How Plastic Material is Made : Extrusion

Plastic injection processors mold material every day .. but do you know how it is produced? There are keys in the extrusion process that can sometimes interpret potential problems on the molding floor. This artticle will explain the process and its relevence to plastic injection.

Material starts as a mixture of powders.. there is a base (such as ABS, Polypropylene,etc.), sometimes additives to improve the blend (in ex: UV to reduce sun damage, or fiberglass to increase rigidity) and pigmented colors to give the material its desired hue.

The extrusion press has similar characteristics of its injection counterpart...it has a screw for blending and moving the material forward. The barrel containing the screw has heater bands to help control primary shear heat. The difference is, the screw does not shoot material...it merely spins. As it rotates, the material is forced out a faceplate with holes drilled into it called a "head". As the material comes out of the holes, it is formed into strands.

These material strands are pulled by the machine operator down a long trough, called a "water bed". The strands run at the bottom of the bed, being held in place by a series of rollers to keep the strands submerged. Towards the end of the bed, the strands are allowed to come above the water through several sets of brushes. The brushes are used to wipe the water off of the strands.

At this point,the strands run through a series of air blowers to remove the remainder of the water. The strands run from the blowers into a large machine similar to a grinder, called a pelletizer. The pelletizer cuts the strands into uniform chunks...the pellet that is familiar to you. Pellet length is controlled by the speed (RPM) that the pelletizer turns. The faster the blades turn, the shorter the pellet size. The pellets also go over a vibrating tray, designed to remove fines.
Pellet diameter is controlled by screw RPM...the faster the screw is ran, the larger the pellet diameter will be.

Now that we have outlined the extrusion process, we will identify elements that can affect the plastic injection side of the equation:

-material changeover: The screw is pulled often when changing material types (such as polypro to ABS) or when going from a dark to light color. This is why it is important to pay attention to lot changes. The beginning and end of an extrusion process is when problems with lots are most likely to occur. When problems point towards a material issue, the lot should be checked for a)whether it was a beginning of lot container and b) material should be checked visually for consistency if it was produced at the end of lot. Pellet diameter and length are important dimensions.

-Base material: The potential for a mistaken base material or improper blend of base, additives and pigment exists. This is generally going to happen at the beginning of a material lot.

-contamination: When running light or clear materials, one of the first things to check when there are scrap issues associated with contamination is a visual inspection of the material being used for contamination within the pellets, and whether the lot being used has changed or is a beginning of run container. The potential exists for an improper screw cleaning, or a contaminated blend receptacle used to blend the powders. The potential for excessive fines (material ran through vibration portion of pelletizer too fast) also exists.

Inspecting for these things can sometimes help you better understand where a problem's roots lie. Most extrusion manufacturers are happy to work with you when problems are found. It takes both extrusion and injection working together to develop product efficiencies.

Scientific Molding:Definition and application

Injection molding came a long way in a relatively short span in the realm of the industrial age. In the beginning, molding was more of an art form than it was a science. John Bozzelli and some of the other pioneers in decoupled and scientific molding changed all of that. They developed new molding techniques based on processing variable replication that revolutionized the methods of an entire industry. Today's article defines the practice of "scientific molding" and will touch on some of the fundamentals that this application supports.

Scientific molding is best defined as an extension of decoupled molding theory. When using decoupled molding procedures, a process is established with the part filling out at 95-98% of being completely filled with pack and hold pressures set at 0. A typical part at this percentage would appear slightly unfilled, or with a sinky appearance. Pack and hold are then added back into the process adding just enough to pack out the part(s).

Where scientific molding takes a step further is the extension of this procedure. The procedure of decoupling is completed, and then all data that can be recorded is taken..in essence taking a "snapshot" of the process while it is in a good running state. Typical molding variables to be accounted for are melt temperature,cushion, fill time, recovery time, peak pressure, cycle time and any other actuals data that can be recorded. The ones listed here are the basics, but I'm a bit more thorough. For instance, I like to get:

- actual mold temps in several locations(running state)
-water pressure(to and from process)
-barrel temperature actuals(each zone, using probe)
-replication of water set up(hard plummed, or water set up sheet, repeating flow directions)
-decomp.aft. rotation actual

The primary point of scientific theory is to repeat the process set-up and actuals to assure that a good process results by repeating the previous values. Because of this, it is only logical that the more data you are able to document when a job is running well, the more likely you will be able to repeat the molding condition the next time around.

That is a basic description...now for a little more advanced version, let's touch on some additional testing procedures...

-In addition to melt temp, a hold pressure study should be completed to determine the gate seal time
-With the shot size set at 85%, back pressure should be raised in increments...stopping the screw after allowing soak time during cycle to perform a melt temperature reading. This data should be recorded on a graph. When the melt temperature stops rising, this is the max setpoint for back pressure. Back pressure should be set at the point in the graph that major temperature increases falter, and melt temperature rises are more subtle in their rise.

-With the shot size set at 85%, velocities should be adjusted from slow to fast, graphing as described above. This time, fill time is the factor to watch. When fill stabilizes and the time stays equal, you have maxxed out your velocities. Again, optimally the velocities should be set at the setpoints recorded that show fill time going from rapid increases to a gradual rise.

