Trunk Roundel Gasket

Ian -- In looking at this, I think the material used to produce the original part has lost its elasticity over time, and the parts have returned to their original round state. I think the parts were designed to be elastic, and to be stretched into place with the lip pressed back onto the side of the gasket by the material attempting to return to its original shape. As to your drawing, I was thinking the base would extend a little more deeply into the gasket, making the angle a bit more acute and stretching the gasket a little more tightly, but I think your profile is very close to what the part should look like.

The gasket I have is pretty good, but it has a few flaws and won't sit flat for making a mold. So, my plan is to have the gasket scanned, clean it up and have a 3d printed representation of the part made. Part of my thought process is to modify the original part so that the prototype is a little bit elliptical. This should yield a consistent edge stretch all around the perimeter of the gasket and a consistent reveal of the edge along the base. As to the finish of the gasket, I believe the only place were fine detail is really important is on the edge. The prototype could be hand finished if need be prior to making a mold.

I've been talking with a company called Smooth On about using a silicons for the mold and for individual parts. They have some materials that they have recommended.

Alas, this means the parts need to be cast by hand. I did a favor for the teenage car guy up the street a little while ago. Perhaps I will enlist his help in casting individual gaskets as payback for my prior generosity ('cause, I am not really that generous).

Markos - why do you think a rigid material is preferable for the mold itself? I was thinking about a flexible mold specifically because of the process of removing the edge of finished part from the mold.
 
Ohmess said:
Ian -- In looking at this, I think the material used to produce the original part has lost its elasticity over time, and the parts have returned to their original round state. I think the parts were designed to be elastic, and to be stretched into place with the lip pressed back onto the side of the gasket by the material attempting to return to its original shape. As to your drawing, I was thinking the base would extend a little more deeply into the gasket, making the angle a bit more acute and stretching the gasket a little more tightly, but I think your profile is very close to what the part should look like.

The gasket I have is pretty good, but it has a few flaws and won't sit flat for making a mold. So, my plan is to have the gasket scanned, clean it up and have a 3d printed representation of the part made. Part of my thought process is to modify the original part so that the prototype is a little bit elliptical. This should yield a consistent edge stretch all around the perimeter of the gasket and a consistent reveal of the edge along the base. As to the finish of the gasket, I believe the only place were fine detail is really important is on the edge. The prototype could be hand finished if need be prior to making a mold.

I've been talking with a company called Smooth On about using a silicons for the mold and for individual parts. They have some materials that they have recommended.

Alas, this means the parts need to be cast by hand. I did a favor for the teenage car guy up the street a little while ago. Perhaps I will enlist his help in casting individual gaskets as payback for my prior generosity ('cause, I am not really that generous).

Markos - why do you think a rigid material is preferable for the mold itself? I was thinking about a flexible mold specifically because of the process of removing the edge of finished part from the mold.
Click to expand...

If you are concerned about cost, brace yourself for 3D scanning costs. A 3D mesh file still needs to be converted to a drawing so you won’t be adding much value by scanning a simple part You can trace the part with a pen and pencil and scan it with your personal scanner (see my thread below). Send that to the modeler or give him/her the part. Any competent modeler can make this part in under 30 minutes, and they likely don’t need the scan. If you do scan the part you need to provide at least one dimension so they can sync the part. I drew on my line drawing after scanning it, snapped a pic with my phone, and included that with the PDF scan. I’ll reiterate, 3D scanning is expensive, and your part is 90% 2D.

Your call on mold material. If you have a one piece mold you will have no problem pulling the seal out with a pick. A hard one piece mold allows you to scrape the seal face flat. If you do that with a sold mold it will flex while you are smoothing our the top. Also, if you have ever caulked your shower you may know that new silicone sticks wonderfully well to old silicone. Make sure you use a good release spray.

Lastly, seriously take a look at youtube. There are eleventy-billion DIY examples of making silicone molds, casting polyurethane, and casting silicone.

Example of me turning a trace into a file.
https://www.e9coupe.com/forum/threads/feeler-triple-weber-t-stat-housing-spacer-50.27971/
 
Yeah, I actually have spent a bunch of time looking at youtube videos. The problem for a rookie like me is that you need to first define the characteristics of what you want to end up with, and seek a material that matches those characteristics. Once you find a universe of potential materials, then you need to consider how those materials are manipulated to make what you want. This eliminates a bunch of materials because the process of manipulating them is too complex or dangerous or expensive or whatever. Then, after you settle on a material, you need to start down the path of defining the process of manipulating the material into the part you want. This entails understanding both procedures and materials, eliminating again based on complexity or expense...

