The Six Questions Are Wrong

Aluminum is roughly 40% of zinc's density. That gap is real and matters — when it matters. The selector treats weight as a constraint the moment the buyer answers "somewhat." In practice, most die Casting Parts and aluminium casting parts outside aerospace and automotive have no specified mass limit. "Lighter is nice" does not drive alloy. A written weight budget on a drawing or specification sheet does. If neither exists, this question should contribute nothing to the recommendation — but it does, because "somewhat" nudges the answer toward aluminum regardless.
The sharper question surfaces the difference in one sentence.

ZAMAK alloys — the workhorses of zinc die casting services — begin to creep under sustained load above roughly 100 °C. Creep is slow permanent deformation under stress that would not cause immediate failure. A part that merely sits near heat and carries no load at temperature is not a creep risk. A part that threads onto a fastener and holds a clamping force at 110 °C is.
"How hot does it run" gets answered with the temperature the part sees. The engineering question is whether load and temperature coincide. A zinc instrument panel bracket that lives in a warm car interior is fine. A zinc bolt-load retention plate at the same temperature is not.

Zinc reaches thinner walls than aluminum die casting — approximately 0.8 mm in hot chamber versus 1.2 mm for aluminum cold chamber under typical conditions. That is an accurate and useful statement. It is also incomplete without flow length.
A 0.8 mm wall fills reliably 40 mm from the gate. The same wall at 180 mm from the gate may cold shut regardless of which alloy is used, because the metal front loses enough superheat to freeze before the cavity is full. Thin-wall capability is a ratio — wall thickness against flow length — and a selector that captures only one dimension of that ratio can recommend zinc for a geometry that neither alloy fills reliably at that specification.

This is the most consequential substitution. The selector uses part size as a proxy for process: small parts lean toward zinc hot chamber, large parts lean toward aluminum cold chamber. The logic has a surface plausibility — zinc tooling does top out near 1 kg per shot, which limits part size — but it runs backward.
The machine follows the alloy, not the size. A small aluminum part still runs cold chamber, because molten aluminum attacks a submerged injection system. A small zinc part runs hot chamber because the economics demand it and the alloy suits it. Part size is an output of the process decision, not an input to it. Asking size to decide between hot and cold chamber is asking the symptom to diagnose the disease.

"Decorative plating pulls toward zinc, weather exposure pulls toward aluminum" is a useful heuristic. It is also the second-to-last thing that matters for most programs — the first being what the part must do structurally and functionally, which determines what the surface is permitted to be.
A sealed hydraulic valve body — a typical OEM casting for fluid control — needs porosity control above everything else. High-silicon alloys — ADC12, A413 — suppress shrinkage porosity more effectively than A380; A380 is acceptable only with vacuum assist. A load-bearing bracket needs datums in a single die half and generous fillets at all wall junctions; alloy choice between A380 and ADC12 is secondary because the UTS values are comparable. A decorative housing needs to plate or anodise cleanly, which is where the surface question finally becomes primary: zinc casting (ZAMAK 3 or 5) for chrome or nickel plating, A360 or A413 if the program is aluminum and bright anodising is required, and not A380 — its copper content at 3–4% prevents a clean bright anodic finish regardless of pre-treatment.
A selector that asks "what does the surface have to do" before asking "what does the part have to do" is reaching for the finish before establishing the structure.

Volume affects alloy selection primarily through economics, not through physics. Zinc hot chamber has faster cycle times and longer tool life than aluminum cold chamber — a zinc tool routinely outlasts an aluminum tool by a factor of four or more. Those advantages only pay back across enough parts to amortise the difference. At low volumes they never materialise.
But "low volume" is ambiguous in a single-number question. A programme with 500 parts per year over ten years looks low-volume in year one and delivers 5,000 parts over its life — which is different from a one-time run of 500 that will never repeat. The annual rate and the programme horizon together determine whether zinc's economic advantages ever become real. A selector that asks only "lifetime volume" collapses the two into a number that can mean very different things.