed resulting in the compaction of the preform to the specified fiber volume fraction. A liquid thermosetting resin is then injected into the mold (typically at high pressure, such as 5-7 bar). The mold and resin can be preheated before injection, or the mold can be heated after injection to cure the resin. Due to the high injection pressures and often high temperatures involved, RTM tools are bulky and costly to manufacture and to process. To aid filling of the mold, vacuum may also be applied to remove trapped air. In addition, vacuum can be used to optimize the resin infusion/injection. This leads to VARTM. More information on RTM can be found in Chapters 9-11. 

Uses Castings used for aircraft glazing, electrical components, decorative applications. Resins used for premix and prepreg molding materials, matched metal molding and hand lay-up molding. ... 

Matched Metal Molding. A reinforced plastics manufacturing process in which matching male and female metal molds are used (similar to compression molding) to form the part. 

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