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Should there be any difference in the hardness of a plastic material that is machined vs one that is pressed from a mold?

This question is in specific reference to vinyl record production where records pressed/stamped from molds are seen to be more durable whereas records cut by lathe are said to degrade significantly each time they are played.

All things being equal (same plastic composition, plastic formed under same pressure, treated equally, equal depth of grooves, etc.) is there any reason a record cut from a blank should be less hard? (I believe I'm using that term correctly, resistant to abrasion from a stylus/"needle")

Is there a search term for this concept so I can read more?

Thanks for your help!

Lukas L
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  • interesting Q. Do you know anything else about the material? I wonder if the blanks used for the lathe-cut records may have different material properties to start with, to facilitate cutting... – Pete W Jan 28 '22 at 22:19
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    Hi Pete, you can cut lathe-cut records onto many different kinds of materials many of which I'm sure many have different properties to the plastic used for regular, pressed vinyl records. However, it is my understanding that you can use the same stock material as pressed records, and still have a "less durable" record that will "degrade more quickly".

    This is put down to the manufacturing method itself as opposed to the material - I would like to understand the material science/engineering explanation for this!

     – Lukas L Jan 28 '22 at 22:36
  • Many factors here, material, pitch, angle of groove wall also number of grooves. – Solar Mike Jan 29 '22 at 03:41
  • The question is really - is there, all other things the same, a reason why a disk of plastic with a molded groove would be harder than a disk of plastic with a groove cut into it. Does cutting from a blank somehow compromise hardness in a way that molding does not? If so, why?

    If material, pitch, angle of groove, number of grooves, and all other factors are kept the same, is there a reason for one to differ from the other?

     – Lukas L Jan 29 '22 at 03:56
  • Is there more concrete evidence for this than audiohile hearsay? Is a cut record even made of the same type of PVC as a pressed record? – DKNguyen Jan 29 '22 at 05:59
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    One possibility is the cutting process generates heat & the heating & cooling process then alters the surface of the cut plastic. Is what you are asking about a feature of wall cut records, which had v-type groove & sound was recorded in the walls of the groove, or in less common vertical cut records, which had straight wall & the sound was recorded in the floor of the groove, or both? – Fred Jan 29 '22 at 10:26
  • Heat from the cutting process altering the properties of the plastic seems like a plausible mechanism that would result in a weaker product all other things equal. I was reticent to bring up that the question was in reference to record manufacturing because at bottom I don't want to discuss the other possible differences between a pressed and a lathe cut record - I want to understand whether the very act of cutting into a blank of a material could result in a less hard product than one pressed from the same material – Lukas L Jan 29 '22 at 14:18
  • Although, come to think of it, I don't imagine cutting PVC at 33rpm would generate very significant levels of heat? – Lukas L Jan 29 '22 at 14:28
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    The cutting edge abrades the face of the cut and creates micro-cracks (breaks) in the surface. Humidity and condensation forms moisture in fine capillaries. This can cause hydrolysis (cleaving of polymer bonds) of the plastic. Also, with cutting you will have interruptions in the as-molded polymer bonds at the surface. These sharp cuts may also weaken the structure by stress concentration. I remember a video of an antique recording by Edison that broke during handling. Maybe the polymer was more fragile with age, but possibly also precipitated by hydrolysis and stress concentration. – Jim Clark Jan 29 '22 at 20:34
  • Thanks Jim! I'm trying to find more information about plastics hydrolysis as a consequence of machining/milling/abrasion but struggling to find more information. The sources I can find talk about hydrolysis-resistant properties of various polymers in the context of age/heat rather than in a manufacturing setting. Could you point me in the right direction? Thanks for your help! – Lukas L Jan 29 '22 at 22:31

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