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Comment by thegrim33 | Hacker News Reader

3D printing options aside, there's no possibility of me replacing random plastic components that break. I'm dependent on some industrial manufacturer producing the random plastic broken part for me, and getting it to me. If something metallic fails, it's much simpler in comparison to fashion a replacement / repair the failure myself. I can work with metal. I can't work with plastic.

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> I can work with metal. I can't work with plastic.

You're right that metal (and wood) are much more amenable to work with in a home workshop.

However:

> 3D printing options aside

I wouldn't put 3d printing aside. The main limitation is the size of what you can print, but if the part is small enough (depending what printers you have access to), it is a game changer. We don't have a 3d printer but my child has access to them at school and watching him fix all kinds of gadgets by 3d printing replacement parts has been very cool.

Even for larger items I’ve often found printing sub-assemblies and gluing together to be a useful technique.

E.g. I needed to replace a shaped plastic cover for a handle mechanism on a motorhome/RV. Breaking the design into two parts and making it so they clip together (rather than permanently gluing in that particular case) meant I could print a complex design without supports.

If you have the tools to work with metal, you can use those tools with few, if any, modifications and do the same thing with nylon.

Have you used polycaprolactone/PCL aka "InstaMorph" for hobby/projects? It's a very tough plastic that can be melted by putting it in hot water, then formed by hand. I think something like a linkage made out of this material could be a fantastic intentional failure point for certain mechanical systems, as long as the temperature requirement is not much higher than human conditions. Also, if you have a hot air blower, you can repair it in-situ.

I'm honestly not sure why we don't see more of this plastic used for consumer stuff. Something that you can melt down and fix stuff or make little ornaments sounds like a great marketing gimmick. It's also generally a pretty bio-safe plastic.

In industry, it's because it's so low-temperature. The benefits of using it aren't outweighed by the potential failure risks in it in pieces not designed to be repaired.

Also, just kinda--it's not well known! You can't even find it as a 3D printer filament without a lot of effort, even though those "3D pens" often use it, because the output is so unimpressive to most people. That's not that it is unimpressive, it's because they don't know much about it, much like how people act like there's a "leveling up" by switching from PLA to PETG to ABS.

While from a materials standpoint, its more of a "what fits the needs", from a printing point of view it's definitely a level up going from PLA to PETG / ABS. Both in terms of skill (PETG can be annoying to dial in) and machine reqs (Hot End, ventilation, etc)

ABS is a level up because it curls like mad off the print bed because of internal stresses, and this will cause prints to fail if you do not have a heated chamber, which is a bit of a challenge to set up over Ender 3 like printers. PETG meanwhile likes to be dehumidified under heat first to avoid excessive stringing, which requires a separate doodad, and it likes all metal hotends that do not include the usual internal PTFE tube, which off-gasses nasty stuff if heated above roughly 250°C. PLA has none of these problems. "Level up" is about printability, not material characteristics of the end product.

> "Level up" is about printability, not material characteristics of the end product.

You think that. The people in forums who go "I never print in PLA" despite it having advantageous material properties for some use cases (it's very stiff, for example! sure, it snaps hard, but it's strong until then) do not.

Printability and usefulness aren't on the same axis, but when it comes to FDM materials, a lot of people do.

If you need heat resistance and can give up reforming and strength sugru is a moldable silicone that I’ve used for a quite a number of repairs. A knife handle I repaired in 2014 is still going strong.

(InstaMorph is new to me - but will certainly get used in the future)

Can it be injection molded? If not, I would assume it would be much much more expensive to mass-produce than an ordinary injection molded part.

> I can't work with plastic.

Why not? If you can machine it from metal, it is easier to machine it from plastic. I fabricate plastic replacements often with a drill, files, and saw.

Yeah, I was gonna say. Not that I'd necessarily want to cut a gear with hand tools even in nylon, considering how exacting the profile would be to get right, but it's doable.

I'd rather hand cut a gear in nylon than one in steel. If you arent doing it by hand, who cares what it is made of.

It's an... unusual skill to have, I'd say. Maybe it's the issue of education or culture, but I'm with GP here: in my mind, metal parts can often be repaired by hand, or an improvised replacement can be made; plastics break too easily, and you can't make new ones without a 3D printer or something.

Yup. Same with wood and fabric. Those kind of parts or components I can replace and work with. Plastic? That's a whole different ball game due to the potentially low tolerances in terms of dimensions and the nature of the type of plastic used. With wood, metal, and fabric it is much easier to gauge the correct replacement material.

