I've worked in product design and manufacturing for about 11 years, did a lot of work with consumer electronics companies and some automotive suppliers, and injection molding is just the default way to make plastic parts at any kind of volume. There's other processes like 3D printing or CNC machining plastic, but once you need more than maybe 500-1000 units, injection molding becomes the only thing that makes economic sense for most applications.
Why injection molding took over everything
The process itself is pretty straightforward-melt plastic pellets, inject under high pressure into a mold, let it cool, eject the part. Repeat. Modern machines can do this cycle in like 15-30 seconds for small parts, maybe 60-90 seconds for bigger stuff. I watched a Haitian Mars injection molding machine at a factory in Shenzhen pump out smartphone cases at about 40 seconds per cycle, and that was for a pretty complex part with multiple internal features. The tooling cost was somewhere around $12,000 USD (the factory manager told me this in 2018, prices have definitely gone up since).
What makes it so dominant is the combination of speed, repeatability, and material options. Once the mold is made, each part costs pennies in material and machine time. A 50-gram ABS part might use $0.15 worth of material, and if the cycle time is 30 seconds that's 120 parts per hour. Even factoring in machine time costs and labor, you're looking at maybe $0.30-0.50 per part on a production run of 10,000+ units.
Compare that to CNC machining where you're paying for tool wear, longer cycle times, material waste, and skilled operator time. I priced out a simple enclosure design once-injection molding came in at $0.82/part for 5,000 units including amortized tooling. CNC machining the same part was quoted at $6.40/part. Not even close.
Medical devices have gotten weird about this
Medical injection molded parts are a whole different world because of FDA regulations and sterilization requirements. The materials need to be biocompatible, which limits your options significantly. Most medical manufacturers use stuff like polycarbonate, polypropylene, or specialized grades of polyethylene. PEEK is popular for surgical instruments because it can handle repeated autoclaving without degrading, but PEEK tooling is expensive because the processing temperatures are so high (around 370-400°C according to Victrex datasheet specs, victrex.com).
I worked on a disposable medical device project a few years back, just a simple injection molded tray for dental procedures. The material validation alone took 4 months because we had to prove biocompatibility testing under ISO 10993 standards. The actual part design was done in 3 weeks. The regulatory documentation was another 6 weeks. This is why medical injection molded parts cost 3-5x more than equivalent consumer parts even when they're simpler designs.
Sterilization compatibility matters way more than most engineers think about. Gamma radiation sterilization can cause some plastics to yellow or become brittle. EtO (ethylene oxide) sterilization is gentler but has absorption issues with certain materials. Autoclave sterilization requires materials stable at 121-134°C with steam exposure. Your material choice gets dictated by how the customer plans to sterilize the product, not just by mechanical requirements.
Automotive parts that nobody sees
Under-hood automotive components are brutal on plastic materials. You've got temperature cycling from -40°C to 120°C+, exposure to oils and fuels, vibration, and long service life requirements (15+ years typically). This is why automotive injection molding uses a lot of glass-filled nylons and specialty high-temp materials.
Intake manifolds used to be aluminum castings but now they're mostly injection molded glass-filled nylon. BMW started doing this in the late 1990s I think, and everyone followed. The weight savings is significant-a plastic intake manifold can be 40-50% lighter than the aluminum equivalent. According to some analysis from a Society of Plastics Engineers conference paper (spe.org, 2015 automotive division conference), the switch to plastic intake manifolds reduced vehicle weight by an average of 3-4 kg per vehicle, which translates to about 0.1-0.15 mpg improvement in fuel economy. Doesn't sound like much but multiply that across millions of vehicles.
Connectors and sensor housings are another huge category. Every sensor on a modern car-temperature, pressure, position, speed, whatever-has an injection molded housing. These need to be sealed against water and dust (usually IP67 rating minimum), handle vibration, and maintain dimensional stability across the temperature range. The tolerances can be tight too, like ±0.05mm for mating features on electrical connectors.
I've seen automotive connector molds that cost $80,000-120,000 because they're family molds making multiple parts, with side actions for undercuts, and hardened steel construction for the million+ part lifecycles required. The per-part cost ends up being under $0.20 though when you're running that kind of volume.
Consumer electronics is where the money is
Smartphone housings, laptop cases, tablet frames-all injection molded. The surface finish requirements are insane compared to other industries. You need A1 or A2 mold surface finishes (SPI standards), which means extensive polishing of the mold cavity. A good mold polisher can take weeks to hand-polish a large cavity to mirror finish.
