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John Kimball

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John Kimball last won the day on April 19

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  1. Testing a scale version of the part out of an arbitrary material won't necessarily give you a data point that you can use as you aren't using that same material or construction that you would use on your 1:1 version. And as composites are hard to scale for testing purposes, I don't see any advantage to building a scale version for strength testing. However, you can build a section of the actual planned structure and test that for stiffness, bending, etc. you will need to extrapolate from there to see if get what you need. This is how aerospace designers quantify the materials they plan to use. you will need to find a test house and determine what tests will quantify your needs. You can also rig up your own static load tests if you know what you are looking for.
  2. I have a spreadsheet calculator, but don't think I can post that type of file on here. But if it is a wet layup or infusion, you can use the same weight as your fiber for a good start. just weigh your cloth, and if it weighs 100 grams, then you can start with 100 grams of resin. With infusion, you will need to add extra to account for the tubing and flow media. Hope this answers your question.
  3. @Anthony Fairhurst, here's the BOM. You can pick and choose what you need for your application.
  4. I’m still working on the BOM, but should be able to get something soon. I really like my PID with Bluetooth. It doesn’t necessarily record, but you can monitor it with a nice graph. However, if you walk away and lose Bluetooth, you lose the record. I use my iPad to record, so I can walk away. The screenshot below is an example of what it can do. Just keep in mind that it can only show 1 thermocouple. The 3 values are: Set point (red), TC value (blue) and power input (green)(% of full power).
  5. Shouldn’t be a problem. It’ll take me a minute to collect it though. the best part is that you could do this same thing with an old kitchen oven if you need larger space.
  6. Welcome to the forum. I can’t wait to see what you do and your thought process along the way.
  7. Loft is the materials ability to rebound after being compressed or in other words, the fluffiness of the material. And the value is the relevant value for the insulation or sound absorption. For instance, more lofty insulation will have a higher “R” value which is a number assigned for how well it insulates. With sound deadening it’s similar, but most sound deadening is foam, but batting can be used as well.
  8. I think the short answer is that you will need at least 2 layers for something like this. One layer would be too flexible, especially at the tab and connection point. I would still be concerned about the heat of the exhaust pipe. Just try to keep the composite clamp below 200F or you could see some issues.
  9. Dan, As you have come to hear me say too many times: “it depends”. Sandblasting is an acceptable way to prep a surface for bonding, but is only the first step. proper cleaning is just as important. Also you consider what blast media you use. For metal, sand is excellent as it won’t damage the metal too severely, but can if you blast too long. With composites, it is plastic and much softer than sand, so it can blast right through the resin and fibers quite quickly. for softer materials like plastic and resin, you can use media like walnut shell or baking soda (you can get a 5 lb bag at Costco). These should pit the surface, but not erode it like sand would. But you will still need to clean it aggressively to get any embedded particles out. You can use a stiff brush to help clean it with IPA.
  10. I’ve been thinking about this today and a few things came to mind. I think you should go with the 4 ply, but maybe consider the orientation of the mold while curing to increase the flow of air to the path of least resistance. maybe up side down or at a 45 degree angle. Also, if your edges are not filled with resin and fiber, then the majority of trapped air will simply fill those voids before it has a chance to leave the mold. For vents, try a couple to start at opposite sides of the tool and tilt the tool at a 45 to allow air to travel up. RTM molds will sometimes fill from the bottom and push air out the top as it fills. What you suggest above should work. Start small and adjust from there. I’ve also heard of people vacuum bagging their compression tools to help evacuate air. Yours is small enough to give it a try. Also,I noticed that you are using a kitchen oven to cure. you have to be careful when doing this as the oven will try to heat up as fast as possible. This can flash cure the surface closest to the heat source and prevent percolation of air in the resin. maybe heat the oven to it’s lowest setting before putting your part in the oven and then slowly increasing the heat by 50F every thirty minutes until you reach cure temp, then hold for 90 - 120 minutes. Ideally, you want to heat the parts at a rate of 2.5 - 5 degrees F per minute. this allows the tool and the material to heat up at the same rate. there’s a lengthy post on how to modify an oven with a temp controller on here somewhere if you’re bored.
  11. Sound deadening is similar to insulation. Loft and volume is needed to increase the value, so adding a single ply of ekoa may provide a small amount of damping (would change the tone of a guitar for instance), but may not provide a noticeable amount of sound or vibration control. It should be noted that Ekoa is heavier and a lot less stiff than Carbon fiber, so there are significant tradeoffs when considering different materials. Adding foam core to the carbon fiber structure would give you better results than adding different fiber materials to the layup. Cored panels are significantly stiffer, less vibration, and add some sound deadening aspects.
  12. Anthony, porosity in compression molded parts is caused by 1 of 2 things. 1. Not enough material to fill the entire cavity. 2. No place for trapped air to escape thus compressing it into the smallest areas avialable. For the first one, you should design a tool that has a slightly smaller cavity gap than the Cure Ply Thickness (CPT) of the material. this is easier with prepreg obviously. For the second, some carefully place vent holes to allow the trapped air and a bit of resin to escape. I will say that it is a very exacting practice and may take quite a few trial and error attempts to get it right. The best material for cavity compression molding is Sheet Mold Compound (SMC). A basic charge is calculated by weight and then placed in the. As the material is compressed ad heated, it will move to the voids and then excess is expelled through vent holes. This method does not allow for fiber orientation, but does provide excellent surfaces and parts. The method to get good fiber orientation and excellent surface quality is called SQRTM. This method uses prepreg, liquid resin, and a pressure pot to inject resin into a prepreg filled cavity to fill any voids left by lack of fiber. Good luck. Can’t wait to see the next iteration. I’d start with adding a bit more material and maybe a few vent channels in the edges.
  13. The tape needs to shrink first, so a quick hit at 150ºF to shrink it, then let it cure naturally and slowly with a controlled ramp. I use a heat gun to do this.
  14. The quick answer to your question is that load test data exists for specific projects, but they may differ greatly from your project. The long answer is a lot more complicated, but is possible given what you want to accomplish. Bonding panels together, especially at right angles, requires much more than just bonding them together. unlike welding, composites rely on surface area to achieve strength when connecting. So you would need some angles bonded to both plates or panels in order to achieve a proper support and bond. This is fairly easy if you only need right angles, but if you need anything else, you have to fabricate the angles first with the correct angles, then you can bond them in place. bolting or riveting them together will never give you the same integrity as an adhesive bond. You can use rivets or bolts to aid in fixturing the parts as the adhesive cures, but in the end, the fasteners will not give you any more strength to the bond area. It’s all about surface area. A bolt will never provide the same surface area fixing as an adhesive bond line will. I’ve often heard fasteners used in adhesive applications as “chicken fasteners”, because the the engineer is too chicken the rely on the adhesive 😅. Another method often used is the bond the plates or panels together the way you want them and then wet lay carbon fiber strips in the corners to mimic a bonded angle. Generally, this works really well with polyester resin parts, but with epoxy laminates, it takes a lot of surface prep to ensure proper bonding of wet layup against cured laminates. This article may help you understand better: https://journals.sagepub.com/doi/pdf/10.1243/14644207JMDA219 It talks about bonding cored panels, but is applicable to non-cored plates as well.
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