To reduce flashing?...

[aspearin1]

I have a client who's top defects include flashing and knife cuts during flash removal. Is it possible to reduce/eliminate flashing on most parts by changing run parameters, or do I definitely need to have the tooling reworked? For some history, this client has presses with barrels and screws which have no record of being serviced for the last 10 years, yet still running 24-7. They say it's reasonable to assume that they could be 0.100"-0.250" out of tolerance. To compensate, they raise the processing temperature. I presume this right here creates enough degradation and viscosity loss to allow it to flash more readily. Also, they recently found their mold cooling channels to be blocked. Would this also contribute to flash, having and overheated mold? In addition, they are using 10-lb barrels and screws to make 2-lb parts, a throwback from where they began, but never made the machine change along with the market. Apparently, the stroke distance for this is very short. I have very little hands-on experience with this, and I only can rely on college courses and engineering intuition. If we fix the mold cooling issues and change the barrels and screws, allowing them to run at a lower temperature and higher shear, can we expect a significant reduction in flash, (assuming there are no grossly exagerated gaps in the fit of the mold? Or does flash hing more on mold fit than process parameters.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee

 

[IRstuff]

Isn't flashing ultimately caused by a gap between the mating parts of the mold?

I would agree that a higher temperature would cause any mold material that gets more plastic to flash more, both from wicking and pressure.

Seems to me that the higher temperature could also induce warping of the mold, aggravating the flashing problem. Likewise, lack of maintenance of the mold could also contribute, particularly if there are burrs or contamination that prevent the mold from mating properly.

Does "0.100"-0.250" out of tolerance" refer to the mating of the molds across the mold cavity or between the mating parts of the mold? If there's really a 0.25" gap, your client is wasting both time and material, since flashing is essentially wasted material and someone has to take time to remove the flashing.

TTFN

FAQ731-376

 

[mauriciob]

.100-0.25" out of tolerance? What is the largest dimension of the part?

If the flash is happening all the way around the parting line, it may be insufficient clamp force. What is the part's material? Has the flash been an issue since the start of the program or after some time in production?

Mauricio Benavides

http://www.injecneering.com

(Free software)

 

[patprimmer]

Your question is way to complex to simply answer.

Flash can be caused by any of or various combinations of:-

Low melt viscosity for various reasons, some being,
To much shear.
Degradation for various reasons.
To hot.

High injection pressure.

High injection speed.

Poor mould fit.

Insufficient clamp.

Flex between the mould plates.

Poor process control precision, like variable melt temperature within the shot or from shot to shot or variable shot size or variable effective injection pressure profile.

To much cusion.

To high melt temp.

To low melt temp.

Wear in tie bars.

etc etc.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

A quick read would indicate much stuff wrong!! (Well, all of it really)

If the tools are in the same state as the presses (i.e. severely neglected), you are on a bit of no-winner. The cheapest option (imho) would be to get the tools fixed. The edges could be built up via laser welding, reground to size on shut height and not affect the cavities (hopefully). You could even try a small length of cavity edge.

Suggest you/they actually get some quotes in and then decide where to start..

What material is it? (generic type will do, e.g.ABS/PP


Good luck


Harry

 

[aspearin1]

Thanks for the replies. By "out of tolerance" I was referring to the wear on the barrels and screws. It's all estimated because no one has measured them in some time. There are barrel and screw suppliers scheduled to visit, and will likely result in a blanket PO to upgrade most of the presses. It still sounds like some mold repair will be necessary. Thanks again.

Aaron A. Spearin
ASQ CSSBB
Engineering Six-S'$

http://www.Engineering6ss.com

"The only constant in life is change." -Bruce Lee