I'd say education and culture might be right on the money. I'm not sure it's occurred to me to take a small block of some type of plastic and cut it to shape using knives, planes, chisels and files like someone could with wood, but now that I'm thinking about it, it seems like it might be considerable easier to work with than wood in some cases, especially with how easy it is to join two parts afterwards with some epoxy or maybe even through heat.

Additionally, it looks like you can possibly re-melt the shavings into another block (I'm not sure if specific plastic types are required).[1] That's like woodworking but being able to easily gather and compress your bits and ends and sawdust into more wood.

1: https://www.youtube.com/watch?v=34wPmcgDRmg

It's because plastic injection molding has different constraints and trade-offs in parts design compared to metal machining. E.g. injection molding, after the mold is done, doesn't really care about machine time, complexity, or the availability of specific cutters and drills. So sometimes the geometry and tolerances of an injection molded part is a pain in the arse to replicate manually -- it's just not made to do it, unlike metal machining, which at scale is still a rough approximation of the manual process.

I get the manufacturing Tradeoffs. What I was responding to was not an issue with part complexity, but part material.

They said "I can work with metal. I can't work with plastic."

Sure, nobody is going to machine a plastic replacement complex injection molded housing. You probably werent going to re-create a complex press-formed metal part either.

IT seems like it is more of a design complaint than a material issue.

How often do you encounter mass-produced consumer goods that include parts made of machined engineering plastics?

I have a pocket knife in my pocket (perhaps not quite as mass produced as what you were imagining) whose handles are made of G10 which is a composite material made partly of epoxy. It has been 3d machined into its current shape.

That's not what Im saying. Machined plastics are rarely encountered, but many molded plastic products can be trivially machined.

Im saying that the fabrication difficult is driven by design, not material.

almost nobody is going to home fabricate a spline gear at home, and it doesnt matter if it is metal or plastic. If something is like a plate or flange, it is trivial to fabricate and doesnt matter if it is metal or plastic.

For any given design, I think refabrication is the same or easier for plastic.

Not sure why we're arguing, but I think we are on the same side. We would prefer that products have parts which are easily replaced by, in order of preference:

1) easily-sourced commodity products like standard screws, washers, bolts, etc. 2) barring that, parts that could easily be fabricated by realistic home production methods (hand tools, FDM printing, possibly simple machining) 3) barring that, parts that the consumer can have easily fabricated by a third party (maybe it requires a 5-axis CNC but all the CAD/CAM files are available to upload somewhere like Shapeways) 4) barring that, easily-ordered at-cost OEM parts

...and in all cases the user manual should require all relevant drawings with dimensions.

The problem is that if you tell an industrial designer to keep costs down, and that they can use injection-molded plastic parts, they will almost certainly NOT design parts that are conducive to 1-3. They could, but all the incentives run the other way, so they probably won't.

I was mostly just curious about the person who said they absolutely cant work with plastic, but can work with metal.

I like some things that are repairable, but don't think everything needs to be. My product choices rarely are willing to compromise cost, function, or aesthetics for repairability.

That's the idea (I assume, not the same guy): you're right that is mostly the form of the part that makes it easy/hard to work, not the material, but nobody designs machinable plastic parts. they design injection-moldable ones. So in practice "plastic part" is synonymous with "impossible to recreate in small quantities". Yes, in theory plastic parts can be of the same shape (and therefore ease of boutique manufacturing) as metal ones, but they aren't, not in the real world.

Plastic welding is a thing, and has been used ever since the discovery of thermoplastics. Solvent glues also work well for some types of plastic.

The only reason why it's not more common is usually due to cost of repair vs replacement.

Thanks to YouTube, you can now find plenty of information on this.

I can work with metal. I can't work with plastic.

How about pot metal, which is what has been replaced by plastic in many applications?

Are you a machinist? I don't understand why you'd dismiss 3d printing and say metal is more feasible to work with. 3d printing is much more accessible to the average DIY-minded person.

You can bend small metal pieces with your bare hands, or with pliers, or you can beat it into shape with hammer (or any stiff object). Plastics get damaged and break when you try that.

I think this is cultural or location based – I know at least a dozen people who can weld and no one who has a 3-D printer, but I live /way/ out and rural America. The Internet would suggest that urban America is the reverse.

I 3D print jigs to hold things together for welding. Mind, blown.

Sure, but if we're talking about replacing machine parts, I'm not sure how far welding will get you. You need to be able to machine custom metal parts, for parity with basic 3d printing capability.

Then just replace the plastic parts with metal, problem solved.

> 3D printing options aside

Aside from the solution, there's no solution?

If you don't want to get into 3d printing then fine, but don't act like that's not on you. you can work with plastic, you're choosing not to.

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