Aluminum Molds

[mjh368]

Hello i recently began working at a company that makes aluminum molds in house to make a large arrangement of parts for use in our machine. Material include PVC, 25% glass filled PE, 30% glass filled, PP, PVDF and a few others.
A lot of the molds run thick parts up to 1/2 with little or no cooling. When the do use cooling, 3/8 lines are used, usually one in one out. We have a coupl of thermolaters that usually run at 100F. Many of the cycles seem lond to me and i wonder it we aren't using the high heat tranfer of alluminum to our advatage. Should i increase line size? layouts? flow from thermolators to increase turbulece? GPM gauges?

I realize that there is not a specific question here. Just hoping to start some kind of discussion.

Another question, i recently got in some test HMWHDPE and UHMW that are injection moldable. They are definatly hard-to-flow(UHMW significantly harder to flow than the HD) materials but they mold. Recently a "buddy" at work told the bosses that my hard to flow material would wear the alluminum mold. I believe that the glass fiber in the RPE and RPP would wear the alluminum more, what do you guys think???

 

[patprimmer]

Molecular weight will have little or no effect on mould wear, other than the extra stress from the extra injection pressure necessary, which might effect fatigue cracks and mould bending, but not wear as such.

Glass fibre certainly does cause mould wear, which I would expect to be substantial in an aluminium mould. I would especially expect the gate and the wall directly opposite the gate to wear quite quickly.

Your cycle times will be terrible with your cooling system.

It is quite simple. Cooling times are directly proportional to surface temperature of the mould, with the most critical regions being where sections are thickest, where the cavity has undercuts and where the ejector mechanism presses against the moulding.

In thick section mouldings, especially for amorphous materials, the cooling time is by far the biggest portion of the moulding cycle, and therefore the area for most gain by improvement.

Regards

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[MikeHalloran]

Your molds are not un-cooled. They are cooled by room air. Long cycles go with the territory. Pushing the cycle time down should be done with caution; parts that warp in service may result. You won't detect those at the molding station. Your time may be better spent choreographing the operation to maintain part consistency, because load/unload times are part of the thermal cycle in an 'uncooled' mold.

I've had many nylon and delrin parts molded with up to 33 pct glass, in laminated aluminum molds in a manual machine that typically had a cycle time around 60 seconds. Mold life was usually several tens of thousands of parts. Relatively low injection speed and pressure probably contributed to the good results.





Mike Halloran
Pembroke Pines, FL, USA

 

[mjh368]

Hey mike i am a little confused on the warpage you spoke of. I would expect warpage only if i was running fast cycles with a cold mold and then heating the parts above the mold temp(increasing crystalization and causing warpage). But how will cutting time of my cooling cause warpage.

 

[MikeHalloran]

Because the molds are, um, partially cooled, the part's interior is still soft when you separate the mold to remove the part. As the parts cool _absent_ the support of the mold, they may move around a bit. It may be necessary to demold the parts and throw them in warm water to support them as the interiors stabilize.



Mike Halloran
Pembroke Pines, FL, USA

 

[mdg123]

Excuse me for jumping in, but this is the closest thread I've found to what I'm looking for. I'm running a very thick inj molded PP+30%glass part and having sinkage problems. I have a lot of experience molding with Nylon+glass and have found a sizable gate, hot mold, and hard and fast shot to usually get a good part, but PP+glass seems to be a different beast.

The part is a 6in dia ring with 3 interior spokes. The avg cross section of ring and spokes is 0.75"x0.5". The best parts I've made so far were run by running 4-step inj over 4.5s (pressure starting at 1800psi and going down slightly over each step). Total process time is around 75s. Gate is an edge gate off one of the spokes close to center and size seems to be about right as I'm not having holding problems. The moving half of mold is being cooled by 65F water.

Not sure if this is much to go on, but if you have experience with thick parts using PP+glass I'd appreciate your comments and suggestions concerning best molding practices. Also do you recommend a water dip to aid in cooling and for how long?

Note that surface isn't showing significant glass so I'm keen not to heat the mold and increase cooling time.
Thanks.

 

[mjh368]

what is your glass %. A simple but somewhat backwards fix would be to up the glass % and decrease shrink.

 

[MikeHalloran]

One time I designed a polygonal ring about 3" diameter to be made of Delrin/glass/TFE. My first attempt made the sections like I-beams to keep the wall thickness reasonable. As usual, I sent the print to my old molder friend, who scratched out all the fancy bits, mostly because he hated fancy bits, and probably because he knew a thing or two. The part ended up with sections about 5/16" square, and I kept the requirement for flat faces on the polygon sides, because they had to slide against mating parts.

The finished parts were perfectly flat.

...

on the outside.

That crafty old guy had created _huge_ sink marks, _on_the_inside_. Sectioning the parts revealed cavities throughout, with walls about as thick as you'd make if you cored the part, but he didn't.

He said he did it by pulling the parts out hot and dumping them in hot water. I think. It's been a while.






Mike Halloran
Pembroke Pines, FL, USA

 

[mdg123]

mjh368, glass is already 30%, can't go much higher!

Mike, I just might try that. Never seen a hot water dip before. The other idea I have is to also inject gas into the part. I'm not too concerned about voids as the part isn't structural and this may also give me reduced cycle times and better surface quality.

Does anyone else have recommendations how to control sink in a thick part with high glass content? I would appreciate any specific molding advice such as how many injection steps are recomended and best temperature of the mold cavities. thanks.

 

[patprimmer]

We make a 45% Mineral/Glass filled PP co-polymer.

It is substantially more stable than 30% Glass, and in fact we have succeeded in replacing PS/PPO in dimensionally critical pump housings.

Warpage can be from 2 basic causes, one being differential shrinkage due to fibre orientation, the other being different crystallisation rates due to different cooling rates.

Various fixes work or fail depending on the original cause.

Cold moulds, long cooling times and dropping into water, all help hold the shape by freezing in stress. This might or might not give suitable results.

One hot surface will allow for all the sinks on that side, but might also cause more shrinkage on that side.

Blowing agents help reduce cycle times, part weight, internal stresses, sinks marks and warpage in thick sections.

Warpage and shrinkage is a complex situation, especially in semi crystalline materials, and even more so in fibre reinforced materials.

It can be influenced by factors such as:-

Mould coolant temperature.
Local mould surface temperature variations.
Section thickness.
Polymer type.
Colours (yes colours).
Fibre reinforcement.
Other fillers depending on particle size and shape.
Gate position.
Flow direction.
Effective pack at various points.
Degree of crystallisation on ejection.
Nature of crystals.
Size of crystals.
Degree of solidification on ejection.
Thermal shock on ejection.
Variations in section thickness.
Force required for ejection.
Physical control or restraint after ejection (water or jigs).




Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.