For all the masses out there at the edge of their seats waiting for the next in the series...Our last post documented our process of ramping up from prototype to production of our carbon fiber centrifuge parts. A quick recap on the project scope- our company, Apex Designs, was hired to develop and manufacture lightweight but exceptionally strong cups to house viles or bags of blood that are spun in centrifuges for medical research. We proposed to build the parts from carbon fiber- specifically bi directional pre-preg carbon. Because our client required a smooth finish on both the inside and outside of part, we opted for a compression molding process that would give us tooled finishes on all surfaces of the part. (See previous blog below on the development of the tooling.)
We use a CAD software called Solidworks for our design and simulation work. Solidworks does a very good job at performing stress analysis on parts and assemblies made from most of the
common materials used in manufacturing- metals, plastics, etc. It does NOT however, allow you to perform stress simulations on composite materials (well, not without an add on $12k non linear simulation program). So if you are designing parts using composites, you're SOL on getting any help from the computer. However, it is still very helpful to perform an analysis on the part by assigning other materials to the part just to be able to see the highest stressed areas or features of the parts even if the numbers dont make any sense.So, in the real world, good old hand calculations are used to give you an idea of where you think you should be and testing is required to verify your calculations. The cool part about composites is that you can orient the fibers of the layup to give you the strength characteristics that you want. The reality is, however, that testing is absolutely necessary to validate what you think is the proper layup schedule. Because we are building parts that have an aluminum ring inserted into the layup- and did not know how that would effect our part's strength, we sent a few samples out for testing to a proper engineering materials testing lab. The tests results showed our prototype cups met the strength requirements set forth by our customer, but because we were going to build several hundred parts, we decided to build our own testing rig to test samples of the batches built in house. This would allow us to verify the consistency of our parts and allow us to sleep at night!
Straight out of college, I worked as a materials
test engineer where we conducted all kinds of materials and product testing- we got to break to alot of stuff- everything from metal roof panels to 25 ft long fiberglass columns under hundreds of tons of load. My background there in ASTM standardized testing gave me the confidence that we could build our own test rig.
To load up our parts, all we'd need is a simple hydraulic ram with a known piston diameter, a good quality pressure gage, and a nice rigid frame to place the cups under load. We converted a 20 ton hydraulic press into our test rig by adding a rigid fixture to hold the cups, and adding a pressure gage to the ram. For peace of mind, we dis-assembled the ram to measure the actual piston diameter instead of just assuming the literature it came with was correct. It was as stated, but if it were wrong, our calculations would be totally bogus.
Pressure = force/ area where pressure is in PSI, force is in Pounds, and area is in inches^2. Figuring the force applied to the cup is a simple matter of converting the pressure registered on the gage and multiplying it by the area of the ram's piston. For our tests, we loaded the cup in 200 psi increments and held the load for 30 seconds before proceeding up the scale. In most tests, a strain gage of sorts is used to measure the deflection of part at each load. The simluation from our Solidworks software showed us the most likely type of deformation, so we chose to measure that deformation using dial calipers. The good news is that the deflections we saw were pretty miniscule for the loads applied. Carbon's yield point and ultimate failure point are very close together so the lack of deflection was really of no surprise to us.
With the testing rig in house, we are now able to validate our part's strength characteristics and it will allow us to try new methods or layup schedules with new products that are surely to follow our medical centrifuge cups. And no - you dont get the numbers and no I'm not ready to divulge how we solved our silicone tooling problems- well not yet anyway! Like my friend Scott says, "a baker's success is in how he protects his recipe" Of course, if I can be of any help, I'd be glad to speak with you. Our contact info is on our website http://www.apexdesigns.net/
-Steve
Straight out of college, I worked as a materials
test engineer where we conducted all kinds of materials and product testing- we got to break to alot of stuff- everything from metal roof panels to 25 ft long fiberglass columns under hundreds of tons of load. My background there in ASTM standardized testing gave me the confidence that we could build our own test rig.To load up our parts, all we'd need is a simple hydraulic ram with a known piston diameter, a good quality pressure gage, and a nice rigid frame to place the cups under load. We converted a 20 ton hydraulic press into our test rig by adding a rigid fixture to hold the cups, and adding a pressure gage to the ram. For peace of mind, we dis-assembled the ram to measure the actual piston diameter instead of just assuming the literature it came with was correct. It was as stated, but if it were wrong, our calculations would be totally bogus.
Pressure = force/ area where pressure is in PSI, force is in Pounds, and area is in inches^2. Figuring the force applied to the cup is a simple matter of converting the pressure registered on the gage and multiplying it by the area of the ram's piston. For our tests, we loaded the cup in 200 psi increments and held the load for 30 seconds before proceeding up the scale. In most tests, a strain gage of sorts is used to measure the deflection of part at each load. The simluation from our Solidworks software showed us the most likely type of deformation, so we chose to measure that deformation using dial calipers. The good news is that the deflections we saw were pretty miniscule for the loads applied. Carbon's yield point and ultimate failure point are very close together so the lack of deflection was really of no surprise to us.
With the testing rig in house, we are now able to validate our part's strength characteristics and it will allow us to try new methods or layup schedules with new products that are surely to follow our medical centrifuge cups. And no - you dont get the numbers and no I'm not ready to divulge how we solved our silicone tooling problems- well not yet anyway! Like my friend Scott says, "a baker's success is in how he protects his recipe" Of course, if I can be of any help, I'd be glad to speak with you. Our contact info is on our website http://www.apexdesigns.net/
-Steve
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