Mold-feed pushing

Developments. A variety of process modifications aimed at improving surface finish or weld line integrity have been described. They include gas assisted, co-injection, fusible core, multiple Hve feed, and push—pull injection mol ding (46,47). An important development includes computer-aided design (CAD) methods, wherein a proposed mold design is simulated by a computer and the melt flow through it is analy2ed (48). 

Two-Stage" Injection Molding Machine. Instead of using a reciprocating screw to melt the plastic and to push the melt into the mold, the extruder screw may feed the molten plastic into a separate reservoir, and then a separate plunger can feed the melt into the mold. 

A typical arrangement for a direct screw-transfer injection-molding machine for thermosets is shown in Figure 2.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold. 

Face (of flight) The front face of a screw flight, oriented toward the feed hopper. The rear face of a flight, which pushes the molding compound, is oriented towards the screw tip... 

The first part resembles in principle an extrasion process with a motor, feed hopper, and a heated barrel containing a screw that melts and positively drives the material. However, operation of the screw is cyclical. It moves forward as a plunger in order to convey a batch of melt into the mold and then retreats through rotation while mixing material and melting a new batch which is then pushed to the front. Actually, prior to use of the screw in injection, a plunger (ram) pushed pellets into a melting zone, in which an... 

Naturally, the anisotropic nature of LCP blends should be taken into account in the product design. The effect of weld lines, which are the weak points for all composites as well, may be diminished with special processing techniques such as push-pull or multiple-live-feed (MLF) injection molding. Moreover, the orientation field of LCP fibers in extrusion of tubular products may be controlled by using a counterrotating annular die [2-4]. 

Injection molding is a forming process. Material (plastic, metal, ceramic, wax, etc) is fed into a hopper which delivers it to the feed section of the barrel and screw. The material is melted usually via a screw that melts or blends the material and then pushes liquefied material (eg, plastic) into the mold, which forms the part. 

Plastic moves from the hopper onto the feeding portion of the reciprocating extruder screw. The flights of the rotating screw cause the material to move through a heated extruder barrel where it softens (is made fluid) so that it can be fed into the shot chamber (front of screw). This motion generates pressure (usually 50-300 psi [0.35-2.07 MPa]), which causes the screw to retract. When the preset limit switch (or a position transducer, on newer machines), is reached the shot size is met and the screw stops rotating. At a preset time the screw acts as a ram to push the melt into the mold. Injection takes place... 

Then it pushes forward to feed the molten plastic through the nozzle into the mold. 

The ram pushes the accumulated material through the die head to form the parison as a shot. This type of machine is obsolete however, there are machines of this type out in the industry still in use. The extruder or plasticizer is stationary and can continuously feed heated plastic into the accumulator. The extruder may be programmed to stop when the die is filled to a predetermined volume and then start again after the parison is extruded. The extruder speed is adjusted to fill the accumulator at the proper rate so the parison is extruded almost immediately after the blow mold has opened and the product ejected. Fig. 13.8 is a photo of an original product blow molding machine of this style of equipment. 

There are several designs for the feed mechanism. The simplest one is the use of a plunger which pushes the material into the mold. However this gives rise to densification of die material. In another design the mix is forced from a cylindrical tank down to the entrance of a screw injection cylinder. The screw conveys the material to the mold at the fixmt of the machine. Short screws are used for thermoset resins, and the compression ratio is below 1.5. Otherwise, heat may be produced and hardening may take place inside the machine. The barrel of the machine needs to be cooled to prevent premature polymerization. The typical temperatures at the rear and front of the cylinder are around 50 and 65"C respectively. 

---