Cost Savings Solutions

Shutdown & Sealing Purging Compounds

It’s time to shut down for the weekend. What do you do? Many processors simply empty the screw and barrel, turn off the heaters, and go home for the weekend. Upon starting back up Monday morning, scrap rates are high, and it seems to take forever to get into production. This is a costly mistake. We’ve been there, and we know how frustrating this can be. We’re here to help. This section explains how to have the easiest start-ups possible.

Difficulty Starting Up After Shutdowns

As companies cut costs, the ability to quickly shutdown or startup a line becomes more and more significant. In response to shifts in demand and general pressure to cut labor, many traditionally 24/7 extruders are now shutting down parts or all their production over weekends and holidays. Most thermoplastic resins are likely to degrade and cause product contamination when left in the barrel for even a short period of time. Running engineering or super-engineering resins raises the likelihood of this occurring. Degradation will even occur if resin is left in the barrel when the heats are shut off – there is simply no effective way to cool that resin down quickly enough to avoid degradation (see ABS example below).

Sealing with Asaclean® has kept our machines in great shape and start-ups from an extended shutdown are a breeze.”

Rodney Davenport , Vice President, CH3 Solutions, Injection Molding, Georgia

The issue here is the misconception that shutting heat off means the resin will just cool down and solidify harmlessly. The truth is that it can take hours for the plastic in the barrel to cool off enough to stop degrading. Over that time, carbon is forming in the barrel and adhering to any metal surface it can find.

abs-over-60mins-880x235

Look at the above photos. We took natural ABS and subjected it to 460° F heat for 60 min. The first image shows the natural ABS at the start. The next one shows the material after 15 min under heat, then 30 min, then 60 min. These images show just how quickly a resin can oxidize and begin turning to carbon. If you were to simply shut the heats off with ABS in the barrel on Friday evening, you would be greeted by significant carbon buildup on Monday morning. To avoid this—and the accompanying losses in efficiencies at startup—seal the screw and barrel with Asaclean® prior to shutting down.

The Benefits of Sealing with Purge During Shutdowns

The term “sealing” refers to using a heat-stable purging compound like Asaclean® during shutdowns and extended downtime events like preventive maintenance (PM) or mold changes. The heat-sensitive resin is displaced by a purging compound that keeps oxygen from entering the screw as the purge cools. At startup, simply run another barrel full of purge through the system and change to the next resin. Before you know it, you will be running good parts. This video shows you in clear detail the difference between sealing with resin and sealing with a thermally-stable grade of Asaclean®:

Sealing with U Grade reduced our start-up time by 60%... Asaclean® Purging Compounds has raised the bar for all purging compounds.”

Mike N. , Process Technician, Injection Molding, Consumer Goods, North Carolina

General Asaclean® Purging Compound Sealing Best Practice for Injection Molding & Extrusion

Injection Molding

Sealing an injection unit with Asaclean® is normally a pretty simple process. The operator should keep the barrel full of production resin to start. Wipe down the hopper and add the appropriate amount of purging compound. When using mechanical purges, it is standard to run the screw slowly until the purge begins exiting the nozzle. Then, crank up the back pressure and screw speed to their respective safe maximum values. Once the purge pile is free of contamination, immediately turn the heats off. The barrel must be left completely full of purging compound so that no air gets into the barrel, as air will cause oxidation, and contamination will shortly follow. Upon startup, get the heats up to operating temperatures. Once the purge is adequately heat-soaked, add production resin, and purge out the purging compound. Once the purge is free of purging agent, begin production contaminant free.

Extrusion

To seal an extruder with Asaclean®, first, purge out the production resin. Increase die temperatures while staying within the safe operating range. Wipe out the hopper and remove the screen pack if using a glass-filled grade. Using a thermally-stable, non-filled purge compound, purge the extruder (per the instructions above) until the purge pile is free of contamination and previous resin. Once clean, slow the screw down to make sure the flights are as full as possible. Then, shut the heats off. When ready to start back up, turn the heats on and purge the purging agent out with the next production resin.

Work with an Asaclean® Purging Expert to develop the best plan for your plant’s shutdown. Be sure to follow the proper sealing instructions for injection molding and extrusion. Never seal with a glass-filled grade of Asaclean® and be mindful of gate/die clearances. Always plug your vents during shutdown for optimum performance. Remember that all purging compounds are different, and it is critical to match the application with the proper type and grade of purging agent. It is also extremely critical to find a purge that works well at the production resin processing temperature to avoid degradation and long purging times.

6 Bonus Purging Tips to Make the Most of Your Shutdown

We know that shutdowns have a major impact on your productivity and many customers ask us for suggestions on how to make the week before shutdowns run more smoothly.

These tips are a good resource for anyone looking to get an edge on the competition and supercharge cost savings during the week before you shut down. These will help you get more machine run-time before shutting down. This will alleviate some pressure on the shift that starts back up.

1. Take your top priority machines and schedule manpower to relieve the operators during breaks and lunches all week and across all shifts. If they don't get shut down during the week for breaks and lunches, you gain a shift of production by Friday.

2. If you run certain machines on certain shifts, then consider having your day shift begin the shutdown late Friday. Day shifts usually have more support staff versus night shifts.

3. With the help of your process techs, mold setters, & material handlers, determine how long it takes for each individual machine to run out of material if the loader is shut off. Then pick the priority of order for all machines to run out.

4. Make sure there are copies of instructions placed on a clipboard at the machine for reference for any special machines that have very specific shutdown instructions. Proper procedures are essential to your success.

5. Don't be afraid to involve more people with your shutdown tasks. Most operators want to move up in the company, learn, and grow. The more people you teach how to do a great shutdown, the better the shutdown will be.

6. Change which shift shuts down and starts-up equally rotating until each shift has completed both. This lets the shift that normally starts the plant up see firsthand what it takes to shut down the plant. This is a big suggestion that often does not get considered. Many good ideas for improvement can be found by doing this. It promotes teamwork and respect between the shifts and shows the company equally values day to day and shift to shift operations.