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Saito2

Injection molding and reinforcement questions

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While, I can fabricate some parts with a mill and lathe, I honestly don't know much about the science behind injection molding. Molded parts, say for instance, the lower front gearbox cover of a DF01, often have various gussets, reliefs, abutments, etc. Are all these strength related or are some more functional for proper liquid plastic flow? Taking the DF01 part as an example, is there durability to be gained by filling in some of the reliefs or voids with an epoxy of some sort? On the other hand, could doing so transfer stresses throughout the part in ways unseen by the layman an actually accelerate failure? I've pondered this for awhile but when the nice aluminum GPM replacement part for said DF01 gearbox was simply too far out of tolerance to fit to my satisfaction ($25 down the drain), it got me thinking harder on the subject and its possible application to other vehicles.

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A very interesting question.

I guess working out which way the stresses act and then working out how to mitigate them would be the first problem.

I saw a program years ago about toy plane makers putting thin layers of carbon on balsa wood with super glue and the whole thing being a lot stronger. Maybe we could do the same?

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I don't think that Tamiya (other manufacturers are available ) -  'crash test' everything before unleashing them on the world . A lot of factors creep in , mainly cost . I guess a lot of parts are made with the least amount of plastic content but still retain integrity to get the job done , for a while - these are 'toys' and replacement parts are sold regularly . In answer , I would say , why not try out 'beefing up ' a part with epoxy or similar to see if you can improve the part .  ' improvise , adapt , overcome ' . I've learnt a lot from this in my life  :)

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2 hours ago, KEV THE REV said:

I don't think that Tamiya (other manufacturers are available ) -  'crash test' everything before unleashing them on the world .

I agree. There is a Tamiya factory video floating around with a short section of them running various durability test. When I saw that, knowing all the flaws some vehicles have had, I thought to myself 'no way they do that for everything'. Did anyone run the Juggernaut 1 for more than 20 minutes before putting it into production and sending it out the door? While ABS construction has its definite pros (and was probably the mainstream back in the day) I would love it if more glass reinforced plastics made their way into the kits. But, as you said, the cost factor creeps in. 

I suppose I also have to keep in mind what design programs were available at what times. In the DF01's case CAD was certainly being used. However, I'm not sure a company at Tamiya's level (or the RC level in general) was looking at stress and fatigue simulation. Perhaps it was still more of a "feel" in the engineer's mind of what worked (and at proper cost) for a certain application. I'm not one for outright casting aside particular aspects of an engineer's design work until I know where its coming from. Engineers are far smarter than I, but sometimes there are improvements to be made simply because said engineers could be hamstrung by cost. 

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11 hours ago, Saito2 said:

While, I can fabricate some parts with a mill and lathe, I honestly don't know much about the science behind injection molding. Molded parts, say for instance, the lower front gearbox cover of a DF01, often have various gussets, reliefs, abutments, etc. Are all these strength related or are some more functional for proper liquid plastic flow? Taking the DF01 part as an example, is there durability to be gained by filling in some of the reliefs or voids with an epoxy of some sort? On the other hand, could doing so transfer stresses throughout the part in ways unseen by the layman an actually accelerate failure? I've pondered this for awhile but when the nice aluminum GPM replacement part for said DF01 gearbox was simply too far out of tolerance to fit to my satisfaction ($25 down the drain), it got me thinking harder on the subject and its possible application to other vehicles.

Here's a pretty good short article on injection molding basics:  https://www.3dsystems.com/quickparts/learning-center/injection-molding-basics

 

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I do a fair bit of injection molded part design and the answer is a little complicated. In general if you are breaking a part in a certain area adding more material will help. If you are adding a material that is different from the original the stresses will be distributed between the 2 based on the stiffness of the 2. The higher stiffness material carries more stress. So wrapping a hinge point which breaks with carbon fiber will make it much less susceptible to cracking as the carbon is much stiffer than the ABS (or nylon) and so it carried the load more. This is why adding a metal or carbon brace to the front of a central hinge pin block essentially stops the block from breaking as it is now carrying the load.

If you just randomly fill areas with something like epoxy (which is pretty stiff relative to abs and nylon) then the epoxy will carry most of the load through those areas, but if those areas weren't a weak part more load will be transferred to the other parts of the part which may fail more easily as the overall loads are higher.

Basically more flexible parts have lower peak loads as there's more distance to dissipate the impact forces. Which is why sometimes aluminium arms and hubs may break the hinge points in the chassis more easily as they are very stiff and the peak forces they transfer are higher.

In regards to molding.

When a part is designed its not good to have really thick sections or thick and thin sections transitioning. So a designer will use ribs to try and stiffen the part. Depending on how the rib is used it could actually weaken the part even though its stiffer. If the rib is on the compression side the part will be generally stronger whereas if the rib is on the tension side it may weaken the part. 

If you can upload some pics of the part I can give you a bit more info based on that particular part. As it is this post is a super long one for a first post

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