So, I know the Smooth On guys are trying to sell me on their products, but they have been very helpful in getting into the details of the process of molding with me so I think I am going to try a silicone molding material and a silicone material for the part (recognizing a good release agent will be essential). I am on the fence about whether to try this at atmospheric pressures or making a pressure tank. I think I can make a pressure tank for under $100 to use in casting the parts, which would ensure the silicone flows all the way into the mold and enable a flat bottom surface.

Your suggestion on scanning is very helpful. Obviously, I need to care about cost otherwise no one will buy these and I'll end up with an $800 gasket on my car.
 
I have the Smooth-On silicone for some other projects I am going to mold. Applying pressure will reduce the bubble when you cast, but silicone is flexible so when you release the pressure, the bubbles will try and return to their normal size which might leave bumps in the gasket. Or the mold. Pressure casting is more for things that set hard so the pressure trapped in the bubbles can't expand them and cause havoc when the pressure is released.

I bought a Harbor Freight paint pressure vessel for use as a vacuum tank to degas the silicone only to find the fittings don't match standard pressure fittings on the tank. I haven't put the conversion fitting together yet.
One note about degassing the silicone, you have to pull a very strong vacuum to get it to work, and that means a pretty good vacuum pump. A small one won't get you there, or will take so long the silicone will start setting up.

The other bit about using silicone as a gasket is that it isn't very hard and may not be mechanically suited for use under the roundel base as it's a metal edge that will cut into it over time. My gasket has shrunk completely under my roundel base or has been cut off by the metal edge.

I've asked my mechanical engineer friend and he suggested perhaps using this sheet silicone material from Mcmaster-Carr. Yes, it's flat so won't have the ridge, but it's harder than the two part silicone with a durometer of 50A and comes in grey. Make a 2D pattern and have a water jet crank them out. Serviceable if not quite perfect reproduction.
He also said: "
Silicone might be OK, but possibly too soft. Such molded rubber parts are usually a neoprene compound or some other compression molded material. I don't think it would be practical to try compression molding at home. And neoprene in any color other than black is problematic anyway.
There are thermoplastic rubbers out there that can be injection molded. Santoprene is a very common product.
There are also several newish rubbery resins that can be printed with some of the new printing technologies. I don't know how they'd hold up to the elements."​

Back to making the mold, assuming we can find a good material to make into these gaskets, and there are many to choose from, I looked closely at DeQ's photo and it seems the raised rim is really not very detailed. It's pretty much a rounded edge. My thought is that we are overthinking the critical aspect of this.

What I suggest is this:

First of all, to get controllable dimensions, a hard mold is essential. The gasket can be made in a flat mold since the material will be flexible enough to bend to the shape of the decklid and will be held there by the roundel base.
The back of the gasket should be flat and smooth, so the back of the mold can be a flat piece of metal. So, a two-part mold, but about as easy as it gets.

Now the critical part, making the mold. The raised ridge will be a groove cut into the bottom of the mold and this can be done with a CNC milling machine, but really the depth is the same for the ridge (Groove) all around so it could be handled by a standard router with a tiny round bit guided by an oval template cut from plexy or MDF. The rest of the gasket can be cut the same way with a flat bottom bit, carefully. I've done some tests with aluminum in the past and it must be done slowly but it is possible. It could be done with other materials as well.

The liquid gasket material is put into the mold and the top is placed on which squishes the extra out. Messy? yes. One at a time? yes. But it's simple, cheap and should work. It's a valid method for low production volumes. The only critical surface is the raised rim so once that's done the rest is much easier. The flash of the extra that squeezes out would have to be trimmed, a minor nuisance.

What do you think?
Roundel Gasket molding diagram.jpg
 
mulberryworks said:
Back to making the mold, assuming we can find a good material to make into these gaskets, and there are many to choose from, I looked closely at DeQ's photo and it seems the raised rim is really not very detailed. It's pretty much a rounded edge. My thought is that we are overthinking the critical aspect of this.

What I suggest is this:

First of all, to get controllable dimensions, a hard mold is essential. The gasket can be made in a flat mold since the material will be flexible enough to bend to the shape of the decklid and will be held there by the roundel base.
The back of the gasket should be flat and smooth, so the back of the mold can be a flat piece of metal. So, a two-part mold, but about as easy as it gets.

Now the critical part, making the mold. The raised ridge will be a groove cut into the bottom of the mold and this can be done with a CNC milling machine, but really the depth is the same for the ridge (Groove) all around so it could be handled by a standard router with a tiny round bit guided by an oval template cut from plexy or MDF. The rest of the gasket can be cut the same way with a flat bottom bit, carefully. I've done some tests with aluminum in the past and it must be done slowly but it is possible. It could be done with other materials as well.

