For a project of which I can not go into specifics, we have issues piercing a lid made out of PP. The only thing I can say is that it is for medical purpose. There is a lid with dimensions height 3,7mm, diameter 5,35mm and the wall thickness of the lid that has to be pierced is 0,15mm.
In the lid an adapter is mounted. This happens in two steps. First step is clicking the adapter in the lid without piercing the lid. After all the parts have been assembled together and a test has to be made, the adapter will be pushed further into the lid and in the process piercing the lid. Adapter is made out of a standard PA6 and the lid we wanted to make out of a PP, but suggestions here are welcome. We are within limits free to choose a material that will break easily without breaking of some of the material of the lid. Distance between the fist and the second click of the adapter in the lid is approx. 2mm. So in that 2mm the lid has to be broken in such a way that a fluid like water will fall through the broken lid into a container due to gravity. There is no pressure in the system. Shot weight of the lid is 0,08 grams and of the adapter is 1 gram. Does anyone have any suggestions about what material is best used for the lid? Any other suggestions are also welcome. Thank you in advance.

After receiving your email relating to Chen Hsong (ones gain thank you) and your mentioning your posting I had a quick look.
It's difficult to give more than some generalities without design details, but here are a few thoughts.
a) Are your actually piercing through a clean cut or more like bursting (i.e. ragged
edged)? Either way your shape of this section of the adaptor will be critical to "pierce"
the lid clean and without lose particle.
b) I would be a little weary about using PA6 to "pierce" through a PP.
Is it possible to use PP with some talc additive (5, 10 or even 20%)?
This would make the breaking/piercing, bursting or perforating much easier (as you
are using 2 relatively pliable materials) and with tests you can easily determine which
would be the best % of filler for optimum performance.
If not possible consider a slightly "harder" adaptor material.
Hope this thoughts are of some use to you.

Roland, Thanks for your comment. We have to pierce through a clean cut. We have made some initiators in the lid so that the break always appears in the same spot. We have also made 8 different geometries for the adaptor to see which one give the best result. We could try and use an Talcum filled PP to pierce through the lid. This is not a problem.We also have made 8 different geometries for the lid. Different initiators in this case. We already saw that one particular adaptor in combination with one of the lids give the best result in breaking, although the hole created by piercing is still very small. We hoped that due to gravity the liquid will still fall into the container where it should end up. Unfortunately the liquid "hangs" on to the adaptor and lid too much and doesn't fall. Still searching.

Angelo, Here are a few more thoughts and maybe somebody else will offer you some more input. As the area is quite small I would use a chisel (V-like) like pre-score and a chisel shaped adaptor shape. Remember putting the V-shape of the area you like to separate opposite the "chisel" point of your adaptor plate. No good driving the adapter plate "chisel" in to the score. Like breaking glass you break not with but against the score. An additional option would be to weaken all around the break through area apart from some section you want to keep to make sure that nothing separates from the lid.
In regards to the liquid being retained try around with different surface structures until you achieve a lotus type effect. I feel that the worst structure you can have in your case would be polished.

Thanks for your input Roland. The idea of a chisel and score is actually something we can implement easily. Could be worth a try. Weak spots are already present in the lid. And yes, polished surface will not help us at all. Let's try some things. In order for us to do this, we need to modify the mold yet again. This will take a while. I will let you know what the outcome is....

Hello Angelo,

suggestion for your products adapter and lid.

We always can see the metal punch shears the metal sheet with a clean edge. there are 2 important factors:

Punch is harder then the sheet.
Clearance is precise

in you case i think it is not the problem of the material. it should be the problem if you can use the adapt " shears " the lid. So the problem is the clearance. I did not see your product, but currently i think you could try to make the a step underneath the lid which its inner diameter is 5.35. And your adapter its outer diameter can be 5.30 ( just a suggestion, it should be fine turned during the test ), when they are assemblied, the adaper can punch the lid...

ray

I will be away for a couple of days, but next week I will try to answer all of you. Thanks in any case already for the comment

Ray, thanks for your comment. In this case I think that making the geometrie differences will not be the solution. Plastic in general has the tendancy to deform relatively easy when applying a force onto it. So to make the inner diameter of the lid 0,05 bigger than the outer diameter of the adapter will not be the solution I think. Besides, the fitting of the two parts should be watertight so it doesn't leak.

Ken, also thank you for your comment. we did exactly what you described above. in our case the PA6 is rigid enough to make the punch. It does not bend nor slide. In our opinion at the moment the real issue is the material fromm which the lid is made from. We will try next week a talcum filled PP with a high melt. The lid should break easy but not splinter. As for the lotus effect, we do not have the lotus effect. Exactly the opposite.
I will post the results of our tests next week.

Thanks anyway.

OK, curious about what the solution will be.

Just my thoughts and of course you might have tried this already and as the pieces are very small the dimensions may still not be enough to avoid the lotus effect of the liquid. Pierce with a three sided box shaped punch to create a flap thus a type of hinge on the remaining uncut side then have a stepped rib in the centre of the punch to hold the flap open. So try to lance it in effect as you do with metal pressing. It may need a more rigid material than PA6 for the punch. But you may be able to have a little raised portion on the PP to prevent the punch from sliding and bending.

If you merely make a three sided punch the lanced flap will partially block the flow through the hollowed out punch. You need to reverse the hollowed out side of the punch after it has gone thro the PP