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Setting Final Cushion…

During some classroom training the other week, a student asked me the following question…

John
What value should you set the cushion at?

My Response
A good rule of thumb is 10% of the initial shot size. For example, if the overall shot size is 100mm, then the cushion should be about 10mm. This avoids pressure losses from too much material in front of the screw, yet provides enough room in front of the screw for normal variability.
Related Posts:
-Andy

Teaching Tolerances…

I was asked this question during a recent on-site visit…

QA Manager
What is an easy was to explain tolerances to new employees?
My Response
Focus on the fact that tolerances are commonly expressed in a way which can be easily broken up into simple math equations.
Dimension: 1.00 ± 0.01
Upper Limit: 1.00 + 0.01 = 1.01
Lower Limit: 1.00 – 0.01 = 0.99
Additional Thoughts
When possible break ideas into their simplest form such as an equation or concept. Keep in mind the issue is very often math not tolerances. In this case, focus the attention on basic math and calculator usage first before moving on to part tolerances.
-Andy 

How Much Training a Week…

As a trainer, this is one of the more common questions I receive…

Manager
How much training a week should I give my employees?

My Response
We typically recommend between 1 and 2 hours a week. It is always much better to provide your employees a slow, steady, & consistent stream of information over an extended period of time. One hour a week for 40 weeks can be much more effective than one 40-hour week in altering the behavior and improving the skills of your employees.
Additional Thoughts
There is always a place for multi-day training sessions, but they are most effective in teaching a specific skill rather than relaying general knowledge.
-Andy

PBT in Hot Runner Systems…

I was asked this question by a molder who was having trouble balancing a hot runner system…

Bob
We have this new mold which is running 16 cavities in PBT. Usually we can balance the mold using the drop temperatures, but that does not seem to work on this mold. Any thoughts?
My Response
Essentially, semi-crystalline polymers with a high degree of crystallinity such as PBT and nylon are not highly affected by temperature. In such a case, you will have to adjust the gates or the hot runner system itself to balance the shear of the mold.
Additional Thoughts
Cold runner systems are the best for these materials. I know many molders who use a press-side low-RPM grinder to effectively grind up the material and re-introduce PBT back into the process.
-Andy

Documentation Takes Too Long…

I was in a discussion recently with a GM and he had this problem…

Manager
My technicians are not willing to take the time to document their changes. They claim it takes too long?

My Response
The issue here is not time, but education. If your employees are not taking time to document, it is likely that they do not fully understand the importance and benefits of good documentation. If they understand the ways it can simplify their job and improve their effectiveness, they would be more interested in compliance. Unfortunately, this is not a simple fix… it require the employees to not only understand the benefits, but also know what to do with the information they are given.
-Andy

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