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For moulds containing intricate impressions,and for multi-impression moulds, it is not satisfactory to attempt to machine the cavity and core plates from single blocks of steel as with integer moulds. The cavity and core give the molding its external and internal shapes respectively, the impression imparting the whole of the form to the molding.
3.4 The Parting Line
To be able to produce a mold,we must have ta least two separate mold halves,with the cavity in one side and the core in the other.The separation between these plates is called the parting line,and designated P/L.Actually,this is a parting area or plane,but,by cinvention,in this intext it is referred to as a line. The parting surfaces of a mould are those portion of both mould plates, adjacent to the impressions, which butt together to form a seal and prevent the loss of plastic material from the impression.The parting line can have any shape, many moldings are required which have a parting line which lies on a non-planar or curved surface,but for ease of mold manufacturing,it is preferable to have it in one plane.The parting line is always at the widest circumference of the product,to make ejection of the product from the mold possible.With some shapes it may be necessary to offset the P/L,or to have it at an angle,but in any event it is best to have is so that itan be easily machined,and often ground, to ensure that it shuts off tightly when the mold is clamped during injection.If the parting line is poorly finished the plastic will escape,which shows up on the product as an unsightly sharp projection,which must then be removed;otherwise,the product could be unusable.There is even a danger that the plastic could squirt out of the mold and do personal danger.
3.5 Runners and Gates
Now,we must add provisions for bringing the plastic into these cavity spaces.This must be done with enough pressure so that the cavity spaces are filled completely before the plastic "freezes"(that is,cools so much that the plastic cannot flow anymore).The flow passages are the sprue,from wherethe machine nozzle contactss the mold,the runners,which distribute the plastic to the inpidual cavities, the wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the molding, there must be no machine marks left which would tend to retain the runner in the mould plate.And the gates which are small openings leading from the runner into the cavity space. The gate is a channel or orifice connecting the runner with the impression. It has a small cross-sectional area when compared with the rest of the feed system. The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back.
4. The injection molding machine process
Injection Mold is installed in the injection molding machine, and its injection molding process is completed by the injection molding machine. Following is the injection molding machine process.The molding machine uses a vacuum to move the plastic from the dryer to it's initial holding chamber. This chamber is actually a small hopper on the back of the "barrel" of the machine。 The barrel is where all the real work is done and it's essentially a large screw housed in a heater which moves the plastic closer to the mold. As the screw turns, the plastic traverses the barrel and reaches a molten state. Only when it's molten can it be injected into the mold with a rapid turn of the screw. As the chamber in front of the screw becomes filled, it forces the screw back, tripping a limit switch that activates a hydraulic cylinder that forces the screw forward and injects the fluid plastic into the closed mold. The gate freezes soon after the impression is filled so that the injection plunger can be withdrawn without the probability of void being created in the molding by suck-back. The tip of the barrel is called the "nozzle" and from this point to the cavity in the mold the material is not heated and is constantly cooling. The "runner" is the cooled/set plastic that extends from the nozzle to the cavity and is process scrap. Actually, the cooled material from the nozzle to the mold is the "sprue" but it's connected to the runner. There are ways around having sprues and runners, but it's beyond the scope of what we're talking about here. Most people have probably seen runners before and not realized it. The most likely place to see them are in model airplane/car kits as the inpidual components are left attached to the runner system. Typically the runners are ejected into a chute below the mold or else they will be picked out of the mold by a robotic arm and dumped into a regrinder. The regrinder chops the runners into bits and prepares them to be moved back into the dryer.