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Strength to weight ratio for thermoplastics

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kursh Mohammed, May 25, 2011.

  1. Kursh Mohammed

    Kursh Mohammed Active Member

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    Hi Guys,

    Is there strength to weight ratio chart available for polypropylene, copolymer and XTX Sprint thermoplastics? And direct milled devices.
    For example if a Pod requests semi-rigid or rigid polypro, how does the technician define what thickness material to use?

    Thank you.
  2. Not as simple as that. The geometry of the device along with the Young's moduli of the materials will determine the load/ deformation characteristics of the orthosis. Then you need to factor in activity levels, footwear, pathology etc. Patients weight won't give all the answers. Think about it: body weight predicted only about 30% of the variance in supination resistance in Craig P's studies.

    Nevertheless, ask an ye shall receive:


    P.S. try googling "thickness to weight ratio foot orthotics"
  3. Kursh Mohammed

    Kursh Mohammed Active Member

    I agree, but we have clinicians specifying rigid, semi-rigid and there is no indication what thickness in polyprop they desire, though you'd think they would put some thought into it. And Polyprop is available in 2, 3, 4, 4.5 and 6mm thickness anyway.
    But a milled device is obviously quite different, true rigid devices begin at 4.8mm and upwards, from which the CADCAM use a weight ratio conversion.
  4. Define "true rigid devices"...

    No such thing.

    And 4.8mm, the mind boggles as to who decided this and why?

    Sounds like a case of picking a number. None-science. Otherwise known as a load of old bollocks.

    You could have two devices: one with a medial longitudinal arch height of say 1cm at it's apex and one with a medial longitudinal arch height of 4cm at it's apex, both constructed with a 4.8mm shell thickness. The load/deformation characteristics will be completely different for these two devices. Moreover, how much deformation do we use to determine and differentiate a semi-rigid form a rigid device under a given load? It's out-moded terminology.
  5. Kursh Mohammed

    Kursh Mohammed Active Member

    I should have written – ‘sturdy’ rigid devices. 4.8mm and above gives you a strong/rigid device in comparison to XT Sprint and TL-2100.
    I understand your argument Simon, but it does work to some degree during the cad cam process. A 3mm milled device is different in characteristics to a vacuum formed 3mm polypropylene device.
  6. In what way?
  7. Kursh Mohammed

    Kursh Mohammed Active Member

    It's a lot flexible. Do you remember Phil stating that they add 0.5mm around the longitudinal arch of the orthotic so it remains a bit sturdy during his presentation last week?
  8. Phil Wells

    Phil Wells Active Member


    I only strengthen this to reduce the incidence/probability of fracture points and not to make the device more rigid.
    We do have a rough matrix to define PP thickness based on arch shape, insole length, heel cup height and wall height e.g. medial and lateral flanges.
    However you need to understand what the practitioner means by rigid etc.

    Even with all of this, we sometimes have to re-design them once they have been milled as this is the only true evaluation you get of the variables above.

  9. Kursh Mohammed

    Kursh Mohammed Active Member

    Thanks Phil.

    What do you mean by the above? Not clearly sure.
  10. Load/deformation of a device is determined by the materials Young's modulus and it's geometry. Thus if we had two devices manufactured of identical material and of identical geometry, their load/deformation characteristics should be identical, whether one is milled and one is vacuum formed.

    In reality, when we take a flat piece of material (lets say a 3mm sheet of polyprop) and we vacuum form it over a cast, the thickness of the material changes from point to point and varies within the finished device due to some areas being subjected to higher tensile, bending and compressive forces during the molding process. If we heated several pieces of plastic and varied the time in the oven between them and then vacuum formed over the same cast, we would likely see variation in load/ deformation in the finished devices due to variation in the shell thickness due to variation in the response of the material during "pulling" in the vacuum former, i.e the hotter the material the lower it's load -deformation characteristics during vacuum forming. Nevertheless, if we cloned anyone of these devices and milled them accurately and identically, provided they were made of the same material, they should have the same load/ deformation characteristics as their vacuum formed counterparts. Unless the heating process changes the Young's modulus of the material.

    I don't want to speak for Phil, but I think what he is saying is that the proof of the pudding is in the eating.
  11. Phil Wells

    Phil Wells Active Member

    There is not a definitive matrix available to get the orthotic strength correct 100% of the time.
    Like Simon sad, you only know for sure once you have milled the device - it is more art than science with orthoses no matter how good the CAD/CAM system or technician.

  12. J.R. Dobbs

    J.R. Dobbs Active Member

    You only know for sure once the patient has worn it.
  13. Phil Wells

    Phil Wells Active Member

    True but then non-compliance or effectiveness is multi factorial and not just about orthotic rigidity.
  14. Kursh Mohammed

    Kursh Mohammed Active Member

    Yes for sure. Well it is guess (experienced guess work) work most of the time. But to have a basis weight of the patient in relation to thickness works well for classifying the type of rigidity they wish. I know it is fundamentally wrong as Simon has clearly pointed out but so is the lab discretion of adding medial plaster addition!
  15. This is true from what I have seen. For a given thickness of polypropylene, the 3 mm thick vacuum formed orthosis will be a little more flexible than the 3 mm thick milled orthosis. My hypothesis is that the heating process of vacuum forming resets the molecular bonds within the polypropylene so that there is a "relief of stress" between the molecular bonds that makes it slightly more flexible than before it is heated. Of course, it could also be that heating and pressing a 3 mm thick polypropylene orthosis over a foot mold slightly thins the device to only 2.8 mm thick (for example) which would also make the orthosis more compliant. Would be a good research study.
  16. Kursh Mohammed

    Kursh Mohammed Active Member

    We also have to understand that the polypropylene cadcam blocks may constitute a different ingredient; they may not be solely 99% polypropylene. The quality is considerably different to thin sheet form also.
  17. Why would you think that the polypropylene cadcam blocks aren't 100% polypropylene while the polypropylene sheets are 100% polypropylene?? Do you have any evidence of this or are you just guessing?
  18. efuller

    efuller MVP

    Different methods of pressing may yield different thicknesses. I've seen poly sheet heated in a form and then vacuumed over the cast (The poly acts as the bladder). The original piece was flat. After pressing the same material has a much longer linear contour, because poly is still attached to the form in its original shape. It must have stretched. I don't see how it could maintain its thickness. It may be different when using a rubber bladder to form the poly over the cast.

  19. Kursh Mohammed

    Kursh Mohammed Active Member

    A guess.
    Though I have recently come across Polypropylene that feels remarkably different when thermoformed in comparison to our previous supplier, even though the material safety data sheet states it’s 100% Polypropylene. I don’t have an answer to why this could be the case.
    I think it’s a matter looking at the MSDS sheet for the polypropylene cadcam block to see what ours may constitute of.
    We can’t buy black polypropylene and it makes you think why it’s only available block sizes, could they used for a different purpose/industry?
  20. Polypropylene is not a proprietary brand of plastic, it is made by many manufacturers. The two types of polypropylene include non-stress relieved (NSR) and stress relieved (SR) which are both 100% polypropylene, are manufactured the same except the stress-relieved polypropylene is heated and pressed after manufacture to relieve the "memory" between the chemical bonds of the polypropylene molecule. In addition, I have heard from others that the addition of dyes to color the polypropylene may affect its material characteristics slightly.
  21. Kursh Mohammed

    Kursh Mohammed Active Member


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