Tolerences for Molding process Parameters

[Maheshkumar Hiremath]

Dear Friends,

I have a question to ask for the tolerance to set for molding parameters like...What are the industry standard tolerance for a better process control.

1.Injection Speed
2.Injection Pressure.
3.Melt Cushion.
4.Holding Pressure.
5.Injection Time.
6.Dosing Stop.
7.Back pressure
8.Cooling time .
9.Cycle Time.

[rickbatey]

This really depends on the type of parts being produced; commodity based items, tight tolerance, purely cosmetic....

For me, I worry more about the fill time as this is a function of speed relative to injection screw diameter. Not so much the actual setting.
I graph peak pressure, and record it in a log, but I don't set a range on it.
With the majority of the remaining items, I have fairly generous ranges (heats +/- 20°C, cushion +/-4 mm, and so on).
All of these are done because based on the light automotive parts I am currently running; we are more worried about part cosmetics than ANYTHING. We have had good tools that are not so close to nominal dimensions that I worry about size. I have to have full parts without flash, knit lines that do not read through on the A-surface. Now on the one dimesional tool, a POM grab handle bracket: the tool steel is such that with the correct steel temp, and pack/hold profile (time with the correct presures), we don't have size issues!

I hate to set small windows that require techs to get the Engineer because they can't stay within the settings and make a good part. Then the forms start, ad the customer has to be notified, and it escalates from there! But that will also depend on the skill level of the process techs.

[Maheshkumar Hiremath]

Thank you ,

We are making tight tolerance parts ..sometimes the tolerance are to be less than 50 microns...and using engineering plastics ...LCP,PBT,PA66,PA46........

[ruhlj]

We all know that when we change one process set point it has the ability to change the process in other areas. As an example, if we change the back pressure, it could affect the temperature of the melt and that would affect the cooling rate of the plastic part. Setting "generic" tolerances on molding parameters is a gamble, you are hoping it will work. Even with very tight tolerance, yo could have a combination of changed set points that could end up making bad parts. The best approach is to do a DOE to determine the best tolerance for each parameter. It's also great to have a pressure sensor in the mold so you have an idea of what the plastic part is seeing too.

[Suhas]

I agree with Ruhlj above. This is the right thing to do.
Regards,
Suhas

[rickbatey]

Well should be more like the process window DOE, but honestly most days you won't have time to do much more than: 1-Tune the mold heats. 2-Shear rate study. 3-Gate freeze study. 4-Process cycle time optimization. THEN you will be onto the next fire!
I haven't molded connectors in many years, but that was the only time we got into process window DOE's until we experienced serious issues running soft resin with intermittant scrap (proved to be residence time) in a 2K process. Many companies won't have the press time, resin, or time before the product is launched for full production!
Rick Batey.

[brentb]

Medical molders will (or should)

Keep on Molding!

brent