Apple famously does stuff with injection molding that other companies struggle to replicate. The unibody polycarbonate MacBook from like 2009-2010 was injection molded in a single piece, which required massive mold making expertise and process control. Most manufacturers would split that into multiple parts because getting uniform wall thickness and avoiding sink marks on something that large is difficult.
Material choice in consumer electronics has shifted a lot over the last decade. ABS used to be the default for everything, now you see way more polycarbonate, PC/ABS blends, and modified PPO materials. Part of this is flame retardancy requirements-the UL94 rating matters for anything with electronics inside. Most stuff needs at least V-1 rating, many applications require V-0. This limits which materials and additives you can use.
Texture and color matching is a nightmare in consumer electronics. I worked with a company that made peripheral devices (keyboards, mice, that kind of stuff) and they had 23 revisions on the texture for one product because marketing kept changing their mind on how it should feel. Each texture revision meant repolishing the mold cavity with different EDM electrodes or chemical etching processes. The mold maker charged like $2,000 per texture revision.
Color matching between different plastics is also harder than it should be. If you have a product with a PC top cover and ABS bottom housing, getting them to be the exact same color when molded is tricky because different materials take colorants differently. You end up with custom color masterbatches and a lot of trial molding to dial it in.
Packaging is the hidden giant
Nobody thinks about injection molded packaging but it's enormous in terms of volume. Bottle caps alone-there's a statistic somewhere that says something like 2 billion plastic bottle caps are made daily worldwide (I can't find the exact source right now but it was in a Plastics News article from a few years ago). Every water bottle, soda bottle, milk jug, shampoo bottle has an injection molded cap.
Food containers, cosmetic packaging, medicine bottles-all injection molded. The requirements are different from other applications because you need food-safe materials (FDA regulations again) and good chemical resistance. Polypropylene dominates this space because it's cheap, food-safe, and chemically inert. HDPE is common too, especially for bottles and containers that need some flexibility.
Thin-wall molding for food containers is its own specialty. You're talking wall thicknesses of 0.5-0.8mm, which requires high injection speeds and specialized machines with good shot control. The cycle times are fast though-like 4-8 seconds for a yogurt container. I visited a packaging manufacturer in Italy that was running 48-cavity molds for small food containers, making thousands of parts per hour on a single machine.
The tooling costs for packaging molds are interesting because they're often family molds (multiple cavity sizes) or multi-cavity molds with insane cavity counts. A 96-cavity bottle cap mold might cost $150,000+ but when you're making tens of millions of parts, the per-part tooling cost becomes negligible.
Industrial and construction applications that last forever
Electrical enclosures, junction boxes, switch housings-tons of injection molded parts in buildings and industrial equipment. The material requirements are different here because you need UV resistance (if outdoors), flame retardancy, and impact resistance. ABS won't work for outdoor applications because it degrades under UV exposure. ASA is better but more expensive. Polycarbonate works but yellows over time. There's always compromises.
Cable management parts like cord grips, strain reliefs, and glands are mostly injection molded. Nylon is popular for this because of its toughness and chemical resistance. I've worked with M20 and M25 cord grips that need to maintain IP68 sealing after being installed and removed multiple times-the dimensional tolerances on the threads and sealing surfaces have to be tight (±0.1mm or better on critical features).
Pipe fittings and plumbing components are a big category too. PVC and PP fittings are injection molded, though the large diameter stuff is often made by other processes. The pressure ratings matter-a fitting rated for 150 PSI needs thicker walls and better material properties than one rated for 50 PSI. The material grades are specified to ASTM standards and testing requirements are spelled out in things like ASTM D2846 for PVC fittings.
Toys and recreational products
Obviously toys are mostly injection molded plastic. LEGO bricks are the classic example-manufactured to incredibly tight tolerances (±0.01mm or something crazy like that) so they fit together perfectly every time. LEGO uses ABS exclusively I think, and they've been doing injection molding since the 1940s or 50s so they have the process completely dialed in.
Action figures, dolls, toy vehicles-all injection molded, usually multi-shot or assembly of multiple molded components. The material choice for toys is constrained by safety regulations. In the US there's CPSIA requirements, in Europe there's EN 71 toy safety standards. Certain plasticizers and additives are banned for children's products, which rules out some material options.
