OVERVIEW: Here are best known practices for bonding and fabricating carbon fiber tube, plate and connection points. You will learn how to join composite parts here. We have put together notes, photographs, images & videos on the best methods of designing and assembly for building your composite Connector System. It is all pretty easy once you get the hang of it. We will break things down by jumping into Fabrication & Surface Preparation, Bonding & Cleanup, Material Performance & Material Properties. Below are further application best known practices. Beyond this section, we will conclude Design notes on how to design using these connectors.
Many of the parts are designed to be used as an air, water, or hydraulic passage. At times, especially with the smaller connectors it is difficult to prevent epoxy from backfilling into the inside of the carbon fiber pipe. We recommend using 10mm OD & Up to insure that you get proper flow.
The image below shows that each connector has a bonding joint and past that is a tube hard stop. We will take you through the process of designing your assembly, then moving through to the final bonding & cleanup.
FABRICATION & SURFACE PREPARATION: You must first have a few hand tools needed to terminate tubing to lengths. Overnight Composites provides a bonding and fabrication kit, yet a good vice, a pair of calipers and a tape rule, sandpaper, alcohol wipes, should work well enough.
With a design that establishes the length of your tube referenced the tube hard stop inside of the connector we can use the hack saw to cut the tube length. We like to measure off of a hard stop. The image below describes setting the length of the cut + the thickness of the hacksaw blade. Add a little extra and we will sand it back after cut. It's easier to stay on the high side of the measurement rather than go to short on the length.
STEP 1: Measure tube with a compensation for the thickness of the hack saw blade. (HINT: Set the tube in the vice and tap it with a small hammer until the dimension is reached. (We add 2 hacksaw blades to the measurement thicknesses, then we sand the tube back until we get the dimension we want.
STEP 2: Use a hard stop on a metal vice. Cut the tube and sand back to the dimension you are trying to achieve.
Once the tubes) are cut, the surface sanding is the dominant variable that will maximize the bond strength. There are (2) Ways to get the length of the sanding section of the tube. OPTION A: We use the depth mic side of the calipers to reach inside of the part to get a depth. OPTION B: We can go to the CAD model and measure to the tube hard stop where the tube will bottom out inside of the connector. We set the sanding back a bit so we do not scratch the visible section of the tube for cosmetic reasons.
Since we are working fast, we should cut, measure and mask all the parts that will be assembled. Before we bond everything together, dry fit the tubes and double check out overall dimensions to insure that you get an accurate bond.
BONDING & CLAMPING: In order to bond Composite Connectors, we will need the following components. NanoTubes with 30 minute Epoxy is included in the consumables. Next, we will make small batches using a flat stick.
MIX WELL.WORK FAST.CLEAN UP: We will show you the original method of mixing nanotubes with epoxy, then rolling it into the glue feature in the connector. Expense
DIRECTIONAL STRENGTHS & MATERIAL PROPERTIES: One must understand that the parts are 3D printed using a (2) head system. It is almost impossible to define the true strength of the part, since 3D printing materials that are monolithic in strength. In other words, one direction of forces are placed on these connectors are considerably stronger than the other direction. See below - working from left ot right where the arrow's pointing up define the direction that the printer lays down material. Our destructive testing consist of 2 flathead screw drivers that were forced outward in the direction of the laying. The part was rotated 90 degrees to make this happen. Description: FMA (Failure Modes & Analysis).
Yes, our steel screw driver is bent. As standardized tests come available, we will publish results here. CCFMA_TestPlan Records.pdf. What further complicates this much needed empirical data is the fact that our 3D printing process can only get fiber placed in the part where we have a 4mm cross section of the part being built. And adding complexity to the solution, the parts have a honeycomb inner core to reduce the weight. Wish we could simplify the subject matter, yet the best information we have on the subject falls somewhere between the graphed out chart below.
We have some data points that might help with your structural analysis directly from our awesome equipment/materials manufacturer here. Our solution is a cross between Onyx and Fiberglass as noted below.
BONDING & STRENGTH OF YOUR BONDS: As we are compiling a series of adhesion tests that will give you a baseline when calculating loads applied cato your Composite Connector design. Bond strength is a function of surface preparation and cleanliness, time & temperature. We are conducting both rotational and lap shear testing. Seeing that we have built a composite 2 part epoxy that includes Nanotubes in the compound.
As we are compiling a series of adhesion tests that will give you a baseline when calculating load capacities of your design. Bonding is only as strong as the surface preparation that has been done to the assembly. We have developed our test plan & protocol and are moving through a series of experiments that will be conducted over the course of the business starting in the Spring of 2018. We will post updates as our findings become available.
SAFETY FIRST: Vaporous We are using 2 part epoxy that is essentially the same as retail ready 5 minute and 30 minute epoxies to bond tubes together. In preparation to using these materials, one should always work in a highly ventilated area and handle these chemicals with safety,(vinyl gloves, glasses, and masks) when handling these compounds. If any epoxy is digested, see immediate medical help. When sanding or cutting tubing, use the same precautions.
DESIGN: All of our parts are up on the storefront at www.overnightcomposites.com. They are categorized by the outside dimension of the part. So all connectors that work with 6mm round carbon fiber tubes are located in the 6mm Round section of the site, etc, etc. Here are 2 buttons that you should be familiar with. Here is a great place to see how we design using the Composite Connector App and the 3D Viewer to build out the shape of your structure.