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Plastic vs. Brass Baffles…

This interesting query was emailed to me the other week…

JM
Plastic vs Brass Baffles?  Do spiral
baffles add any cooling efficiency?

My Response
The plastic baffles are new, and the jury is out… I can see the obvious benefit of not stressing the mold base as they are installed, as well as the textured surface to increase turbulence within the water channel. Spiral channels, as a rule, tend to provide consistency of cooling around the core. These designs also help to reduce the potential of baffle deflection.

Additional Thoughts
Personally, I would love to see some independent studies comparing the spiral design baffle in brass vs. a turbulent promoting design in plastic.

-Andy

Transition Zone Length for Amorphous Polymers…

I was asked this question via email the other day…

JP
We process a large amount of amorphous polymers including ABS, PS, and PC. We are looking to purchase come new screws for these materials, the screw manufacturer suggests using nearly half the screw for the transition zone, does this make sense? If so, Why?

My Response
Your supplier is making the correct suggestion. To avoid over-shearing the material, it is best to use a long transition zone to slowly heat and soften the amorphous polymer chains. This will help avoid un-melted gels from reaching the metering zone and the molded part.
Additional Thoughts
In a 20:1 L/D screw, amorphous polymers require approximately 8-10 diameters of transition to ensure a proper melt.
-Andy

Optimal Baffle Length…

This was a second part of a question I received the other day…

Jim
What is the standard baffle length to baffle
channel length?  How much clearance at the between end of baffle and
bottom of channel?

My Response
There is no standard for baffle length, though it is critical that you balance the baffle flow (and restrictions) with other coolant lines. As for the baffle end, you do not want to restrict flow in this area, this could cause water to flow around the baffle rather than over it. This might cause it to bend to one side. Generally, this clearance is 0.5-1.0 times the diameter of the hole.

Additional Thoughts
After installed, you should always test the balance of the cooling system with a flow meter to ensure the baffles are actually receiving coolant flow. I have seen many molds where 6 or 8 baffles are installed in series using one cooling line in parallel with a straight water line. In such a case, the water will avoid the baffles and take the straight water line path of least resistance.

-Andy

Distributing Handouts…

I was recently in a discussion with a fellow instructor about how annoying it was for people to be reading ahead while he was trying to teach…

Bob
How do you keep the attention of student who like to read ahead of the presentation?

My Response
Although there are many reasons why people start reading ahead of the presenter, but the most obvious reason is because the materials are there. In my experience, if you give people the materials you are presenting, many will read ahead… especially if you present the same material they are reading. My recommendation is to avoid distributing directly-related materials to the participants until after the material is presented. This allows them to focus on your presentation, and then refresh themselves with the handout materials.
Additional Thoughts
I am not adverse to providing materials to the students ahead of time, especially if it is a reference such as a worksheet of manual. You just want to avoid giving a handout which contains the exact information you intent to teach.
-Andy 

The Affects of Scaling…

This question came via email this morning…

Jim
What is the effect of scale build up/rust build up on cooling channels?

My Response
The actual losses in efficiency are due to the difference in heat transfer from scale and the mold base material. Therefore, the more conductive the tool steel, the more the scaling will diminish cooling. So, for example, a 420-SS mold base will not be as greatly affected by scale as a BrCu core would be affected. In general, the excessive scaling will drop the efficiency 10-25% for steel.
Additional Thoughts
Some people look at a 15% drop in cooling efficiency as a minor change. In most processes, this can result in a 10% increase in overall cycle time, resulting in a 5-10% drop in overall productivity, efficiency, and plant capacity. In today’s market where every penny counts, this is an avoidable drop in productivity which most companies cannot afford to bear.
-Andy

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