Molding methods liquid injection

Different casting processes are used. They tend to overlap and could be identified by other processing methods. An example is liquid injection molding that can be identified as injection molding (Chapter 4) or reaction injection molding (Chapter 12). Many decades ago the reaction injection molding process was initially called liquid injection molding. 

The latest trend in foamed composites is automotive structural parts produced by the SRIM (structural reaction-injection-molding) process. The SRIM process is a method of the reactive liquid injection molding (i.e., LIM). LIM includes RIM (reaction injection molding), RRIM (reinforced RIM) and RTM (resin transfer molding). IX M (Liquid Composite Molding) is a relatively new technical term which includes SRIM, RRIM and RTM (Resin Transfer Molding). 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mixture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

A third construction method involves creating a form in three dimensions by casting. Casting is a technique that requires a mold, into which a material such as plastic, metal, or clay in liquid form is poured or injected. (See Figure 4.3.)... 

Liquid colors can be used in injection molding and extrusion. Thin films usually require large amounts of color and so are not usually made with liquid color. Thick films and sheet are suitable applications for Liquid. Usually, 1-3% is the practical limit for a letdown ratio, but liquids have been used up to 8% in special circumstances. For letdown ratios of 1 % and less, liquid color is often the preferred method of coloring or introducing additives. 

In the 2P process (4), a thin photopolymerizable liquid layer is cured in situ between a smooth substrate (cut from a polymer sheet or molded) and a master and stripped off to effect pattern transfer. The liquid layer ensures accurate, defect-free replication. Examples of solvent-free formulations include multifunctional acrylates with adhesion, wetting, and release agents. Free radical and cationic polymerization processes have been described (9). At the present disk volume levels, this method is claimed to be cost competitive with injection molding. 

The injection molding specifics of inhibited composites consist first of all in the necessity of accurate metering of components and preparation of the extruded composition containing a liquid-phase ingredient. The simplest and nonetheless most efficient method of preparing the composition is biending of the polymer powder with the liquid PI followed by addition of granules of the same polymer [139]. 

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