Commercial Purge Compound Use Improves Color Change Efficiency

Andrew Reeder, Moulds Plus International

Introduction

The number one goal of companies in the thermoplastic processing industry, just like any industry, is to maximize profits. There are many working parts to a plastic processing plant, thus, the means to maximizing profits are endless. One of the fastest and easiest ways for plastic processors to maximize profits is to reduce production costs during dark to light color changes. Plants that are processing different types of dark and light colors often struggle with excessive downtime and scrap when transitioning out of a dark color and into a light color. Downtime and scrap can be very costly for companies over a long term basis. One solution to reduce downtime and scrap is to use a commercial purge compound for all dark to light color changes.

Background

Companies in the plastic industry have been performing dark to light color changes for decades. Plastic processing companies would typically use either natural resin or regrind to purge their machines when performing dark to light color changes. Some companies came up with their own in house solution for purging out dark colors such as Coca-Cola or Tide. Using resin to purge machines or homemade purge compounds was not an issue decades ago because business was very good, resin prices were relatively low and the profit margins were very high. Therefore, companies did not have to be concerned with reducing the production cost during dark to light color changes. Creating extra scrap or having excessive downtime was not imperative to a company’s well-being. However, times have changed and the market has become very competitive in the thermoplastic processing industry forcing companies to cut costs wherever they can find. Over the past twenty years manufacturers of commercial purge compounds have been educating companies on why they should be using a commercial purge compound, but like anything in life old habits are hard to break. Also, there is a psychological barrier of paying the higher price per pound for a commercial purging compound compared to resin, however, many production managers and purchasers have overcome this barrier and determined commercial purge compounds to be beneficial. For the most part plastic companies have warmed up to the idea of using a commercial purge compound, especially in the injection molding sector of the plastics industry. Commercial purge compound use in the extrusion, blow molding and blown film industry is less common but more companies in these industries are starting to evaluate commercial purge compounds.

Colorants

Commercial purge compounds may be used to clean all types of pigments and dyes for all varieties of thermoplastic processing machinery. Certain types of colorant can be more difficult to purge out than others. For example, organic pigments that are insoluble tend to accumulate in the microscopic crevices on the screw. Using resin to clean out these imperfections tends to take a long time and be inefficient. Thus, in order to expedite the cleaning process, an efficient purge compound should be utilized to clean these difficult to clean areas.

Color contamination typically takes place in one of four places, the screw/barrel, nozzle, die, accumulators or hot runners. A purge compound may assist the processor in cleaning these areas of the equipment. Additionally, color contamination may also be a result of issues taking place before the plastic and color enter the feed throat area of the machine. Thus, situations such as these are not a plastic processing purging issue, but more of a systems and/or procedural issue. For example, here are some of the color contamination issues that a commercial purge compound will not be able to remedy: colorant not cleaned out of the hopper, contaminated resin, and colorant hang up in the conveying system. Issues such as these should be fixed with either change in procedures and/or replacing or modifying the equipment.

Why is it Difficult to Remove Color?

One of the basic reasons that colorant cannot be easily flushed from thermoplastic processing machines revolves around the principle that plastic has a laminar flow as opposed to a turbulent flow (see figure 1.a). Fluids that travel in separate, parallel layers with no mixing, such as thermoplastics, are considered to have laminar flow. Conversely, turbulent flow fluids, such as water, are constantly being tumbled and mixed as it moves forward. Hence, if thermoplastics flowed in a turbulent pattern it would be much easier to clean color out of plastic processing machines.

 

Plastics flow in a laminar pattern which results in the material moving very slowly and having virtually zero shear along the wall of the channel in which it is flowing (see figure 1.b). This principle holds true wether it be the melt channels in hot runners or the screw and barrel. The plastic flowing at a lower velocity next to the wall results in the color being more difficult to be pushed from the machine. Thus, the differences in velocities between the outer edge and the middle of the flowing plastic is one of the main reasons it can be difficult to perform a fast color change.

Why Use a Commercial Purge Compound?

Companies that choose to use a commercial purge compound do so for one reason, to save money. The goal of saving money is accomplished by a combination of reducing the downtime and reducing the scrap. Accordingly, reducing production costs allows companies to become more competitive in the market place resulting in a more successful company.

