by Steve Johnson, ToolingDocs
As overlooked as vents can be in the design phase of molds, the ongoing condition of vents also typically is ignored – just as much as the lowly water line when molds get into the production environment. And, just like the poor water line, vent integrity usually only comes under a spotlight after it begins to cause part or processing issues.
In a perfect world, perfectly placed vents would never get hobbed (typed) in or washed out and the depth always would remain at the original specification. But, in the real world, mold vents take a pounding from being slammed shut, hobbed in from excess tonnage due to incorrect set-up parameters or run in presses too large for the job. Or, they become washed out from abrasive resin off-gassing, overzealous use of Scotch-Brite or the application of other cleaning methods using hand stones, sandpaper and glass bead blasting. Tenths of a thousandth of an inch can affect the performance of mold vents.
Another consideration is the finish in the vent. Many processors feel vents need to be polished to an SPI A3 finish to allow the compressed air to escape more easily and to make the vent “self-cleaning” by allowing the vent residue to exit into the vent dumps instead of sticking in the vent itself. While I can’t attest to ever measuring any decrease in the cavity pressure during filling of cavities with polished vs. unpolished vents, I can attest to the residue not loading up as quickly in polished vents, and this allows molds to run longer in between PMs. Polishing vents as an afterthought – especially small ones – is a task best left to skilled technicians as it is easy to roll critical edges and take the vents too deep.
Keeping vents fresh and breathing easily relies on an ability to adequately measure the depths at specific periodic frequencies. While not something that needs to be performed after every 30-, 60- or 90-day run, vent integrity should be validated before burning and inconsistent backpressure issues are allowed to surface.
Monitoring of vent depths should be performed using a tenth indicator and base on a surface plate, and the results mapped by cavity and vent location. Parting line vents being crushed in specific locations can point to unequal pressure being applied by out of square or hobbed press platens and other tooling stack issues that can affect mold face shut-offs.
Vent residue buildup between cores, sleeves and other dynamic tooling, as well as internal lubrication levels, is one of the prime considerations when determining PM schedules for molds and should always be noted before mold tooling is cleaned. Vent dumps that fill up will force the gas/residue into areas where it shouldn’t go, such as between a core and sleeve or other moving and close-fitting tooling. This becomes a big problem simply because typical residue is gummy and has been the root cause of many molds galling up.
As usual, all the mechanical and physics wizardry in the world won’t work for long without a solid maintenance plan that keeps tabs on the critical elements of molds being run.
Steve Johnson is the operations manager for ToolingDocs, a provider of maintenance training products and services. His more than 35 years in the tooling industry includes eight years as senior tooling engineer for Abbott Laboratories and 24 years as a toolmaker at Calmar Inc., repairing and rebuilding high cavitation, close-tolerance, multi-cavity molds. Johnson also has designed and developed MoldTrax, a leading documentation software system for tracking mold performance and maintenance, and he authors articles for several plastics industry magazines. For more information, call 419.281.0790 or email [email protected].