by Brittany Willes, contributing editor, Plastics Business
For molders, purging refers to the process of removing traces of old material and contaminants from molding equipment. With a plethora of purging compounds to work with, which compound is most efficient when it comes to color and resin changeovers? What are the benefits of using a purging compound at all? As with most things, the answer depends on the particular job or situation.
Mechanical vs. chemical compounds
The first step in reducing material waste is to identify which purging compound is most suitable to a given job or process. Many types of purging compounds are on the market today; however, they are not equal. Selecting the wrong compound can lead to a slew of problems, including inefficient cleaning and mold growth or damage to equipment.
Each process should determine which purge compound is right for each situation, stated Corey Henley, senior technical service specialist for Chem-Trend, Howell, Michigan. You would not want to use a standard polypropylene purge on a high-heat application, nor would you want to use a white, highly abrasive purge to clean the hot runners on a highly polished clear lens mold.
Mechanical purging compounds tend to work based on material affinity and viscosity differences, using the machine to perform the work. All things being equal, you want to be more viscous than the material youre trying to get out, explained Eric Procunier, product development manager for Asaclean-Sun Plastech, Inc., Parsippany, New Jersey. At the same time, if you can match the base resin in the purging compound with the material you are purging, youre going to have much more success because the material will be attracted to the purging compound and youll be able to pull the material off the screw and barrel much more easily. According to Procunier, typically there are scrubbing fillers in most compounds – whether mechanical or chemical – but theyre not such that they would do any damage. Mechanical compounds can differ on a wide range from a mild foaming agent to cracked acrylic or glass fillers.
By contrast, chemical compounds call for a chemical reaction to take place inside the machine, reducing the viscosity of the material in the barrel and making it easier to move out. A chemical purging compound creates a reaction within the contamination, breaking the molecular bonds that hold it to the metal surface, stated Henley. Once the bonds are broken, the contamination can be easily removed from the system.
Naturally, there are pros and cons to each type of compound. For instance, unlike mechanical purging compounds, chemical compounds tend to have foaming agents in them, which allow the purge to expand into areas that might be low-flow or poorly designed, reaching areas that a mechanical purge might not have access to.
On the other hand, chemical compounds can result in greater downtime. Chemical purging compounds have to soak under heat for a period of time in order for the chemical reactions to occur, said Procunier. This can take anywhere from five to 30 minutes. Mechanical compounds can just be put into the machine and run straight through. Using a mechanical purge cuts down on the time spent just waiting.
Whether using a mechanical or chemical compound, the most important thing is to be sure to use each purging compound the way it was intended. All compounds are not the same and, therefore, will not work the same. Running a chemical purge like a mechanical purge runs the risk of damaging the machine or mold, resulting in lost time, lost profits and potential risks to employee safety.
As mentioned earlier, one of the benefits of purging compounds is reduced downtime. When used correctly, purging compounds will clean a screw and barrel (as well as hot runners and dies) far faster than virgin or regrind resin will, while greatly reducing resin waste, stated Procunier. This allows for much faster color and job changes.
For instance, if purge is not being used on color changes then no cleaning – mechanical or chemical – has taken place. The only thing that is taking place is trying to displace previous material and contaminant buildup with resin, Henley affirmed. This is a highly ineffective method, as it can take up to 10 times the amount of natural resin for the system to be clean enough to make good parts. In addition, scrap will continue to build up. Just the time it takes to process this material equates to a significant amount of downtime when viewed over every machine for the year, he said.
Machine maintenance also can be affected by the use of high-quality purging compounds. Carbon buildup and other contaminants will eventually require machines to be down for maintenance; however, a quality purging compound, used regularly, can help extend the amount of time that systems can run before tear-down becomes a necessity. Henley asserted, A good chemical purge can keep carbon low and remove buildup from even hot runner systems. In this way, a purge compound is not something you just purge on the floor and throw away; it is an integral part of your process (by limiting downtime) and maintenance plan (by limiting frequency of tear-downs).
