iMac base: extruded, cast, or bent aluminum?

I’m stumped on this one. Any expert opinions out there that know how Apple manufactures this part?

Also, which of the three processes do you think is the most inexpensive (including all the secondary ops, like the cable opening, etc.)…no guarantee that Apple went the cheap route on this…

I found a couple of other views that reveal a bit more detail about this part. The second image is really big, but it does show important imformation.

We see:

  1. The base tapers; the bottom is wider than the top; so it’s not strip material.
  2. The material thickness is very uniform; so it’s probably ‘plate’ or ‘strip’ stock’ material.
  3. The surface texture is essentially smooth, with a ‘honed’ finish; another characteristic of ‘plate’ or ‘strip’ materials (the finish being added).
  4. The bottom of the base has a recess for a non-slip material; machined.
  5. There is a through-hole in the back; machined
  6. The edges of the part are ‘sharp’; again, characteristic of flat stock.
  7. The edges of the part show no draft; not cast or molded.

Based on these features, I would say, that starting with flat stock:

  1. the round hole, and the recess (for the non-slip) are first CNC milled; the endmill having a small ‘round over’ of .020" (?) to remove the sharp edges. Then,
  2. the perimeter is milled, freeing the part.
  3. It is then formed to shape, and then,
  4. tumbled, or blasted, (or both) with soft shot (walnut shells, plastic shot, etc.) to achieve the final surface texture. And finally,
  5. Clear anodized to protect the finish.

Casting would be, by far, the most expensive way to manufacture this part.
The volume of the mold would have to encompass the entire part(s), and a slide would be required to form the hole, and recess. Cycle time to allow the part(s) to cool, enough to remove from the machine, would be measured in minutes. Excess flash would need to be removed, and the surface treatment applied. Without coring it is difficult to cast parts without creating unattractive ‘sinks’ (dimples). Castings require ‘draft’, somehwere, to get them out of the mold (unless they are sandcast). This thin, flat-sectioned, part is just not economical for casting.

The extrusion process is generally used to form linear products; channel, flat, ‘angle’ , ‘rounds’, I-sections, tube, etc. Any feature(s) perpendicular to the direction of the extrusion requires secondary operations (milling, drilling, punching); in this case essentially the entire part … the perimeter, the hole, and the recess. This part may, in fact, start with an extrusion; a flat piece of strip stock.

That pretty much leaves us with a ‘bent’ component; generally termed ‘Formed.’ While the perimeter, and the round hole, on this part could be cut out using laser, or water-jet, technology, the recess on the bottom would still require a secondary milling operation. Some material would be saved because the parts could be ‘nested’ tighter on the flat stock. I suspect, however, that the added expenses of handling incurred to move the blanks to a milling station would offset those savings.

Now, having pontificated on this process, we may yet learn that these components are a naturally occurring object that Apple mines out of the ground on Saturn, and grades for size, and texture… … .

My guess is they nest the parts like you mention above, and hit it with a CNC mill all in one shot. You can mill shapes and cut through them in one pass. The next stop would be a die set up to form the pieces in one shot. A standard roller would have to spend too much time per bend, so a one shot die would make the most sense. I haven’t seen the finish in person in a while but my guess would be also media blasted (walnut shells) or sand blasted.

Thanks for the insightful explanation, Lmo! That process definitely sounds like something not too many companies (besides Apple) can pull off.

Question about the extrusion process…most parts that I can immediately identify as extrusions have some of the tell-tale signs you describe: long, linear forms; profiles like channels or sections; absence of draft; sharp edges, etc. I was guessing that this part could possibly be extruded from a die that has the shape that we see in the side profile. No forming necessary, just machining. Of course, that would be a pretty large die, much larger than most extruded parts I’ve seen. But possible, perhaps??

Another clue maybe can be found in what’s not visible in the picture (we’ll have to use our imagination, I guess), where the base enters the monitor. If it’s an extruded part, I can easily see a cylindrical opening that would accept whatever support rod it pivots around…

I think bent or formed plate, not extrusion. Don’t large extrusions risk deforming out of the tool?

If a formed plate was continuously milled, the hole would be a freebie in the process. This would also account for the edges being devoid of draft or the telltale bulge you’d get from the inside-bent areas.

