Quick Color Change: 3 to 7 Minutes

Quick Changeover: The 3 to 7 Minute Color Change

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In lean manufacturing, changeover times are a vital component of shop performance. Minimal down time assures that multiple changeovers have little effect on a plant’s available machine time available. After all, less down time in the production equation offers more opportunities to take on more work and increase company profits. This article outlines specific approaches towards color changes that will support the potential of completing color changeovers in 3 to 7 minutes. This may seem like an impossible task to some, but with the proper planning, methods and equipment,  it is an attainable and repeatable goal.

Assessment: First of all, it is important to point out that this method applies to a specific set of circumstances. Many color changeovers can be optimized for maximum efficiency, but there are conditions that must be met before these techniques can be applied in the ways described. Here is a list of questions that must be asked during the development of fast changeover procedures:

  1. Does the mold being used have a hot runner system? Hot runner molds do not completely adapt into the quick change over system described, but with some modification to the procedures the change over times can be reduced dramatically. The primary drawback is that not only do the screw and barrel require purging, the hot runner manifold itself must also be converted over to the new color.
  2. Does the material being used require drying time? If so, what is the throughput used per hour? The methods described in this article cover the use of dried material, but part size and material throughput could affect timing and equipment needed for fast changeover.
  3. Is a grinder being used? Grinder cleaning can add significant time to changeover without proper planning. One way to remove the grinder from the changeover equation is to remove the grinder for cleaning, then collecting the new runners for the new color to be ground after the cleaning is complete. Another method is to have spare grinders available. This allows for grinders to be switched out by cleaning the replacement grinder prior to the changeover.
  4. Is the process being worked with stable? The primary goal of a process engineer is to develop a process that offers stability with any color. Stability is defined as a process that can be used to mold most if not all colors with minimal scrap and only small adjustments if any.

 

 

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Preparation & Equipment: This is the first stage of fast change development. In NASCAR, a pit stop is a planned event that utilizes a well trained crew who have all the tools needed to perform their job quickly and methodically. The same principles apply to color changeovers. Your team needs to be properly trained in the fastest approach procedure to perform the changeover quickly. They also need to have all the tools available to them that help them to perform their duties easily. This section outlines the tools and methods needed for color changes to be maximized:

  1. Hopper- Shutting down the press to clean a hopper is ineffective and adds to overall down time. The best way to avoid this is to keep the press running as the hopper is cleaned. There are a couple of ways to remove the hopper from the downtime equation:
    1. Satellite Hopper-The more expensive approach would be the use of a satellite hopper. This hopper would be mobile, allowing for the next color to be staged prior to changeover. When the job completes, the loader for the feed throat is removed from the stationary hopper and attached to the satellite hopper. The press is restarted, using material from the satellite hopper while the stationary hopper is cleaned and reloaded. When the stationary hopper is ready, the mobile unit is removed from the area, cleaned and prepped for the next changeover. This method works extremely well for materials that don’t require dry time. However, when material requires drying, the throughput of the satellite dryer needs to allow for drying the new material. If the satellite hopper only holds 2 hours worth of material, but you need 4 hours to dry the material in the hopper, you still have 2 hours of unnecessary downtime. Plan your equipment and approach. The goal of any facility that practices lean manufacturing ideologies is for color changes to typically take between 3 to 7 minutes, 15 minutes if there are complications such as hot runner use or above normal purging needs.
    2. Press-Side Loading– This method is far more economical, and easier to implement. The hose is disconnected from the hopper, and draws material from a bucket or small container while the hopper is being cleaned. Something to remember when utilizing this approach is that material taken out of a dryer can be used for up to an hour without drawing in enough moisture to affect your process. Based on this information, material can be drained from the hopper 1 hour before the order is complete so the hopper can be cleaned and prepped for the next color. Again, it is important to note that material that requires drying time can affect your ability to quickly change colors. One way around this is to use material that is vacuum packed in a foil bag. In most cases the material was packed hot, which removes the moisture in the pellet and because of the packaging technique moisture isn’t able to get back in. Calculate how many pounds you will need for proper drying time and fill the hopper using the foil bags first.
    3. Combination: In this method, material is dried at another location in preparation for the changeover. The hose is removed from the hopper 1 hour prior to the changeover to allow for cleaning of the dryer to take place. The press continues to run out of a bucket or barrel until the order is complete. Dry material is loaded into the hopper prior to the changeover. It is important to note that the amount of dry material loaded depends upon the amount of dry time required for the wet material that was loaded on top of the dry material. It is also recommended that you load enough material to supply your machine with 1 hour extra drying. This helps to compensate for non-dried material that funnels through the center of the hopper.
  2. Screw & Barrel: Part of quick changeover depends on having presses outfitted with one shot loaders. Hose lengths should be limited to only what is needed to attach to hopper and one shot, allowing enough slack in hose for moving barrel back and forth. Hopper side of hose should be outfitted for easy conversion to a material wand, and vice versa. In preparation for establishing a changeover procedure, shut the material loader off with the press running the job that you are creating a procedure for. Make sure the loader is shut off with the crew having rotated back, waiting for the mold to open. After the shot in the mold is ejected, begin counting how many cycles the press runs before the screw runs dry. This gives you the shot throughput. Record this number for later use which will be outlined in this article.

