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Injection mold venting

I would like to share an old story about injection mold venting. Years back mold makers kept a Perfume Atomizer on their bench. This was the old style with a rubber bulb that you squeezed. It however was filled with Kerosene. Prior to adding venting channels to a new injection mold, the mold maker (at the press) would squirt the atomized Kerosene in the cavity area just prior to closure and subsequent shot. When the mold opened for inspection it showed the burn marks at each spot where the last fill spots resulted in "Dieseling", thus identifying the best placement for adding vents.
Injection mold venting
Today we have the software that will identify flow patterns and point out the ideal vent positions. However, once a flow front passes a vent it becomes useless and I can't count the times that I have had issues with molds that had vents in the wrong place. This is why I always insist on running a "Short Shot" series of parts to validate the flow pattern and vent locations. I make it a point to keep these Short Shots stored for reference should any issues with that mold show up at a later date, sometimes minor mold revisions change the flow pattern and the venting efficiency.

Perhaps I'm just in the mood to share tooling tips this morning and the subject of "Venting" is a key contributor to many molding issues. For the past 25 years I have been an advocate of Vacuum Cavity evacuation-----and I've heard many Nay-Sayers Pooh-Pooh this process enhancement only due to their lack of understanding. The features required in the injection mold as venting channels are quite simplistic and don't add any significant cost to the mold build. I have resolved many cosmetic issues by using vacuum and have shown A to B dynamic improvements that have changed people's minds on this. Getting back to basic venting (to shop atmosphere) I focus on the design of vent channels. Aside from placement, what size (Depth and Width) do you specify? Depth is typically a function of the material (resin viscosity) used, and for many resins like Nylons, the vent depth is very shallow, perhaps .0002/3 " deep as featured on a parting line surface. Depending on the actual "Shut Off" area on the Core and Cavity Blocks which are standing "Proud" above the actual Mold Base and considering the "Clamping" force being exerted on that surface area, I can assure you that in most cases that after the mold sees the first 1000 cycles the vent depth will be significantly reduced (if not gone) due to the break-in or "hobbing" of the steel and certainly more problematic when using softer materials such as used in lower cost injection molding. This early break-in always requires a touch-up to refresh the vents to proper depth. It is quite common for vents to be Ground or EDM'd to the same depth for rather long distances and I have found that it is a significant advantage to maintain the vent depth for a limited distance from the actual parting line, perhaps .160"(4mm) and then having an angled falloff to perhaps .040"(1mm) depth for the remaining length of the vent channel. This featuring of the channel eases any resistance to the air/gas flow and keeps the mold cleaner, thus the time between "In Press" cleanings becomes greater. The Width of vents must also be considered. I would consider widths less than .375" (10mm) to be inadequate for having the ability to capture the air/gas in considering processing variations that may shift the perfect point of vent location, many injection molds having narrow vents do not perform as expected.
hi how are you.i am aware that vent should always placed opposite of the gate.However do u have any certain formula to calculate the vent depth and width based on the cavity size or based on the gate opening.Thanks hope to hear from you soon.

Sincerely,
Engineer
- - - -> by: jeeven

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