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Correcting for Cavity Imbalance…

I received this inquiry the last week…

Kevin
I have been using the intelligent moulder worksheets and have found poor fill balance in our hot runner tooling. I have had variations from 14 to 22%. How do you get to 6% or
less?

note: The Dynamic Cavity Imbalance Test measures the imbalance by comparing the part weights from each cavity using three different injection speeds. 


My Response
You should first look for inconsistencies such as blocked or damaged vents and gates.

You should review your hot runner system and determine if any gates can be adjusted. Often an adjustment can be made, or a him can be placed behind the gate drop to reduce flow in the mold cavities which are not filling well.

If the runner system has sharp turns, you can first try rounding the corners to reduce the stresses.

Additional Thoughts
Each mold can pose it’s own unique circumstances. For branched runner systems, the ‘Melt Flipper’ can often be used, even with hot runner molds. This technology, licensed by Beaumont Technologies Incorporated, uses strategically placed inserts at runner branches. The inserts rotate the polymer melt to help manage the uneven stresses resulting from runner turns. I have seen this become very helpful in many different applications.

-Andy

“Steel Safe” Runner Designs

I was recently asked the question…

Jeff
We are about to build a 2 plate family mold but are not sure which molding machine we will use it in. We may also process two grades of material in the mold. How will these factors effect the size we should cut our runners?

My Response
Often, the feed system design is uncertain. You have mentioned two common issues which do effect the runner size.
The capability of your molding machine to fill the mold with a material whose flow characteristics are unknown should always be taken into account. If the runners are cut to large your machine may not be able to fill the mold.
There may be variation between the melt flow rates and pressures to properly balance a family mold, especially when two grades of material are being used.

In this situation I suggest you use a runner dimension that is “steel safe”. “Steel safe” means your design should call for the removal of less mold steel than may be ultimately required. You may want to round down your runner dimensions by one or two standard sizes than what you calculated as nominal. This will result in a lower volume feed system with a greater pressure drop. There is a chance the smaller feed system will work and you will save on scrap material. However, if the undersized feed system is not optimal, this approach will allow you to easily machine the runners and improve the performance of your mold.

-Andy

The Need for Safety…

I am travelling overseas and witnessed a roadside welding facility where the workers were not wearing any type of eye protection. As most people know, this is a sure-fire way to lose your eyesight permanently…

My Concerns
Regardless of the country or region in which you live, it is critical that you promote safe working standards for your employees. I understand that different cultures take varying positions on worker safety, but some situations are just unsafe no matter where you live.
-Andy

Different Training for Different Employees…

I was asked this question by a colleague recently…

PG
Some of our managers would like to pick and choose what training certain employees take. In essence, one technician who we feel is weaker will take most of the training assigned to their job position while another tech will take only a few courses.  Other techs will take varying amounts of training based on our estimation of their skill level, is this the best course of action?

My Response
The best way to approach your employee training is as follows… If you believe that certain knowledge or skill is critical to the success of a particular job description, then all employees at that level should take that training. On the production floor, employee training should always be divided by job description, not personnel.
This is the best way to ensure consistency in both the training of the employees as well as the support of newly trained employees by those above them. This situation will create more conformity, better communication, and increased loyalty throughout the workforce.
Additional Thoughts
In my experience, breaking up the training on an individual basis always creates division and resentment. In most cases, people who get excluded from a particular training initiative often ask to be included or complain about being left out.
-Andy

Calculating Cooling Time…

I just received this question…

Vishal
How do I estimate the cooling time? I located a chart referencing a proportionality constant of a polymer, is this the right way?

My Response
There are many helpful charts and factors, but these use too many assumptions and are just ways to avoid proper circulations.
Proper cooling time estimations contain the following considerations:
  1. Cooling time is is proportionate to the thickness squared
  2. Cooling time is inversely proportionate to the thermal diffusivity
  3. The temperature of the mold, melt, and part at ejection
One of the most common calculations are as follows:
tc = [(h^2)/(\alpha\,)(π^2)] ln |(4/π)[(Tm-Tw)/(Te-Tw)]|

h = wall thickness
\alpha\, = thermal diffusivity
Tm = melt temperature
Tw = mold wall temperature
Te = part temperature at ejection

If you perform a search on the internet, you can find automatic cooling time calculators which will quickly take all these factors in consideration once entered.
Additional Thoughts
Keep these factors in mind…
  • Always use your thickest portion of the part for these calculations
  • Assume your actual mold surface temperature will be higher than the temperature setting
  • Give yourself a 10-25% buffer based on the level of geometric complexity and amount of plastic shrinkage

-Andy

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