-  Week 14  +

composites

Tasks

  • Read and follow the instructions mentioned in material saftey data sheet and technical data sheet for resins you are using.
  • Design and make a 3D mold and create a fibre composite part in it.
  • Coupon

    Material Exploration
    Before initiating design process, I decided to make a -test coupon- to check strength and limitations of composite, and also various possibilities it could offer. Since, composite should have a materials that could take compressive as well as tensile foreces, I choose to go for natural fibre and in resin matrix. So, I procured burlap of two kinds, one with thinner fibre and other slightly thicker. Observing their permeable nature, I thought of combining them with khadi fabric which is comparatively less pervious than both types.

    Least Dense and most pervious amongst all three

    Most commonly used burlap, less premeable than the previous one.

    Khadi - Good amount of density, yet quite porous.

    Standard Epoxy Resin + Hardner

    Layering, Infusion and Compaction
    To make the coupon, I took 25cm x 10cm rectangular pieces of all three types and layered them alternatively one over the other. The coupon I am making has nine layers of fabric in epoxy resin matrix. The reaction ratio of epoxy resin to hardner is 100:80 by weight.

    All three types of fabric are 25cm long and 10 cm wide

    All nine fabric pieces have been layered alternatively with top and bottom layer of Khadi and rest are of burlaps.

    Then I quickly applied resin with the help of wooden tongue depressor, one by one, as the shelf life of the mix is nearly half an hour. As even contact throughout suffices to ensure proper cure, I clamped it tightly and kept it aside for 10 hours in order to achieve handling strength as mentioned in the data-sheet.

    Firstly, I prepared two pieces of MDF to be used as mold, then I wrapped them up tightly with kling wrap.

    As per the information gathered from datasheet, I poured 100gms of resin.

    And proportionately, I poured 80 gms of hardner.

    Then, I mixed them evenly with the help of wooden tongue depressor.

    Next, I moved out of the lab and went to the clay workshop for applying resin to the burlap layers. One by one I applied the mix to all the layers in sequence shown above. I ensured that every layer absorbs sufficient resin

    After applying resin, I used compaction technique to fuse resin and fibers. To do so I used C-Clamps and tightened them as much as I could.

    This is how it looks like after I removed the moulds after 12 hours of curation period. Its texture looks pretty good!

    After I removed the extra edges in the workshop. I realised that its so important to use optimum resin. It was tough to get rid of that extra super hard resin.

    Size of the coupon is smaller than the mould. I should ensure in the next try that there is less wastage of fibre material.

    Cross-section through the coupon showing evenly fused burlaps in resin matrix. The thickness of the composite varies between 3.95 mm to 4.05 mm. It is a result of even compaction.

    Starting with coupon is a good beginning, its has turned out to be a good example of natural fibre in resin matrix. The video below tries to show how elastic in nature this tough composite is!

    Lesson Learned
    I should have applied lesser quantity of resin in the composite. To ensure the composite is evenly infused with resin, I applied too much of resin, casuing excessive resin to overflow from its sides.

    Mold design and construction

    3D Designing
    For this exercise, I am using my week 2 - 3D design created in Rhino. It is 320mm x 320mm in size and is something I wish to make composite of. It seems a challenging task to me and not really sure if I am going to succeed or not. But I am still giving it a try.

    Since the 3d that I made is appropriate as the inside mold, I will have to now create it's outer mold that would ensure compaction. I learned from the coupon I made that nine layers of natural fabric when infused with resin yields a surface that is roughly 4 mm thick. So, I have spaced my outside mold accordingly.

    To create the mold I simply had to offset the surface by 4mm usign OffsetSrf Command. Then, I created a reactangle of size 320mm x 320mm using Reactangle Command. Now, using Move command I shifted it by 55mm. The using Loft Command I created polysurfaces between the two geometries. The next step is to join all the surfaces, for that I used my favourite command BooleanUnion. Once the closed single model is there, I flipped it upside down to place both inside mold and outside mold side by side. For detailed design workflow, please check following commands I employed for creating the mold.

    Following 3D shows both the molds that I will be milling next.

    Preparing Drawings for Laser-cutting the fabric

    To cut fabric that could take the 3D form shown above, it is first required to unroll the surface of the 3D and then laser cut them. To do so, again I used rhino 3d. I simply used UnrollSrf Command.

    First select the object and type "unrollsrf" and push enter to get the 3D surfaces unroll in one plane. Its this simple.

    Then I exported it as 2D drawing which I then opened in Autocad. Since the unrolling of surface yeilds polysurfaces arranged linearly, it cannot be used right away for laser cutting. It needs some more work to be done. Once the file got opened in autocad, again I had to a set of simple commands to prepare it for futher stage. I decided to keep the small rectangle at the top, to be at the center connecting all four sides, as it's limbs. First I created the rectangle of 150mm x 1500 mm using Rectangle Command, then I moved all the four sides using Align command and aligned the shorter side to the rectangle I created. Later, I removed the central rectangle, which I drew as a reference to bring all the unroll surfaces together. Now at last, I copied it using Copy Command and pasted it as many times as I wanted. The profile can be seen in the following image:

    RDWorks and Laser Cutting

    As demonstrated in week 3- laser cutting exercise, I religiously followed all the steps that are required for successful laser cutting. Following are the parameters that I had set to cut the fabric.

