Proper venting procedures can help you eliminate or minimize over 26 of the most common defects. read the article to see how.
Author Douglas Bryce
Venting Can Eliminate or Minimize 23 Defects
Proper venting methods will help reduce your processing costs.

Venting Methods Are Often Overlooked When Building Molds.
In the "old" days, we usually built a mold and then ran it in a sampling run to determine where vents might be needed. We would then cut a vent wherever we discovered a black soot (charring) on the plastic part that was molded. This indicated that air was being trapped and actually compressed by injection pressure to the point of igniting and burning the surrounding plastic. Thus we knew that venting was needed to allow that air to escape instead of burning the plastic. We didn't know any better. This had been the practice at least since the late 1940's in the injection molding industry.

But today we have a much better understanding of how the molten plastic flows through the mold and how trapped air can be present in many areas other than the visually obvious places. In fact, air can be trapped within the flow front itself and get buried within the plastic walls, never to be seen but causing voided areas and physical weakness in those areas.

However, even though our industry knowledge has been expanded to include the understanding for the need of proper venting, many mold makers today are reluctant to cut vents in a new mold without first sampling it to look for those telltale burns. And, even after the vents have been determined this way, the mold will never run properly because the flow fronts are being forced to compress trapped air and this results in many, if not all, of the 23 most common defects we find in plastic parts.

The 23 Most Common Defects
I have listed the 23 most common defects in my book titled "Troubleshooting: A Guide For Injection Molders" (available HERE). They are as follows: black specks, blisters, blush, bowing, brittleness, bubbles (voids), burn marks, clear spots, cloudy appearance, contamination, cracking/crazing, delamination, discoloration, flash, flow lines, gloss (low), jetting, knit lines (welds) nonfill (short shots), shrinkage, sin marks, splay, and warpage.
That's a pretty extensive list and most defects require 1 or more adjustments before they are eliminated or minimized. But only one single adjustment produces a positive result for all 23 defects and that is adjusting the venting.

Vent The mold When It Is Built!

I'm going to go against the grain of most molders and mold makers when I make the statement that the mold should be vented while it is being built, NOT after it has been sampled. Venting is now a science. We can easily predict where venting is needed by using common sense. In many cases, a mold can be built and vented, and then sampled and never require additional mold work. Please read that sentence again. In most situations today the mold is built steel-safe knowing that adjustments will be required after sampling. That is probably a good practice and I won't argue against it (although I could present a pretty good case). But please understand that venting will absolutely change your processing settings. For instance you will use less injection pressure, lower heats, shorter cycles, and lower ejection forces. These are all used to determine the actual dimensional results as well as physical properties of the molded part. So, if you vent the mold before you sample you will get more meaningful results on your first sampling run and not have to keep going back and forth fine tuning it.

So...How Should We Vent?

The concept of venting is simple. Place vents where they do the most good. In my book I explain the actual dimensions and such, but I also tell you that 1 vent should be placed at every inch along the parting line of the parts shape. So, measure along the perimeter of the part. Then figure having a vent at every inch along that perimeter. For example, if you measure 8 inches on the perimeter you should have "at least" 8 vents. Each vent should be between 1/8" and 1/2" wide, and between 0.0005" and 0.002" deep. It should run from the edge of the part all the way out to the edge of the mold( the atmosphere). Extra vents should be placed in blind holes and in cavity corners and on ejector pins. It is safe to say "you can't have too many vents". Exact vent dimensions are dependent upon the plastic being molded (these are covered in my book) with easy-flow materials requiring thinner vents. If you are using surface runners they too should be vented. Any air you get out of the runners won't get into the mold in the first place.
Take a look at your mold and see how many actual vents you have and where they are located. It might surprise you. If you want to see how venting can help you can easily put small pieces of masking tape at various places around the perimeter of the part shape (say every inch or so) and run a cycle or 2. The tape should be placed such that a vent is formed. The tape won't last long of course but you will see immediate improvements in your molded part. It may be flatter, denser, flash-free, crack free, etc, but it will be better.

Now, take a moment to imagine how proper venting can help your bottom line. Your material supplier may be able to help you with venting principles. But if you would like a totally unbiased expert approach please consider my book, "Troublehooting: A Guide For Injection Molders."

Simply go to Troubleshooting for details and order info.