Can anyone give me some information on how to control the glas-resin percentage when using a FRP Chop System?
There are a variety of things you can do to vary the glass to resin ratio. Tip size on the spray head regulates how much material can possibly flow through. Fluid pressure (regulated by air pressure on the pump)introduces variability to the flow. Fluid pressure gives you some control over the amount of resin coming out of a given tip size.
Resin viscosity is also a variable. Thinner resins are obviously going to flow easier than thick resins will.
The number of roving strands will also come into play. When spraying, a certain amount of resin will come out of the gun based on the previously listed variables. One strand, two strands, three strands all have to use that set amount of resin. The speed of the chopper head also becomes a variable, although most people I know run the chop head wide open.
So to sum it all up, it’s all variable. There is no way I know to tell you to set air pressure, add roving, pull the trigger. It takes a bit of practice to make it work well but a good gun operator can really put out some good product.
Thanks Roger. I know how to set the different variables but the main question is how to determine the glass/resin ratio upfront. I now weigh the glass and resin use after producten so I can determine the glass (weight-) percentage after the product is finished. I would like to use a technique or method however to set my equipment PRIOR to spraying knowing that I have the correct glass percentage.
I understand what you want to do, I just don’t think that there is a way to do it to an exacting standard. Chop is imprecise. I suppose you could capture a bunch of data (resin temp and viscosity, mold temp, air pressure on the fluid pump, number of rovings through the gun, etc), create a spreadsheet, and apply what you’ve learned to the operation of the gun and modify every hour or so as conditions change but I think, in the end, you won’t do much better than a competent guy who just gets out there and pulls the trigger every day. It’s been my experience that chopping and rolling the material is your best data.
If you do find a good way to quantify it, please let me know. I love adding control to operations. Repeatable is money in the bank.
Roger, thanks again. I fully understand your last sentence. I come from the metal industry where everything is standardized and controlled, and find the composite industry still a bit of a “gray zone” ;
Much more control can be had in other composite processes. For instance, we have equipment for RTM/Lite RTM that controls resin flow, resin volume, resin temp, catalyst ratio, injection pressure, etc. Also has an RFID system that we can assign recipes to particular molds. The recipes can include all the previously mentioned parameters. It also has data capture so we can go back and look at the data if we have a problem.
Chopper gun operations are very much in that grey area you mentioned. However, when you have a good and experienced operator, you can build fairly consistent parts. Every system has it’s pros and cons.