Overall, purge compounds enhance machine efficiency in several ways. We see better labor utilization, scrap rate reduction, increased uptime and better response to changes in customer demand, said Procunier.
Mechanical or chemical, there are many benefits to using a high-quality purging compound, some of which tie directly into savings benefits. There are three main areas in which the majority of cost savings are realized when using a high-quality purge compound: cost of usage, scrap and downtime, stated Henley.
He went on to explain that the cost of usage is calculated by the cost of the purge multiplied by the amount of purge used. In most cases, the more expensive purge compounds will require less to do a better job. For example,
Natural Resin: $0.95/lb x 50lbs
(amount needed to do a color change) = $47.50
Purge 1: $1.95/lb x 20lbs
(amount needed to do the same color change) = $39.00
Purge 2: $3.35/lb x 10lbs
(amount needed to do the same color change) = $33.50
In each of these, the cost per pound is increased, but the cost per use has reduced, he explained.
Purging compound also can help reduce the amount of material waste from scrap parts that are unable to be sold as a result of color change defects, such as color mix or black spot contamination.
According to Henley, A high-quality purging compound will reduce the amount of scrap parts produced between color changes. Using just a natural resin can run up to 20 to 30 scrap parts before good parts are produced. A cheap purge may reduce this to around 10 parts. A high-quality purge compound targets zero to two color-mix parts per color change. The cost of the scrap is determined by the cost of the no-good part. This cost has to include the cost of material to make the part, the machine time, the operator time and, if needed, the overtime cost to remold the parts. Assuming each parts scrap cost is $5, then the natural resin running 20 scrap parts would cost $100 per color change vs. $0-$10 with a high-quality purge.
As mentioned earlier, purging compounds also can reduce the amount of downtime required between color or resin changeover. This results in another type of cost savings, separate from that which is gained through material savings. Essentially, savings can be found in the time it takes to go from last part to first part, whether scrap or good (not counting purge parts if molding with purge compound).
Every machine has a burden rate, and every operator has a cost. As Henley explained, each minute the machine is not producing parts and the operator is idle is costing the company money that otherwise would be passed on to the customer via the goods sold. Using the amount from the example above, it would take five times as long to process the natural resin as it would to process Purge 2. If it takes 10 minutes to process Purge 2, then it is easy to see that it would take 50 minutes to process five times the amount of material. This would equate to 40 minutes of downtime. If a company had 10 molding machines running, two color changes per day, five days per week, 48 weeks a year, this is 3,200 hours wasted per year. If the burden rate for the machine and operator combined is $60/hour, the result is $192,000 per year wasted in downtime. Keep in mind: If downtime is counted as the time from last sellable part to first sellable part, then the machine burden rate and operator cost must be removed from the scrap part cost or this cost is counted twice.
Using those numbers, a cost savings calculation from natural resin to a high-quality purge is shown in Table 1.
Total savings of a high-quality purge vs. the natural resin is $144. If there are two color changes per day on 10 machines, five days per week, 48 weeks per year, the annual cost savings is $691,200.
Of course, this is not an actual situation, Henley remarked, but it is a good estimation based on many processes from many injection molders.
Final advice for using purging compounds
Over the last decade, purging compounds have improved dramatically. A lot of time and energy has been spent on research and the development of new technologies to make processors jobs easier, which is why suppliers like Chem-Trend and Asaclean encourage processors to take advantage of it. However, purges are not interchangeable. Therefore, it is especially important for processors to follow the appropriate procedures.
The biggest issue we see is when operators move away from the purging procedures created by the suppliers, stated Procunier. Always follow the advice of your purge supplier. You want to be mindful of the smallest restriction in the system – be it gate size, die orifice size, screen size, etc. When instructions are followed and grades are properly chosen for the application, there is no risk to equipment or molds.
Before using any new material, Henley added, always be aware of potential hazards, either due to a chemical reaction or just as a danger to the machine or mold. If you have questions, ask. If your sales representative does not know, then ask to speak to a technical service representative. Get answers before processing a new material.