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The Print Farm

Logans project was ultimately to help Prusa Research evaluate its 3D printing manufacturing mode and investigate injection molding as a replacement manufacturing technology for the current production scale. To do so, Logan needed to build a cost model for the print farm. The first principle, Logan determined, was that only costs that differ between 3D printing and injection molding should be considered, namely the capital expense (CapEx), material, energy and direct labor for manufacturing the plastic parts in a printer kit. On the other hand, procurement of other parts, inspection, sorting and packaging, management, R&D, marketing, and shipping are not to be included because the same costs would still be incurred even if all plastic parts were injection-molded. Logan also estimated the facility cost (e.g., rent for the space for production), but found it to be magnitudes lower than other types of costs and negligible.Logan considered the information he had collected. As of February 2018, the print farm had 300 printers working 24/7 to ship 6,000 printers monthly. The Prusa i3 printers retail for about $1,000 each (assembled), although the production cost for each farm printer must be a lot less. Logan felt that a 65% gross margin should be in the ball park, so that he could assume that each farm printer costs Prusa Research $350. Joseph had said that it takes slightly more than one full day for a printer to print all plastic parts of a new printer. The productivity can be estimated in another way: 6,000 printers shipped monthly means that 300 printers on the farm produce 200 printers in 24 hours, which translates into a printer printing another printers plastic parts in 1.5 days. The calculated productivity seems lower than what Joseph had said, but the difference may well be explained by overheads such as loading materials, removing finished parts, and other occasional downtime. Logan learned that farm printers on average last two years, which was rather impressive given that they work 24/7.The Prusa i3 printer has an average printing power consumption of 100 watts (W). The energy cost in the Czech Republic is $0.2/kilowatt hour (kWh). The raw material for production is Polyethylene Terephthalate Glycol (PETG) 3D printing filament, which costs $14 per kilogram (kg). Each Prusa i3 contains about 50 plastic parts weighing 0.5kg in total, all of which are currently 3D-printed. The parts are arranged to be printed in batches tightly occupying the entire printing bed so as to reduce human intervention. The printing process can be sloweasily taking more than 10 hours to finish a full bed of partswhich is good news for the 15 employees tending to the print farm: they can schedule long print jobs overnight and work regular 8 hour shifts even though the print farm runs around the clock. Logan also learned that the 15 employees work 5-day shifts to cover all seven days of the week, and estimated a $15 hourly wage for them.

Injection Molding

In order to compare 3D printing to injection molding, Logan also needed to investigate the latter. Injection molding is the most common process for manufacturing anything plastic. Plastic pellets are fed into a heated barrel. The melted plastic is then injected into a mold. Once it cools, the plastic hardens and the parts are ejected from the machine before the cycle repeats itself. A mold may house multiple cavities connected by narrow tunnels, and as such can produce multiple parts or multiple copies of a part in one cycle. The more cavities, the costlier the mold becomes due to the size and complexity, and the more powerful and expensive machine is required.Once a mold is installed and the machine is up and running, a large number of parts can be produced in relatively short periods of time. A medium-sized injection molding machine that would be suitable for Prusa Research typically has a 20-second cycle time. If a mold has 10 cavities, the average time to produce each part would only be 2 seconds. An injection molding machine can be fitted with different molds, but the process of switching molds is cumbersome and slow.Since Prusa Research had not produced plastic parts using injection molding, Logan had to make educated guesses about relevant operational details. First off, for simplicity and comparability purposes, Logan assumed that the exact same parts (design and material) as being 3D-printed right now would be produced using injection molding[i]. This means that each printer still requires 50 plastic parts made of PETG weighing 0.5kg in total. However, unlike 3D printing which requires precisely manufactured PETG filaments at $14/kg, injection molding takes PETG pellets with a much lower cost of $4/kg as input.Of the 50 parts, about 10 are large parts (5-15 cm in largest dimension) and the rest are smaller. Logan estimated that the most economic design was to fit the parts in five 10-cavity medium-sized molds. Because of injection moldings high rate of production, Logan assumed that investing in one medium-grade injection molding machine would be enough, which would cost $100,000, last 10 years, and consume 2kWh of energy per cycle. He did some back-of-envelope calculations to see if the assumption made sense. In order to achieve the same level of productivity (6,000 units per month), the daily production target would be 300 units (assuming 5 working days in a week), or 15,000 parts. If production is never halted, producing 15,000 parts in 10-cavity molds would require 1,500 cycles or 8.3 hours. Overheads due to switching molds and cooling and warming up the machine will easily push the daily working hours to 10. Therefore, one machine would fit the need if it worked 10 hours a day, 5 days a week. In addition, Logan assumed that during the 10 hours, one person is constantly needed for loading the pellets, monitoring the machine, collecting the parts, and switching molds, again at a $15 hourly wage.Finally, the costs of molds are significant. Each 10-cavity mold would cost roughly $20,000 to machine, and can last 500,000 cycles of production. Since each cycle takes 20 seconds, 500,000 cycles would translate into 278 10-hour workdays, or roughly one year, if it is constantly being used. However, Prusa Research would switch between five molds on one injection machine, therefore each mold is only used one fifth of the time, thus would last 5 years at the Prusa Research factory.In reality, both 3D printing and injection molding have limitations and advantages, and engineers may optimize design and material for each manufacturing technology.

3D printing cost model Injection molding cost model CAPEX Cost of a farm printer ($) Lifespan (years) Cycle time (days/unit) Average cost ($/unit) CAPEX Cost of machine ($) Lifespan (years) Total output Average cost ($/unit) Cost of mold ($) Lifespan (years) Total output Average cost ($/unit) MATERIAL PETG cost ($/kg) PETG usage (kg/unit) Average cost ($/unit) MATERIAL PETG cost ($/kg) PETG usage (kg/unit) Average cost ($/unit) ENERGY Power (W) Energy usage (kWh/unit) Energy cost ($/kWh) Average cost ($/unit) ENERGY Energy usage (kWh/cycle) Cycle time (sec) Cycle output Energy cost ($/kWh) Average cost ($/unit) LABOR Staff Wage ($/hr) Hours per day Days per week Weekly total cost ($) Weekly output Average cost ($/unit) LABOR Staff Wage ($/hr) Hours per day Days per week Weekly total cost ($) Weekly output Average cost ($/unit) Total unit cost ($/unit) Total unit cost ($/unit) 3D printing cost model Injection molding cost model CAPEX Cost of a farm printer ($) Lifespan (years) Cycle time (days/unit) Average cost ($/unit) CAPEX Cost of machine ($) Lifespan (years) Total output Average cost ($/unit) Cost of mold ($) Lifespan (years) Total output Average cost ($/unit) MATERIAL PETG cost ($/kg) PETG usage (kg/unit) Average cost ($/unit) MATERIAL PETG cost ($/kg) PETG usage (kg/unit) Average cost ($/unit) ENERGY Power (W) Energy usage (kWh/unit) Energy cost ($/kWh) Average cost ($/unit) ENERGY Energy usage (kWh/cycle) Cycle time (sec) Cycle output Energy cost ($/kWh) Average cost ($/unit) LABOR Staff Wage ($/hr) Hours per day Days per week Weekly total cost ($) Weekly output Average cost ($/unit) LABOR Staff Wage ($/hr) Hours per day Days per week Weekly total cost ($) Weekly output Average cost ($/unit) Total unit cost ($/unit) Total unit cost ($/unit)