I noticed many kitchen utensils or stand mixer attachments are made with injected aluminum. Aluminum (~\$20/kg) is usually more expensive than stainless steel (~\$10/kg) and is not compatible with dishwashers.
For instance, I have a peeler made with anodized aluminum and a KitchenAid attachment with some aluminum parts that I cannot put in my dishwasher. I don't understand why engineers are still making such utensils in this material.
Is there any good reason for this?
Yes, steel (even some stainless) may be cheaper than aluminum, but the material cost of an item is seldom the majority of the total cost, especially a small item such as a potato peeler.
Making aluminum parts with complex, curved shapes can be fairly easily done by casting. Aluminum pours at around 1500 °F, which can be achieved with low-cost furnaces and contained by low-cost crucibles. The molds can be easily made, and you'll find lots of instructions for doing so, e.g. using lost-plastic casting from 3D-printed templates done in the home. Aluminum can also be easily machined.
Stainless steel, however, has a much higher pour temperature (above 2500 °F), which makes melting it, keeping it uncontaminated, and casting it much more difficult. This is why you'll seldom see a consumer item made from cast stainless; if it involves stainless it will generally be stamped or machined, either of which seriously boosts the price of the item. In a market where price is king, and speed to market is queen, the fact that the results aren't strictly dishwasher safe is a minor consideration.
I looked up 6000 grade aluminum to be about \$2/kg, while 304 stainless steel is about \$3.40/kg.
This means stainless steel is about 70% more expensive than aluminum, which is consistent with my experience in doing mechanical design.
I'm not sure where your numbers came from that aluminum is 4x more expensive than stainless steel.
I also have a KitchenAid stand mixer, and I put the whisk attachment in the dishwasher and it came out oxidized (black-ish). I was pretty pissed too to find out that I spent $300 for a thing that needs to be hand washed.
Re-tooling and re-equipping a factory to use a different metal can be very expensive. If the required rate of return from re-tooling/re-equipping cannot be achieved be it simply isn't done.
The production rate from some utensil manufacturers may be low and the profit margins small resulting in no major new investing in changing to other metals and re-equipping.
Engineering is more than just science, design, materials and efficiencies, economics and profit also plays a significant role.
I'm guessing that shipping cost could be a reason. For a pallet (or several) of utensils, shaving a few percent weight off of each one could amount to a significant savings in shipping cost, especially if the trade route is between China and Europe or North/South America.
Having spent several years in product development at Kitchen-Aid's parent company, I can guarantee that if there were a less expensive way to provide the same function to the consumer, that is how it would be done. They have whole teams of engineers tasked only with cost reduction.
The reason that items are marked as "hand wash only" often have more to do with the detrimental effect of the dishwasher environment on components of the attachment other than the base metal.
Beside very cheap products, the only aluminum parts I have found in my kitchen tools are either injected or molded. Why this?
Because the fusion point of aluminum is much lower than steel, plain steel molds can be easily used. The opposite is not true and for steel casting, it requires disposable sand molds. Thus, the process of steel casting is more complex and way more expensive.
I realized that it is not easy to find stainless steel injection molded parts apart from very big stuff such as car rims or machine frames. Almost all steel parts are either machined, laminated, forged or heat pressed, but never injection molded.
So engineers will use stainless steel or plastic for any part that can be simply laminated, forged, heat pressed or injected. However, for more complex parts the only remaining choices are: injection molded or CNC machined. The latter is too expensive for high volume production for both steel and aluminum.
Steel injection modeling is neither a solution because of the process complexity.
KitchenAid is probably focused on cost reduction and will choose the most affordable. Using aluminum is the best compromise in terms of cost because aluminum casting is not much expensive and offers good mechanical performances.
Making the same part in stainless steel would make the part five to ten times more expensive. It is worth it, just because you can safely put it in the dishwasher? Certainly not...
The aluminium has different temper, for the utensil usually use 1 series aluminum alloy which is much softer, so it's easier to cast. And aluminum is light, also aluminum can be recycled which is environmentally friendly. Most of all, for business, profits are very important.
