Hello everyone,

I am still baffled by most manufacturing techniques, so excuse my ignorance, but here goes:

I was wondering… can a rotationally molded part look almost seamless if it is properly deflashed or is deflashing not a guarantee that you won’t see where the two molds joined to make that part.

In other words, how is it that this Magis Puppy is so seamless yet it was rotationally molded? It’s an amazing finish. or were the seams photoshopped out? :slight_smile:

Interestingly, you can see the parting lines on the non-lacquered PE version (the green one).

Thanks guys

I would say that your red image is a rendering based off of a computer model were in draft and part lines were not taken into consideration.

Chevis W

Oh i see. I didn’t take that into consideration; that it could be a 3D model used in their promotions.

Makes sense. :slight_smile:

Just curious though; can a plastic rotationally molded (or any molded parts for that matter) part be deflashed well enough to conceal its parting line, to make the part appear seamless? I just read online that some rotationally molded parts can come out completely seamless. Not sure if this is always the case.


Hiding parting lines is certainly possible, but it’s tough and just requires a lot of control of the tool.

For rotational molding, and when working with large tools that becomes quite a bit harder since you have a lot more metal, and a lot more risk that something won’t be perfect and flash will occur.

The top image is definitely a rendering, and the bottom one you can see the parting line. Seeing the parting line that results due to your draft angle is more likely than worrying about flash, but painting the part after the fact can help a bit.

Ultimately anal control for something like that is going to depend on your tooling vendor. If you show up to most tooling vendors and tell them you want a gigantic rotomolded part with no seams they may tell you itll be 5X the normal tool cost, or laugh you out of the building. This is why when you read about Apple suppliers so many of them refuse to work with them after a project due to the insane requirements that most customers don’t demand.

“Ultimately anal…”

This is also a great example of the 3d geometry not using curvature continuous, or better, blends. You can see where the reflections break in the rendering, and also on the green molded parts, despite the heavy mold texture.

The Magis chairs are at the public library where my kids borrow books. The part lines are very clean and look as if they have been considered a feature to display:

The part lines are very clean and look as if they have been considered a feature to display:

I kind of doubt that, but certainly a “detail” that warrants consideration … the flash might constitute a child-safety concern if left too sharp so “softening” the P/L would be a necessary step.

You might want to keep in mind that roto-molding is a relatively crude porcess; most tools are cast, or fabricated, aluminum (bolted/unbolted (for filling/part removal)) each cycle), and the process itself is border line Neanderthalic.

Not exactly high tech, but perfectly suited for the intended use.

(couldn’t believe this image popped up)

In this video they are molding vinyl (the resin is in a liquid state); pelletized material is the norm. The process itself is generally the same, regardless of size.

here’s pic of one (I have one of the larger size dogs).

You can clearly see the parting line if you look for it, but at 5’ away or so it’s not noticeable. t’s not sharp to the touch.


For the original poster - even if you could tune in the tool and begin shots that had almost perfect alignment, you’d soon find subsequent shots with part lines beginning to reappear.

Pressure, material flow, tool wear - it all plays a part and all creates slight differences in part appearance.

That’s only the factory side of it though. From the design side one can adjust draft, curvature continuity and texture specifications to minimize part lines.

Thanks for all the input, guys. Lots to learn; as I am still trying to learn the basics of the most used processes. Even with all the manufacturing/materials books I have such as “Making it”, there are things still not mentioned in detail, but I assume as I progress in my future career in ID, this will all come with experience. I just hope that for entry level, interviewers don’t expect me to have engineering-level knowledge. I am trying my best to inform myself though for that first interview; to appear knowledgeable.

Have a splendid Sunday.

Does every designer have one of these rotomolded doggies? Lamps, chairs and i-___ are out, doggies in? I’m so out of touch…

im really curious how they can make this lumia 920 with no parting line at sight and… glossy

High polished surfaces are the easiest to do zero draft on for a clean shutoff. If you don’t have draft and you have a well made tool hiding the parting line is easy. They may also go through some post processing or coating that helps hide any marks. I had a 900 for a week but I never inspected it closely.

so for the lumia 800-900 body, do they inject-mold it or cnc it? because i’ve seen video about n9 and they are showing cnc cutting a 80% done body case at the back part for the cameralens frames which i think it dont need cnc cutter to make the hole.
if they mold it, all the inner structure (where they put the components) are being form with sliders?
Thanks for answers, im trying to grasp on how&why the production goes :slight_smile:

I believe the main form of the housing is injection molded, and then the small details are CNC’ed where molding them is not feasible. IE you can’t properly mold speaker holes due to the thin material flow between them, but you can mold it solid and cut the holes out easily.

They also most likely use the CNC to create the undercuts on the inside around the display.