Definition: Vent: 1. to exhaust unwanted air or fluid from an area. 2. A passageway between the mold cavity and outside edge of the mold face which allows air to escape as molten plastic is injected into the cavity.
How many vent does one need? Where to place the vents?
The answer to the second question above is not "put the tool in and test shot and where the plastic burns or does not fill we well place a vent there!" if in fact as I recently read if a tool maker actually states this to you start looking for a new one. While correct in one way, is the tool maker paying you to test shot the mold and all the time and effort this is going to take? The question of course is "were vents specified in the tool drawing, and who created the tool drawing?"
In answer to the first question it comes down to a few other questions? Hot runner or cold runner and is the cold runner vented? If only a hot runner and or a cold runner with vents than approximately 20% of the perimeter of the cavity should be vented. If in fact it is a cold runner with no vents on the runner than 30% of the perimeter should be vented.
Why vent the runner? The reason being is that it allows air trapped in the runner to be exhausted prior to being forced into the mold cavity and then having this extra air to exhaust from the cavity. Further by venting a runner, one may use a deeper vent that may even flash a little down the vent as it does not affect the part or runner performance. In some cases the author has noted that just by venting the runner, the issues with venting in the tool cavity had been resolved. Yes not all the air in the runner is exhausted through these vents, but when one calculates the volume of the air in a cold runner system it can sometimes exceed the volume of air in the actual cavity, thus creating issues.
In placing vents on the cavity, as explained above the total length of the perimeter of the cavity should be calculated and then multiplied by 0.2 or 0.3 (20% / 30% respectively) dependent on type of system and other vents. This than results in a total vent area and then when divided by the actual vent widths results in number of vents of say 3mm width (0.125") that are necessary.
Example:
Perimeter of part is 300mm (11.91")
300mm x 0.2 = 60mm (2.38")
60mm / 3mm = 20
Thus there is a need for 20 vents at a width of 3mm for this part if it is a hot runner filled and or the runner is vented.
The placing of the vents should be started at end of fill, critical areas, last place to fill and areas which if one uses a mold filling software are easy to see, and if one uses experience of how the plastic flows, it becomes apparent where to place.
The other area for vents is on the back side of flow around pins and opening in the part. Since the plastics flows in a parabolic flow pattern as it goes around a pin for example the knit or meld line that forms usually creates a void at the junction of the pin / open area and meld /weld point that than back fills to the pin and or opening form. Since the plastic has sealed the cavity and trapped air in the mold as it continues to fill this air has to escape, thus these areas should be vented. If a pin or insert it becomes easy as a vent is cut into pin or insert and located at this point of meld. If a solid with the tool than it can be difficult but a possible solution is to now place a vent at the shutoff areas and vent through the form if it allows it.
The example of a part above shows the following:
Gate area: this is where the gate is and from this one can than determining the flow of material up and around the part to fill all areas.
Vent area: these are the obvious places to install a vent. The one area left out which is important is the top of the part between the two radiuses' which are indicated at having a vent.
A : this is an inserted pin, thus it can be vented, and due to the large area C which is cored out the flow of plastic may not be in direct line with the gate but more to the inside of the part.
B: this is an insert which again can and should be vented and most likely this would be to the inside toward the center line of the part.
C: This area was created by milling around it thus this area was created not by an insert but by cutting away steel which creates the cavity. Thus the venting would be on the parting line on the face of the mold, and would now have to exhaust down a hole, drilled into this face area and out, or through a pin.
The part is somewhat complex though simple, and it shows that venting has to be thought out, and though some may overlook venting on the inside of the part in areas, A,B, C in the long run these areas would than become issues with cosmetics, and or breakage points.
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