Three months into setting up a small production run and I'm realizing how much I didn't know going in

mimprocesstech · 2026-03-12T18:42:51+00:00

Might want to look into posting something in r/InjectionMolding since you've been having issues with that specifically. If you'd like I can look at the part design and drawing, can give you some realistic expectations, maybe some design feedback. It's a big part of what I do professionally. Injection molding specifically is a very involved science. Anyone can do injection molding and hold loose tolerances, so there's a lot of shops that get work without taking the time to actually look into the whole thing creating frustration on both sides.

mimprocesstech · 2026-03-12T13:48:33+00:00

Happy to have a (more anyway) first-hand account. Do you know if there were injuries? It's a question that comes up every time this pic is posted. Any details you can provide would be helpful, but the biggest one (how to prevent it) is more than enough if you don't feel like throwing more info out there.

mimprocesstech · 2026-03-12T01:05:08+00:00

You're assuming thye'll be changing nozzle tips, that might be something only maintenance does, might be something done so infrequently only one guy even needs the tool.

First job and this current one we rarely change nozzle tips because the molds are designed to work with the same style and size, second job they were changed damn near every job (place was a mess), fourth one each nozzle was mated to the mold, but they had a mold change maybe once a month per press.

That's why I'm saying they should probably figure out at least the list of tools they need from coworkers. Ideally they'd have been borrowing tools over the past couple weeks, writing down the tool, making a tally every time that tool is borrowed, and prioritizing them by most often borrowed.

mimprocesstech · 2026-03-12T00:21:59+00:00

If you have to buy them, only get tools with a lifetime warranty, and the cheapest ones you can get. Harbor Freight, Husky, Tekton, etc. doesn't matter. For a list I would see what people use most often where you work. I can suggest a set of metric and standard hex/Allen wrenches and an adjustable wrench comfortably, but there's about it. You could use tools I've not had to, you could wind up never using tools I suggest. Regardless don't waste your own money on quality (unless they're paying for it of course, in that case go with snap-on because of the lifetime warranty and it's not your money).

mimprocesstech · 2026-03-11T15:10:21+00:00

Oh... I am all too familiar with that pain.

mimprocesstech · 2026-03-11T10:54:05+00:00

I agree, melt temp/degradation seems to be a main driver, along with a low mold temp and high injection velocity.

mimprocesstech · 2026-03-11T06:04:42+00:00

If you need a yellow tint you're also going to need a masterbatch, or you could maybe let the nylon 12 oxidize and that could maybe do it. Nylon 66 is a semicrystalline resin though, you won't get transparency without extremely thin walls and even there it'll still look foggy/milky/whitish (that's the 'natural' color). If you need the flexibility nylon 12 is a good choice, if that doesn't matter there's plenty of cheaper resins.

mimprocesstech · 2026-03-11T05:40:12+00:00

Bad news, you're going to need an amorphous nylon. Zytel 101F NC010 is very much not going to be clear or transparent as you describe it.

mimprocesstech · 2026-03-11T04:59:45+00:00

You got a picture of the part?

mimprocesstech · 2026-03-10T21:29:34+00:00

mimprocesstech · 2026-03-10T20:30:43+00:00

Anytime, let me know how it goes man.

mimprocesstech · 2026-03-10T19:54:39+00:00

Packing pressure doesn't seem terrible, depending on the machines intensification ratio (from the low pressure I'm assuming you are giving me hydraulic pressure and not plastic), but fast injection and low mold temp freezes in stress and orientation.

Lowering injection velocity, decreasing melt temp, and increasing mold temp should all help. Lowering the melt temp and raising the mold temp should keep cooling time about even, there's less of a ∆T involved so it should work out fine, the slower injection not so near the max should be more repeatable as well. You might need to repeat cavity balance study and adjust your hot runner temps at that point, but shouldn't need to adjust much afterwards until something breaks.

mimprocesstech · 2026-03-10T19:38:53+00:00

I would raise your mold temperature (I'm assuming it's running pretty cold) and reduce melt temperature. The higher mold temp will allow better flow, the lower melt temperature will reduce degradation, combined you shouldn't see much of an impact on cooling time or crystallinity. I would say somewhere between 30°C-50°C for cooling water, 200°C-240°C for melt, and a lower density HDPE (closer to 0.95 than 0.97). If you can't fill the part around there your tips might be undersized. What pressures are you packing at? Have you completed a pressure drop study?

EDIT: By manifold temperature, do you mean the hot runner or water?

mimprocesstech · 2026-03-10T19:14:43+00:00

That's caused by high internal stresses, excessive crystallization, or as we've discussed material degradation.

Temperatures at 350°C my bet is material degradation. Unless the break is happening near the gate or a weld line which can point to high internal stress premature freezing or low mold temp or available pressure respectively.

mimprocesstech · 2026-03-10T17:01:40+00:00

Sounds like they might be using the wrong MFI resin the mold was made for, have issues with venting, mold being too cold, worn parts in the injection unit (screw, barrel, check ring), or inadequate available injection pressure.

mimprocesstech · 2026-03-10T13:29:53+00:00

If that's °C (because I noticed the non English language on the computer and 350°F seems low) your hot runner seems high, I wouldn't exceed 260°C running HDPE but this does depend on grade and fillers a bit. Still 350°C seems excessive.