The liquid gasket material is put into the mold and the top is placed on which squishes the extra out. Messy? yes. One at a time? yes. But it's simple, cheap and should work. It's a valid method for low production volumes. The only critical surface is the raised rim so once that's done the rest is much easier. The flash of the extra that squeezes out would have to be trimmed, a minor nuisance.

What do you think?
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Lol - I think that is exactly what I suggested in the middle of page four. :D I'm glad that you arrived at the same conclusion.

Chris - I have a call with my modeler at 12:30 PST on Monday. I have a half hour of modeling time credit and I'm happy to have him model a mold, just like Ian described above. You are welcome to listen in on the call if you wish. I don't have any plans to make any gaskets, but you are free to try the mold. It will likely cost about $30 to print depending on how sturdy we make it. The benefit of printing the mold is that if/when you screw it up you can simply print a new one.

Again, you can make your own mold for a few bucks worth of MDF and polyurethane. You make a small box. Glue your gasket lip up in the box (all corners sealed). Pour in the resin. Let it set. Rub some vaseline or brush some wax on the seal to help it release.

Ian - Grey High Temp Loctite 5699 has a Shore A hardness between 45 and 70. Keep in mind that you get air bubbles from mixing silicone. If you just buy a tube of 100% silicone it isn't really going to have a lot of bubbles in it. The downside is that you can't control the mixture, and it is obviously more expensive.

http://www.loctite.sg/sea/content_data/93788_5699EN.pdf

Z-B-_-fo5oy.JPG
 
Grey High Temp Loctite 5699 looks interesting. However single component silicones take a while to cure. The spec sheet shows it would take two days to reach 50% strength which would slow production greatly. My idea of having a top on the mold to create a flat smooth back for the gasket would result in very slow curing. Leaving the back open would allow a much faster cure. Striking off the excess sealant with a straight edge dipped in a suitable solvent should give a reasonably flat surface. This is where a hard mold would be helpful.

Using a two-part silicone would speed curing a great deal since it's not dependant on the atmosphere. Bubbles can be avoided by applying the silicone to the mold with a brush.

If there exists a perfectly preserved OEM gasket it can be used as a model and a mold created directly from it, making several would be short work and that would speed things greatly. I'm skeptical since they all seem to shrink to some extent.

Otherwise, a model could be mocked up using an appropriately sized O ring stock as the lip to create the gasket shape on the bottom of the mold box. No power tools needed.

The possibilities are almost endless....
 
Great discussion guys.

Ian - I think the lip is important, so for now I am not going to pursue anything like cut neoprene. I am interested in your comment that the silicone may fail where the edge of the base meets the lip. I know I place my hand on the emblem when I close my trunk, so there will be stresses on the gasket. Are you suggesting a higher durometer material would be preferable for this reason?

Another question you raise on the molding process relates to the bottom of the gasket. I was thinking a low viscosity material could be poured into an open mold on a level surface to create a flat bottom, but one of the other reasons I was considering using a pressure tank was the thought that consistent air pressure across the open gasket surface would produce a flat surface. Thoughts?

Markos - I will be in a car traveling Monday, but would like to join your call if possible.
 
I agree about the lip, it's a small but important feature. Neoprene is too soft anyway. I'd think something about 60A to 70A durometer hardness would be about right. Smooth-on has some in that range, but not in grey. I suppose they could be colored, but that's another variable, and cost. It might be worth trying a builder store to see what gasket made from a tube of silicone caulk ends up looking and feeling like.

As for your excellent thought of having an open back mold, the viscosity is proportional to cured hardness so the tougher stuff will not want to lay down for you. At least not as much as you had hoped. Increasing the temperature should increase the viscosity and you could put the filled mold into a bucket and give it a few whirls around your head to let the centrifugal force have a go at increasing the flow out. Regardless, it looks like the cure time is basically overnight up to 24 hours so it would be a slow production process.

No, a pressure tank will not increase the flow out. Solids and liquids are not compressible and the only change would be if you had any air bubbles, they would be compressed. Of course, you have to hold the pressure until the silicone or other material is set. Again, at least overnight in most cases. There are accelerators, but that's more cost and it reduces your working time greatly.

I'll have to take off my roundel and check out the remains of the gasket on my car to see if I can get a better idea of what is OEM. Since I have the harder (high temp) Smooth-On silicone, I may just make a quick one to see how it behaves. It's been a while since I bought my silicone kit and I didn't realize they state it has a fairly short shelf life. That could be an expensive oversight.
Best to test it out and see if it's still good.

