Welcome!

This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

VISIT OUR STORE
VISIT OUR WEBSITE

Appropriate Sensor Clearance…

This question highlights an often overlooked concern when using pressure sensors mounted underneath ejector pins.

Milan
How much of a gap should be used between the pressure sensor and the ejector pin?

My Response
The general recommendation from most manufacturers is between 0.25-0.5mm or 0.010-0.020in.
In reality, this specification refers to the clearance between the top of the ejector pin head and the counter-bore in the ejector plate. Most mold makers use a similar ejector pin clearance even when a sensor is not being used.
Additional Thoughts
The purpose of this clearance is to ensure there is no preload on the sensor, especially if any dirt gets beneath the sensor. Such a preload will destroy the sensor and provide erroneous pressure readings.
-Andy

Regrinding Sandy Plastic…

I received this interesting question recently…

Joy
I have a client with ten tonnes of material awaiting regrind and re-processing. Unfortunately, the material is contaminated with sand.

I have a couple questions. First, is it unethical to regrind and re-process the material since it is contaminated? Second, what are the financial costs and ethics associated with regrinding ten tonnes of material?
My Response
In general, once a material is contaminated with a hard material such as sand, it will become highly abrasive to the grinder, screw, barrel, hot runner, and mold. To regrind that much material in such a state will most likely cause significant damage, requiring repairs and possible replacement of many components. Likewise, such an abrasive material would damage the screw and most likely require a new check ring after re-processing.
If the reground material is sold or any products are sold molded with this material without the customer’s approval such an act would be unethical and most likely illegal.
Since many of the costs associated with regrinding this material depend on the material, grinder, region, as well as repair costs, it is impossible to estimate with any degree of accuracy.
Additional Thoughts
This is one of the reasons it is critical to teach your employees the importance of proper handling of materials, parts, and regrind to ensure that contamination does not occur.
-Andy

Understanding Pressure…

Someone was reading one of my archived blog entries and asked me this follow-up question…

Tan
Could you please define pressure in a way that I can explain it easily to my technicians?

My Response
Pressure is a measurement of force spread out over an area.
Pressure = Force / Area
This means, if you apply the same force on a smaller area, you get more pressure.
Additional Thoughts
An easy way to demonstrate this by holding up a piece of paper.
First, ask the employee to push on the paper with the palm of their hand. Explain that they can apply a lot of force without the paper breaking. This is because the force they apply is distributed over the area of their palm.
Second, ask the employee to push on the same piece of paper with a pen or pencil with the same amount of force. Explain that the paper breaks because the force is applied over a much smaller area resulting in a much higher pressure.
-Andy

Moving a Mold from One Machine to Another…

I just got this question the other day…

MJ
When I want to move mold to another machine, which machine parameters should I check?
My Response
This question brings up the importance of machine independent documentation that includes part specific data such as part weight, fill time, coolant and melt tempertaures as well as the actual pressures applied to the polymer.
When you do this, you can ensure the actual process output from two different machines match. The most important factor is not the parameters that go into the machine, but the parts and associated outputs that result from the process.
Additional Thoughts
This topic is also addressed in the following blog: The Symptoms Of Machine Dependent Documentation
 
-Andy

Understanding Velocity-Control…

I received this question from a previous contributor…

Rafael
When you write velocity-controlled injection, what do you mean?
My Response
A velocity controlled process uses a velocity setpoint to control the rate at which the polymer is injected into the mold. Usually this is represented on the control panel as in/s or mm/s. 
On a hydraulic machine, the hydraulic pressure will adjust to maintain the desired screw velocity. On an electric machine, the power applied to the injection servo-motor will be adjusted to maintain the desired velocity.
This is much better than injection using pressure control where a specific pressure is maintained, and the screw slows down as the mold fills.
Additional Thoughts
For more information on this, please visit: Open Loop vs. Closed Loop Controls

Andy

  • Recent Posts

  • Recent Comments

  • Archives

  • Categories

    • © 2016 All Rights Reserved | Powered by Wordpress | Made with ❤ by Themely