Question Regarding Material Manufacturer Specifications

[RobertPHale]

Hi all,

Something I have wondered for awhile now, when a resin manufacturer has their recommended Back Pressure, why do they specify in PSI and not PPSI. It is hard for me to believe that they mean PPSI on their data sheets because one particular resin I am looking at (Celanex 2004-2) states that 0 - 50 PSI be used. If this is supposed to be Plastic Pressure, then I would be afraid of risking un-melted pellets getting through, It's hard for me to believe that is enough BP. We currently run this material between 800 to 1000 PPSI. What are your thoughts on this? Thanks in advanced, and Keep On Molding!

Bobby

[Suhas]

Hi Bobby,
You are absolutely right that they should give you in ppsi and not psi. Where do you think they get these numbers from? 'Someone told them'. I do not think they do any tests for that. The numbers you are using are perfect. I do have a procedure for scientifically optimizing it. I will publish it some day.
The same is true with other values also. How can they specify injection pressure? I am not talking units, I am talking actual value ranges. I think a thick part will require less pressure to fill and a thin part will require high pressure. How about screw speeds? They give values in rpms. They should give mm/sec or in/sec. A 50 rpm on a 60mm dia screw will give you double the shear at 50 rpm on a 30 mm screw.
A data sheet is only a reference point. Hope this helps.
Suhas

[RobertPHale]

Suhas,

That is exactly what I was thinking, these guys probably send there resin out for physical testing, and that molder will give them back this arbitrary information that doesn't mean a whole lot to the general public. But if you have the exact make and model of the machine, the same screw size, the same lot of resin, and the same sample mold, then these may work for you. Thank you for the quick reply, look forward to having more discussions on here.

Bobby

[MTUHusky]

Hey Robert,

This is an issue in our industry. I find that the data sheets are almost useless in a lot of cases. I find that when I go to compare numbers between materials it is almost impossible because different testing labs use different standards (ISO, UL, and ASTM are common). I have also found errors such as nylon 66 and nylon 66 G50 having the same max moisture when I know glass absorbs no water so it should be half with 50% glass.

With regards to your comment about un-melted pellets, I just wanted to point out the misconception that raising back pressure will raise melt temperature. It shouldn't raise it more than a few degrees in reality. A lot of people feel that BP will raise temperature because they see it in the field. What I have found is that raising the BP will raise the temperature indirectly because when someone raises the BP, the rotate time goes out of tolerance on the high side. When their rotate time goes out of tolerance, they raise the extruder RPM's to compensate and that is where the extra heat is coming from.

I typically use BP to create a more uniform melt and cushion.

[moldingppm]

Who uses data sheets for processing? :)

[RobertPHale]

Moldingppm - more of a general question on the legitimacy of the resin manufacturers, I only look at these sheets for a baseline of recommended temperatures, however sometimes I find this is incorrect as well.

[iautry1973]

I just use them as a starting point, but as others have said it is not always correct.