1) Keep Track of Processing Data: To really understand what your purging costs are, you need to measure all aspects of a purge. Measuring only the cost of the purge compound or scrap gives you only a part of the whole cost. To calculate the true cost of purging, keep the following in mind:
Good part to good part: The time a purge takes is not the time required to run the purging agent through your machine. It is the time it takes to go from your last ‘good part’ on your previous run, to your first ‘good part’ on the subsequent run. Of course, if your new run is interrupted by black specks or color streaks that were not removed from the prior run, this time and scrap must also be taken into consideration when calculating purging times.
Weigh your purge patties: While you can probably ‘eye-ball’ the purge piles to determine whether there is more or less scrap than before, this estimate cannot be translated into dollars. Unless you can calculate the true cost of your purge (which requires accurate measures of purging compound used and scrap created) you will never know how much money you are saving, if any at all. Even relatively small differences in time and pounds can mean thousands of dollars saved (or lost!) over the course of a year.
- Consider lost opportunity: The cost of the purge should also include the profit your company would have made if it had been producing good parts instead of purging. Two hours of downtime is not only a cost in terms of machine running costs and scrapped resin cost. It could be hundreds or thousands of dollars of profit lost as well.
2) Follow directions: Your purging agent supplier spends considerable time and money developing instructions to maximize your savings. While you are likely to realize savings without strict adherence to your manufacturer’s recommendations, you will almost certainly benefit even more by following instructions. Once you have become familiar with the proper use of your purge compound, you may benefit by trying to increase your savings by experimenting with using less purge compound, decreasing soak time, etc. With this trial and error, you may be able to improve upon your supplier’s instructions. However, you are strongly encouraged to start by following instructions.
3) Preventative maintenance approach: Most of us only use purging compound when the need is very clear, such as on difficult color or material changes, appearance of black specks, etc. However, in some cases, you might save considerable time in the long run by adopting a proactive approach to purging. For example, if you are working with a resin that is prone to degradation, such as PVC, and you have long production runs, you might benefit from purging mid-run. If you remove the layers that build up in your machine before they degrade into carbon, you will greatly shorten your downtime later due to cleaning. You may even avoid the necessity of a screw pull.
4) Shutdown and sealing: Most purge compound manufacturers provide grades that can be used to seal your machine when not in use. This can be a huge time and money saver if you have difficult start-ups. Before an extended shut-down, purge your machine according your supplier’s instructions. Once the machine is clean, turn down or turn off your heats while the machine is still full of the purging compound. Having the machine full of the appropriate grade of purging compound will prevent oxygen from entering the machine. This will greatly reduce the opportunity for carbon to form as the machine cools. (Note: Glass-filled purging compounds are not recommended for sealing. Please consult your supplier for more information).
Before startup, turn the heats back up to completely melt the purging compound inside the machine. Remove the purge agent according to your supplier’s recommendations, and start production without black specks or any other contamination. This can also be done with hot runners and when storing molds.
5) Hot runners: Given the cost of hot runner systems, it is understandable that many processors are reluctant to use a purge compound in them. However, most manifold makers specifically recommend commercial purging compounds for cleaning their hot runners. Consult your hot runner builder, as well as your purge supplier for the right grade for your application.
“Closed mold”: if your purging compound allows, it is very effective to simply mold parts. This reduces time and often improves the cleaning power of the purge compound.
“Open mold”: If you cannot mold the purging compound for some reason, you can still clean the manifolds by simply keeping the mold open. Of course this option is not available with every type of mold, and requires a safe method to remove the purge pile.
- Shutting valves: if you have a multi-cavity mold and can shut the valves individually, you can help increase the cleaning power of most purge compounds by shutting some of the valves, and purging through the others. (Be sure to keep an eye on your pressures when doing this.)
6) “Disco purge”: When using a mechanical purge compound for an extrusion application, it is often recommended to stop the rotation of the screw for a minute or two several times during the purge. This will help increase the scrubbing action of the purging agent.
7) Reducing the die opening: If your die is adjustable, it may help the effectiveness of your mechanical purge compound to reduce the size of the die opening. This will increase the pressure inside the machine which helps the purge compound ‘scrub’.
8) Clean the hopper: Basic, right? Yes, but often missed. We have seen operators on many occasions run a purge trial, or execute a color or material change without properly cleaning the hopper. Not only should you blow the hopper clean with compressed air if available, but you should also wipe it thoroughly with a clean rag.