Overview of Loading Fiberglass Into the Tool
John Moore from JHM Technologies, Inc. explains the ins-and-outs of properly loading fiberglass into your tools, how to choose the fiberglass that is right for your application, and some critical areas to pay attention to.
Video Transcription Below
Hi, I am John Moore. Today I would like to talk about loading the fiberglass into the tool, and what is critical. The things that we are going to look at are the types of fiber and then how to decide. Obviously, the fiber is giving us the greatest amount of strength in the overall performance of the composite. Yet, the loading of the fiber into the mold is most critical to the process in the ability to control the resin flow path.
Important Considerations When Choosing a Fiber
When we are deciding on a fiber, we have to know what is out there, what is available to us. Let’s first discuss what is important when we are considering [the fiber]. Obviously, the overall strength; we have to concern ourselves with the final product. How much fiber content is there? Some are considering [the content] by volume, others by weight percentage of the total laminate. But when we are thinking about the actual molding process what we are most concerned with is that the fiber fit uniformly, cross-sectional. The thickness of the part, the mold cavity has to be filled from the one surface to the top surface. What I am showing you here is the cross-section of a part (the same part we will show in a discussion on loading). We want that fiber to fit uniformly in that cross-section.
Here is where we look at the different materials. The first thought would be a woven fabric. [Woven fabric] would give us the greatest strength. However, it is the least desirable for conform-ability. If we think about trying to [mold] woven fabric we try to tuck it in here and it is pulling there and vice-versa. It is a very difficult product to form. The other issue is there is no loft. If this were the material of choice, we would have to design in the calibration of the mold for the part thickness wherever there was an overlap. [This includes any] darts, cuts, or over the top [access in the fabric]. The mold needs to be sympathetic to that because the laminate must be uniform in thickness in respect to the fiber. If the fiber has to overlap the mold will have to compliment where that overlap is and that overlap will have to be in the same place each time. That is the disadvantage to the ridged second half of the mold. Unlike if we had a nylon bag, silicon bag, [etc.] for the vacuum infusion process. There is the one advantage it has, wherever the overlap falls it doesn’t matter in the next one if it was different.
A woven fabric could be put in, but we would put it in selectively and we would have to have it in combination with something that has fluff or loft to it. [Historically] we only had continuous strand mat. [Continuous stand mat] does not stretch in either direction [and] it is a continuous filament. For years this is all we had. It does have loft, so if it had an overlap it was tolerant of that when we are molding and there was a uniform cross-section in the mold calibration. But we [still] had to fit the [mold] as best we could and then dart and cut to have the overlaps fall into play, as well as holding it in place.
In the mid 90’s there came about some materials that had a sandwich construction (this may be difficult got you to see). But they are chopped with a core in the middle and then chop on the bottom and then this is stitched together. The beauty of it is that it stretches and takes shape and holds that shape (memory). They come in two varieties; one is glass woven in the center (a knit all to its self). In fact let me get a piece of that knit fabric center. Here is what is in the center of the glass type, when you have glass cores. You have a knitted fabric of this type stretches in each direction nicely and then this has chop applied to it top and bottom and [is] then stitched. You end up with a product like this. That is also the type of product where you have chop, you have a felt (typically a polyethylene felt) and then more chop and it too is stitched as a sandwich. For fire retardant applications it is generally respectful to use the glass core and the chop on each side. The beauty of these types of products is the ability to form. So when we are loading it is critical that the glass takes the shape and holds it in the final product. We are looking for the glass to be complimentary to the shape and fill the cavity up in all areas.
What I have here – to speed this operation up – here I have taken that sandwiched glass and I have shaped it. Now right away you’re saying where do I get a preform like that? There are ways to make preforms this was done for this demonstration by spraying a coating on the surface of the mold so it would hold it in shape. What I am trying to illustrate is the fact that these formable materials take the exact shape of the mold, and that is what is critical. It is also (which you cannot appreciate in the video) thick just as the part thickness is. The fiber is the same width or greater than the part thickness, so when the mold closes we have glass touching both sides of the mold and the resin is flowing with a bit of controlled resistance.
The key to the Process
That is the key to the process. Having the ability to predict the resin flow resistance at the leading edge and that will force the air ahead of it out towards the vent. It is when the resin has the ability to scoot up a radius because there is glass pulled tight to the inside against the male side then it will racetrack down that radius that is when you will entrap air or have dry spots in the part.
In summary, when selecting a fiber we will base it on how the fiber content must be, we may be restricted to some areas that need additional strength so we put in some woven fabric for that strength selectively. Ideally we want a type of fiber that has loft and has conform-ability. That is how you decide.