Molding your own engine mount isolators

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
717
Location
Brisbane
trimtab, I have molds for things and found split molds made of Delrin easier when making complex shapes, I machine the Delrin surfaces to get a good mating surface, bolt them together and then carefully turn out the required shape. However a bit overkill for some things where the steel inserts are required and the shape is fairly simple.
George
 

wktaylor

Well-Known Member
Joined
Sep 5, 2003
Messages
320
Location
Midwest USA
Interesting...

Caution1...

With potential for leaking engine fluids [hot oils, coolant, etc] I would NEVER recommend silicone rubber for engine mount application. I would only recommend fluorosilicone rubber or special grades of synthetic rubber [nitrile] compositions in vacuum-degassing molding procedures. And I'd NEVER recommend use of an open flame to 'polish-up' surface defects [chips, tears, roughness] of any elastomer parts. But hey I'm just a dumb aero-materials engineer.

Have You though about die-cutting rubber doughnuts from thick high quality sheet?

For something this critical and unique and available with an established supply chain, why make them?... or are they simply NON procurable?

Caution2... But then, its your call...
 

Gary K

New Member
Joined
Sep 11, 2020
Messages
1
Nice to have less expensive options for engine mounts but need to be careful with elastomers, especially urethanes since they degrade. Some turn to liquid or crumble under higher humidity and temperature as low as 130F. Saw an example of Flexible shaft joints crumble after less than 5 years . Also had a sea container of urethane cores melt from combination of humidity and temperature. If you go this way be sure to inspect them frequently.
 

trimtab

Well-Known Member
Joined
Apr 30, 2014
Messages
189
Location
rocky mountains, rocky, usa
Silicone rubbers are at the top of my list for fuel and oil resistance, elevated temps, and adhesion to steel. Few other compounds will suffice. The only downsides are a) non Newtonian behavior, b) moderate strength.

They work very well in the mounts.

Urethanes are often tougher, but there are so many kinds with so much variation in physical characteristics it requires navigation. Urethanes are way less expensive. Silicones are way less than fluorosilicones.

The torch is used to clean the fine residues off the metal parts to allow good adhesion to the metal. It's fine. The parts are generally made of low carbon steel.
 

Flow

Well-Known Member
Joined
Apr 27, 2016
Messages
110
Location
Auckland
If Silicone had better tan delta it would be the winner for physical properties.
The esters tend to chalk up without use and hydrolyze. I think a program of covering with Silicone grease might improve longevity of the esters which do seem to have the best visco elastic properties.

Did anyone do double mixing, vacuum degassing and or pressure casting with their efforts?
 
Last edited:

trimtab

Well-Known Member
Joined
Apr 30, 2014
Messages
189
Location
rocky mountains, rocky, usa
Did anyone do vacuum degassing and or pressure casting to help with bubbles?
Of course. Nobody doesn't use degassing. It's easy and cheap. You can also make your own low pressure injection system or do vacuum injection to manage pack out and bubble formation. I use it for plastic resins, elastomers, adhesives, etc.
 

Flow

Well-Known Member
Joined
Apr 27, 2016
Messages
110
Location
Auckland
I would only recommend fluorosilicone rubber or special grades of synthetic rubber [nitrile] compositions in vacuum-degassing molding procedures.

Have you thought about die-cutting rubber doughnuts from thick high quality sheet?
Cheers, do you know of any viton or nitrile rubbers that have equivalent or better dampening coefficients as the polyester urethanes in 55 Shore A.

Please see DMA data attached.
 

Attachments

Flow

Well-Known Member
Joined
Apr 27, 2016
Messages
110
Location
Auckland
Just making sure I understand this correctly, for a given tan delta and impulse energy the mount with the lowest durometer absorbs the greatest energy from the system as its compressive modulus is lower and thus movement is over a greater range?

So tan delta / compressive modulus is equivalent to energy absorbed for a given impulse, preload compression, frequency and temperature?
 
Last edited:

Basil

Well-Known Member
HBA Supporter
Joined
Aug 19, 2017
Messages
45
Location
Countesthorpe UK
Just to followup on my previous post, here is a picture of the very simple mold for my engine mount rubbers. It will do 8 in a batch.IMG_20210516_093416.jpgno
 
Top