Definition: Gate: 1. opening permitting passage, 2.moveable barrier to passage, 3. any means of access or entrance
Generically gates come in all shapes, sizes and types. We have cold runner gates, hot runner gates and valve gates, and than the actual type within those classifications.
The purpose of any gate is to act as the entrance or door to our part within the feed system of material that is used to fill the part. The gate in itself is the orifice that allows material to flow into the part, and is that piece of the runner system between the runner and part. The other factors of this gate area, is that it composed of a length (referred to as land) and a height and width unless of course it is a round opening than only a diameter.
The size of this orifice is determined by many mathematical formulas, practical experiences and or check list and crib sheets that various suppliers, tool makers and others have gathered and or developed over their years of experience.
Some really big rules of thumb may be to start with a height and width of 40% of the wall thickness of the thickest section in our part. This can also be used as the diameter of the gate at 40% of the thickness. The key to the preceding statement was thickest area.
The freeze off of the gate, is set to be as quick as we need and also as long as we need. A simple test can be performed to determine the time it takes for a gate to freeze. There are multiple programs for sale that well plot these points and or develop a graphical analyst of this aspect of the mold. It is also possible within a multicavity and family mold that different cavities / gates have different freeze time, an issue in itself to production of quality products.
The question to ask is what is a good time, since in fact what is sold in the molding business is time. Is it long enough or too long. An example of how long may well be a statement made to this author at a conference years ago when someone stated that a said material needs 5 seconds of pack time for each millimeter of wall thickness, to develop the said properties of the material. My question at the time was not answered and it was what happens if we don't, and how much of a decrease in physical properties would we see. As a molder this data would allow for knowledge that cycle time has to include this said pack time.
Now from the processing point alone some key attributes can be associated with gate freeze time.
1- Shrinkage/ part size.
2- Cycle time
3- Stress
Shrinkage is a big one in that if the gate freeze is too quick we may not see the proper packing of the part and thus produce parts that shrink too much or more than was calculated for within the mold build.
This is best explained by comments that the part is too small, sinks and or voids are found in the part.
If the gate takes too long to freeze than size may still be an issue but may vary shot to shot or run to run with the same set up conditions. This due to the pheoumn of the part is filled and then material is allowed to leave the cavity when the screw rotates and pressure is pre relieved from the mold cavity.
Cycle time is critical, and if the gate freezes quickly we can drive the screw back faster, thus reducing the time but also if the gate is slow to freeze than we run into a long cycle time to assure that we are frozen. This time to freeze or not is driving the cost of our production and the amount of time that is needed.
Stress, this is a multi component issue in that it is most associated with pressure use in the molding process and also other conditions within the process. But given that if the gate freezes quickly we may need to have more pressure to assure full parts this can lead to stress in the gate area, or if the gate freezes slowly and a high pressure is used to maintain the pressure than the area of influences may be greater or have a more varied pressure profiles across our part leading to stress within the part. This may be seen in the field as parts cracking or crazing around the gate or even weld lines.
The key to determining what is going on within the gate is multifunctional. The calculation of gate freeze time should always be done on a new tool trial. Further the calculation of shear through the gate should be done, this is either mathematical, and or via a shear viscosity test. The last test should be the pressure drop test to see how much pressure is lost while traveling through our gate area. It should only be than that the decision be made to increase a gate diameter, though in truth and practice most well just open up the gate and then struggle from that point forward.
