Definition: Flash gate: a long, shallow rectangular gate extending from a runner that lies parallel to an edge of a molded part, along the flash or parting line of the mold.
The flash/film gate though not used commonly it is used in cases to fill long parts and or avoid issues with fill. The gate itself usually parallels an edge of the part for distances which can be short to start but than can be increased if needed. It can be consider as a long rectangular-type edge gate and it is typically used for large, thin walled components to assist in the production of warpage free products. The design normally extends across the complete width (narrow side) of the molding, but in some cases may be along the length of the part dependant of wall thickness and fill.
In most cases the runner will T into another runner section which than feeds the gate into the part. Balance in this area can be an issue in filling the part properly and resulting part quality.
The gate is similar in function to a diaphragm or ring gate in that it provides for a large flow area and results in a quick fill time. The gate depth is usually less than the edge gate.
From a math stand point a starting thickness for the gate height is 0.7(nT).
T = wall thickness
n = factor from the following list:
|
Materials |
n |
|
PE/PS |
0.6 |
|
POM/PC/PP |
0.7 |
|
PMMA/PA/ |
0.8 |
|
PVC |
0.9 |
Thus in the example of gate height for PE we would end up with a 0.7(0.6 T) or .42T from a reference. If the wall of the part was 0.100" than our gate height would be 0.042".
This formula is a starting point and should be considered just that. In many cases the gate is extremely thin, so as to snap off. At the thickness we have suggested in the example we may need to cut it off, but if we start thinner we can always make the gate area thicker.
The function as explained above is to fill a part quickly but in so doing to spread out the force and rate of fill so as to also not impinge upon the pressures within the part so the resulting part is flat.
In the gate design we still have a land area, which when considering the design and our tool steel may be longer than usual given the thin area between our runner feed and part. The land length should be minimal to 0.050" (1.27mm) and than the height of the gate as determined above. The width is determined after some trial and error in that we choose which side of the part we are going to gate from. The runner parallels the side and the gate is from the edge of the runner to the part. (See example) As would be expected with this gate we may have some premature freeze off and other issue going on depending of the overall feed system design. It is important to envision how the material is flowing through and across our feed system. A short shot study is highly recommended with results being logged for future reference.
The other issue with this type of gate is how we trim it from our part and the cost associated with it. These are things that need be considered.
Though not a commonly used gate it can be beneficial to know of it and the proper design especially when using materials of differential shrinkage, in parts that cannot be center gated.
SLSilvey
