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For good part quality, you’ve got to have good mold venting
Just like Yellowstone’s Old Faithful, molds need to blow off steam. When questions about nonconforming part quality arise, often the answers can be found in inadequate mold venting. Even if the mold is vented, a lack of good venting can contribute to a multitude of sins regarding part quality, including weldlines, flow lines, bubbles, waviness, sinks or what appears to be sinks, burning, and dimensional stability, to name a few. It often seems there is no apparent reason for these quality issues in the cavity and core steel.
October 8, 2009
6 Min Read
Just like Yellowstone’s Old Faithful, molds need to blow off steam. When questions about nonconforming part quality arise, often the answers can be found in inadequate mold venting. Even if the mold is vented, a lack of good venting can contribute to a multitude of sins regarding part quality, including weldlines, flow lines, bubbles, waviness, sinks or what appears to be sinks, burning, and dimensional stability, to name a few. It often seems there is no apparent reason for these quality issues in the cavity and core steel. That’s when a close examination of the venting might be beneficial.â€¨â€¨
Problems of filling and obtaining uniform part quality lead some processors to resort to tweaking the molding process, making changes to the various processing parameters in an effort to find the problem. If that fails to produce the desired results, they often fall back on an old remedy: Increase the processing temperature to reduce melt viscosity. This strategy may help, but may also result in a longer cycle time to get the extra heat out of the part. More importantly, this approach may be addressing only the symptom and not the root cause.â€¨â€¨
People are frequently surprised to learn that many part quality issues can be traced to inadequate venting of the mold. Plastic cannot flow into the cavity unless and until the gas/air in the cavity is able to escape. The time required to fill the cavities and the pressure to do so are quite often controlled by the ability to get the gas/air out. Although it is not generally recognized, improving the venting of the mold will promote a faster fill rate at lower injection pressure, which translates into better part quality at an improved cycle.â€¨â€¨
Most molders assume that the moldmaker has provided proper venting of the mold. This may be true, but moldmakers are often very conservative when it comes to venting, due largely to the fact that if the moldmaker gets too aggressive with the primary vent depth and the mold flashes, the moldmaker has a major problem on his hands. This problem is one that is not easily remedied in a timely or cost-effective manner.â€¨â€¨For example, if the design calls for 0.0006 inch ±0.0002 inch on the primary vent depth, the moldmaker will usually try to hit a number between 0.0004 and 0.0005 inch. Rarely do you find a moldmaker getting aggressive on the primary vent for the reasons mentioned above. But the actual primary vent depth is only one area to consider when determining adequate venting. There are actually four critical areas to consider when evaluating the effectiveness of the mold’s venting:â€¨
• The primary ventâ€¨
• The vent land (the area between the cavity and the start of the secondary channel)
• The depth and size of the channels that allow the gas/air to escape from the mold
• Adequate projected surface on the parting line so as to prevent hobbing, which will ultimately reduce the ability to vent adequately
If any one of these areas is inadequate or neglected, nonconforming parts could be the result.
Taking each of these critical areas one at a time, we can examine the function and the required steps to provide optimum venting. The primary vent is the area adjacent to the cavity, and is the area through which the gas/air flows as the plastic fills the cavity. This is very important because if the primary vent is too small or too shallow, it will prevent adequate venting. If the vent is too deep, flashing will result. â€¨â€¨
The vent land should be long enough to be robust but not too long, as this will increase the backpressure and offer more resistance to venting the gas/air out of the cavity. Next, beyond the vent land should be adequate, generous channels that lead out to the atmosphere. Think of these channels as a river and just as a river flows through its channel to reach the ocean, all vent channels need to be routed through the mold’s complex maze to the outside environment.
Additionally, as two rivers flow together to become one larger river, so should the channels in the mold become larger to accommodate the increased gas/air flow. If the length and size of the vent channel are not taken into consideration, there is little chance that the gas/air can flow out of the cavities effectively. Simply stated, once the gas/air passes through the vents at the primary vent land, the vents should increase in size sufficiently to allow the gas/air to be released into the atmosphere without backpressure. â€¨â€¨
Increasing the vents at the parting line is one way some processors try to overcome insufficient venting. However, caution should be taken as this method may make the problem worse long term, even creating a situation in which much of the surface area is removed that supports the tonnage. This could result in hobbing of the parting lines and actually make quality problems worse.
If problems occur, first examine the mold and the mold drawings carefully to determine the source of the difficulty. Check to see if the primary vents are deep enough to allow the gas/air to escape the cavities, or to confirm that the vents are actually there. Use a toolmaker’s 0.0001 indicator in those areas that are intended to provide venting passages to see if they are touching when the mold is closed and thus shutting off the primary vent passages. Next, thoroughly check the other critical areas discussed above to ensure the vents are working correctly.â€¨â€¨
Poor part quality could be nothing more than trapped gas. When plastic experiences backpressure during the filling process, it does not flow into the cavity in a uniform manner. Pockets of trapped gas/air slow the fill in some areas and accelerate it in others, resulting in weldlines, waviness, dimensional stability issues, or, in some cases, burning.
When experiencing part quality problems, I recommend a thorough examination of the vents before changing processing parameters, such as increasing the processing temperature. Improper venting is often the root of the problem. â€¨â€¨
Vince Lomax is VP of Tech Mold Inc. (Tempe, AZ), an independent, privately held mold manufacturer specializing in high-cavitation, precision molds for Fortune 100 and 500 companies in the medical disposables, consumer products, packaging, and pharmaceutical markets. Tech Mold offers mold design and developmental tooling through its R&D facility: fully designed, high-volume production tooling, and integrated manufacturing cells in turnkey operations for its global customers.
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