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We need to cut a very thin crack into a polymer plate for a research project. The closest thing to what we are looking for is Wire EDM but since the polymer we are using is not conductive we cant use that method. Any suggestions? Is there an insulator version of Wire EDM?

Edit:

a is the crack length and we want the crack thickness to be less than 1 mm. It should be as thin as possible.

enter image description here

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    $\begingroup$ Use a laser! $\endgroup$ – Chris Mueller Apr 4 '16 at 12:03
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    $\begingroup$ Do you have a drawing showing what you need the crack to look like? $\endgroup$ – grfrazee Apr 4 '16 at 16:43
  • $\begingroup$ I uploaded the drawing, have a look. @grfrazee $\endgroup$ – Murad Nazari Apr 4 '16 at 18:54
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    $\begingroup$ Crack width generally doesn't matter as much as crack tip radius in the loading condition you have, provided the aspect ratio is large (L/W). What material property are you trying to determine? Fracture toughness? Modulus of rupture? Is your polymer generally brittle or ductile? $\endgroup$ – wwarriner Apr 4 '16 at 20:24
  • $\begingroup$ A scroll saw blade would be able to cut a very narrow slot. I have a 0.2 mm scrollsaw blade that I use for acrylic from time to time. To get a perfectly straight cut would require a jig of some kind to keep the blade from wandering. $\endgroup$ – user6335 Nov 2 '17 at 10:02
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Since you used some specific words in your question, I will respond a certain way. Cracks are generated by mechanical action, and cannot be machined in. They may, in certain brittle materials, be a byproduct of machining.

Using a method like EDM will alter the properties of a polymer nearby, as it will heat the polymer. If you are interested in introducing a crack for the purposes of fracture testing (see the reference to ASTM D5045 below), then it is imperative that you create a "sharp" crack that is representative of the material (and thus EDM is not a good choice). The method for doing this involves machining a notch in the plastic at the test zone, and then using mechanical load to precrack the specimen. The precrack needs to be measured, and then the fracture testing can commence.

If, when you said "crack" what you actually meant was that you need to cut a small opening in the polymeric material, I suggest you change your wording to "machine a hole or slit". In that case, I suggest a CNC end mill with a very small diameter bit, which will achieve tolerances approaching an EDM wire. If kept cool enough, this will not alter the local properties of the polymer. If this is not fine enough, then you are most certainly in need of a CNC laser cutter, which again will locally alter the material.

For method reference, see ASTM D5045-14: Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic Materials

General information about material fracture: https://en.wikipedia.org/wiki/Fracture

EDIT: Now that you have posted the drawing, I suggest an end mill. Bits are readily available that will machine that, particularly if it is not a hard material. You will want to work with a machine shop that does precision machining, and ask them specifically about tolerances. Also, I suggest you have them cut some samples prior to fabricating the final article so you can evaluate their tolerances.

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  • $\begingroup$ +1 for referencing known material testing standards. I don't know the entire body of ASTM, but I will suggest that "plane-strain" may not be applicable for a thin plate, which would more likely experience plane stress conditions. There are a number of plane-strain ASTM docs (E1820) which are intended for metallic materials and may potentially be modified for polymers. OP should check out various ASTM tests and decide which is most suitable. $\endgroup$ – wwarriner Apr 4 '16 at 17:41
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    $\begingroup$ Absolutely concur @starrise, you can't go wrong delving into test standards. I am afraid that the question was intended to understand how to cut a hole in plastic, but the use of the word crack drove me to give a material science answer. $\endgroup$ – tillmas Apr 4 '16 at 17:57

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