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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.

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

Boss Cracking on a Polysulfone Part

A recent question was received via e-mail…

RZ
We are encountering cracking at the bosses on a polysulfone part. We press fit inserts on the bosses but we don’t see cracking during insertion. It is not until a day or two later that we see the cracking. Any thoughts?
My Response
The occurrence of cracking and crazing in this case is most likely due to the factors of molded-in stresses and strain.
Since polysulfone has a high degree of semi-crystallinity and rigidity, the resulting parts have many molded-in stresses… especially in the case of a boss. Many injection molders will anneal the parts with a heat cycle to reduce these molded-in stresses. Doing this before and after the insertion would significantly reduce these stresses.
Reducing the strain during insertion will also reduce the likelihood of cracking and crazing. Using a barbed, threaded, or trilobal insert may reduce this strain. The use of ultrasonic insertion or in-mold insertion will also reduce this.
Additional Thoughts
It may also be a good idea to mold the part in a translucent material so that you can see where the stresses are present under polarized light.
-Andy

Troubleshooting Voids… In The Sprue

I recently received an email from a friend of mine regarding voids…

Ron
How do you troubleshoot voids in the sprue?
My Response
The presence of voids in the sprue is very common, but the need to troubleshoot this is unique to sprue-gated parts. Voids in a sprue gate are a little different than a typical sink or void at the gate.
There are three ways to approach this… Cooling, Pressure, and Temperature.
The first way is to improve cooling at the sprue with additional cooling in the sprue area or mold cavity at the gate region. Beryllium copper inserts or a sprue made from an alternative material would improve the cooling in this region.
The second way to reduce this would be to increase the pressure applied during second stage time. As this pressure is increased, the time required to seal the gate will actually increase. Since the increased pressure causes a higher tendency for the polymer to flow back through the sprue.
The third way to improve the situation would be to increase the mold temperature… assuming sinks in the region are acceptable. Since higher mold temperatures increase the tendency to for sinks over voids.
Additional Thoughts
Such a situation is difficult to correct… and typically requires much trial and error to determine a correct process. Again, as with any other process, you really need to document the process thoroughly once the issue is resolved.
-Andy