|
The following are questions
posed by participants who attended John's webinar presentation on "Scientific Molding Strategy for Troubleshooting" on June 8, 2007.
Q. How does screw L/D effect voids or bubbles?
A. First, Length to Diameter (L/D) of a screw will only influence bubbles, not voids. Short L/D screws literally can pump air bubbles into parts. Especially new screws and barrels with short L/D’s. The problem is the length of the feed section is too short (the air between the granules is trapped and cannot be vented out the feed throat) to prepare the resin for the transition section and the transition section is too short to melt the plastic granules properly. You get both unmelted solids and air bubbles especially with new equipment using a large percentage of the barrel. Also, remember the feed throat is a vent, make sure you know what you are doing before “looking” into the hopper. Talk to a few processors and someone will tell you stories of the hopper being blown to and through the roof.
Q. How do you troubleshoot weld or knit lines? Is there a way to process them out?
A. Usually you can spot a weldline or knitline by noting the flow path of the plastic filling a part and seeing where two or more flow fronts meet. It is usually a weak section of the part; it often breaks in a brittle mode. Sometimes there is a cosmetic blemish, like scratch or grove, non-fill, burn or blush. To find out, short shoot the part by change position transfer under velocity control, do NOT do this by adjusting 1st stage pressure. To process them out is very difficult but here are some tips: VENT the area where the flow fronts meet, vent core pins, increase injection speed, remove the filler if a filled resin, try a lower molecular weight resin (but make sure your performance is not sacrificed), make sure you are running under velocity control with a delta P, form the weldline during first stage, provide a flow tap at the weldline (this will have to be removed later and will look like a gate) and finally redesign the part or change gate location to put the weldline in a non-performance area of the part… if that is possible.
Q. On the Arburg Silogica you are able to set the pack/holder velocity to zero, is that doing the same effect as zeroing the pressure? The screw does not bounce back with velocity set at zero; if pressure is set at zero the screw moves backwards.
A. Do not set the pack velocity to zero. Set it to the value you will be using during production. You need to see the over-travel after the screw reaches the stroke transfer position. It is OK to have some screw bounce back, especially during the development of the viscosity curve.
A. Injection regulation on Arburg's puts pressure on both sides of the hydraulic ram; and it acts as a breaking mechanism. Most hydraulic machines do not have this breaking capability. It acts like an electric machine.
Q. On the velocity curve study form is it necessary to switch the units to psi from bar?
A. No, any pressure units are fine.
Q. Is the 99% full part determined by first making a "full" part?
A. Some folks do it that way, but it is not my recommendation. Do it by sight (volume). In addition, the part can be 90% to 99.9% full. For some high cavitation tools, some parts will be quite short and some full. For thin wall or living hinge applications go to 99.99% full by sight.
Q. What machine pressure and velocity is used to perform the pressure loss analysis?
A.. First, do the viscosity curve. From the viscosity curve, you determine the fill time for production use that fill time. Pressure required is NOT a set point, the machine must be allowed use whatever pressure it needs to achieve the velocity. That is the 1st stage must be set up like cruise control on a car. You set velocity; you do NOT set how much energy from the engine the car needs to go the speed you want. Many processors do not know how to set up a machine to function this way, but it is the best way to setup and run a process in injection molding.
Q. How do you run velocity curve when using amorphous resins that do not respond too well to shear rate?
A. It is my understanding that this question relates to polycarbonate (PC). For nearly all resins, 99.2% of all that are out there, do the viscosity curve as you would normally do it. Run with a delta P, change only velocity and take the data from as fast the machine can inject, making a short shot to as slow as 10 -15 seconds fill time on normal nominal wall parts. For thin wall, this may only be 3-6 seconds due freezing of the flow front. By the way, PC does respond well to shear rate, perhaps not as much as some other resins but still it changes viscosity by orders of magnitude.
Q. Where can you purchase the pressure sensitive paper?
A. To purchase the pressure sensitive paper visit Sensor Products, SensorProd.com; 1-800-755-2201. Tell them, I sent you and that they ought to treat me better. They also have a “Tactilus” method but more expense: www.sensorprod.com/dynamic/injectionmolding.php
Thank you all for your time.
I hope to see you at my next seminar on July 17 titled, Scientific Injection Molding for Six Sigma!
-
John B.
June 10 , 2007 |
