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Some Basic concerns between the two systems
Definition: Hot runner: 1) a system which keeps the sprue, runner and secondary runners molten thus eliminating / reducing material usage. Cold Runner: 1) typical molding system which produces both parts and the feed system to produce parts of which the feed system is considered waste.
It is generally understood that the feed system of an injection mod transfers material from the machine nozzle to the part/ gate area, and that a cold runner means that we shall have a solid or waste material produced with the parts and a hot runner means that we eliminate waste and only have a parts. There are also some processing issues to consider between the two systems.
A note or comment here for those whom may run thermoset materials and or LSR molds. The terminology for what is a hot runner in thermoplastics is called a cold runner in these materials, this due to the fact that when heated up these thermoset materials cross link and cure. Thus in these materials the runner system is kept cold so as not to cross link and cure.
The first difference in hot vs. cold is the cost of the hot runner system. This is a capital cost, and must also include the cost of a controller if a plant does not have one. The controller controls the temperature of the hot runner system, and in today's market these can be a wide price range dependent on what one wants or needs for control. It should be noted that once purchased these controllers are available for use on other hot runner systems, as long as the cable connection are compatible.
The second difference is the actual shot size on the machine. The cold runner tool shall have a larger shot size, this due to the runner system. What this than means is that screw recovery shall be longer on a cold runner tool than that of hot runner, also that the volume of material is larger. The difference now is that less material is used in a hot runner, and less time is necessary for screw recovery, so a savings in actual part costing exists both for material used, and possible reduction in cycle time.
Melt temperature / residence time. In a cold runner tool one may look at the barrel and shot and note how many shots they have and how long. This is also true in a hot runner with the additional point that they then also have to calculate out the turnover rate of the hot runner system itself. What this referrers to is the amount of material in the hot runner manifold, how many shots does it take to refresh all the material in the manifold, less than one or greater than one.
Pressure drop, while a cold runner does have its pressure drops throughout the system as does the hot runner system, both must be evaluated so as to know what one is working with in the production of parts. Opening up a runner on the cold runner tool is much easier than redesigning a system on the hot runner. But actually knowing the pressure drops and where one loses the pressure is important in knowing the difference in systems, and begin able to place the molds in capable machines for theses mold systems.
The other major difference in pressure is how it packs out a part. In a cold runner system the runner gradually cools down and this in essence decreases the pack/ hold pressure to the part/ gate. While a hot runner feeds a molten path to the gate/part, it does not cool down or lose pressure but maintains it. Thus the pressure at the gate is held constant, resulting in a slight difference in how the part is packed, and possibly the effect of stress in parts.
Startup procedures are very different for both a hot and cold runner system. While in a cold runner one makes sure the mold is up to temperature and melt is correct, with the hot runner system it is both of these plus the sequenced startup of the hot runner system itself. While each manufacture of systems has their specific sequence generally it is noted that the manifold is brought up to temperature first and then the drops are brought up to temperature after the manifold has equalized. This sequencing is time dependent and may add time to the changeover of tools, and startup of the press while waiting for the temperatures to come up.
SLSILVEY
20112014.01
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