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Definition: 1) A hardened steel insert in an injection mold that contains a heated passage for material to flow from the machine nozzle seat to the parting line of the mold (2-plate), also be used in hot runner systems, 3 plate molds. 2) A system for transferring material in a mold without waste so as to eliminate a cold runner sprue.
The heated sprue bushing is a great tool and works well until it fails or people do not understand it. Basically it is an extension of the machine nozzle in that it conveys molten material to the runner system or if a large part directly to that part. In using a heated sprue bushing there are a few parts to be aware of:
1- The body or outside of the unit
2- The internal flow channel
3- The tip section
4- The heater component and how it operates
5- The thermocouple
6- The external control box.
There are other properties or facts that are greatly overlooked in selecting and or using the heated sprue bushing, and the biggest one is what the volume of material in the unit is. The material is transferred from the machine nozzle to the heated sprue and then to our runner path / part, some of the material now has a certain residence time in our heated sprue bushing. It also has time to reshape the molecule structure from any shearing that occurs with our machine nozzle if by chance the diameter of the orifice is greatly smaller than our orifice on the heated sprue bushing.
To calculate the volume of the bushing it can be done mathematically, or by placing a color pellet in the O of the bushing and allowing the pellet to heat up and then injecting material into our mold and or by changing completely the color of our material and allowing the previous color to sit in the bushing. What happens is that upon injecting material into our mold/part we can see the changeover or where material is located within the flow system.
To know the change location may be beneficial in troubleshooting cosmetics and other issues within the molding of our parts. It also allows the processor to see what if, for temperature change in the bushing.
Another aspect of the bushing is the pressure loss that can occur. This is simple calculation by injecting into air with a purge and recording the amount of pressure required and then by injecting into the mold but only to just to start to fill the runner or part (whichever is allowed and well eject). By subtracting the air shot from the part/runner shot the pressure loss within the bushing can be calculated. The loss should be minimal and by adjusting temperature it may be found that temperature setting has to be increased to get good flow from the bushing (to be discussed later). Note that the pressure loss is relative due to current pressure limitation of injecting into air and into mold.
An example of the above explained process tool was a large case in which the blush around the gate was not acceptable (direct gate from sprue bushing). By showing the exact transfer area of material in the bushing to material from the barrel, and that the area of concern was all in the bushing allowed an engineering change to the bushing to eliminate the issue. This was accomplished because it was not thinking or guessing what that area was but showing what it was by data, thus no one could dispute the issue.
To be continued...
SL Silvey
silveysplastics@hotmail.com
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