I’d look at manufacturers of architectural panels.

Now, how is Apple getting the draft-free plastic parts, like shown in the iPod dock above, or the shuffle? Are they just benefiting from the gloss and letting shrinkage help in the mold release?

I’ve never physically laid eyes on either of these products. Hell, in the shop I still listen to AM radio …

But based on Apple’s published information that the 60GB iPod is a little over 1/2" (.550") [1.4cm] thick, and the two images of the iPod & Dock that are posted, I’d guess that the Dock is no more than 5/8" (.625") [1.6cm] thick (tall). Proportionally, the Dock appears to be a bit thicker/taller.

If this is, in fact, the case, it is a shallow part and may not need any draft; the high gloss exterior surface would certainly help.

But there is still the issue of the INTERIOR; mounting bosses, ribs, etc. to consider. Remember that as the part shrinks away from the “exterior” side of the mold, it shrinks onto the “interior” side of the mold (the core). With no draft to assist demolding, the ejector pins would have to apply a greater amount of force on the part to demold it … possibly leading to the part being damaged. Wear, as a result of this, would lead to decreased mold life.

Given the shallow, high-gloss, and uncomplicated nature of this part, I would suggest that there is draft on this part, just not very much; 1/2 degree, or so.

See this draft angle chart;
1/2 degree of draft would only be .006" [.152mm] off vertical (at .625 high). As a reference, a business card is roughly .015" (.38mm] thick.

haha- Apple designers must have some serious clout. I can just imagine the confrontation with the engineer. “No stupid! I said zero draft! and…what the… is that a fillet!! I want zero draft and zero fillet! jeez- do I have to CAD it myself?”

The plastic part ends at the end of the radius, so I don’t think draft is really an issue there. The metal part has the flat wall, seems like a very low draft, and the interior pieces on the metal back that the plastic snaps to are secondary metal pieces that appear to be welded in. It’s possible the metal wall maybe flat, it’s really thin and flexes easily, no discernable ejector marks on the inside. Not sure if any of that info helps at all, just happen to have an open one here


Lmo’s got it, except I reckon it’s probably water jet or laser cut rather than milled. And no radius tool on the edges- they’d have to flip it over to get the back side, which would be too much of a hassle for something like this. It’s possibly tumbled, or more likely just a heavy pre-anodizing acid etching would knock the corners down a few thou. Only looks like .005-.010" max radius to me, and the cord hole has a pressed-in plastic grommet, so they don’t need a big radius there.

It’s clearly bent into that final shape (not cast or extruded). You can pretty clearly see how the material thins out in the lower bend. That’s characteristic of the stretching you get in bent material.

I couldn’t stand it anymore; that is, commenting on something I’ve never physically seen. So, I went over to the local Mac Superstore to look at a new G5, and waited patiently while the sales person explained what a ‘mouse’ was to an elderly gentleman.

I was surprised by two things;
1) the edge ‘radius’ over the entire perimeter of the part was much sharper (< .01" [.25mm]) than I had expected and,
2) the plastic ‘grommet’ in the hole in the back appeared to be flush with the back surface (almost like it was seated into a counter-bore), and the ‘inside’ (the side facing toward the monitor case) of the grommet snapped over the surface (like you would expect a grommet to) but it isn’t depicted on the profile-image above.

Unfortunately, the 60G iPod Dock was temporarily out-of-stock, so I didn’t get to take a look at it.

it would’ve looked better if the radius on the bend was smaller like maybe in a tighter geometric proportion with the monitor’s. i don’t think structurally would have mattered - also would’ve looked better in combination with the top bend.

i can’t see very well from the jpegs but do the rounds on the top of the monitor differ from the bottom ones?

This just in!

While at a client’s place, I stumbled across his gorgeous 23" Cinema Display. I was surprised see the aluminum base part that we all analyzed earlier, but with a new feature where it pivots. I can’t believe I never noticed it before…I guess I thought it was the same as the iMac.

Although he looked a little upset when I touched it without asking (sheesh!), he was kind enough to let me take the pix you see here. This seems like a much more complicated part to bend now…what do ya think?

So that is why those monitors are so expensive!

That feature changes everything. So we are now dealing with features in at least two axis - the grommet and the pivot.