What is a Purging Compound?

In the thermoplastics industry, a purging compound is a compound specifically designed to efficiently clean thermoplastic processing machinery of leftover resin and other contaminants that build up on interior processing surface. Purging compounds most commonly use a resin base which allows them to be loaded and processed similarly to resin when performing the purging process. Effective purging compounds are designed to work on a variety of machinery, over a substantial range of temperatures, and with a wide range of materials. Purging compounds are designed to require the least amount of material and time to effectively purge leftover resin and contaminants. This has allowed purging compounds to replace existing costly and time-consuming methods of cleaning thermoplastic processing machinery used in modern plastics manufacturing. Purging compounds are a cost effective and time efficient method to maximize the uptime of your machinery while minimizing wasted resin and defective plastic parts.

Types of Purging Compounds

Purging compounds can be separated into two main types based on their mechanism of action. Some modern purging compounds, such as those in our Purge Right™ series, use a combination of mechanical and chemical interaction to most effectively purge processing surfaces.

  • Mechanical Purging Compounds – Mechanical purging compounds primarily utilize shearing force to purge contaminants from processing surfaces. Mechanical purging compounds can also utilize differences in material viscosity or bond with residual resin and other contaminates to purge them from processing surfaces. Many mechanical purging compounds are abrasive which means they are expected to wear processing surfaces during their cleaning action. As purging compound manufacturers work towards providing the most cost-effective solutions for purging many non-abrasive mechanical purging compounds are now available. These non-abrasive purging compounds still cause wear of processing surfaces, but this wear is within the range the machinery would be expected to experience from normal operation.

  • Chemical Purging Compounds – Chemical purging compounds operate by utilizing heat activated chemicals to break down bonds between the various contaminants and processing surfaces. Chemical purging compounds often require a short period of “soak” time, commonly between 5 and 30 minutes, to most effectively purge contaminates. While almost all mechanical purging compounds are solid you can expect to find both solid and liquid chemical purging compounds.

Uses for Purging Compound

  • Clearing Residual Plastic Material Between Batches – The most common usage of purging compound is to remove remnant plastic material between batches of resin. When processing resin for thermoplastic manufacturing not all of the resin ingested by the hopper makes it all the way through processing and can carry color and other properties into subsequent batches of plastic parts. Purging compound can produce clean articles free of discoloration and other properties from previous resin in as little as a few cycles.

  • Clearing Contaminants – When doing a lengthy production run with many loads of the same batch of resin you may need to utilize purging compound to resolve contaminant buildup. Contaminants build up on the processing surfaces of thermoplastic machinery as a result of the chemical changes in plastic material which does not fully pass through the system as it continues to heat up. Purging compound can clear these deposits of contaminants such as carbon buildup in addition to cleaning any plastic material. Keeping your processing surfaces free of contaminant will keep black spots and other defects that result from the buildup of contaminant out of your plastic parts.

  • Contaminant Prevention – Depending on the operating specifications of your machinery and the resin materials you work with you may find the need to clear machinery of any leftover resin or contaminant before or after shutdown. When residual plastic and contaminants are left to cool after machine operation, they can harden and become difficult to remove. Once this hardening has occurred this can increase the difficulty of resolving buildup through the use of purging compounds. The hardened contaminants can also lead to a reduced lifespan of parts and can require disassembly and maintenance of the machinery for a full cleaning.

Thermoplastic Machinery that Utilize Purging Compound

  • Injection Molding– Injection molding is the least restrictive thermoplastic manufacturing process and can be easily adapted for use with a wide variety of parts of varying complexity and size. Injection molding often utilizes molds with multiple parts creating many narrow channels between those parts known as runners. These narrow runners can quickly fill with contaminate and become a bottleneck requiring disassembly and maintenance. Regular use of purging compound keeps narrow channels free of contaminate and removes the need for maintenance just to clean these problem areas.

  • Hot Runners – Hot runners are injection molds that utilize a heat manifold to feed multiple injection nozzles that equally distribute plastic material to different areas of a mold. The plastic material is heated to a high temperature in the heat manifold to aid in even distribution even during quick cycles and then quickly cooled once injected into the mold. This combination of having higher temperature plastic material, multiple points of injection, and accelerated cooling allow for quick cycle times to be obtained. This thermoplastic manufacturing process is particularly effective with more complex molds as it allows the plastic material to evenly distribute within the mold various cavities.

  • Blow Molding– The blow molding process injects plastic material into a mold with a single large cavity while also maintaining an interior cavity within the plastic material. This internal cavity is then filled with air forcing the plastic material into the shape of the mold. Though they are most commonly used to create plastic containers such as bottles and barrels any object with a significant cavity can be a candidate for blow molding.

  • Extrusion – Extrusion is used to produce linearly uniform parts with a continuous profile. Extrusion can produce parts with complex cross-sections, and works well with brittle, hard materials. Purging compounds effectively clean screws, heads, and barrels without the need for costly disassembly.