It is important to realize of course that sometimes your process will require adjustments to be made in order to get a process running good parts again. Whenever possible, the technique above should be followed as closely as possible to assist you in running your processes at optimum efficiciencies.

When a part requires velocity profiling, each stage of the profile should be treated as a single velocity. Adjust the speed for each position from slow to fast, again monitoring the fill time as described above.

Coupled with set up standardization, adding scientific method to your process will greatly improve your control of each of your processes. It is important to note, that when a process has been established as consistently performing well it is a solid framework and should be followed closely with only minor changes to process. If you see a drastic change in process variables, it is important to realize that something changed. All of your recorded variables should be examined for change and when a change is discovered it should be analyzed for potential causes in the state of that condition(in ex: melt temp change due to a heater band going out).

Look for more on this subject in later articles....

Plastic Material Problems: Plastics Drying, Defects, Lot Changes, Material Changes, Properties and Solutions

A company's material handling methods and supplier choices can be their best friend or biggest nightmare. This article addresses typical material problems and solutions for approaching issues as they arrise. As I'm sure many of you are well aware, this is a monstrous topic...and my being able to identify and offer solutions to every material topic in a single article are slim to none. I will touch on a few topics at a time, and as time goes by I will expand on these in greater detail.

This particular article will touch on the various issues that plague our daily operations and offer some brief information on potential causes and solutions.

It is important to understand the variety of properties that are associated with different material categories. There are many different grades and types of plastics, and each grade has its own set of rules and potential defects. It is important to always be aware of what type of material you are running and you should always refer to your MSDS when issues arrise that could be directly related to a material issue. A good material supplier performs extensive testing on the materials they produce, and you should always be able to get resource information from them regarding the molding properties associated with the material they are providing you. Some examples of data they should be able to offer you are as follows:

• Mean melt temperature
• Melt Window
• Specific gravity
• Temperature settings (Window
• Potential Problems
• Velocities
• Back Pressure/ Hold Pressure settings
• Mold Temperature setpoints

These are the basic ones of importance...there are others that sometimes need to be addressed as well. You can get some pretty intensive information regarding a specific material's properties from the following website:

http://www.ides.com

Materials sometimes require drying, and even removing the moisture for extended periods beyond manufacturer specs can lead to an array of other problems, such as splay and process variances. It is important to know what the minimum/ maximum drying times for your material should be, and when choosing a dryer you should calculate what the throughput of that dryer will be. There is a throughput calculator on the site that can help you determine this.

Drying temperatures are also important. I've seen situations where a material seemed to be performing incorrectly, and the cause was an individual trying to run at a higher temperature on the dryer for a shorter length of time. Sure, this may work on occasion...but it is not a good approach. Material specifications exist because people put a significant amount of testing and time determining how a material performed optimally. I will say that there are times you can push those specs to their limit...but any such action should be approached with caution and as a sample first. Changing your material procedure can affect your end product's functional, aesthetic and dimensional properties.

Materials that are susceptible to moisture should be considered when there are changes to your normal process. It is important to know what moisture level your material runs best at (moisture analyzer) and one of the first things you should check when moisture-specific defects arrise is the moisture content.

Another thing that can result in a drastic process change is a change from one lot of material to a new lot. I cannot stress how important it is for you to track your lots! There have been a number of times that I have witnessed a huge crisis that was identified as a change in lot properties.

I've just barely scratched the surface on this topic. Material can make or break consistency and profitability in a company. Buying the cheapest does not concrete being the most economic approach. If there are huge variances in your processes (resulting in scrap) that can be resolved by buying a better brand or a specific blend, then this may be the best approach to a long term solution.

Can't we all just get along?

I commented this blog written elsewhere..in response to a very lopsided post based on environmental hysteria, rather than data..here's the link:

http://theonlyonewehave.wordpress.com/2007/05/13/70-of-americans-dont-know-plastic-is-made-with-oil/#comment-405

So blatantly incorrect, I had to respond:

Just like you… I too am concerned by the current environmental situation…enough so to leave an engineering position on the production side of things and jump fence to the recycling side. I intend to use my skillsets to make a difference…not my voice to create a panic.
I have been in the process of gaining data…for the purpose of building a recycling center in Michigan. One thing I know is this…I am not biased about who’s data I use…but your statement that plastics production accounts for 10% of the worlds oil consumption is blatantly incorrect…the numbers I’m seeing are in the 3-5% range…with improvements made over the last couple of years… and a trend towards making things right.
I will say that I agree with you fully..there is a very strong need to educate the public. But education is NOT defined as propaganda…it is a clear representation of both sides and ALL issues..so that educated decisions can be made based on the facts…not ropes hung in a medieval witch hunt.
I run a plastics website…and I have a section devoted to environmental concerns. This section exists for the sole purpose that I as a person who loves this planet have the same concerns regarding its misuse as you do.But the products I produce have been in your best interest…I’ve been breathing the fumes you fear for 23 years now…and I am as healthy as a horse.
Bags an issue…you betcha…marine aquatic issues…certainly…but that is why I’m here. Because I care, and want to make a difference. I am glad to help…I’m more than willing to listen. But remember…there are two sides. And they live and breathe the same air….

I'm telling you, folks...it is time for us to step up and educate those who are operating on mistruths and assumptions.

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