The downtime, in regards to color changes, is the total time in between making the last good dark color part and the first acceptable light color part that may be sold to their customer. The downtime costs should take into consideration lost production, energy costs, and labor. Also, downtime costs vary greatly depending on the industry and type of part being produced. For example, the downtime cost typically falls between $75/hr - $300/hr (depends on product) in the injection molding sector of the plastics industry.

Scrap and Downtime

Every pound of plastic that is run through a plastic process machine that is not made in to a sellable part that can be sold to a customer is considered scrap. In regards to dark to light color changes the scrap produced will have remnants of the previous color contaminating the light colored part that the processor is transitioning into. Depending on the type of machinery that is being operated the contamination presents itself in different forms. For example, color that is hung up in a hot runner gate will contaminate the part with a distinct streak leading from the gate to an outside position of the part (see figure 2.a). Conversely, color that is lingering on the inside of the nozzle will spread throughout the part in wavy lines causing contamination. Color hanging on the flight of the screw will appear in the part as a uniform color/tint. A well-qualified commercial purge compound technical sales representative should be able to diagnose the source of the color and suggest appropriate measures. Furthermore, it is important to note that any type of purge pile that is produced should also be counted as scrap. There are many companies in the injection molding industry that only include rejected molded parts as a part of the total scrap count. Purge piles produced cost just as much to make as contaminated molded parts and should be included in the total scrap rate.

Many thermoplastic processing companies can either use a granulator to grind up their scrap and use the regrind back in production or sell their scrap to a scrap dealer. While this may be true, there is still a net loss due to the produced scrap. Thus, companies that do their own in house grinding and reuse all scrap are still using energy to run the granulators and convey the material back and forth which becomes costly over time. There is also typically a certain amount of labor that is required to sustain an in house grinding program. The processors that choose to sell the scrap never receive the same price that they originally paid for the virgin resin and are typically losing many cents on the dollar.

Scrap is definitely the most visible sign of inefficient color changes for plastic processors, however, typically the largest monetary loss during a color changes take place in the lost production time. Lost production time is not as noticeable on the production floor as a big pile of scrap; therefore, many companies overlook the downtime cost. Unlike scrap, which as we discussed in many scenarios can be either recycled or sold, the lost production time can never be recouped and is lost forever. Companies that are generating a higher rate of revenue per hour will find that the downtime is much more costly compared to plastic processors generating less revenue per hour.

Cost Analysis

The best way to do a proper evaluation for the different methods of purging is to put together a cost analysis. A cost analysis is a statistical means to determine the most cost efficient way to purge a thermoplastic processing machine. By using a cost analysis, a company has the ability to compare one commercial purge compound to another and it is also possible to compare different purging methods such as using regrind or an in-house produced purge compound. In order to perform a proper cost analysis it is important to collect the most accurate data possible on a handful of attributes for all purging methods that is to be compared. The crucial pieces of data that need to be recorded during a trial are as follows: (1) amount of commercial purge compound required, (2) scrap produced, and (3) downtime. Using a data collection form is recommended to assist with collecting the proper information when performing a purge test (see figure 3.a). Other pieces of information that are not related to the testing, but will be required in order to put together a cost analysis are: (1) price of resin, (2) price of commercial purge compound, and (3) downtime cost per hour. The cost analysis will be put together after all of the trials have been run and all the data has been collected.

The easiest method in creating a cost analysis is to generate a simple excel spreadsheet as seen in figure 3.a. Most commercial purge compound manufacturers supply cost analysis calculators to their customers to assist with evaluations. Figure 3.b represents a real life cost analysis comparing purging an injection molding machine with Ultra Purge (Moulds Plus International is the manufacturer of the chemical purge compound called Ultra Purge) to virgin HDPE. The test was conducted on a 450 ton press with a 6 cavity hot runner system. Using the Ultra Purge compared to HDPE reduced the downtime by 64% and reduced the scrap by 75% resulting in a savings of $567.10 for this particular color change.

