Frequently Asked Questions

To assist you, we have identified some of the more common questions asked by plastic processors. Please click on your question to see the answer.

How much commercial purge compound is needed to clean my injection molding and extrusion equipment?

There are many different types of commercial purging compounds such as chemical (liquid/solids, concentrates, pre–blends) and mechanical (concentrates and ready–to–run, abrasive types and hybrids). Each has its own formula to calculate the volume required. Please read the processing instructions that accompany each product. Only by using the compounds as recommended will you get acceptable results.

Dyna–Purge offers a variety of non–abrasive purging compounds that come pre-blended and ready to use. No machine adjustment or soak time is needed. Since Dyna-Purge is pre–blended, there is no need to mix it with your own resin. There are no hidden costs!!

As a rule of thumb, the recommended amount for injection molding and extrusion is 1 to 2 times barrel capacity.

How much you will need may vary depending on the following:

    • The color — some colors are more difficult than others.
    • The type of resin and additive package.
    • The viscosity of the resident resin and the viscosity of the following resin.
    • The type of the preventive maintenance program for the plastic processing equipment. The better the program, the less time required.
    • The age and condition of the processing equipment.
    • The size of the machine.
    • The skill of the operator.
    • The uniformity of purging procedures among the operators.
How do you locate the root cause your of streaking?

Often a color can get trapped in a negative flow area. There is not enough physical turbulence to move the color. Therefore, the next material processed picks up trace amounts of the color and causes a streak in the part. This streaking will continue until the color that is trapped is “flushed” away by the next resin.

If color streaking occurs in the same location on the part, then the entrapment is likely close to the nozzle or die. If the streaking is a moving streak on the part, then the entrapment is likely to be in the barrel and/or feed zone of the machine. Streaking often generates considerable waste.

Purging with Dyna–Purge purging compounds will help remove the entrapped color and flush it away. Dyna–Purge scrubs the color out, reducing the amount of transition resin required, thereby enhancing productivity.

What is the cost of purging vs. not purging?

There is a considerable difference in price among the various types of commercial purging compounds. Almost all are a higher price per pound or kg than basic resins.

There are many factors that need to be considered to develop an accurate cost to purge. See our form Savings Calculator for help. Some include:

        1. Material
        2. Downtime
        3. Scrap Material Generated

We believe it’s more important to consider the cost of “not purging.” Without a purging program in place, the processor will generate more scrap, have reduced productivity, and will incur substantial opportunity cost.

We encourage all plastic processors to use an effective commercial purging compound that is formulated for the process/resin /temperature needed to purge. Dyna-Purge offers many different products to service injection molding and extrusion applications.

Are commercial purging compounds safe to be left in the barrel?

Purging before shutting down makes for an easier start–up. Dyna–Purge offers thermoplastic non–abrasive purging compounds that can be used for shutdown and start–up. We recommend the machine be thoroughly purged with Dyna-Purge and then shutdown with the barrel completely filled with Dyna–Purge. Dyna–Purge is very stable, therefore it will not degrade upon start–up. Dyna–Purge ensures that start–ups will be easier and faster than with other types of purging compounds.

Are any commercial purging compounds FDA approved?

In a word, no. The FDA does not offer approval for purging compounds. Purging compounds are an intermediary product — a maintenance product that is never intended for manufacture into finished plastic products. Purging compounds are not FDA regulated. They can be formulated with components (resins, additives) which may be FDA “compliant”. Dyna-Purge is formulated with FDA compliant components.

Dyna-Purge is successfully used in hundreds of medical, food, pharmaceutical and other critical applications where FDA and other stringent third party approval are required. Please contact us to learn more about Dyna-Purge for these applications.

What are the different types of purging compounds?

There are three main types of commercial purging compounds (and also hybrid versions of these types).

Chemical: These types come in liquid and solid form, are often concentrates that need to be let down into your own virgin resin or are supplied as ‘pre–blended’ with a carrier. This type of purging compound typically requires soaking in the barrel for a specific length of time, with adjusted heats and without the screw turning to allow the chemical reaction to occur. Heat activates the chemical reaction. This chemical reaction modifies the molecular chain of the polymers that need to be purged, making it easier for the next material to flush it out. Chemical purges can be effective but may be costly from a cost per pound and also a “ROI” basis. Purging may take longer because of the soak time, machine adjustments and added resin costs. Some chemical purging compounds release fumes and are known to leave a residue.

Mechanical/Abrasive: This group of purging compounds is often supplied in concentrated form, but can be supplied as pre-blends. The carrier system is loaded with a high percentage of glass fibers or beads and/or inert fillers, including clay, calcium, and TIO2, etc. These purging compounds clean by aggressively scouring the barrel and screw. Their aggressive content may cause excessive wear on the critical tolerances of the screw and barrel, and may result in premature screw replacement.

Included in this group of purging compounds is the “non–melting” type. These can cause “mini-explosions” when they are processed at high temperatures or through tight nozzles or die clearances. They can be hazardous to equipment, because of high–pressure buildup, and to operators, potentially exposing them to polymer burn hazards.

**HYBRIDS — Share the same risks and drawbacks mentioned in numbers 1 and 2 in some proportion but they also offer many benefits.

Thermoplastic Mechanical/Non–abrasive: This group of purging compounds is the easiest to use and the safest for the equipment and machine operators.

Most of the Dyna–Purge products fall into this group. Dyna–Purge is a thermoplastic (non-chemical) blended purging compound. The Dyna-Purge formula includes polymer carrier systems with unique “soft scrub” scrubbing granules. The scrubbing granules soften on the outside, but remain solid on the inside, so that the metal surfaces of the screw and barrel are scrubbed softly and thoroughly without wear. The specially formulated carrier system flushes the old color, degraded material and contamination out of the machine. They come pre-blended and ready to use, with no hidden costs.

Are commercial purging compounds safe for use with injection molding hot runners?

Among the three main types of commercial purging compounds, several can be used with hot runner systems. The question that the processor should ask is: “Which is best for my equipment and machine operators, and which will deliver the best results?”

Dyna–Purge has multiple thermoplastic products that are safe to use throughout hot runner systems and are safe for the machine operator.

It is important that the right product be matched to the process and application. See our Product Selection Guide to learn more about the different Dyna–Purge products.

Are commercial purging compounds ever mixed with other resins?

Among the three main types of purging compounds, there are several grades that are offered as concentrates. They are designed to be mixed with a carrier- either a commodity resin, or the next material to be processed. The main issues with concentrates are a) adding the right amount, b) hand mixing so blend is uniform, c) stocking the carrier resin and d) flushing resins thoroughly.

Please note that when using a concentrate and letting it down into the processor’s own carrier resin, the cost of the resin used should be included in the cost of purging. We refer to this as a hidden cost and often discounted by the user — the same can be said for the mixing time required.

Dyna–Purge is a pre–blended, ready–to–run thermoplastic purging compound, uniform and consistent. It is easy to use, with no hidden costs. Simply scoop or pour Dyna–Purge from the package right into the machine.

“These are the answers to your most commonly asked questions. If your questions are still unanswered, please contact us!”

Kelli Ropach

International Business Manager, Dyna-Purge

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