Benefits of Purging Compound Usage

Purging compounds were formulated primarily as an efficient alternative to existing methods of cleaning thermoplastic processing machinery. Before purging compounds were introduced into the market the next batch of virgin resin or reground plastic, also known as regrind, was most commonly used to clear any residual plastic material left in the machinery from the previous batch of resin. When a new batch of resin was introduced plastic parts were visibly checked for defects and discoloration until the machinery was thought to be sufficiently cleared of the previous resin. This method is very wasteful of otherwise useable resin and results in considerable downtime while still requiring machine operation. This method also does little to prevent any buildup of contaminants that form as a byproduct of processing such as carbon deposits. These byproducts would eventually require disassembling and cleaning the machinery to properly maintain them.

  • Ease of Use – Modern purging compounds have been widely adopted because of their ease of use. We provide products that easily fit into your existing workflow and work with existing equipment without modification. Most purging compound use a resin base, so their use differs little from how you would normally load and process resin. As they are so easy to use purging compounds are able to seamlessly integrate with your existing production procedures.

  • Increase Machine Uptime – Using purging compounds greatly reduces machine downtime compared to using virgin resin or regrind by reducing the time required for effective cleaning. When cleaning out residual plastic material between batches the reduction in time is significant. Though exact timing varies due to variables such as machine capacity, material properties, processing temperature, and interval between cleanings you can expect purging compound to take between 5 and 15 minutes to effectively clear remaining plastic material and contaminate buildup. Purging compounds also significantly reduce downtime by allowing contaminants to be cleared without requiring disassembly and maintenance. Our customers typically see an overall reduction in machine downtime of 60% – 80%.

  • Cost Savings – If you are currently using virgin resin or regrind to clear machinery purging compound can offer a cost-effective solution for your business. Though the cost of purging compound may be more expensive by weight then virgin resin and is often more expensive then regrind utilizing purging compound is still an overall cost savings. When comparing the amount of purging compound needed to effectively clear plastic material to the amount of virgin resin or regrind needed purging compound offers a cost savings. The real cost benefit of purging compounds however becomes apparent when you consider the cost savings that result from the clearing of contaminates. Every time you run purging compound through your machinery you will be clearing contaminants that would otherwise require disassembly and cleaning which can have significant costs due to labor and machine downtime. Additionally, many parts can see an increase in lifespan due to being more consistently cleaned which results in further cost savings.

  • Cleaning Efficacy – In addition to their advantages in cost and speed purging compounds also do a far more complete job of clearing thermoplastic machinery than existing methods. Though using virgin resin or regrind will eventually clear much of the remnant plastic material from a previous batch it does little to clear contaminants that build up over time. Using virgin resin or regrind also simply replaces existing plastic material with remnant plastic material of the current batch. Purging compounds can effectively clear contaminants in addition to completely clearing plastic material and not just replacing the existing material with new remnants.

  • Reduction of Defective Product – Contamination may pose a risk to the long-term life of your machinery and components – but the more common concern is in plastic part contamination. Black specks and streaks are common reasons to reject plastic parts. When using dated methods of clearing machinery of contaminants, they are thought to be an unavoidable part of the process. In addition to visual defects residual plastic material and other contaminants can cause structural weakening of parts due to effects such as the uneven filling of mold cavities. These structural weaknesses can cause issues with your product that are hard to verify from visual inspection and require extensive and costly testing. With regular use of purging compound during long production runs in addition to purging between batches of resin you can significantly reduce these defects in most applications. Our customers have generally experienced a 40% – 50% reduction in scrap produced as a result of defective product.

Choosing the Right Purging Compound

If you are considering seeing if purging compound is right for your business the first step is to find the right purging compound for your unique application. We make this easy as our Purge Right ™ line of compounds is designed to cover the widest variety of machinery, operating temperatures, and materials that we can without sacrificing quality. We are happy to work with you from helping you choose the right product for your business to formulating new compounds that fit your unique needs. If you don’t see a compound that fits your exact needs, or you have any questions please send an email to gary@purgeright.com or fill in the contact form at the bottom of this page.

  • Purge Right PR-5000Our most popular product that includes both mechanical and chemical cleaning agents. PR-5000 utilizes four distinct chemical agents allowing it to cover a temperature range of 340F to 610F. PR-5000 can be used effectively for injection molding with hot runner manifolds, blow molding and all types of extrusion processes.

  • Purge Right PR-6000This compound offers all the benefits and ease of use of our PR-5000 compound but with an added agent for additional mechanical cleaning. This product works great for applications that need some additional cleaning power like heavy colorant using liquid colors, worn components like screws and barrels, and complex molds with difficult to reach problem areas.

  • Purge Clear PRC-7000 Formulated specifically to work with applications using clear resin. PRC-7000 uses a water carrier for the most effective means of preventing cloudiness, black specs, or hazing in your clear products.

  • Purge Clear PRC-8000 – The second in our Purge Clear™ line of products has the same great performance of our PRC-7000™ compound with an acrylic carrier. Developed specifically for use with machinery that processes clear acrylic such as headlight enclosures.