3a.
  Units Trial 1: Other Purging Compound Trial 2: Ultra Purge
Quanity of Resign Used lbs    
Quantity of Current Purging Compound Used lbs    
Quantity of Ultra Purge Used lbs    
Time Required for Purging Minutes    
3b.
Color change/Material Change Units HDPE Ultra Purge
Total Purging Material Cost USD 182 64.90
Time required for purging Min 240 60
Down-Time/Lost Production Hourly Cost USD/hr 150.00 150.00
Total Down-Time Cost USD 600.00 150.00
Total Cost per Purging USD 782.00 214.90
Total amount in USD saved per color change using Ultra Purge USD 567.10

Reduction Purging Material

64%

Reduction Down Time

75%

Comparing the cost of different purging methods is as simple as adding the total material cost to the total downtime cost. The total material cost is calculated by multiplying the total amount of commercial purge used by the cost per pound of the purge and then adding this to the total dollar amount of scrap resin produced. The downtime cost is calculated by multiplying the total minutes of downtime by the machine output per minute. The definition that should be used for downtime is the amount of time it takes to go from the last good dark colored part to the first acceptable light colored part that may be sold to the customer. The total between the downtime cost and material cost is what is considered to be the cost to purge the machine for that particular method. Thus, obtaining this value for all purging methods allows a company to quantify the most cost efficient method for purging their machines.

Commercial Purge Compounds

Two basic types of commercial purging compounds exist in the market today, chemical and mechanical. Chemical purge compounds work at the molecular level breaking the bonds of the polymers, which results in an increase in melt flow index. The reduced viscosity of the resin allow for the resin/color to be easily displaced by the next resin. Mechanical purge compounds work by scrubbing the machine clean. A good analogy for the two types of purge compound is that a chemical purge is similar to using soap and water to wash dirt from a person’s hands, while conversely a mechanical purge is comparable to cleaning a person’s hands with a brush. It is important to note that all commercial purge compounds have unique procedures. Using the proper procedures are imperative in order to maximize the performance of each purge compound.

Depending on the machinery, colorant and resin certain commercial purge compounds work better than others. Due to the fact that there are so many variables involved, it is important for thermoplastic processors to test a wide variety of different types of purging compounds. For example, not all commercial purge compounds are hot runner safe. Manufactures of commercial purge compounds typically offer free samples in order for companies to evaluate the performance of the respective purge compound. Most manufactures of purge compounds have multiple grades that are designed to purge specific types of resin and machinery. The data that is collected from the various commercial purge compound tests can be inputted into a cost analysis spreadsheet and the most economical solution can be easily selected.

Systems Approach

Using a commercial purge compound is unequivocally the most cost efficient method to purge thermoplastic processing machines. However, performing a fast and efficient color change is more involved than just adding a purge compound to a machine. Fast and efficient color changes are affected by many variables that companies can control. For example, the simple task of manually cleaning out the previous color from the hopper with a rag can be the difference between a thirty minute color change and a one hour color change. Figure 4.a represents the different variables that may affect a color change beginning with conveying the resin to the machine and ending with the hot runner design. Optimizing the system by using the correct screw/design, proper hot runner technology, a good quality colorant and a good performing purge compound will result in the most cost efficient color change that is possible. Not only will using a commercial purge compound assist with dark to light color changes, but it will also help clean the machine in a preventative maintenance fashion. Every time the equipment is purged to remove color, the compound will also remove the degraded material that is beginning to build on the screw or in the hot runners.

Conclusion

Companies in the thermoplastic processing industry are always striving to gain the competitive advantage in the global marketplace. Competing with different parts of the world that require lower costs to manufacture goods demand companies to look for alternative ways to reduce production costs. Plastic processors that process a variety of colors should seriously consider using a commercial purge compound in order to reduce downtime and scrap during dark to light color changes. Commercial purge compounds typically cost more per pound compared to resins; however, the majority of companies that use a commercial purge typically save enough money to pay for the purge and save the company money on top of this. Companies that have converted to using a commercial purge have saved tens and even hundreds of thousands of dollars a year. Thinking outside of the box and reducing production costs is what makes a good company great and it is also what allows companies to weather the storm when economy weakens.