Sports equipment has a lot of molded parts too. Ski boot shells, inline skate frames, bike components, protective gear. The mechanical requirements can be demanding-ski boots need to be stiff enough to transmit force efficiently but not so brittle they crack in cold weather. This usually means polyurethane or polyamide materials, and the wall thicknesses get heavy (3-6mm in some areas).
What doesn't work well with injection molding
Very large parts get challenging because you need massive machines and the tooling costs explode. Automotive bumper fascias are about at the limit of what makes sense-those require 1000+ ton clamping force machines and molds that weigh several tons. The capital investment for that equipment is millions of dollars so only large manufacturers can do it.
Super small parts with tight tolerances are also difficult. Medical micro-molded parts like catheter components or microfluidic devices push the limits of what's possible. You're dealing with gate sizes under 0.3mm, cavity dimensions measured in tenths of millimeters, and process control that requires active feedback systems. The reject rates can be high even with perfect molds because tiny amounts of contamination or process variation cause issues.
Parts with very thick sections don't mold well because of cooling time and sink marks. Anything over maybe 6-8mm wall thickness starts to have problems. The outside cools and solidifies while the inside is still molten, and as the center cools it shrinks, pulling the outer surface inward and creating sink marks or voids. You can sometimes compensate with packing pressure and longer cooling times, but it's always a fight. Better to redesign the part with ribs or hollow sections if possible.
Materials that aren't thermoplastic don't work at all. Thermosets like epoxy or phenolic resins need different processes (compression molding or transfer molding). Metals obviously can't be injection molded, though metal injection molding (MIM) exists as a related process for powdered metals.
The economics when you're trying to make a decision
For low quantities (under 1000 parts) injection molding usually doesn't make sense unless you need specific material properties or surface finishes you can't get other ways. The tooling cost dominates the per-part cost at low volumes. A simple prototype mold might be $3,000-5,000, but you're spreading that cost over a small number of parts.
The crossover point where injection molding becomes cheaper than other processes depends on part complexity and size, but it's usually somewhere between 500-2000 units. Below that, 3D printing or CNC machining tends to be more economical. Above that, injection molding wins.
I usually tell people to think about it this way: if the tooling costs $10,000 and parts cost $0.50 each to mold, you need to make 20,000 parts before the per-part cost drops below $1.00 ($10,000/20,000 + $0.50). If you only need 1,000 parts, your per-part cost is actually $10.50 ($10,000/1,000 + $0.50). That changes the calculation completely.
Production volumes in the millions are where injection molding really shines. The per-part cost can drop to ridiculous levels-like $0.10-0.20 for simple parts. The tooling cost gets amortized over so many parts that it barely matters. This is why disposable consumer products are almost all injection molded-razors, pens, bottles, containers, whatever. The margins work when each part costs pennies to make.
There's way more applications than I covered here (furniture components, appliance parts, optical lenses, musical instrument pieces, probably a hundred other categories) but those are the main industries I've dealt with. The versatility of the process is what makes it so dominant-you can make a 1-gram precision component or a 5-kilogram automotive panel with basically the same fundamental technology, just scaled up or down.
There's way more applications than I covered here (furniture components, appliance parts, optical lenses, musical instrument pieces, probably a hundred other categories) but those are the main industries I've dealt with. The versatility of the process is what makes it so dominant-you can make a 1-gram precision component or a 5-kilogram automotive panel with basically the same fundamental technology, just scaled up or down.
If you're getting into this field, spend time understanding how to design injection molded parts properly because the parting line injection molding strategy affects everything downstream. I've seen too many engineers order injection molded parts without considering the cosmetic specifications of injection molded parts, then complain about the cost for injection molded part when revisions are needed. Common defects in injection molding parts usually trace back to design issues-things like injection molding thick parts without proper cooling channels, or attempting large part injection molding without understanding how material flow works in bigger cavities. The expertise required varies wildly too: plastic injection molding automotive parts demands different knowledge than injection molding for aviation parts, and specialized processes like reaction injection molding parts or metal injection molding parts are completely different skill sets. Even post-processing matters-annealing injection molded plastic parts can reduce residual stresses that cause warpage issues months after production. Whether you're working with a basic injection molding mold parts setup or evaluating plastic parts produced by an injection molding operation, understanding the fundamentals prevents expensive mistakes later. The technology isn't going anywhere-injection molded parts and plastic injection molding parts will keep dominating manufacturing because nothing else scales like it does.