Plastics, what technology? - 20-07-05 012.jpg (0/1)

Hi, Mark:-

Although they probably could be (with tooling an order of magnitude more expensive), those parts are not injection molded, IMHO. And I don't think that they are nylon, maybe some flavor of polyethylene. Probably rotomolded. This is a low-pressure, low-volume process that is probably suitable for domestic manufacturing in Spain. Tooling cost, precision, and production rate are all relatively low, parts can be quite large (like a tank or a whole kayak).

Best regards, Spehro Pefhany

Sorry to be a pain but after my previous post, some doubt has been cast over whever I am talking about Plastic Injection Moulding or Not. I am looking to have a similar product to the one shown in the attached photo, made either here in Spain or in "China" or maybe a combination of the two (tooling there, moulding here). I have attached some pictures, (one by one so not to overload anybody) please could you have a look at the item I am trying to emulate, and look at the plastics and even more closely at the way the two parts are "welded" together. There is a couple of the plastic parts (for example where the mains switch is, which looks like it has been inserted into the outer case. The plastic is not brittle but feels like some sort of Nylon. I am totally new to plastics and would really appreciate your positive input. If there is a better place to post this subject, then please tell me and I will be gone, thanking you in anticipation.

Mark Scotford snipped-for-privacy@markXscotford.com (remove the X to reply)

Thanks Spehro

Appologies to everyone, I have quickly put the pictures onto my web site instead, they can be found at

Mark Scotford

Google up on "Rotational moulding". Basically they pour a quantity of plastic powder into a hollow metal mould and then rotate it whilst heating. Process is ideal for thin walled hollow structures. The mould tools are quite cheap, as no pressures or high temperatures are involved. Product quantities in say the hundred or so can be economical. Don't know where you're from but in UK there are quite a number of manufacturers. regards john

Rotomolding = rotational molding. John Jardine described the process.

For hollow product of smaller size and with a more tooling $$ you can also consider blow molding, where a molten plastic slug called a 'parison' (of varying cross section, in general) is extruded out, and then inflated to fill the mold with a blast of compressed air. This method is used to make millions of plastic bleach containers etc., as it is relatively fast.

Best regards, Spehro Pefhany

They do indeed look like polyethylene.

What's this 'rotomoulding' ? I'm curious. Might be useful to me too.

Graham

From what I can seee, it looks like plain old ordinary polyethelyne.

For something that size, you might be better off blow molding them, or even may want to consider fiberglass.

Good Luck! Rich

Thanks Lads, I took some semi-professional advise and they also said it was Rotomolding, but they advised that the quantities we will be buying may necessitate the use of more than one mould to get the throughput. We need in the first year 1000 pieces and ramping up to

Mark Scotford snipped-for-privacy@markXscotford.com (remove the X to reply)

We were looking at rotomoulding something the size of a vacuum cleaner. Some double curvatures with the odd internal strengthening rib. Material was to be a high density, cross linked Polyethylene. Tool estimated at £6000. Looking at the other tools in use I queried where the £6k estimate came from, as the tooling seemed as technologically sophisticated as glorified biscuit tins. Quantities were about a 1000-2000 a year.

I'd go the Roto route. Two tools?, no problem, the costings still balance. Roto gets you moving and buys you time to look around and see if anything else fits the bill for future higher quantities.

I'd suggest it's pointless carving the design in stone, shelling out say £25k for a fancy precision tool and then 6 months down the line finding you would like to alter the plenum chamber to a Mk2, to improve performance and reduce costs. By all means invest the money but only after you get some sales history in the bag. regards john

My customer (new to designs from scratch) wants pricing before he will commit, the product in question will need 3 parts (1 common part and