In a continuous process like extrusion blow molding, contamination has a detrimental effect on your business’s bottom line. Blow molding tends to have bigger issues than other processes. Once contamination begins, it’s almost impossible to stop it without aggressive intervention — usually through a screw pull or use of a purging compound. Effectively purging your thermoplastic processing equipment is critical if you want to stay competitive. Not only do material changes occur more frequently, but waste disposal is more difficult (and more costly) and a greater number of customers demand just-in-time, customized products. For many blow molding plants, reducing waste, time and cost are some of the most significant factors for staying competitive. Many customers have more than a 70% reduction in costs with the right purging compounds, saving them up to tens of thousands of dollars each year.

Until recently, the most popular method for purging extrusion blow molding equipment had been to use the next processing resin or regrind. However, this approach results in longer purge times, higher scrap rates and greater residue buildup over the long term.

Here are seven quick tips for using purging compounds in order to keep your purging operations as efficient as possible:

Tip #1: Don’t use production resin to purge your machine

Production resins are not designed to clean machines and are not a substitute for commercial purging compounds. Purging your blow molding machine with the next resin or regrind is time-consuming, wasteful, and ineffective at removing color and carbon contamination. When you use regrind or virgin resins to purge, the material creates another layer over any existing resins, color deposits or carbonized material within the barrel and on the screw. In time, these layers become additional sources of contamination, making purging even more difficult. Once your machine returns to production, carbonized materials eventually break free and contaminate your products.

Tip #2: Select the right purging compound for blow molding

In many cases, blow molding changeovers are dramatically shortened with the use of a purging compound because they eliminate residue and contamination. Selecting the right grade makes all the difference. Extrusion blow molding heads and accumulators are, by design, low-pressure areas. The low-flow or low-pressure environment of extrusion blow molding leads to difficulty cleaning certain areas. Since mechanical purging compounds require maximum pressure, they’re not always the best choice for this application. Instead, try using a chemical purging compound. When cleaning screws for extrusion, it’s important that you use the right product and purging process. Using the wrong chemicals or tools may ruin screws or die heads and lead to serious production problems.

The product grade you’ll need depends largely on the resin being purged and its processing temperature. If contamination is an issue for your facility, your supplier may recommend a different grade. In addition to the processing temperature, the compound should have a similarly low melt index, which is better for resin compatibility.

The following are typical melt indices (MI or MFI) for blow molding resins:

LDPE: <2g/10 min HDPE: <1g/10 min PP: <2g/10 min

Another option is to use a Purging Concentrate like our PLUS Grade. PLUS Grade super-charges your processing resins with Asaclean’s strongest cleaning ingredients. You are guaranteed to match processing temps and melt flows because you’re using your own production resin.

Purging procedures for the extrusion blow molding process vary depending on the purging product you’re using. If you’re unsure on how to clean your extrusion blow molding machine most efficiently, it’s best to reach out to your purging compound supplier.

Tip #3: Practice preventative purging procedures

Many plastic processing companies only using purging compounds when it’s absolutely necessary. But engaging in this type of purging “process” leads to reduced profitability and efficiency. Adopting preventive purging practices not only saves your facility from machine downtime, it also reduces your scrap rates, customer rejects and line shutdowns. You can’t afford to wait to run purging compounds until the last minute because, by that time, you’re already suffering from contamination issues or color streaks.

Using a purging compound at regular intervals prevents contamination from the beginning. Typically, it only takes one to two system capacities of purging compound to prevent contamination. Most operators would rather have planned purging-related downtime than an unplanned screw or head pull. There are three main benefits to implementing a preventive maintenance program and purging process at your facility:

• 1- Preventing the buildup of color and carbon contamination

• 2-Reducing excessive purging time and material waste at the end of production runs

• 3-Limiting the frequency of screw pulls and minimizing the time and effort required to pull and manually clean your screw

Tip #4: Use the right machine settings

To maximize purging performance, follow your supplier’s recommended machine settings. The settings are very important. Asaclean’s Chemical Purging Compounds are a great option for blow molding because they work by an endothermic (heat-activated) chemical reaction that causes the material to expand into those low-flow areas. Without the correct settings, you miss out on the benefits of these compounds. Using the right settings will activate your chemical purge for optimal cleaning power.

How a Midwestern Blow Molder Reduced Scrap by 21% and Downtime by 30%

One of our Purging Experts recently managed a cost-savings purging project with a manufacturing engineering supervisor at a Midwestern EBM plant. They were blow molding large, bladder-type parts out of translucent Polyurethane (processed at low temperatures) for both residential and commercial applications. They were chronically struggling with random carbon specks throughout their parts on starts ups from weekend shutdowns.

Depending on the production demand, they were shutting down their machine and restarting it multiple times per month. Their goal was to reduce machine downtime and lower the quantity of scrapped pounds on start-ups. They conducted four separate trials with Asaclean E Grade purging compound over a 2-month trial period, comparing its performance to a well-respected competitor. After these trials were conducted, the manufacturing engineering supervisor delivered us a detailed data-driven report, yielding great results.

Using the average of these trials, his report indicated that they had about 180 minutes and 215 lbs. worth of scrapped parts on start-ups when using our competitor. After switching to E Grade, they saw major improvements and dropped down to 130 minutes and 170 lbs. worth of scrapped parts. Ultimately, they saved in the neighborhood of $4,640 for this one application for each machine in their plant.

Tip #5: Increase heat in troublesome equipment

For difficult-to-clean pieces of your machinery, increase the heat in that area to 500°F (if the production resin is not heat-sensitive). When using an ASACLEAN chemical grade, this will allow the endothermic reaction to take place where you need the cleaning most.

This is common in extrusion blow molding systems with accumulators, the accumulator head is the most troublesome area. First, be sure to pinch the gap on the accumulator head as tightly as possible to maximize the mechanical cleaning effect. Additionally, increase the pushout speed to the maximum safe setting. Fill and empty the accumulator four to eight times to varying levels between 25% and 100% to ensure the accumulator is clean. Lastly, be sure to keep these settings when displacing the purge material to decrease the amount of displacement resin. When all the purge appears to be removed, return the head gap, fill height, and pushout speed to production settings.

Tip #6: Seal your machine with Asaclean during every shutdown

Aside from regular and preventative-maintenance purging, use a heat-stable purging compound for temporary or extended shutdowns and sealing, especially over weekends or holidays. During a shutdown, oxygen in the barrel causes residual polymer to degrade and carbonize, delaying startups and resulting in extended machine downtime and scrap. A heat-stable purging compound creates an “airtight” environment in the barrel and promotes further cleaning during the shutdown.

Tip #7: Purge every extruder in multilayer systems

A common mistake made in extrusion blow molding is to only purge contact layers and not the barrier or inner layers. However, the materials used as barrier or inner layers can be more susceptible to degradation and contamination. Some processors use regrind as fillers or thickeners for an inner layer, and regrind can cause contamination due to multiple heat cycles on the resin. Be sure to purge every layer during every changeover to maximize quality and performance properties.

Using commercial purging compounds keeps your blow molding machinery running clean and smooth, but only if you use them correctly. Keep these seven quick tips in mind and your blow molding plant will have no problem staying competitive.

Take advantage of our remote consultations, trials, and training via Skype, FaceTime, or WhatsApp. Call 800.787.4348 or visit asaclean.com to request your FREE Purging Consultation.

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