Plastics Technology: December 2009 Archives

Understanding Torque...

Andy Routsis
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I received this question the other day via telephone...

Steve
Can you explain the concept of torque...? and why using a torque wench is so important?

My Response
Basically... Torque is a measure of rotational force. In other words, the force being applied to rotate something is considered torque.

The way torque is calculated is by multiplying the force being applied times the distance it is being applied. This is typically represented as Newton-meters (N-m) in the Metric system or foot-pounds (ft-lb) for Imperial measurements.

For a more detailed definition of torque, please feel free to visit wikipedia:

Technicians and mechanics often use torque wrenches to measure the rotational force being applied to a screw when they are being tightened. This helps prevent the platen threads from becoming stripped or damaged.

For a more in-depth discussion on this topic, please visit my previous blog:

Proper Torque Value for Clamping Mold to Platen


Additional Thoughts
Engineers are often concerned with the amount of torque or rotational force being generated by a servo motor. Since the energy usage of the servo motor is measured in AMPS... it will directly relate to the rotational torque being produced.

-Andy

Producing Hollow Parts...

Andy Routsis
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A molder in China just sent this unique question about producing hollow parts...

CJ
We are a molder in China and our customer asked us to prototype a decorative hollow part for a consumer application. What is the best method to do this?

My Response
In the prototype phase, the quickest, and least expensive way to do this is using either rotational molding or blow molding.

Rotational Molding: The material clamped within a mold, it is rotated about multiple axis while it is heated to melt the material and cover the circumference of the interior. The mold is then cooled and the part is eventually removed. This process is very slow, but is great for very low production prototypes.

Extrusion Blow Molding: This process uses an extruded, tube shaped, parison which is clamped within the mold and blown to size using pressurized air. The mold is more expensive than a rotational mold, but you are able to manufacture many more parts than with rotational molding.

You can learn more about Extrusion Blow Mlding with our blow molding series of training:

Additional Thoughts
You can also produce the part using injection molding along with an assembly process such as ultrasonic welding... but this seems far too expensive for a prototype of a basic decorative consumer part.

-Andy
One reader asked this question regarding grinder selection...

Allen
I am considering the purchase of a low RPM grinder for nylon sprues and runners, is there any reason I should reconsider?

My Response
Note: High RPM grinders typically rotate a set of blades at a very high speed. When the rotating blades pass by the stationary bed, they cut the part giving the familiar rat-tat-tat sound. The motors on these grinders tend to be over sized... and they often consume a significant amount of energy relative to the amount of material they consume since they have to maintain a very high RPM to work effectively.

In most cases, I prefer the low RPM grinders because they consume less energy than high RPM grinders... especially if it is running constantly. Low RPM grinders use a heavier set of stepped blades which operate more by maintaining the momentum of the blades to steadily chew up the plastic. Over time... theses grinders tend to be less expensive to operate and maintain... especially if they are in constant use.

Additional Thoughts
Most grinders are best used in applications where the amount of material being fed into the grinder is close to the maximum amount it can consume. A grinder which consumes only 10-25% of it's capacity can waste a large amount of energy over time.

-Andy




I was on-site the other day and was asked a common question from the tool room manager...

Ron
It seems like our molding technicians know very little about tooling. As a result, they always use the wrong name for components such as gibs, lifters, and actuators... which makes the toolmakers laugh. What is the best way to improve this?

My Response
Believe it or not, this is actually a very common situation. The best way to improve this is to have the tooling personnel help cross-train your employees. This makes them partially responsible for the technicians knowledge  of tooling. This gives them a vested interest in helping the technicians rather than insulting them.

A great opportunity to do this would be during mold maintenance. If you have the technicians assist in the mold breakdown or re-assembly the tooling person can explain the name and purpose of each component during the process.

Additional Thoughts
Much of this comes from the defensiveness on the moldmakers part since they seldom have a good knowledge of processing. So... don't forget to reverse the process and have the technicians teach your tooling personnel about processing.

-Andy




This question just came in regarding energy savings and electric molding machines...

Kyle
I know that having a short shot can provide a more reliable process, but my technician says a fill during first stage saves electricity since we can use a lower packing pressure. Is this True?

My Response
First, you are correct that your process will be more solid and reliable if it is short during first stage. This is critical to compensating for material variation... which will always save you money over the long term.

Second, your first stage injection pressure will show an increase as the material reaches the end of fill. If you look at the injection pressure integral for the entire cycle, you will find that the energy applied to the polymer will be virtually equal in either case.

Additional Thoughts
You should always approach your processing in a logical manner... regardless of whether the machine has an all-electric, hydraulic, or a hybrid design.

