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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.

Self-Paced vs. Self-Taught…

I was just asked this question on the phone…

JD
If I leave the training up to my people, it will never get done. I have many books, magazines, and articles available to my employees, but they never use them. What is the difference between that and self-paced training?
 
My Response
Self-Taught: This expression implies that the employee actually guides, develops, and executes their own learning program, and is motivated enough to make it work. Although you have occasionally heard stories of those who are ‘self-taught’, these highly motivated employees are extremely rare.
Self-Paced: This expression refers to employees who are given specific training, tasks, exercises, and direction, and they follow this at a pace suitable to themselves. In cases where employees have reasonable goals and objectives,  virtually every employee has the ability to learn and grow professionally… especially when they understand how it benefits them professionally.
Additional Thoughts
Basically, every employee wants to succeed at their job, but not all employees receive the training necessary to do their jobs well.
-Andy

Preheating the Polymer…

A customer asked me this question the other day…

KL
We are using a screw specifically designed for our grade of PP, we have performed the tact temperature study, and have optimized both injection and packing. All these have helped, yet our cycle time is still limited by the screw recovery time, do you have any suggestions?

My Response
Once the process and screw have been optimized, the next step you can undertake is to preheat the polymer. Even though this material does not need drying, a simple hot air hopper dryer can be used to preheat the material to reduce he amount of heat necessary to melt the material. The more heat that is added to the material before it enters the barrel, the less energy that is required by the screw and heater bands.
Semi-crystalline polymers benefit most from pre-heating since the polymer will typically endure more preheating due to their sharp melting point.
Additional Thoughts
Always be cautious of your temperature in both the feedthroat and rear zone of the barrel. Since preheating hastens the melting, you may need to repeat the tact temperature study.
-Andy

Dynamic vs. Static Cavity Balancing…

After answering some follow-up questions from the last post, I decided to clarify the difference between Dynamic cavity balancing and Static cavity balancing with repsect to either injeciton molding or injection-blow molding.

Dynamic Cavity Balancing
When the mold cavity is filling, the polymer flow front is considered dynamic, or in motion. The balance of two or more mold cavities at this stage primarily relates to the consistency of pressure loss and shear balancing between the mold cavities. This relates to both hot and cold runner systems. The more the cavities are dynamically balanced, the more consistent the process will be in the long term.
Static Cavity Balancing
Once the flow front reaches the end of the mold cavity, it becomes static. At this stage, part weight differentials are primarily based on the variability in pressures and temperatures. This information is important for final part quality, but can be significantly different than the dynamic balance.
-Andy

Cavity Balance for Blow Molding…

I received this question over the weekend…

Ponce
What is the right procedure for performing a cavity balance test for a blow molding machine with an injection, pack and hold?
My Response
Basically, the procedure is similar to that of an injection molding machine. You would turn off the second and third stage, and then compare the shot weight of the different cavities.
Additional Thoughts
To calculate the % variation is as follows:
100% x (maximum weight – minimum weight) รท (maximum weight) = % Variation
-Andy

DIII Process and Part Dimensions…

During an on-site consultation, I was recently asked this question…

George
When troubleshooting a Decoupled III process for variations in overall part dimensions, what parameters should we investigate.

note: A DIII process has many specific attributes, but the key aspects include a velocity controlled 1st stage fill (90%), 1st to 2nd stage transfer by position, velocity controlled 2nd stage pack, 2nd to 3rd stage transfer on cavity pressure at the gate, and 2rd stage hold by pressure control.
My Response
The parameters and process outputs which affect part size in a DIII process are commonly:
  • Melt Tempertaure
  • Transfer Setpoint
  • Packing Speed
  • Hold Pressure
  • Mold Temperature
Additional Thoughts

One of the best ways to approach dimensional issues with a DIII process, would be to compare the pack rate and cooling rate to the documented standard and return them to the standard. These are the upward and downward slopes of the in-cavity pressure curves representing the rate of packing and cooling within the cavity.

-Andy