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Purging compounds reduce machine downtime and increase productivity for compounders

(October 2002; Reprinted from Plastics Additives & Compounding)

Effective cleaning of plastic processing equipment is vital for keeping the machinery running productively and free from trouble. Josephine Komarmi of Dyna-Purge^® Div. of Shuman Plastics Inc. discusses the issues involved when comparing manual cleaning or in-house remedies to the use of commercial purging compounds for compounders.

There are several occasions that call for purging and cleaning processing equipment, including colour changes, and routine shutdown and maintenance. Ensuring that equipment is cleaned thoroughly and effectively is critical for quality control and in extending the operating life of the machinery. Although breaking down a large extruder and painstakingly cleaning it by hand is thorough and effective, increasing pressures in the marketplace can make this method a strain on productivity. For example, Just-In-Time delivery demands result in greater frequency of resin and colour changes. In addition, there is always the need to manage bottom-line costs to stay competitive and profitable. Machine downtime is neither competitive nor profitable to a compounder's operation.

The use of commercial purging compounds is helping compounders meet today's pressures by minimizing machine downtime and boosting productivity. A purging compound is introduced to the system to expunge a colour, resin, or formulation before a changeover or shutdown. Purging compounds are particularly helpful in preventing streaking, caused by bits of a previously run colour becoming trapped in a negative flow area. Since there is not enough physical turbulence in the area to remove the colour, the next material processed picks up trace amounts of the colour and causes a streak in the new product. The streaking will continue until the trapped colour is flushed away by the next resin.

Dyna-Purge purging compound

Compounders using purging products may still break down a machine for manual cleaning; however, pre-cleaning with the purging compound saves a significant amount of time, frequently 50 per cent or more. This bolsters the company's capability to meet the requirements of its customers.

In addition to cleaning out the previous colour, resin, or formulation, purging compounds are also effective in removing black specks (carbon buildup). These can be caused by any number of reasons such as hot temperature spots in the machine, degradation of polymers, separation of additives, fillers and colour concentrates from the carrier, dead spots or negative flow areas in the barrel, mixing area or die, regular start-up and shutdown, or a lack of regular preventative maintenance. Like shale rock, the carbon buildup will break into pieces and be moved forward by the screw, only to be displayed as unwanted black specks in the finished pelletized product.

According to Shuman Plastics, Inc., manufacturer of Dyna-Purge purging compounds, the scrubbing granules used in their formulations safely penetrate into the dead spots and hot spots in the machine to break away and remove carbon buildup, including layers of separated additives and degraded polymers. This mechanical, non-abrasive action will not damage the surface of the screw, barrel, or any metal components or tolerances of the plastic processing equipment. Although there are various hybrids sold, commercial purging compounds are available in three primary types.

Chemical

A chemical purging compound is a liquid or solid concentrate that is typically let down into a universal carrier (PE or PP) or the next resin. There are also solid chemical purging compunds that are supplied as ready-to-run pre-blends. The purging compound is introduced into the machine and allowed to soak in the barrel for a specified time period at adjusted temperatures. The screw is stopped and a chemical reaction occurs that modifies and shortens the molecular chain of the resident resin, allowing the now altered resin to be flushed out. This can be a very effective purging process, but chemical purges can take longer because of soaking time, and usually entail a higher cost per kg/lb. than other types. Some chemical purging agents may release ammonia or other fumes and polymer breakdown can expose equipment to caustic acids that can cause pitting and wear. If the chemical needs to be let down into the resin, the carrying cost of the resin and blending time, as well as machine adjustments, are considered hidden costs.

Mechanical/abrasive

Mechanical/abrasive compounds are concentrates or pre-blended, formulated with glass and/or inert fillers such as clay, calcium, talc or TiO₂. These purge aggressively and effectively by scouring the barrel and screw. But the fillers can cause wear on the critical tolerances of the screw and barrel.

Mechanical/non-abrasive

Commercial purging compounds can help keep extrusion machinery running at optimum performance.

Shuman Plastics' Dyna-Purge is a mechanical/non-abrasive purging compound. Purging prior to a shutdown is recommended because it makes for easier start-up. Chemical purging compounds are active only for a specified period of time during the purging process, and become unstable when subjected to inconsistent heat and extended residence in the barrel or when used beyond their shelf life. Consequently, all chemical residues should be removed before shutdown for the protection of the equipment. The same is true for mechanical abrasives. If left in the barrel, these purging compounds may freeze, which can cause damage to the screw on start-up. Mechanical non-abrasive purging compounds do not degrade, and can be left in the barrel during shutdown to help ensure faster, easier start-up.

Commercial purging compounds, including Dyna-Purge, were introduced to the market more than 20 years ago. Despite two decades of performance and acceptance, many processors still use their own resins and regrinds as a home remedy in their efforts to purge economically. In fact, this is usually more expensive when bottom-line costs are computed. Regrind resin is not designed to clean processing equipment. It may appear to remove previous resins and colours, but it will not clean carbon deposits or negative flow areas.

Furthermore, the cost to purge is not just a comparison between the cost of a commercial purging compound weighed against in-house regrinds. All factors affecting productivity and operations must be considered (see Cost-to-Purge Worksheet).

Commercial purging compounds are available in a variety of formulations designed for specific processing equipment and temperatures. They work faster and clean more thoroughly than regrinds, which reduces machine downtime. More scrap is produced using in-house resins, contributing to a higher cost of waste disposal.

Taking Dyna-Purge as an example, there are no hidden costs associated with using thermoplastic purging compounds. Shuman Plastics recommends using 2.2 to 4.5kg per 2.5cm diameter of the screw (5 to 10 lbs. per diameter inch) to purge extrusion equipment.

Some colours, such as blue, and various types of resins are more difficult to purge, requiring the higher recommended quantity of purging compound. In addition, the L:D ratio may affect the amount needed for extrusion.

Typical instructions for using purging compounds, for example for Dyna-Purge, are:

• Select the appropriate grade formulation based on the process, running temperature and equipment.
• Maintaim running temperature and RPM of the machine (no adjustments are required).
• Empty the barrel and starve the screw.
• Measure one scoop at a time of purging compound and pour it into the empty hopper or side port.
• Flush out purging compound with the next material.

Other factors can contribute to the efficiency and effectiveness of purging such as the viscosity of the resident resin and the viscosity of the following resin, the size of the processing equipment, and the age and condition of the machine.

Quality practices also play a role. For example, these include whether there is a Standard Operating Procedure for purging in operation. Other issues include whether there is regular machine maintenance or purging is done only when time permits, or during an emergency.

Although the skill levels of operators may vary, a Standard Operating Procedure can help ensure uniformity of purging procedures among operators to achieve better, more consistent results.

Calculating the actual cost to purge must take into account a comparison of the cost of the purging compound, the cost of the carrier resin, and the amount of time for pre-purge, purge and post-purge.

Using purging compounds is less expensive because of the significant time savings and reduced scrap. Comparisons among different purging compounds typically favour the pre-blended ready-to-use mechanical brands (see Cost-to-Purge Worksheet).

For the plastics compounder, breaking down the machine and cleaning it thoroughly, is a time-honoured and acceptable way to prepare for changeovers or handle routine maintenance. With today's increasing pressures to do more with less, operators do not always have that luxury. Commercial purging compounders, combined with the operator's experience and skill, can help keep extrusion machinery running at optimum performance and give compounders a competitive edge.

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