The OEM gasket was clearly injection molded as indicated by the seams in the part. That would be the best way to reproduce them, but the cost of the mold is the big drawback. However that gives a much larger range of material options. I have a couple of other things I want to injection mold, so if I can find the time, I'm planning on building a small plastic injection machine. But that may wait till next year, realistically. If you have a mold, it is possible to buy time on big machines, but I don't know what that ends up costing on a per part basis without further investigation.





plastics hardness Scale.jpg
 

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I don't think production time is a concern. If you want to produce more, make more molds.

DB3D06EA-89ED-40BC-9BD7-6D548DC22859.jpeg
 
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Chris has been very patiently waiting on me. I had an oval gasket modeled. It is a good 10mm taller than it is wide. It is a tad thinker than the original and the lip is a bit chunkier than the original. I’m sending a nylon version to test fit, and it should arrive at the end of next week.

Chris will take it from there and let me know if the model needs updating. This includes but may not be limited to making the lip more triangular like the original. Once complete he can use a print to make the mold.

Thanks you Chris for your patience.

FF343A65-BD8D-4DED-945B-7A757054FE43.jpeg


I think that the 2mm lip will be too big, but I am more concerned with test fitting the seal at the moment. I will probably need to cut it down to 1mm and make the lip triangular. Chris was already thinking the same thing.
two-millimeters.jpg
 
Chris,

If the price is reasonable I would like to get 4 of them.

Regards, Jon
 
Markos said:
FYI - I am not making gaskets. I am just helping Chris with the “master” gasket with the proper elongation, since none seem to exist
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The elongation was something I decided should be done. The original injection molded gasket was round, and was then stretched to fit the base. All of the examples of the original I have been able to identify have lost elasticity and have returned to their original round state. I don't want the new parts to be trying over time to return to a shape that differs from the installed shape.

While the prototype Markos and I are developing is progress, I still have not identified the process for production. I am looking at whether 3d printing may work, although I have concerns about finding a flexible enough material that will provide a nice finished part, allow the proper color and stand up to UV exposure. Some type of molding would allow for a wider range of materials, but then we have to go from a prototype to a mold. And injection molding is probably too expensive for such a small volume of parts...
 
I could ask my Australian manufacturer
I will only take a couple of years for turn around and he might lose the first sample you send to him...:eek:
 
Stan said:
I could ask my Australian manufacturer
I will only take a couple of years for turn around and he might lose the first sample you send to him...:eek:
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Yeah Stan -- I was looking at the list of tips n tricks we put together for newbies seeking to work on their coupes and thought maybe we need a list of tips n tricks for rookies trying to get into making parts!
 
I'm a bit behind, but I finally submitted the print. It should arrive on the 29th. I kicked off one in the white nylon that I use for my parts. I also printed one in ASA, which is a more UV stable form of ABS. They have an option in dark grey as well. I'm keeping my hopes up for this one. The plastic is only 1mm thick on the mating surface and 2mm at the lip. If this material checks out it may be usable as an end-product. I would apply a light coating of grey rtv above and blow the gasket for sealing.

Here is the spec sheet on ASA.

ASA matches or exceeds the properties of standard ABS and has higher UV stability. Ten colors are available. A strong "go-to" prototyping material. Light infill parts are filled with a double cross-hatch patten. A great way to lighten thicker parts.

http://s3.amazonaws.com/xometry.craft/files/specsheets/datasheet-asa.pdf

https://rigid.ink/blogs/news/175845063-the-difference-between-abs-and-asa
You’ll have noticed quite a bit of mention in this article about ASA filament’s excellent ability to retain it’s shape and colour even in rough weather. This, more than anything, would be the deciding factor if you wanted to make the change from ABS to ASA.

If you are printing indoor items, then ABS is as good as ASA in terms of life and colour/shape-retention. However, if you want that magnificent statue of David to last for years in your front yard, in all his naked glory, ASA is the way to go. It will last longer than ABS in the sun and doesn’t yellow, at least not noticeably, under UV radiation.

Now, the decision is yours to make; ABS or ASA. And it shouldn’t be a hard one, as we’ve already discussed that the biggest influencing factor should be what you are printing and where you want to put; inside or outside the house.

If you are thinking of buying ASA filament, it is worth bearing in mind that it is slightly more expensive than ABS, but that makes sense due to it’s sturdier outdoor design and purpose.

We advise you to buy from somewhere that offers excellent support along with excellent high quality filament. Ahem… If you were looking for either of these materials, you can view our selection of ABS here and ASA here.
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I would appreciate input from the forum as to the need for a sealant underneath the gasket. The setup BMW now sells does not provide a water tight seal against the trunk, and I am not sure that one is required. As Markos notes, however, it does appear that the original gasket did provide some level of a seal.
 
I don’t think sealant is needed, I don’t have a gasket now and water has not been a problem. The biggest issue is making sure the clips hold it tightly to the trunk. I never close the trunk using the Roundel.
 
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