The Designer's Edge: More on Venting

This is not the prefered method of parting line venting, as it takes away too much parting line bearing surface.

There are industry specs for vent depth in relation to the plastic material being used. These specs are a starting point, but dependent upon the process, flow lengths, wall stock and part design. For example, PC-ABS materials require tools with vent depths that cannot exceed 0.0007 because of part design and plastic pressure. These are thinner-walled parts with longer-flow lengths. To maximize venting, some parts can go 0.002 deep when using that same material.

This is important, as some people like to use industry specs per the material as the standard. Also, there are cases in which vents flash. And not because they are too deep, but as a result of tool deflection from the lack of support or support pillar pre-load.

There are times when venting is critical and maximum depth is required. For this,  I've had to deepen the vents to a point where they flash and then weld them up to the maximum depth so they would not flash.

Vent land is another area on which my opinion has changed since working in the tool shop.  I used to think that if the vent land was too thin, it created a weak condition on the tool. My starting point for vent land is 0.080 maximum, but when maximized venting is needed I go down to as little as 0.030 - 0.040 land.

After years of using this method, I have not experienced any disadvantages. Some people think you are at risk for flash, but the opposite can be true with venting. I have had situations where too much land results in inadequate venting or vents plugging up too quickly, which causes erosion and what appears to be flash.  I have many examples of thinner lands reducing erosion because they allow gases to escape and vents to remain free from buildup.

My view on specs for parting line vent design has changed also.The most common standard was running vent tracks the width of the cutter every two to three inches around the part. This would take away too much parting line bearing surface and does not provide adequate venting. Knit lines can move around at times and if the vent track is not wide enough, you can lose venting. I prefer to run vent track two to three inches wide along the part with one exiting track. I am also a proponent of peripheral venting.

My June 12 blog will cover parting line vents and parting line bearing surface.

For more on this and other Designer's Edge topics, catch up with Randy the Amerimold Expo on June 17 for his talk on "Overcoming Repair and Part Quality Challenges with Design." Click here to register to attend.