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Acceptable Variation in Fill Time…

I corresponded with a frequent blog contributor late last week…

Milan
If my fill time varies 0.02 seconds, is that a machine problem? I have a similar machine with 0.00 seconds variation.

note: after a few emails, it was determined…
  • 200 ton hydraulic machine
  • 38% shot size
  • 0.55 second fill time
  • Possibly a pressure limited process
My Response
As a general rule, a 3-4% variation in fill time is not unusual with a hydraulic molding machine.
Although common, one way to improve this variation is to tailor the process to the machine’s capabilities…
1) Make sure the machine has adequate maximum injection pressure. Also read: Providing a Buffer To Accommodate for
Variation

2) Make sure the hydraulic valves and transducers are in good functioning order. Also the hydraulic fluid should be sent to a lab every six months to check for contaminates and breakdown. Also, the filters should be checked and replaced routinely.
3) Your machine may have a hard time during velocity transitions, causing a pattern or cycle of overshoot and undershoot. This may also be seen at the beginning and end of fill. In these cases, a stepped velocity profile will adjust the velocity to one which is easier for the machine to maintain.
Additional Thoughts
Although closed loop controls are very helpful, it is often necessary to adjust the process to compensate for a machine’s shortcomings. Keep in mind, once you identify an issue with your molding machine, you should work with the manufacturer to better understand the cause. They might have encountered the same issue with another customer… and might already have a fix which you might not have considered.
-Andy

The Purpose of Vent Drops

This question arrived in my e-mail the other day…

Jason
In a previous blog you mentioned the term ‘Vent Drop’. Could you explain how these are typically incorporated into the mold design?
Note: Jason is referring to the post Venting Issues which states:
One commonly overlooked aspect to venting is the vent drop. Behind any well designed vent is a substantive vent drop.  These drops are deep grooves, which channel the gas from behind the vent to the outside the mold. Think of the vent as a gate designed to transfer the gas from the mold to the vent drop.
My Response
One of the most common vent drop is a deep channel machined around the perimeter of the mold cavity. This method is often preferred because it is easy to add more venting since it will automatically transfers the gas to the vent drop.
Another common vent drop method is to machine a channel 5-10 times deeper and at least 2 times wider than the actual vent.
If runners or actions prevent the vent drop from reaching the to the perimeter of the mold through the parting line, holes are often be drilled through the core to vent the air to the ejection housing.
Additional Thoughts
Many mold designers make the mistake of assuming that the air will vent away from a core block which is ‘proud’ (sticks out from the base a little). When clamped, most of this metal is designed to focus the clamp tonnage, and does not provide enough clearance to properly vent the gas from the mold during injection.
-Andy

Near Field vs. Far Field Ultrasonic Welding

A student just e-mailed me this question…

JH
I am a little confused about the difference between Near Field and Far Field ultrasonic welding. Is this referring to the size of the part or the horn?
My Response
Actually, near and far field refers to the distance from the ultrasonic horn to the weld joint. Typically, when this distance is less than 0.25in or 6mm it is considered ‘Near Field’ or ‘Close Range’ ultrasonic welding. Distances greater than that are generally considered ‘Far Field’ or ‘Long Range’ ultrasonic welding.
As a general rule, the greater the distance, the more difficult it is to get the vibrational energy of the horn transferred to the point of the weld. Materials which are softer often get less polymer chain entanglement at the interface since much of the vibrational energy is absorbed by the part before it reaches the joint.
Additional Thoughts
Parts being designed for ultrasonic welding should take factors such as horn location and joint design early in the design process to ensure success.
-Andy

Turning Off Pack and Hold…

This is a question similar to others I often receive…

MJ
If I’m using a DII process, why should I turn off pack/hold time when I am filling 98% during first stage?

My Response
Turning off pack and hold is a great way to verify your fill is properly established, especially when you document the fill weight.
This is typically done when you are establishing a process… or troubleshooting.
Additional Thoughts

Usually when I develop a process… I document the fill (short shot), pack (full with sinks), and then hold (gate seal).
I also like to take a picture of the 1st stage short shot to document the appearance.
-Andy

Feed Problems With A Reverse Profile…

I received this follow-up questions from a previous blog…

Milan
If I use a reverse profile, could I have problem with melted pellets or trapped air?

My Response
Basically, the purpose of a higher feed temperature in a reverse profile is to ensure the pellets soften enough to stick to the barrel so they convey to the transition zone for melting.
You should conduct a test to ensure what feed zone temperature is best for your material as different heater bands, screws, and barrels behave differently.
There are many times I have found that the same temperature can be used for both the feed and middle temperature zones.
Additional Thoughts
Processing guides can be a great starting point when setting your temperature zones, but do not always reflect the best settings for your machine.
-Andy

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