A 16-cavity mold on a 22-second cycle produces 2,618 parts per hour, against 164 parts per hour from a single-cavity mold on the same cycle, a 16x throughput swing that dictates every per-hour cost calculation on an automatic molding line [S7].
Hourly economics split into three line items: mold cost amortized across the run, machine hourly rate multiplied by cycle time, and material cost per shot. Mold cost alone runs $2,000–$100,000+ depending on cavitation, steel grade, and undercut complexity [S3]. Machine hourly rate spans $11 on small presses to $85 on mid-tonnage cells, with cycle times of 19–32 seconds typical [S4][S6]. Material is 30–50% of the unit cost and rides resin price, which sits at $1.0–$2.0/kg for PP/PE, $2.5–$6.5/kg for ABS/PC, and $25–$70+/kg for PEEK [S4].
Hourly Output: Cavity Count and Cycle Time Drive the Equation
Parts per hour = (3,600 seconds / cycle time in seconds) x number of cavities. A single-cavity mold with a 22-second cycle yields roughly 164 parts per hour; a 16-cavity mold on the same 22-second cycle yields 2,618 parts per hour [S7]. Drop cycle time from 19 seconds to 18 seconds on a single-cavity tool and the production run shortens by more than 5 hours for a 20,000-part order, freeing the press for the next job [S5].
For a sample 5,000-unit run, a 4-cavity mold at a 32-second cycle completes the lot in 11.1 hours, while the same lot on a single-cavity mold at 32 seconds needs 44.4 hours of press time [S6]. The longer the press is occupied, the more the hourly rate of $11–$85/hr compounds, which is why cavitation is the single largest lever on molding line throughput cost.
Machine Hourly Rate: The Hidden Multiplier
Shop hourly rate is the second lever and the easiest to overlook. Public rate cards put small-tonnage cells at $11/hr and mid-tonnage presses at $85/hr, with one U.S. molder charging $36.64/hr and another $44/hr across all presses on the same shop floor [S4][S6][S9]. That gap exists because the rate bakes in machine depreciation, labor, electricity, and overhead, not just kWh draw.
For a 100-ton press running a single-cavity mold at a 30-second cycle, processing cost lands at $0.05–$0.15 per part, assuming the mid-range $30–$60/hr band typical of that machine class [S4]. Halve the cycle to 15 seconds and the same press halves the per-part labor/machine slice, without buying new equipment. The same logic applies to a mold base decision: a hardened P20 or S136 steel mold base carries higher upfront cost but supports the cycle-time compression that drives hourly output.
Mold Cost Amortization: Fixed Cost per Output Hour
Mold cost is paid once, then divided across every part in the run. Across most projects, mold cost ranges from $2,000 to $100,000 or more, with the spread driven by part complexity, undercut features, and required cavitation [S3]. A typical mid-range mold lands at $3,000–$15,000, and at 50,000 parts the amortized mold cost per part is $0.06–$0.30 [S4].
Translating to per-hour terms: a $10,000 mold amortized over 50,000 parts at 164 parts/hr (single cavity, 22-second cycle) is $0.061 per part, while the same mold amortized over the 2,618 parts/hr case is $0.0038 per part, a 16x difference that mirrors the throughput swing [S4][S7]. This is the part most procurement teams underweight when comparing casting mold or injection mold quotes, because they compare mold sticker price without converting to amortized cost at the planned cavity count and cycle.
Material Cost: Resin Selection Moves More Than Steel
Resin price is the largest single line item at 30–50% of unit cost, and it is the only line item the production engineer cannot change by tuning the press [S4]. General-purpose plastics (PP, PE) sit at $1.0–$2.0/kg, engineering plastics (ABS, PC) at $2.5–$6.5/kg, and high-performance plastics (PEEK and equivalents) at $25–$70+/kg [S4]. A 60-gram ABS housing at $3.5/kg is $0.21 per part in material, rising to $0.25–$0.30 per part with color masterbatch or glass-fiber additive.
Hot runner vs cold runner is a second material lever that ties to hourly output. A 2-cavity cold runner producing 20-gram handles with an 8-gram runner per shot consumes 1,200 kg of material for a 50,000-part run; converting that mold to a hot runner drops material to 1,000 kg, a 200 kg saving that pays back the hot-runner premium on medium-to-high volume runs [S5]. Hot runners also tend to shorten cycle time, which compounds the hourly-output gain already captured by cavitation.
Total Cost per Output Hour: Putting the Levers Together
For a 100-ton press at $30/hr, single-cavity mold, 30-second cycle, 50-gram ABS part: hourly output is 120 parts, machine cost is $0.25/part, material is roughly $0.18–$0.33/part at $3.5–$6.5/kg ABS, and the amortized mold cost at $10,000 over 50,000 parts is $0.20/part, giving a unit cost around $0.63–$0.78 before post-processing [S4].
The same part on a 4-cavity mold at the same 30-second cycle pushes hourly output to 480 parts and drops the machine slice to $0.0625/part; the amortized mold cost on a $25,000 4-cavity tool is $0.50/part, so unit cost lands near $0.74–$0.89, more expensive per part but with one-quarter the press hours, which matters when the press is the bottleneck [S4][S6]. This is the sand casting mold versus hard-tool tradeoff in numeric form: higher cavitation wins when press time is scarce and order volume is high, single-cavity wins when order volume is low or design is still moving.
Decision Criteria: Cavity Count, Run Size, and Press Hourly Rate
Three numbers decide which configuration wins: planned annual volume, available press hours, and hourly rate. Under roughly 10,000 parts per year, single-cavity molds at the lowest hourly rate minimize cash outlay even with longer press occupancy. Between 10,000 and 100,000 parts, a 2-to-4 cavity mold on a mid-tonnage press is the typical sweet spot. Above 100,000 parts, 8+ cavity molds on the fastest available press win on amortized mold cost and machine cost per part combined [S3][S4][S7].
For a deeper look at how high-volume cycles translate into parts-per-hour on real lines, see the breakdown in Automatic Molding Line Molds-per-Hour: What High-Volume Cycles Actually Deliver. For changeover and SMED numbers that protect the hourly rate assumption, the Automatic Molding Line Pattern Changeover: 2026 SMED Numbers and Setup Pitfalls piece pairs cleanly. Regional resin lead times that gate the material line item are tracked in Engineering Plastic Compound Lead Times October 2026: Regional Stock vs Offshore.
Limitations and Failure Modes
Three failure modes consistently break these per-hour cost models. First, cycle-time assumptions built from the 2D CAD often miss cooling-channel optimization; a mold designed for 30 seconds that actually runs at 45 seconds silently adds 50% to the machine slice [S8]. Second, hot-runner conversion costs are real, and below roughly 50,000 parts the cold-runner regrind path is cheaper even with the labor cost of grinding and refeed [S5]. Third, logistics swings 1–10% of unit cost, and air-freight at $5–$15/kg from China to the US/EU can erase the per-part saving from a cheaper offshore mold once the run is rerouted [S4].
The published rate spread ($11/hr to $85/hr) also means a quote built on $36.64/hr cannot be compared to one built on $85/hr without normalizing the press class, tonnage, and overhead basis [S9]. For new programs, request the hourly rate band, the cycle-time assumption, and the cavity count in the same document before comparing mold quotes.
Trackable signals for the next planning cycle: 16-cavity mold cycle-time benchmarks under 20 seconds on commodity resins, hourly rate disclosures that separate depreciation from labor, and hot-runner vs cold-runner breakeven curves for 2-cavity molds at sub-50,000-part runs. Each one of these will sharpen the per-mold-per-hour cost number on the next automatic level quote review.