Mold Requirements

The injection mold need not be made of noncatalytic metals any high grade tool steel may be used because the plastic cools in the mold and undergoes Httle decomposition. However, the mold requires good venting to allow the passage of small amounts of acid gas as well as air. Vents tend to become clogged by corrosion and must be cleaned periodically. 

Some draft is required in side walls to facilitate the easy removal of the product from the mold. Female molds require less draft since products tend to pull away from mold walls as they shrink during cooling. With female or male tooling, for most plastics the draft on each side wall should be at least 1 degree (Fig. 3-39). 

RM is a simple, basic, four-step process that uses a thin-walled mold with good heat-transfer characteristics. Its closed mold requires an entrance for insertion of plastic and, most important, the capability to be opened so that solidified products can be removed. These requirements are no problem. Liquid or dry-powder plastic equal to the weight of the final product is put into the mold cavity(s), which rotates simultaneously about two axes located perpendicular to each other (Fig. 8-68). These two rotation speeds can be varied to permit more... 

Injection molding requires the barrel temperature to be about 350°C with a barrel pressure in excess of 138 MPa. The mold is maintained at 110°C to ensure uniform flow and high definition, and to discourage an uneven index of refraction, birefringence. The CD is about four one-hundredths of an inch (0.5 mm) thick. For prerecorded CDs, the PC is compression-molded on a stamper imprinted with the recorder information. This takes about 4 sec. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc. Then, a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic surface and the CD is complete. This process is described later in greater detail. 

Thermoset molding compounds, when contained within a hardened steel mold, require heat and pressure to be polymerized into a solid mass. Molds may be heated by steam, electricity, or hot oil to temperatures of 280° to 425°F, depending entirely on the type of material and method of molding. Molding pressures may vary from a low of 50 p.s.i. to 15,000 p.s.i. Epoxy materials will mold at 50 p.s.i. whereas, phenolic fabric-filled material may require excessive pressures. Again, the method of molding dictates molding pressures. 

The selection of optimum conditions for rotary molding requires extensive experimental investigations of the various processing regimes. Recently, another way has been developed, i.e., simulation of the flow of a liquid system polymerized in a rotary (rotating) mold.178... 

When the clay piece is removed from the mold, Additive Method, separation and removal from the mold requires... 

Pouring liquid metal into the mold requires a steady hand and constant attention. When the mold containing liquid metal is placed in a centrifuge, hot metal can fly out of the centrifuge. This is why it is preferable to use the vacuum casting method, for which a centrifuge is not needed. 

The cavity surfaces should be finely sand blasted to prevent formings from sticking in the mold. The mold requires a multiplicity of vacuum or vent ports, and these should be distributed across the forming... 

With backpressure the process is performed in conventional injection molding machines (IMMs) (Chapter 4). The cover stock is inserted and located in an open mold. A shear edge mold permits draw-in of the cover stock during the closing cycle to avoid wrinkles and damage by stretching of the fabric. Molds require special attention. They generally use a hot runner system with its shut-off nozzle(s). All mold elements such as ejector, core pulls, and slides have to be on the injection side mold half. 

The type of process used also indicates the addition of polymer modifiers. Blow-molding requires that the polymer flows easily, while retaining good cohesive strength when molten, to avoid tears and voids. Melt flow modifiers are typically used for this purpose and include fatty acid esters, oleoamides, and adipates. Compounds of this type often also serve to prevent blocking (adhesion of one side of a bag to the other). 

In the second chapter, all the problems set by the process of heat transfer are considered, with convection at the solid-fluid interface, and with conduction through a solid, which can be the rubber or the mold. The situation of heated rubber after the extraction out of the mold requires an application of the heat process connecting the convection and the conduction. 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

In the long fiber RP markets applying Pushtrusion to profile extrusion, as in injection molding requires only a little modification. Technically,... 

Molds for autoclaving must be more substantial and can carry more detail. Depending on molding requirements and conditions, they can be produced by electroforming nickel. To cure large parts size restrictions and availability of autoclaves and ovens can occur. Additional heat is obtained by incorporating electrical heating elements in the molds. 

Special mold required Not recommended Only flexible material Only direction of extrusion Possible with special techniques Fusing premix/yes Rated 1 to 5 (1 = very smooth. 5 = rough)... 

Compared to both compression and transfer CM, injection molding requires shorter vulcanization and cycle times as well as better automation. On the other hand, mold costs are so high that only large-series production is economical. 

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