MANUFACTURING WITH PLASTICS Machinery's Handbook, 31st Edition
588
Vacuum Bag Cellophane or polyvinyl acetate is placed over the layup. Joints are sealed with plastic; vacuum is drawn. Resultant atmospheric pressure eliminates voids and forces out entrapped air and excess resin.
Contact Molding Resin in contact with air. Layup normally cures at room temperature; heat may accelerate cure. A smoother exposed side may be achieved by wiping on cellophane.
Modification of the pressure bag method; after layup, the entire assembly is placed in steam auto- clave at 50–100 psi (345–690 kPa). Additional pressure achieves higher glass loadings and improved removal of air. Autoclave Pressure Bag Tailored bag − normally rubber sheeting − is placed against layup. Air or steam pressure up to 50 psi (345 kPa) is applied between pressure plate and bag. Fig. 16. Variants of Hand-Layup Method of Reinforced Plastics Molding with Cloth or Mat Provide Improved Control of Final Product Qualities. and clamping force. They range from machines for forming micron-sized gears up to meter-sized garbage dumpsters. Injection molding is capable of making identical items with very close tolerances on dimen - sions, so it is ideal for the production of interchangeable industrial parts destined for complex assemblies. Undercut shapes can be molded with the aid of side-draw mold sections. For ex- ample, millions of bottle caps are molded in automatic unscrewing molds. Producing mold cavities and their male halves in fine tool steels is costly, so one should be quite certain that the part design is accurate before finishing the mold cavities. Successful production requires the inputs of all persons connected with the project: the product designer, the mold designer, the mold maker, the molder and the customer. Because of the high investment in the mold, injection molding is seldom considered for very short runs, especially in view of available 3D printing technologies. For more on this topic, see Metal Additive Manufacturing on page 1555. However, inexpensive molds that may serve to make a few hundred parts have been made from metal-filled epoxy casting compounds, while CNC machining of aluminum and other rapidly produced metal molds is still widely used for production of tens of thousands of parts that have tolerances and surface finishes that exceed the capabilities of most current 3D printing processes. Design for Injection Molding.— Product analysis is a good starting point in the design process for plastics molding. Make a detailed checklist of the functions the new part will have to perform and the operating conditions it will encounter. If the part will replace an older design that required fabrication from other materials and, perhaps, subassembly with fasteners or adhesives, the new design may be able to take advantage of injection molding’s versatility to meld the several functions into a single new part design. Properties Checklist: Identify potential candidate materials that can fulfill required functions. Preliminary analysis using relevant properties should reduce the list to a few materials that can do the job and the required volume (in 3 or cm 3 ) of each.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online