Tips of the trade: Effective shutdowns and start-ups are key to profitable injection molding

Anyone who has experienced a full plant restart can tell you it can either be a smooth or a tragic event. As much as we wish that machines, tools and equipment would just cooperate, this 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 facility. Here are some of the key components that lead to poor starts, as well as what you can do to prevent these factors from negatively affecting 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 shutdown procedures are one of the biggest contributing factors in how effectively your team brings the plant back into a new production workweek. Here are some of the primary focuses in plant shutdown:

1. Material Handling: There are many material-related failures that can cause poor starts. At shutdown, 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 causing unneeded downtime.

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. Mold chillers 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 polycarbonate 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 the screw sucked back to about 1.00 to prevent drooling and to leave the 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 you can 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 inert 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 that team with a list of scrap / downtime issues that occurred during the last shift of the week, so the team's members are fully aware of potential problems.

Press shutdown provides the perfect opportunity for a post-production preventative maintenance inspection. Tiebars, belts, hoses, oil levels, lubrication systems, etc. should be inspected for proper working conditions.

Molders, start your facilities

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: Plastics that require drying are a key consideration to address when returning the floor to production. Material moisture can wreak 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 ensure 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 minutes. Not doing so could lead to damage to 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 thoroughly 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.

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In closing, consistency in your shutdown/ start-up procedures cannot 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 downtime will result. By incorporating standardized procedures in your shutdown, 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.

About the author: A consultant, process & robotics engineer, scientific/ decoupled molding trainer and founder of Plastics411, Garrett MacKenzie has spent more than 25 years in plastics processing including experience with U.S. and Japanese automotive OEMs, Johnson Controls, Summit Polymers, Stanley Electric and more. Contact him at mailto:[email protected].