One of the trickiest materials to mold with is Nylon. The material itself has great molded in properties, such as strength and when used properly visual aesthetics. Using the material itself, however can sometimes require a strong molding approach to prevent molding variances and defects. Nylon processing does not have to be frustrating. Applying some basic molding rules to nylon can help to prevent processing headaches.
This article will address some common problems that occur in nylon molding. It will identify many of the normal molding conditions that are common with this material, as well as provide some insight into processing approach that can help to reduce, and even eliminate scrap events.
Gassing One of the most common problems with nylon molding is gassing. Gas causes such defects as shorts/ unfill, poor/ chalky appearance and poor filling in rib/ detail areas. There are several reasons that gassing can occur. First, the higher the barrel temperature is, the more gas will be produced. There are things to consider when trying to correct a gassing condition. First would be the initial process set up. Always start at the lower end of the melt window as established by the material supplier.
A processor’s goal should always be to keep the heats as low as possible. Higher temperatures reduce gas. However, it should also be noted that a temperature that is too low can reduce flow front performance leading to shear and burns or unfill. Take the time to find the appropriate temperature that allows nice flow with reduced gassing. In addition, verify that gas has ample opportunity for escape through venting.
Poor venting and venting approaches can lead to major headaches. End of fill venting is imperative to assure process control. When vented pins are used, the mold will need to be analyzed historically to develop a strong preventative maintenance approach. It is also important to monitor gassed vents, to determine the appropriate mold cleaning schedule. The general rule is that a mold running nylon should be cleaned once per shift, but some might require more frequent servicing.
Barrel throughput is another variable that can be considered when gassing is an issue. The longer nylon stays in the barrel, the more prone it is to producing gas. In some cases, the process might require lower temperatures in the rear zones of the barrel. It can become a tricky situation, however. If barrel temperatures in rear are too low, you can expect increased shear in the metering section. This of course can cause splay. Use car to reduce barrel temperature, while maintaining proper shear to provide proper melt and mixing.
Moisture Moisture is another key to nylon production. Poor drying can cause gassing and other potential scrap-producing defects. It is important to note that of all material, nylon is the most hygroscopic and can quickly take in moisture. When starting a press, it is good practice to drain at least 25 pounds material from the dryer cone to assure that the material being introduced to the loader is sufficiently dry. In addition, make sure the loader line and loader receptacle have been cleared of material that has sat for any time period over 5 minutes.
Purge the barrel amply, to allow the barrel time to heat soak. Learn what appearance the purge puddle should have to identify that the material is ready for injection. In addition, it is important to note that nylon is not only affected by poor drying/ moisture conditions.
Over drying can also lead to multiple defect conditions. Pay attention to long-term drying occurrences, and bank the dryer appropriately to prevent over drying. Material that is overbaked tends to take on a yellowed appearance. If defects such as splay or poor appearance occur, Dry time and dryer heat should be reviewed and considered as suspect.
Shrinkage Nylon is one material that is more prone to shrinkage. This can affect dimensional properties. In addition, many nylons are semi- crystalline. Increased or poor shrinkage can result in reduced strength, poor color and warp. Shrinkage needs to be controlled through barrel and mold temperatures. It is important to remember that any added heat increases cycle time. In some cases, that increase is essential to improve part properties.
It is also important to note that although the general rule that part shrinkage is achieved within one hour, there are occasions where nylon stasis might require up to 24 hours for shrinkage to be complete. Nylon can be a very temperamental material, leading to frustration, high scrap and low efficiencies.
With the proper handling, care and approach nylon is capable of the same consistencies of materials that require less maintenance. Taking a strong approach through tool design, proper material handling procedures and concentrated processing set up are the best approaches to assure successful nylon molding.. and a company’s success and profitability.
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