visible seams

Is it possible to achieve “perfect” hollow plastic ball without visible seams where two halfs meet?

also, probably have to say that I don’t have prior knowledge in processes and materials when it comes to plastics.

It might be possible with an EXTREMELY accurately machined metal mold; which = $$$$

The “seam” (called a witness line) is actually that very small anount of material that works it’s way into the void created by the matching surfaces of the mold halves not quite being perfectly matched.

Even if this extremely, accurately, machined, metal, mold were achievable, as the mold wears from use the witness line will increase and become more noticable.

Blowing a sphere would be “more” perfect but would still leave a mark where it was attached to the blow pipe.

“Perfect” is a tough order.

What are you trying to achieve with this invisible witness line?

thanks for information.

It will be used on chair as seating top and back also. I’m afraid that those visible seams (witness lines) which are sharp and raised, could tear cloths off and or at least could make seating uncomfortable.

i can’t imagine sitting on a ball to be very comfortable, but i think mrfsrf is on the right track…a ball could be blow molded (like just about any plastic bottle you can think of) however the thicker wall sections needed for strength would indicate a rotational molded part…the parting line from where the tooling separates could then be sanded smooth…

…sorry lmo was the respondent

rotationally molded, for sure, especially if it is of any signficant diameter.

cycle times for this process are notoroiously slow and “shot weight” is up to the discretion of the operator – which can be a good thing, or a bad thing.

if he takes his job seriously he will carefully weight the amount of material he dumps into the mold and your parts will come out fine (if necessary he can add more, or less, material to acquire the “feel” you need) … but if he doesn’t, you end up with really thick parts (which cool and warp inconsistently), or you end up with thin sections that don’t perform as your design specified and they end up in the dumpster.
either way you’ll end up paying for them … but i digress.

maybe you could use a tool something akin to a potato peeler – that you could just drag along the “flash”.

maybe the parting plane could be oriented such that it didn’t make contact the the sitter (sittee??)

I would expect a secondary action to buff-out the flashing.

How 'bout dropping the whole thing into a giant abrasive polishing drum?