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Plastic Injection: Effective Shut-Downs / Start-Ups Are a Key to Profitability

When the weekend ends, and the shop floor is once again revived.. a molder can learn to dread a full plant restart. There are many potential failures that can occur as the plant is returned to a production state. The following article addresses many key components that often lead to poor restarts. It also provides insight into some of the methods that can be implemented to not only reduce system breakdowns.. but can also offer smoother start-ups with fewer problems…

Anyone who has experienced a full plant restart can tell you.. it can either be a smooth or tragic event. As much as we wish that machines, tools and equipment would just cooperate.. it simply is not always the case. It is important to understand the key components of start up to be able to counter with fail-safe procedures on restart. Rushing through start-up often leads to failures within the plant system. Here are some of the key components that lead to poor starts, as well as what you can do to prevent these factors from effecting your shop:

Shutdown: One of the first key considerations to plant restart is shut down. Shutting your shop down is best viewed as preparation for plant start-up. Your facility’s shut-down procedures are one of the biggest contributing factors in how effectively your team brings the plant back into a new production work week. Here are some of the primary focuses in plant shutdown:

1.Material Handling: There are many material-related failures that can cause poor starts. Make sure your gaylords and material containers have been covered to prevent contamination, and excessive exposure to moisture. Dried material in hoppers or one-shots above the press should be removed in anticipation of the restart, and to prevent wet material from being a start up concern. As the press is shut down, run the barrel dry. This prevents material from bridging in the feedthroat of the press, and unneeded downtime. It is recommended that if dryers are to be left on over the weekend.. their temperatures should be banked (lowered) to help prevent overdrying.

2.Tooling: Molds are a key focus.. improper shutdowns can be a primary reason for poor start ups. Molds on chiller that are prone to sweating and should be cycled 5-10 minutes prior to full shutdown. This helps to reduce sweating, a primary cause of rust. Tooling should be cleaned and sprayed with rust preventative, and moving components should be inspected for wear and proper grease. When working with clear polycarb or light colored materials and a hot runner mold, it is good practice to shut the hot runner off and allow the machine to cycle until the mold no longer produces parts. This keeps material from baking in the manifold, and reduces contamination at start up. Mold should be left with mold halves touching (not under pressure) to protect the mold faces. Look for signs of water leaks on top of the mold. If they exist, fix the leak and soak up water to prevent it from getting inside the mold. Main water should be shut off and relieve the pressure from all water circuits.

3.Press Side: The barrel should be left empty, and screw sucked back to about 1.00 to prevent drooling and to leave screw in a neutral state. As mentioned above, mold halves should be left touching to prevent exposure to environmental exposure to moisture while in stasis. Check the bushing, purge tray and press bed for material to prevent unneeded downtime during start up. The production area should be left clean, and ready for the team responsible for beginning the production week. Heats should either be banked at 300F, or a perform a complete press power down.. depending on your facility’s start up approach. Materials prone to degradation (such as nylon, acetal, ABS, etc.) should be purged out using a purging compound or an innert material such as polypropylene. Verify that drying sources have either been set to banked (reduced) temperatures or have been powered down. Thermolators should be turned off, and the pressures on them released. Make sure all auxiliary equipment is turned off, and that the production area has been prepared for the start up team’s duties. Be sure to provide their team with a list of scrap/ downtime issues that occurred during the last shift of the week, so they are fully aware of potential problems. Press shut down provides the perfect opportunity for a post-production preventative maintenance inspection. Tie bars, belts, hoses, oil levels, lubrication systems, etc. should be inspected for proper working conditions.

Start Up: The key to a successful plant start-up is preparation. Clean starts are the result of careful planning and prudent procedures that prevent problematic production restarts. The following section addresses some of the primary focuses the start up team should address prior to and during the start up event:

1.Dryers: Polymers that require drying are a key consideration to address when returning the floor to production. Material moisture can reap havoc on a start up team’s goal of low scrap and fast and efficient restarts. If the dryer temperatures were reduced to a bank temp, then less drying time would be required to assure that material moisture is at an adequate state. Moisture content can be quickly and easily verified using a moisture analyzer. In addition, the first 25# in the hopper should be drained due to poor circulation of heated air to the bottom of the dryer cone. As the material is drained, hold your hand in the material exiting and drain until it is consistently warm to the touch. Do not load to the feed throat until just before you begin production.

2.Molds: Inspect the mold for signs of abnormal conditions or rust. Cycle the clamp a few times to inspect for proper operation and clamp set up. Fully clean the mold and inspect any moving mechanisms or contact points to assure they are properly lubricated. If the mold is normally heated, then turn the water and thermolators on.. allowing the tool to heat soak with mold halves touching for about 15 minutes to assure the mold faces are consistent in temperature. Inspect water hook ups for signs of leaking, and repair these to prevent moisture from being a cause of scrap. Also inspect core lines for excessive wear and/ or leaks. Hot runner molds should be preheated and allowed to heat soak. Check all water valves to assure they are in the “open” position and feel lines for turbulent flow..

3.Screw & Barrel: Make sure that the barrel has been brought up to temperature, and once temps have been achieved heat soak the barrel for 20 mins. Not doing so could lead to damaging the screw/ barrel assembly. Review each temperature zone to assure that heater bands are working properly and each zone is at the required temperature. The screw should be purged thouroughly prior to beginning the first cycle to assure that all purge compound and degraded material has been removed and that fresh material is what you will be shooting into the mold.

In closing, consistency in your shutdown/ start-up procedures can not be stressed enough. All individuals assigned to these tasks should be trained to perform these duties uniformly. The more variance there is from press to press (or even job to job).. the more likely that poor efficiencies and down time will result. By incorporating standardized procedures in your shut down, you improve your capability of solid starts. Your start ups require a lot less thought and are stronger.. with less scrap and a much smoother return to full production.