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Applications for Cashew Gates

I just recently received this question from a mold maker…

KG
When would you recommend the use of cashew gates?
note: Cashew gates, or banana gates, are semi-circular gates which tunnel under the parting line to gate on the underside of the part. During ejection, they detach from the part and curl out of the mold. Since these gates cannot be machined directly into the core, they are difficult to machine. As a result, the gate has to be machined into a pair core inserts, each insert contains half of the cashew gate detail. This is done using a CNC milling machine or a die sinker EDM machine.
My Response
There are two complications with this gating system which must be overcome before it becomes a viable option.
1) The material must be flexible enough to curl out of the mold during ejection, yet it must be strong enough to not break. Very soft elastomers, for example, will generally break off, leaving pieces of debris inside the gate during the gate removal process. This can either block the gate… or create solids which may act as contaminants for the next shot.
2) The underside of the part must not be cosmetic. Since the gate enters the mold cavity perpendicular, the gate removal process often leaves a large and inconsistent amount of gate vestige behind.
Additional Thoughts
When implemented properly, cashew gates can be a good way to disguise the gate location.
-Andy

Calculating Injection Speed When Changing Machines

I received a series of follow-up questions from yesterday’s blog entry. I will do my best to paraphrase and answer…

Blog Reader
One machine has a 30mm screw (3100 bar max pressure), and one has a 40mm screw (1800 bar max pressure). Both have the same size injection cylinders. When I transfer from the 30mm to the 40mm machine, do I need a higher pressure pump to move the screw the same distance.
My Response
In reality, the larger diameter screw changes all the aspects of pressure, velocity, and distance.
Since the surface area of the 30mm screw is 56% of the 40mm screw, you need to approach the process with the following assumptions:
  1. To maintain the same injection volume, the 40mm screw will need to travel approximately 56% of the distance.
  2. To maintain the same injection rate, the 40mm screw will need to travel at a speed approximately 56% of the 30mm screw.

In most cases, you will need more pressure only if the machine is maxed out… and you must run a pressure limited process to make a good part.


Additional Thoughts
Basically, this is why it is critical to document your process based on process outputs such as fill time, part weight at the end of fill, and plastic pressure at transfer. Such an approach will eliminate the need to make such conversions.
-Andy

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

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