Part size small on larger machine

[processtech]

Hello all,
Been a while since i posted on here,
You may be able to help me with a problem i am having with size on a part we mould
It is a round thin wall,flat part approx 3.35 inch diameter , 1.75 inch diameter hole in the middle with a tolerance of +/- 0.015 inch. Slightly raised section around the circumference of the inner hole, with a heavier wall section
On a smaller machine (100 tonne) part size is at around the nominal spec, but i am having difficulty getting this size on a 160 tonne machine.
I think i have exhausted all avenues/methods of trying to improve the size,
Packing them out more improves size but not to a great extent.

The largest influence can be noted by increased cooling time up to greater than 20 seconds, our current cycle is around 21 seconds, so increased cooling time inst an option, current cooling at 7 seconds.
So to match the effect of this, or at least i thought it would, i lowered the mould temperature, but still didn't see an increase in part size as great as expected
I also tried slower and faster fill, but still no great improvement. current fill time is pretty good, approx 1.1 seconds, managed to reduce this to 0.9sec
I also increased and decreased barrel temperatures by 30°C, again to no effect.
The max i am getting is 1.74 on this machine,generally around 1.738
Is this a general problem that you would have seen?

Thanks

[pjhall]

What is the material? Try reducing the tonnage of 160 press to 100 tons. Does the part molded in the 100 ton have any flash on the od? More info needed.

also, 21 second cycle with 6 seconds cooling? are you using 10 seconds hold time????

[brentb]

Along with what PJ is saying: What was clamp tonnage used on machine making
good parts"? What was melt temp? What was part temp at ejection? What was part weight?

Sounds funny but we have "grown" parts by lowering clamp tonnage.

KOM

brent

[Suhas]

Those are interesting solutions that PJ and Brent have given. I am curious and so please post the results.
Thanks,
Suhas

[rickbatey]

I want to know what the screw sizes are between the two machines and maximum injection pressures? I think the larger press has a much larger screw diameter resulting in a much lower cavity pressure. Also I've seen a mold run perfect in a hydraulic clamp press only to have burns and air trap in a toggle press. Same tonnage in theory but the actual applied was very different across the mold face.
Rick.

[processtech]

Hi,
Im back on this again,thank you for your suggestions, sorry for not replying sooner, we moved that mould back to a smaller machine again and with one thing or another i didn't get to look at this.

pjhall, the material is ldpe, parts are good on the smaller machine, with no flash , its just a capacity issue for running on larger machine at the minute.
It is back on the larger machine, and i have the same issue.
Cycle Time 21 seconds
Hold Time is 7 seconds,
Cooling time is 7.2 seconds
Inj. Time 1.2 seconds

Barrel Diameter on larger machine is 40mm
Max Injection pressure is 225 bar (hydraulic)

Barrel on smaller machine is 36mm
Max Injection pressure is 180 bar (hydraulic)

So not a significant difference? or maybe it is?

brentb,
something i did notice previously was that shot weight differed between the two machines, lighter on the larger machine.
Something i took note of on a smaller machine was the shot weight at each stage of fill and hold, and i tried to mirror this to the larger machine, but couldn't achieve the same final shot weight.

I will look at dropping clamp tonnage, that is interesting that you say, you were able to improve size this way? maybe along with this see does fill speed(fast/slow) influence part size

Melt set points are similar between both machines, only i haven't taken the actual melt temperature from each to compare,
A thought on this is that if there was a differ in actual melt temperatures for each machine, would i not have captured the effect of this in a trial i completed of increasing and decreasing melt set points by 30°C? for which i saw no significant change in size.

Thanks again for your suggestions, and I hope to put this issue to bed on this run.

[Jorge Villegas]

Hi,
Im back on this again,thank you for your suggestions, sorry for not replying sooner, we moved that mould back to a smaller machine again and with one thing or another i didn't get to look at this.

pjhall, the material is ldpe, parts are good on the smaller machine, with no flash , its just a capacity issue for running on larger machine at the minute.
It is back on the larger machine, and i have the same issue.
Cycle Time 21 seconds
Hold Time is 7 seconds,
Cooling time is 7.2 seconds
Inj. Time 1.2 seconds

Barrel Diameter on larger machine is 40mm
Max Injection pressure is 225 bar (hydraulic)

Barrel on smaller machine is 36mm
Max Injection pressure is 180 bar (hydraulic)

So not a significant difference? or maybe it is?

brentb,
something i did notice previously was that shot weight differed between the two machines, lighter on the larger machine.
Something i took note of on a smaller machine was the shot weight at each stage of fill and hold, and i tried to mirror this to the larger machine, but couldn't achieve the same final shot weight.

I will look at dropping clamp tonnage, that is interesting that you say, you were able to improve size this way? maybe along with this see does fill speed(fast/slow) influence part size

Melt set points are similar between both machines, only i haven't taken the actual melt temperature from each to compare,
A thought on this is that if there was a differ in actual melt temperatures for each machine, would i not have captured the effect of this in a trial i completed of increasing and decreasing melt set points by 30°C? for which i saw no significant change in size.

Thanks again for your suggestions, and I hope to put this issue to bed on this run.


Hello.

I´ve had this same problem before. Did you check the switching point? do a partial fill of the part to se how much are you filling before aplying the holding pressure. Also try increasing the injection speed if you have enough pressure left.

If you see differences when you change the clamping force, then it could be related to the mould venting.

[processtech]

Hi,
fill is 92%
increased speed along with higher hold and longer hold time show a positive effect on size.
shot weight does increase with lower clamp, but not effecting the dimension in question, looking at my notes i am seeing a 2g difference between shot weight on a 100t vs. 160t, with the larger machine resulting in a lighter shot weight.
by dropping clamp tonnage, the max the shot weight increases by is 0.5g?
thanks

[Josue]

Check mold temperatures, You should use a diferent MTC from one mach to other,, check if the mold temp its the same as previus machine,, I mean mold surfaces temp not termolator temp,, check water circuits, are they connect as same or previus cond?

what abut part weight ? same as previus,,,