    Following parameters are for laser cutting of khadi fabric:

  • MinPower - 60%
  • MaxPower - 60%
  • Speed - 50mm/s
  • Following parameters are for laser cutting of thinner burlap:

  • MinPower - 50%
  • MaxPower - 50%
  • Speed - 50mm/s
  • Following parameters are for laser cutting of thicker burlap:

  • MinPower - 70%
  • MaxPower - 70%
  • Speed - 30mm/s
  • Basically, for laser cutting a fabric, I ensured that the power is not too much and the speed is not too less. For khadi and thinner burlap the power to speed difference is kept close to avoid burnt edges, where as in case of thinker burlap 'jute' the difference is increased. This was done beacuse jute has numerous thick fibres which needs more power to cut them through. Below two images are of laser cut burlap.

    Video of laser cutting in progress of Khadi fabric

    Video of laser cutting in progress of thinner burlap

    Video of laser cutting in progress of the thicker burlap

    CNC Machining
    11mm tk MDF x 5no.s are glued together to create the base material for 3D milling of mold-1, where as base material for mold-2 is made out of 18mm tk MDF x 3no.s.

    MOLD- 1

    MOLD- 2

    a. Toolpath: Following are the screenshots of the toolpath that I created for roughing, finishing and cutting the material.

    Orientate and Size Model: In partworks 3D, I defined the orientation of the model by selecting the correct face of the .stl model as the top surface. And then I specified that machining is to be to be done only on the top side.

    Material Size and Margin: In this screenshot I defined the origin point, the depth of the model from the top surface and the bottom surface of the model, which is known as a cut plane.

    Roughing Toolpath: Key parameters in the roughing toolpath are-

  • spindle speed - 14000 rpm
  • milling bit - 1/4" stright Cut
  • feed rate - 1.5 inches/sec
  • plunge rate - 1.5 inches/sec
  • stepover - 30% (the most important parameter)
  • Finishing Toolpath: Key parameters in the finishing toolpath are-

  • spindle speed - 10000 rpm
  • milling bit - 1/4" ball nose
  • feed rate - 3.0 inches/sec
  • plunge rate - 3.0 inches/sec
  • stepover - 10% (again an important parameter)
  • Cutout Toolpath: It meant to cut out the actual model from the stock material, if it is bigger than the model. Following are the key parameters for it-

  • spindle speed - 15000 rpm
  • milling bit - 1/4"straight end
  • feed rate - 1.5 inches/sec
  • plunge rate - 1.5 inches/sec
  • pass depth - 0.12 inches
  • I made the cut out toolpath but did not use it for the mold-1 (inside mold), because after finishing toolpath was done, irealised that I would need the exessive material for handling. But I have used similar settings for cuting the mold-2 (outside mold).

    Preview Machining: This stage of creating toolpath provides with a simulation of roughing, finishing and cut out toolpaths, ensuring if the desired milling is going to happen or not.

    Save Toolpaths: To this page saves toolpath in .sbp format that is good for milling.

    b. Milling: After Setting up the machine, and mounting the base material on the sacrifice board, I started milling on CNC. Following are the final milled molds to be employed later for compaction.

    Milled Mold-1

    I used similar process of creating toolpath for mold-2, (outside mold) and was successfully able to mill it. The only difference lies in mold-1's milling and mold-2 is the use of cut out toolpath. As I mentioned above that I made cut out toolpath for mold-1 but did not actually applied it so that I can handle the mold. In case of mold-2, cut out toolpath could have easily been avoided, but I wanted to see the process and learn. So I used the for mold-2. Mold- 2 toolpaths are included in the week14 files.

    Both milled Molds, 1 and 2

    Layering, Infusion and Compaction

    Coupon making process helped me get hold of the entire process of composites, I followed same steps that I employed for coupon. Following are a few pictures of the process. I am using the same epoxy resin that I used for coupon building. So I mixed resin and hardner in the same proportion of 100:80 by weight.

    At first, I took the inside mold, wrapped it with kling wrap and started resin matrix to the natural fibre. The very first layer is of Khadi then thin burlap and than thick.

    Than again thin burlap layer is applied. This time I made small batches of resin mix. It is beacuse the shelf life of the mix is very low. It instatly needs to be applied.

    A closeup of the composite in progress.

    This is how It looks like after I had applied resin to all the layers one over the other with last layer of khadi again.

    Then I wrapped kling wrap around the outside mold as well.

    Now finally, I placed the outside mold over the inside one. Next step was to wrap the inside and outside mold along with natural fibre in resin matrix in between them. Now I provided compaction using C-lamps ensuring that the pressure is good enough to allow excessive resin to come out.

    Final Object

    This photograph was taken right after the composite taken off the molds. The molds have come out absolutely clean with no trace of any resin deposite. It can definitely by used again for another composite.

    This is the rear side of the composite. It is as good as the other side. Now, I am worried about the edges which just have resign in it and less to no fibres. This has happened may be becuase I did not provide enough overlap to the edges. So in the process of compaction the edges got pushed away allowing resin to fill in the gap.

    Links

    Week 14 Files
    Saftey Data-Sheet and Technical Data-Sheet for Resin
    Araldite Website