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a refresher, start continued...
Definition: Runner: the feed channel between the sprue and gates in an injection mold. Cross sectional shape: The shape and or design of an extrusion, perpendicular to its length/flow direction.
The cross sectional shape of runner is the flow path of that runner and contributes to the flow of the material. In a cold runner mold this is waste, basically this material shall be ground up and reused hopefully back into parts and or sold if the material cannot be used within this plant. Thus by optimizing the runner design (shape of runner) one can hopefully minimize the waste of the runner.
The author should state here that if this is a high volume part that long term the costing or what is the waste should be done, and then to calculate out the cost benefit of the use of a hot runner system.
In many cold runner systems it is prior history and plant preference as to what is used. While this may work in many cases, as the competition increases the optimization of new and or existing tools should always be taken into consideration, not only to reduce waste, but also to improve cycle time.
In deciding where to start with diameter of the runner, it should be noted what is the cross sectional thickness of the part at the gate area, and what is the average wall thickness of the part. The reason for this is that minimally the diameter flow of the runner must or should at least be the equal to the wall thickness. In many cases it even increased to 1.5 times the wall thickness. The reason for this is that the material in the runner must fill and then pack the part, through the gate, (the restrictor in many cases). The other concern is that what is the flow length of the runner, if short than possibly the diameter may be less while if long it may require even greater diameter so as to maintain pressure. This is where prior history and use of computer models come into play. In the end though it is the processor whom must run the mold, and he must have the data produced from the models so that they may confirm it works or that it does not per the model. (Pressure drop; vol. 7 No. 4)
While the use of various software programs today help in assisting the design of the runner and cross sectional size, other consideration come into play. The cross sectional design should be that which is a minimal surface perimeter area to diameter flow, or stated differently an efficient runner design is one which provides a maximum cross-sectional area from the standpoint of pressure transfer, and a minimal contact area of the perimeter from the standpoint of heat transfer. The best is the full round runner. Other typical runner designs are trapezoid, modified trapezoid, ½ round, rectangular. The ½ round and the rectangular runner are not recommended as the cross sectional area, as compared to the perimeter of the part may cause too fast of cooling and greater pressure drop, and once these are overcome by typically the increase in size the runner consumes too great of material, increasing cost and possibly cycle times in that screw recovery becomes too great, and or waiting for the runner to cool.
The following shows the cross sectional shapes of the various runner designs.
In constructing the tool, the full round must be placed in both half's of the mold, meaning that we have to be sure it is mated correctly, and that it shall have its own parting line. The tool maker should be able to perform this, yet in many molds with these types of runners it is often found that there is a slight mismatch, and that after numerous grinding of the parting line that the runner is no longer round but oval and this may or may not effect the pressure drops and or process to produce parts. Also in the creation of this type of runner, there are two setups on the machine since both the A and B half of the tool must be machined. The smaller the diameter of the runner the greater the increase in mismatch has been observed.
Thus in many molds, the modified trapezoid is used, as it may be cut in only ½ of the tool and this allows for an easier time for the mold maker, and may lower the cost of build a bit as there is only one set up to cut. The volume of the runner is only approximately 14% greater than a full round, of equal diameter flows. Also the modified trap is recommended for complex molds with various parting line configurations, as a full round would become more difficult to match.
Ejection of the runner and gate types shall also play into which design is best for the mold. Whichever design is chosen, it should always be remembered that the function of the runner is to supply the material to the gate, then to the part and to maintain a pressure so as to pack out the part. If the runner cannot provide both pressure and flow, than it becomes the issue.
SLSILVEY
25082014.01
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