The molding machine has the function of injecting molten plastic material into a tightly closed mold where the shape of a product is formed. The mold is kept closed for a specified time, the cure time, during which the fluid material becomes solid and rigid. A coolant circulates through passages in the mold, so that heat from the fluid plastic is transferred to the mold and from there to the circulating fluid, a process that accelerates the curing (solidification) of the part. At the end of cure time, the mold is opened, and the parts are ejected, ready for packaging or other operations if required. At this point, a new cycle begins. Now, let us see in detail how the machine carries out its job. (See Fig. 2-65.) 

          The cavity half of the mold is attached tothe stationary platen (7), where it is centeredby means of the locating ring. The core halfof the mold is mounted on the moving platen(8). When the press gate in front of the moldis closed, a hydraulic circuit is activated thatcauses the main ram (9) to move forward at afast rate. This movement is brought about bysupplying a large volume of oil from pumpsdirectly into the booster ram (10). This oil exerts a pressure on the body of the main ram(9), causing it to slide over the booster ram(10) and move forward until at a designatedposition the moving main ram actuates a limitswitch that sends a signal to the hydraulic circuit ordering the high-volume pump to dump its oil at low pressure into the prefill tank (ll), while at the same time, the low- volume pump keeps supplying its oil to the booster ram (lo), thus causing slow main ram movement. 

          The pressure at which this slow movement takes place is controlled by a mold protection valve. The pressure of this valve is set at a low figure (around 200 psi), so that the pressure exerted on mold halves, if something is caught between them, will be low and not cause damage to the mold. The space vacated in the clamp cylinder housing (12) is filled with oil by gravity from the prefill tank (11) through the opening of the prefill piston (13) in its retracted position. The mold halves make contact at the low speed of the ram movement, and at this point, another limit switch closes the prefill piston (13) and activates a high-pressure pump (2,000 to 3,000 psi), which will apply its full pressure over the main ram (9), holding the mold halves tight and resisting opening when plastic material is injected into the mold at pressures up to 20,000 psi. This second limit switch also initiates the movement of the injection ram (14), which injects the plastic into the mold. 

           Injection is carried out by the front of the screw (2), which contains a shutoff valve (15) that prevents any possible backflow of the fluid plastic. The screw is firmly attached to the injecting ram (14), whose movement takes place at a fast rate (usually in about 1to 2 sec for the full shot capacity). 

           The injection time is controlled by a timer(the injection high timer), and the ability torespond to the timer setting is determined bythe pressure of injection and fluidity of thematerial. 

           The speed of injection can be varied bymeans of a flow control valve that can bypass a desired amount of the pump oil andthereby reduce the speed. This valve usuallyhas 10 bypassing positions, thus providing aconsiderable degree of injection speed variation.

            Once the shot is completed, the high-volume oil injection pump is ordered by a sig- nal from the timer to dump its oil into the prefill tank (11) at low pressure; at the same time, a low-volume pump (hold pump) maintains pressure on the material in the cavity until the gate through which the material was fed freezes and prevents back flow to the cylinder. (Back flow can be caused by the pressure within the cavity if the feed gate is open.) The hold-pump duration is set by  the injection hold timer. At the expiration of  this timer, the screw starts rotating, picks up material from the throat in the cooled chamber (16), and moves, compresses, and shears it in the extruder chamber (3), where it absorbs heat and liquefies before entering the measuring portion of the injection chamber. 

            The extruder barrel is heated by strip heater bands (18). A group of heaters is divided into zones, with each zone having a pyrometer for controlling the temperature. There are usually three or four zones on the extruder chamber. The extruder work-represented  by feeding, compressing, and shearing of the material-partly shows up as heat induced in the plastic. The heat needed to fluidize the plastic is derived partly from the work of the screw, the balance coming from the strip heaters of  the extruder chamber. 

            As the material comes off the extruder screw (2), it creates pressure on the front face of the screw, causing it to retract so that a space is created for the incoming material required for the shot. This backward movement of the screw makes it necessary to push oil out from behind the injecting ram (14). 

            The displaced oil passes through  a controlled valve, which can be adjusted to provide varying degrees of resistance for the screw’s backward  travel. This resistance, known as the back pressure, is utilized to provide good mixing and homogenizing of the material in the injection chamber. When a slight temperature adjustment is needed for the material that is to be injected, a small increase in the back pressure will accomplish this requirement. The duration of screw rotation is determined by a limit switch, which is activated by the backward-moving screw at  a position where the necessary volume of  material required for the shot has been reached. The screw limit switch may also start a melt decompress timer, which will cause continued limited backward movement of the screw. This additional screw movement creates a space in front of the screw that permits the built-up pressure to decrease enough that, when the mold opens, no drooling of plastic takes place.  

            The final stop of the screw movement usually coincides with the expiration of the curetime as determined by the correspondingcure timer. On a signal from the cure timer,the press starts opening the mold. This isaccomplished by feeding oil from a small-volume pump into the space behind the rambushing (17). This causes the press to startopening slowly; then another limit switch isactuated by the ram movement, which ordersa large volume of oil to be fed into the space soas to shorten the press opening time. Since the area between the clamp cylinder (12) and the main ram (9) is small, and this area multiplied by the pump pressure gives the force for mold opening, this force is small in comparison with the clamping tonnage (usually around 5% of the clamping tonnage). Before stripping (ejecting) starts, the ram is slowed down by actuating still another limit switch for gentle action of the knockout pins, to prevent the pins from punching through the parts while pushing them off the cores. With hydraulic ejection, the slowdown can be so delayed that no banging takes place when the ram returns to the starting position. After ejection, the parts are removed from the press, and the cycle starts all over again. All limit switches have  numbers that  tie  them  to  specific actions.