 

  1. Color Planning: It is important to understand what to expect when changing from one color to another. This next section outlines a method to analyze what steps must be taken from a cleaning standpoint:

Think of colors in terms of shades. This section will use 4 degrees of color shades going from light to dark. The color examples used below are white to black, white to dark blue, white to dark green and white to dark red. The labels assigned to each shade level are X1, X2, X3, X4. See the chart below for examples of these labels in use:

 

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X1 to X2, X2 to X3, X3 to X4: These light to dark variations are simple color changes. Material conveying and storage equipment can be quickly cleaned. The new material  can be used to purge to the proper color and shade. Purging compound in most cases will not be necessary.

X1 to X3, X1 to X4: These are simple and quick changeovers. The instructions outlined above apply to this level of color change as well.

X4 to X3: The rules above generally apply here as well, but there is the potential that the barrel might need to be purged with a purge compound if scrap occurs in the form of color swirls.

X3 to X2: In some cases, cleaning the storage and conveying equipment more thoroughly may be required. Purge compound might also need to be used.

X2 to X1: Going to white from even the lightest color can be tricky. Testing is the best way to determine what the results will be. If going from the lighter shade can be completed using no purge compound, this can optimize change over time. Thorough cleaning will be important to prevent contamination. Be sure to wipe off the nozzle tip to prevent spec contamination.

X3 to X1, X4 to X1: Purging the screw and barrel will be necessary to assure that screw buildup has been removed. Thorough cleaning of the material storage and conveying equipment will be necessary to prevent scrap resulting from cross contamination.

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The 3 to 7 minute color change: Similar to NASCAR, this type of lean manufacturing change over requires planning, preparation and efficiency. Here are the steps for performing it:

 

The following information will be used as example data for this procedure:

Material throughput(Shot Size): 50 lbs p/hr

Material type: Nylon

Shot Throughput: Screw runs dry in 8 cycles

Parts per hour: 60 pph

Hopper size: 300 lbs

Beginning of 1st shift (6AM): 320 parts to finish order before change over

Changeover type: X1 to X3

Planning: Planning is the first stage of fast changeover, and the data above is used to develop the plan. Here are the data points to analyze and how they relate to plan development:

  • At the beginning of the shift, 320 more parts are needed to finish the order. The press is running 60 parts per hour. Based on these variables, the press will be ready to change in 5.33 hours, or around 11:20 AM respectively. It is important to remember that if the press is running scrap, that will need to be calculated into the run time. For instance, a press running 3% scrap must be calculated at 1.03 {Parts needed / parts parts per hour  x (1 + scrap percentage)}
  • The material running is Nylon, which of course is a material that requires drying. The hopper being used is a 300 lb hopper with a 50 lbs p/hr throughput. Based on this information, the hopper would run out of material in 5-6 hours. Based on this, material should stop being loaded in 4 hours which will leave the hopper nearly empty at the time of the change. It is important to note that scrap rate adds to run time. Consider this as you prep. Use the formula above to calculate additional time.
  • Nylon is a material that draws moisture, but it can be used for up to an hour outside the dryer. Based on this, drain the hopper into a container 45 minutes prior to the time of changeover. The material throughput for part size has been calculated as a usage rate of 50 lbs p/hr. The container you are using must hold at least 50 lbs, and it is highly recommended that it have a lid designed to accept a material wand to prevent moisture from accessing the container. Modify the feed throat loader hose for wand use. Load the material in the container next to the press to keep the press running as the hopper is cleaned. Changing over the loading system should be performed without shutting down the press.
  • Pull the grinder from the press 30 minutes prior to the changeover time to be cleaned. Runners can be dropped into a catch container while finishing the run. The runners are either disposed of, or saved in a container to be ground at a later time.
  • According to the data above, the barrel runs dry 8 shots after the loader is turned off. A container with dry material is placed press side just before it is time for the change.When the order has 6 parts to go, turn off the loader and prep to clean the wand and one shot. This occurs as soon as the material is out of the one shot and positioned at the top of the feed throat. Once cleaned, put the loading equipment back into ready positions.
  • When the machine is molding the last shot, put the press in semi automatic, put the wand in either purge compound, or the new material based on the chart above. Turn on the loader and start loading for the changeover.
  • When the press stops, quickly pull the carriage back just enough to give you access to the purge droppings. Moving the carriage too far back adds time to the changeover.
  • Changeover with purge compound: Run the screw, watching the melt flow from the tip until the material exiting is nearly the same color as the purge compound in natural form. Quickly transition the wand into the container of new material. Purge the new color through until the melt exiting the tip is the right color with no signs of swirl or specs. Make it a habit using a rag to wipe the nozzle tip. This will help to prevent specs and swirling on restart.
  • New Color Change Over: Purge the new color through until the melt exiting the tip is the right color with no signs of swirl or specs. Make it a habit using a rag to wipe the nozzle tip. This will help to prevent specs and swirling on restart.

Remember, the procedures outlined in this article are merely starting points for your facility. You may need to make adjustments to these procedures based on the materials you use and material characteristics and requirements. Continuous improvement is a primary goal in the science of lean manufacturing. Design your approach using the experience of your team, and provide them with all the tools and knowledge required. Optimum performance is best achieved through proper planning, analysis and preparation.

 

 

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