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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

Fill Time, Viscosity & Molding Economics

A friend emailed me this great question the other day regarding the relationship between fill speed and pressure…
JM
The pressure required to fill a particular mold went up from 950 psi at 1 in/s to 1050 psi at 3 in/s. 

I understand the higher pressure needed to hit the higher speeds, but shouldn’t this be offset by the significantly reduced viscosity?

Does it hold true that anytime you increase the fill velocity, you see an increase in fill pressure… all things being equal?
My Response
The pressure required to fill will increase because the viscosity change will not completely outweigh the pressure losses… The overall energy consumption does drop considerably during fill since the pressure to fill is being appllied for only one third the amount of time!
It is very likely that a rheology curve would demonstrate that the 1 in/s fill is on the left hand side of the shear thinning transition region and the 3 in/s is on the right hand side of this region.
Think of it in vehicular terms… Your 1 in/s is like pushing a large pickup (higher resistance to movement) at 20 miles per hour, while your 3 in/s is like pushing a compact car (lower ristance to movement) at 60 miles per hour. It takes more gas per minute to move the smaller car at the higher rate, but it takes less time and you consume less gas overall getting to your final destination.
Additional Thoughts
Always keep in mind…the reduced viscosity due to shear thinning will actually save you money… making your processes much more economical since the overall energy consumption to fill the mold as well as the time are reduced.
Additionally, the drop in viscosity will also reduce the pressure required to pack out the mold cavity during 2nd stage. In the long run, all these changes can make a big difference in the productivity and efficiency of your facility.
-Andy

PET Screw Design For Injection Molding

I received this blog topic request yesterday…

Samer
Concerning injection molding, is there a specialized screw used for PET?
My Response
Yes, the goal in PET processing is to (1) maximize output, (2) control shear heat, and (3) minimize melt tempertaure… yet ensure the polymer is fully melted and as homogeneous as possible.
(1) The need to maximize output is due to the fact that most PET injection molding processes are for high-cavitation high-speed molding PET preform machines.
(2) The need to control shear heat is due to the fact that a higher temperature polymer requires more time to cool… and may increase the development of semi-crystalline regions. Also, inconsistent melt temperatures will result in inconsistent cooling and crystallinity… resulting in profit losses due to scrap and troubleshooting.
(3) PET also has very poor thermal conductivity… making it great for drink containers, but terrible for part cooling. The goal in processing PET is to process the material as close to the melting point as possible, while ensuring all the pellets are melted and mixed.
To address all these concerns, different machine manufacturers have devised some great machine and screw designs.
Whether it is a reciprocating screw machine, or a two stage ‘shot-pot’ machine, most PET single screw designs incorporate a barrier screw. In such a design, the screw channel depth remains the same from the feed into the transition zone, but the channel width becomes progressively smaller. As this channel width decreases… an adjacent shallow channel, separated by a barrier flight begins. Ultimately, the melted material flows over the barrier flight into the shallow channel until it transitions into the metering zone. Such a design reduces shear heating and allows stingy pellets more time to melt.
In most single screw designs, a distributive mixing element is typically incorporated on the screw or a static mixer is placed within the nozzle to ensure a more homogeneous melt… especially if a colorant is added to the polymer.
Some of the more ambitious machine manufacturers are working with twin screw designs for two stage molding machines. These systems show great promise since a good twin screw design would provide the best control over both shear and melt temperature.
Additional Thoughts
PET processing is one of the more advanced and refined fields of injection molding, making it very competitive. I strongly suggest anyone getting into PET molding do some significant research to ensure your strategy is well suited to take advantage of today’s advances.
-Andy

