PROCESSING | PURGING COMPOUNDS
Right: Carbon specks captured in a Dyna-Purge purging compound
An effective purging strategy also helps in labour cost management. “Global labour costs are rising and talent is at a premium. Therefore, as quality improves, the value and optimisation of labour is better rationalised,” she says.
Purge analysis Dyna-Purge says the first step towards achieving these benefits is to conduct a purge analysis. “Before starting with a cost-to-purge analysis, it is a good idea to spend some time establishing the current metrics,” says Ropach. She says the process is straightforward. The first step is to record the amount and cost of the material used to purge and post purge, adding them together to determine the total material cost of the current method. The next step is to calculate the purging time in minutes, including any soak time, and multiply that by the machine hourly rate. The same should be done for any post purge time. Adding the two time metrics together reveals the total cost of downtime. Finally, the amount of scrap production measured in terms of material cost and time is added in. Ropach says this information sets the benchmark for the current purging programme metrics, allowing a comparative trial to be run to track the metrics for alternatives. “The next step is depend- ent on what the current purging situation is. If existing purging is with an in-house resin, there will be a lot of room for changes and improvement. If the in-house resin is natural PE, PP or regrind to purge, for example, then the comparative trial can be used as an opportunity to test a commercial purging compound.” In most cases, a significant improvement will be seen when using a CPC over an in-house resin, according to Ropach. “It is highly likely the scrap rate is already being tracked at your company. However, this can be taken to the next level by establishing a programme to reduce this rate from the information gained in the cost-to-purge analysis. Utilising the expertise of a purging compound company, set up a plan to implement a purging program. This may require a visit, purging information posted at each machine and buckets with pre-measured amounts, for example.” The reason that commercial purging com- pounds typically yield better results is because they are specially designed to clean the machine, according to Dyna-Purge. The company says that using the same in-house resins that are processed in the equipment continues to add layers of contamination; purging compounds will remove it. It says its Dyna-Purge products are designed to provide a mechanical non-abrasive purge that
62 COMPOUNDING WORLD | September 2020
flows to the boundary layers and cleans on the first pass. They are said to be able to reach and remove layers of contamination in stagnation areas.
Twin screw challenges Compounding extrusion presents some unique challenges in terms of purging and is an area of particular interest to RapidPurge at present. “These machines are generally harder to clean than single screw extruders and injection moulding machines,” says Joe Serell, President and Chief Executive Officer. “One of the primary reasons for this is that twin screw extruders are starve-fed. This means that the barrel is not completely full of material and the purging compound may not reach all surfaces for cleaning. Even if it does, the lack of pressure inside the machine reduces the effective- ness of mechanical purge compounds.” The RapidPurge solution to these challenges is to use chemical purge compounds for twin screw applications, supported by a foaming element. “The chemical ingredients in RapidPurge break down the resident resins and contamination on a molecular level. We do not consider foaming agents - often used effectively in purge compounds - as a chemical purge, although they are activated by heat to cause a chemical reaction. Our product is the only purge compound that works via depoly- merisation,” Serell claims. “In our case, a foaming agent is utilised for two purposes. Firstly, it creates pressure inside the machine that helps the scrubbing agents do their job. Secondly, it delivers the chemical agent into the low-pressure/low-flow areas to thoroughly clean along with the mineral scrubber. This is especially important when trying to clean behind the flights. The final component of RapidPurge’s twin screw purge is a high viscosity resin carrier. This helps push
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IMAGE: DYNA-PURGE
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