-Andy
I recently received this from an extruder manufacturing PP film...

RB
We are processing a 1.1mil 3-layer PP film with a random copolymer.

Initially, we were processing at 210ºC with a kinetic COF (coefficient of friction) = 0.5 initially, and 0.2 after two weeks.

Currently, if we process at 210ºC, we get a COF of 0.8, so we must process at 225 to get a COF of 0.5.

Do you know why we are getting this increase in COF?

note: After further questioning, it was discovered that the quantities of slip, AB, and MB anti-block additives have not changed. Also, they do not currently collect any MFI (melt flow index) or viscosity data from the supplier.

My Response
It is very likely that your material supplier has changed material characteristics on you. The efficiency of your additives relies on their ability to 'bloom' or migrate to the surface. Additionally... it appears that overall morphology of the polymer, including crystallinity, has changed since the material no longer exhibits the two week drop in COF as it once did.

If a significant change in either molecular weight, or molecular weight distribution has occurred, then it will change the COF of the base polymer as well as the additive's ability to 'bloom' or migrate to the surface.

You should require the supplier to provide certification for each lot that you receive. This should include some basic data including the Melt Flow Index.

note: the same effect occurs with injection molding with molded-in lubricants and internal release agents.

Additional Thoughts
You should consider performing some basic material tests at your facility... this should include the MFI (Melt Flow Index), but may also include the DSC (Differential Scanning Calorimeter) and capillary rheometer.

Some companies actually purchase a benchtop extrusion line to test and understand the processing characteristics of incoming material lots prior to actual production.

-Andy
As a follow-up to one of my recent posts, I received this question...

Matt

My Response
Regarding the screw - Most screw manufacturers will recommend that you use an outside micrometer. Since the flights do not match up, you should lay a block gauge block across a couple flights on one side of the screw and deduct the thickness of that block from the overall measurement. For consistency, you should note the distance, from the end of the screw, that the micrometer contacts the screw. As a result you should have a table with lengths and corresponding diameters.

Regarding the barrel - Using an inside micrometer, most companies will follow a similar procedure as with the screw. The diameters should be take at specific distances down the barrel and be listed in a table of lengths and diameters.

Additional Thoughts
You should avoid using a surface plate with a height gauge to measure the screw wear... This will often mask the areas of the screw with high wear.

-Andy

Measuring Screw Wear

Andy Routsis
Vote 1 Vote
A friend of ours recently asked this question about screw wear...

Jim
I can get maintenance to measure the screw, but their philosophy is that there’s no reason to do so. Typically we replace barrels and check rings, but do not know what the state of the screw is.  I think we should know, and measure, but I can’t say technically why.  Can you help?

My Response
The purpose of routinely measuring screw and barrel wear to see the trend over time.  This practice allows you to preemptively correct for wear by scheduling repairs and replacements. for example, let's suppose a screw is still working, but having some mixing or melting issues, you can document approximately how much wear, and clearance, indicates such complications. If you see a similar situation developing on another machine, you can schedule a screw replacement or repair, rather than react after you produce a bunch of scrap. You can also avoid certain jobs with sensitive materials or strict colorant requirements if you know a machine is suspect.

For comparison… top drag racers completely dismantle and re-assemble the engine after each race, replacing everything from pistons to pushrods (takes about 90 minutes). The components they remove are later measured and a determination is made whether to re-use, refurbish, or discard each component. Each team has its own set of records derived from experience to identify when a component is useful or useless. If the team fields different classes of cars, then the acceptable tolerances would vary from machine to machine. 

Additional Thoughts
There are always rules of thumb, but your mechanics need to think about their jobs more like crew chiefs knowing the driver (mold) is stepping into a machine that is ready to do the job.

-Andy
I receive this question very often, and feel it would be great for the blog...

Larry
Can you briefly explain melt flow index, and how processors use it?

My Response
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. The melt flow index (MFI) is the measure of how many grams of polymer pass through a standardized capillary under a standard load over 10 minutes. The value obtained through the melt flow index test is a single data point. The melt flow index only tests the material at one shear stress, and temperature. 

In general, a higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials.

Many processors use this data to qualify incoming materials and to help anticipate changes in the process. For example, if the lot of material you are processing has a MFI of 10, and a new lot has an MFI of 15... you can anticipate issues such as flash, over packing, or overweight product and make the appropriate adjustments.

Additional Thoughts
To obtain more accurate and relevant viscosity data... it is better to perform rheology tests using a capillary rheometer or a parallel plate rheometer. Many companies will also perform in-mold rheology tests using actual production molds.

-Andy

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