The Pitfalls Of Classroom Training

I was recently asked this interesting question…

Customer
I see that you teach at a university, are you planning on providing classroom training?
My Response
Classroom training is not the most effective method available for training your production employees. Some seminars provide 4-8 hours of training a day… which is far too much information to absorb in one week…never mind in one day!
Lengthy classroom training sessions tends to be very disruptive to the production environment. Sending employees out for training can often cost 1-2 times more than the training itself. Likewise, sending a consultant to your plant for on-site training classroom training can be both costly and distracting to your employees. In either case, you still have the underlying issue where the employees do not learn well in the classroom environment.
As an adjunct professor at the Plastics Engineering Department of the University of Massachusetts Lowell, I have successfully blended labs, interactive training, simulation software, product testing, new technology, and outside exercises. Since these classes are comprised of plastics professionals, this helps ensure my participants actually learn the information and understand how it applies to their role as production employees. At Routsis Associates, I have also used this approach successfully in distance learning situations using a variety of media such as interactive training, on-the-job exercises, testing, and short 1-hour training sessions.
Additional Thoughts
As I blogged previously, ask yourself… ‘What Do You Hope To Achieve Through Training?’. To paraphrase… training should be purchased to help meet and sustain specific employee development and competency goals. Keeping this in mind will help you evaluate whether a training method develop competent employees, or just give them a bunch of information.
-Andy

Buffered Pressure Limit Settings

This question came in last month. Although I could not get any specifics about the machine such as age or model, I believe the question still makes a good discussion…

E-Mailer

Today I tried to adjust limit of injection pressure on Demag machine. I adjust maximum of machine 210 bar actual injection pressure was 160 bar, so I decided to decrease limit to 190 bar, but actual pressure decreased to 150 bar. I tried to decrease limit to 170 bar, but actual pressure decrease again to 130 bar. 
I would say this is not a good thing.  It seems like that this is built into the programming of the Demag machine.  Might be automatically trying to create a pressure differential, but by doing it this way it moves the process back to pressure controlled rather than velocity controlled.
My Response
Although I cannot speak specifically about that particular machine, there are some general reasons for this behavior.

Most machines do have a buffer to ensure the machine does not overcompensate when approaching the maximum setting. Sometimes this buffer compensates for line pressure or pressure losses in the system. 

Often machines set the limit near the pump and take peak pressure measurements at the cylinders… which does not compensate for losses in the system. Certain machine types even use a pressure differential to control velocity, which could cause this difference.

Another cause for this could be dynamic breaking… to compete with the accurate controls of electric molding machines, machines are applying a small amount of hydraulic pressure to the front of the injection cylinders to better control the velocity transitions during injection. The displayed injection pressure will most likely be calculated as a differential between the front and back of the cylinders.

In any of these cases, to test if you are actually hitting this peak limit, increase your max pressure and see if the peak pressure increases.

You may also want to perform a Dynamic Load Sensitivity Test on the machine to ensure the machine controls are functioning properly and there is not a problem with the hydraulics or controls.

Additional Thoughts
As the molding machine technology increases, the specific functionality of molding machines is becoming less obvious. This is why it is very important to good relationship with the technical representatives at your machine supplier. If the machine is an older model, a machine re-builder can also be a great resource as they tend to have great knowledge of your machine’s shortcommings.

-Andy

‘If I Train My Employees… They Will Leave’

I very often hear people justify not training their employees with the age old argument ‘If I Train My Employees… They Might Leave.’ One of the funniest responses to this I have heard is from Zig Ziglar who says ‘If you don’t train your employees… they might stay!’.
All humor aside, ignorance is never a good employee retention policy. For example, one company we worked with had a 75% turnover. This meant that 3 out of every 4 employees leave each year. When we discussed employee trianing, it turns out that they never train, nor do they promote from within. They went silent when we asked them… ‘So, am I correct to understand that everyone at your company is in a dead-end job’.
Unless your company has a true culture of learning and advancement, sending only one or two people to specialized training provides them credentials not applicable to your production environment. For example, ‘5S’ training provides great information… but sending one employee out for a couple days of training will not transform the attitude and actions of your entire workforce.
Ask yourself two questions… (1) do you have enough ‘key’ people on your production floor…? and (2) are any of your production employees in ‘dead end jobs’…?
The goal here is not to sell…  but to help people understand that you need a plant-wide focus on improving the skills of the entire workforce so ultimately, everyone shares the load.
-Andy