Out of gas

10-12-18 Mould gas trap

Gas and air entrapment can cause a variety of problems in injection molding, including part burns, short shots, voids and blemishes, even clogged vents. Vacuum venting can solve all these problems, and increase output in the molding process.

Proper venting is essential to molding a defect-free part. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. Trapped gas is one of the most common causes of part burns, and can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. Moreover, trapped gas can cause residue buildup in vented pins, which then necessitates frequent production interruptions to clean the tool. It can even cause corrosion of the tool steel, thereby increasing tool maintenance costs.
Most injection molders know that, of course. But what do you do when conventional venting methods just don’t work?
The most common response to the problem is to slow down first-stage fill time to provide more time for the air or gas to dissipate. But that can be an expensive proposition. If you have to increase your first stage fill time from 1 second to 1.5 seconds, what is the cost of that additional cycle time?
There is a better way. Vacuum venting can alleviate all these problems by instantaneously evacuating air and gas from the mold cavity as it is filled. There are companies that can provide that. Moreover, proprietary technology may also include a “blowback” circuit that purges vented pins to reduce maintenance on the mold.


What Parts and Materials Require Vacuum Venting?

Sustems can be used on a wide range of thermoplastic and thermoset materials including ABS, acetyl, nylon, PEI and PEEK, PPO, PP, phenolics and more. But, the need for vacuum venting isn’t nearly as much about the material as it is about the requirement to consistently produce high-quality parts when outgassing or air entrapment is a problem.
Where vacuum venting shines most is when part quality requirements are stringent and molders don’t want to sacrifice processing speed to meet those standards. High-return applications include:
• Engineered resins such as glass-filled nylons, glass-filled PBT and acetals
• High-speed injection of thin-walled parts
• Optical parts such automotive lighting, light pipes and lenses
• Medical applications where part clarity is important and “air bubbles” are common.

Does Vacuum Venting Require Tooling Modifications?

If the problem is gas or air entrapment the tool will require minor modifications to properly configure the vacuum venting channel.
With a parting line installation, an O-ring and chimney are cut around the cavities. The O-ring seals the closed mold to help maintain the vacuum, and the chimney creates the passage for air or gas to exit the cavity.
On long, thin wall sections with an insert, it’s often best to vent as close to the bottom of the insert as possible. In such case, it’s recommended that insert walls in contact with the mold base be coated with grease or RTV silicone to prevent vacuum leakage.
Another method is to vent through the ejector pins of a mold. To maintain maximum vacuum, it may be necessary to seal the ejector pins with a heat resistant Viton seal.

Will such solution, improve every injection molding process? Not necessarily, especially if a process problem is not related to air or gas entrapment. But where outgassing is a problem, vacuum venting is an extremely effective—and cost-effective—way to consistently deliver high-quality parts without sacrificing cycle times and with fewer process interruptions for mold maintenance. That’s technology that is very easy to justify.


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Source PT


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