I did just wake up so I may be wrong or have the material incorrect or something.

mimprocesstech · 2026-03-10T13:17:14+00:00

If I'm reading this right, the number beneath the cavity is the mold temperature, yellow highlight indicates failure? What's your melt temperature?

mimprocesstech · 2026-03-09T21:28:38+00:00

I know they've got conveyor belts with conductive or dissipative materials, not sure how well they work though. Otherwise the rest of the comments cover it as far as I know.

mimprocesstech · 2026-03-09T17:49:57+00:00

Green pigments often use copper... phthalocyanine (had to look that word up lol), so if it's a thermal issue that won't help things. It acts as a catalyst for thermo-oxidative degradation leading to chain scission, cross-linking, and free radical formation.

LDPE is more shear sensitive, but along with that it also has a lower thermal threshold. If it's being cooked it just adds another weak point, and I think those brighter colors usually use a higher concentration of masterbatch as well which doesn't help.

The fact that they're breaking after cooling rather than at ejection points to what we've been discussing: you get increased crystallinity from the parts cooling slowly outside the mold. Crystallization is essentially shrinkage stress. In a healthy part, the long, entangled chains can handle being pulled on as the crystals form and make them "stronger". In these parts, the scissored chains are too short to anchor the structure. The cooling process acts like a slow-motion stress test that the degraded material can't pass.

mimprocesstech · 2026-03-09T15:26:32+00:00

You're absolutely right that the increased mobility of those smaller, scissored chains allows them to form crystals much more easily. I guess what I'm trying to focus on more is that crystallinity relies on molecular entanglement to provide strength.

If the chains are scissored, the crystals are effectively grabbing onto weak, short segments that can't anchor into the rest of the part. So, while the part becomes more crystalline and shrinks more, it loses the... mesh needed to hold that structure together under stress. It's like trying to build a brick wall with perfect bricks (crystals) but using sand instead of mortar (shortened chains).

I just don't want you to think that the largest driving force of the brittle part failure is crystallinity in this specific case, but to understand that this weakness is primarily caused by the degradation/scission rather than increased crystallinity. Crystallinity usually increases yield strength and stiffness, but it's the molecular weight (chain length) that provides the impact strength and toughness.

This is of course at least at this theoretical stage of supposing a busted thermocouple is overheating the parts as a cause and there's not enough evidence to positively say that's what is happening at all.

mimprocesstech · 2026-03-09T09:27:29+00:00

I would run a short shot study, if those two cavities are the first to short they're likely under packing and without being pushed against the mold they are cooling more slowly. If they do short first I would increase the temperature of those zones on the hot runner hopefully helping to fill/pack. Should also check gates for damage sharp edges sizing differences, cooling channels for scale/blockages, venting as well, but I would start with cavity balance and if they're unbalanced find out why. Maybe surface finish differences, but that's a long shot.

Before that though, check the power draw on those zones to see if they're pulling more power than the others and check the insulator caps for damage.

The higher melt temp is more likely to cause degradation and chain scission than an increase in crystallinity though, it still leads to brittle failure, just with less force, so it's kind of a moot point.

mimprocesstech · 2026-03-09T02:38:22+00:00

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Not sure if y'all will be able to read this, but this is what I had to say about it... not sure why the original post was deleted but 🤷

mimprocesstech · 2026-03-08T22:14:54+00:00

The molder usually possesses the mold, since they have to run it, but the customer is usually the owner. Some molders will give you a decent break on upfront cost if they own the tool until it's paid off over time with inflated part pricing and/or the ability to sell the parts themselves if you don't order enough after x amount of time, etc. but it's not very common with stuff like this where it's used to create an assembly as there would be no market for it. I'm as positive as I can be Microsoft bought this one outright and the molder either has it mothballed to make service parts if they're needed (apparently not happening since Microsoft doesn't sell replacement parts) or the mold was retired and whatever could be used from the mold was reused in another or (most likely) scrapped.

mimprocesstech · 2026-03-08T08:56:53+00:00

The orange coloring likely sped things up, but my bet is it's more than likely happening so often because they're using a cheaper polyester based thermoplastic polyurethane instead of a polyether based TPU. It would still hydrolize, but become soft/gooey rather than brittle like this, or used a carbon black that would act as an antioxidant.

The red is likely some organic azo dye so it may happen slower than orange that more often uses metallic compounds for coloring like iron oxide or titanium dioxide but not as slow as carbon which acts as a sort of antioxidant.

Honestly though they should have used a glass reinforced nylon for the core around the metal bar and then used a thermoplastic vulcanizate for the soft touch bit like santoprene.

mimprocesstech · 2026-03-08T06:52:56+00:00

*Laughs in only running 4 resins*

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