Tuesday, November 30, 2010
These diagrams above are obviously simplistic but illustrate what to watch out for. The first illustration will be fine to use a one part mold; the second is marginal but will probably work ok as a one part mold if the part or mold has any flex to it. The third illustration will definitely require a multi-part mold. Again, this is simplistic and in the real world the part shape will be much more complex. All it takes is one little concave ridge or spot to ruin a mold. The good news is that we have options to deal with these small areas which we will address later. Another piece of good news is that most production parts are stamped which also requires that a part not be concave.
BTW, when I made my first roof mold I did not think that the concavity of the window ledge flanges relative to each other would be a problem. They were and it cost me a carbon/honeycomb part and a complete mold to learn this lesson.
This is a good time to address physical mold release. Another problem I ran into with the roof was that the shape did not allow me to insert release wedges up into the main central area of the roof. So there will be situations where even though the part is not concave, you may want to make a multi-part mold. This is exactly the case with the interior door structure for this part. The shape would not allow me to get wedges into to large flat section so I split the mold down the middle of the roof.
For my door there were several issues that I had to get my arms around up front: How much door did I want to make and if I was going to make an interior structure, how much to make? How was I going to attach the door? Do I want to use the stock latch? How strong does the door need to be in flex and in impact? At what point do I sacrifice the above for weight?
From here I came away with two options. One, make as light a door as possible - i.e. just an outer skin - and two, make one with a stock matching interior structure but with a smooth interior panel that would create a completely enclosed structure. Ultimately I chose to make the molds such that I would not have to make this decision now. This would require a single mold of the exterior panel and a separate mold of the interior structure. If I wanted a part with an interior structure I would simply lay-up carbon parts in both molds and bond them together (more on this later).
The first challenge that jumped out at me was the shape of the stock door’s interior structure. As you can see in the picture above, it is quite complex with lots of tight curves and angle changes. I wanted a large flat panel on the inside so I had to create a shelf for a panel to sit on. You can clearly see in the picture how I was trimming the inner structure to accomplish this. I also have to work out how the inner carbon fiber structure was going to bond to the top outer carbon skin. More on this later.
The next item to address was the door handle and lock areas on the outer skin. As the doors I am making are going to be in the 5-6 lbs. weight range I decided to eliminate these parts and to make the door smooth completely smooth. To do this I will filled these areas with clay on the stock part which keeps them from being transferred to the mold. More on this in the next phase.
After studying the door for literally hours I finally came to the conclusion that the inner structure would require a multi-part mold to guarantee release and that the seam between the molds should bisect the large flat section of the middle of the door. I chose to seam it vertically to make the mold halves square instead of long rectangles. That way the seam would be smaller and the mold halves would have more structural integrity. Also, I could have chosen to seam it along the edge of the inner panel but that would have been much more difficult and as this was not an exterior panel I chose the faster route.
Phase 2 - Part Preparation
Now that I had a plan it was time to make the initial mold. The first step in this process is to completely clean the part inside and out. By completely clean I mean 100% free of grit and completely smooth. For this I love to use one of the clay bar cleaning bars. If your stock part has any major defects then this is a good time to address them using common body repair methods - welding, body filler, hammer, etc. Just get the surface perfectly smooth with a minimum of porosity as that will prevent the mold from sticking to the part. Remember any imperfections in the part will be in the mold and any imperfections in the mold will be in the finished part.
BTW, my original door was in pretty good shape except for an ‘outward ding’ that came from the removal of the side impact door bar. This I chose to address on the mold as it would be an inward depression. More on this later.
One thing I haven’t discussed yet is flanging. When vacuum bagging it is, in my opinion, highly desirable to have a flanged mold. This does two things, principally it gives you a smooth surface that surrounds the part to enable you to bag up the part (this will make more since later) and it also makes the finished part oversized which makes it easier to release from the mold and allows you to trim the part back to match the original parts dimensions.
For flanges I shoot for a minimum of 2” and prefer 4”. Much bigger than this and you are wasting fiberglass and gel coat. To make the flange I like to use strips of 6” wide x .016” aluminum and glue them to the underside of the edges of the part. Often this will require trimming and radiusing of the aluminum to insure that the flange is continuous and has good adhesion to the part. I will then trim the excess flange of to give me my desired 2-4” size.
Above you will see a great example of the beginning of the flanging process. I held the aluminum under the part and trace the profile with a marker. I then trimmed along the line which resulted in a part that was ready to apply to the inside door flange. BTW, you must make sure the aluminum follows the contour of the part in every plane by pressing the aluminum up to meet the stock part continuously. You absolutely must make sure that there is no floating gap between the flange and the part. If there is then resin will seep into the seam and cause all sorts of problems.
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