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Conditions For The Dynamic Check Ring Repeatability Test

I was recently asked this question about weighing parts…

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I received
this inquiry the last week…

Karl

I have a single cavity mold with a
part varying in wall thickness from 3 to 5 mm. The part is warping near the
transition of the thin to thick section. What can I do to overcome this problem?

My Response

Like all
warpage yours is caused by differential shrinkage. In this case it sounds like
you have an uneven temperature gradient throughout the wall thickness of the
part.

You should
start by checking your coolant temperatures. If the cooling lines enter on the
thin side of the part and exit on the thick… the water temperature will already
be higher on the exit side of the mold. I suggest hooking up your cooling lines
on the thick side of the part first.

Also, compare
the entering and exiting water temps to ensure the difference is not to great.
If your exiting water temperature is fairly greater than your entrance
temperature you may want to consider getting a more powerful chiller.

A chiller
with a stronger pump and higher volumetric throughput will have better cooling
uniformity and water temperature throughout the mold.

You may also
want to try increasing the packing pressure to reduce the amount of material
shrinkage, increase the pack time until the part weight no longer increases, or
profile your packing pressure to obtain a more uniform shrinkage across the
part.

-Andy

I was just asked…

JW

What factors should I take into account when calculating the capacity of a new dryer system?

My Response

When calculating the capacity of a dryer system you need to determine your total dried material consumption. Your dried material consumption can be calculated by multiplying the number of machines by their respective output in pounds. If you have 10 machines consuming 4 ounces of plastic a minute you are consuming 150 lb/hr.

Once you determine your material usage you must take into account the drying time of your material. Most plastics will vary in the type, extent, and duration of drying needed for the application. This must be determined before the dryer capacity can be calculated. Ask the dryer manufacturer typical drying times for the material you are processing. Multiply your hourly material consumption by the drying time and your result is the minimum dryer capacity. For instance, in the example above if the material drying time was 2 hours the dryer capacity would have to be at least 300 pounds.

Additional Comments

The example above was for the minimum capacity needed. If the dryer manufacturer offers a 400 pound capacity machine you may want to choose that instead, in case you decide to expand your production… or if the material takes slightly longer to dry.

-Andy

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I was recently asked…

George

We just received new material and started getting burn
marks on our parts near the end of fill. What do you think is causing this and
how can it be fixed?

My Response

Burn marks usually appear when high temperature gas combusts
in front of the melt during mold filling. In your case the new material may
have a slightly higher flow rate and the mold is not venting quick enough with
your current injection velocity. You should check and make sure your vents are
clean and able to vent properly.

Many polymers gas-off in the molten state releasing
particles that can clog the venting system. You may also want to check the melt
flow rate and compare it with data from previous batches of material. If
possible you may want to increase your fill time a bit to allow for longer venting.

-Andy

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