Injection molding works by melting plastic pellets and forcing them under high pressure into a shaped metal mold, where the material cools and solidifies into a finished part. The entire cycle, from clamping the mold shut to ejecting the final piece, can take anywhere from a few seconds to over a minute depending on the part’s size and complexity. It’s the process behind nearly every plastic product you touch daily, from bottle caps to car dashboards.
The Six Stages of the Molding Cycle
Every injection molding cycle follows the same sequence: clamping, injection, dwelling, cooling, mold opening, and part removal. Each stage flows directly into the next, and the machine repeats this loop continuously during a production run.
First, the clamping unit presses the two halves of the mold tightly together. This force has to be strong enough to keep the mold sealed against the incoming pressure of molten plastic, which typically ranges from 10,000 to 20,000 psi. If the clamp can’t hold, plastic will squeeze out along the seam and create thin, unwanted edges called flash.
Next, the machine injects molten plastic into the mold cavity. A rotating screw pushes the material forward through a nozzle at a controlled speed. Once the cavity is full, the process shifts to the dwelling (or packing) stage. Here the machine maintains pressure, usually between 8,000 and 15,000 psi, to pack extra material into the mold and compensate for the slight shrinkage that happens as plastic cools.
Cooling is by far the longest part of the cycle, accounting for roughly 80 to 85 percent of total cycle time. The plastic needs to solidify enough inside the mold to hold its shape without warping when it’s released. Channels running through the mold carry water or oil to pull heat away from the part. Once it’s firm, the mold opens, and ejector pins push the finished part out. The mold closes again, and the cycle restarts.
Key Parts of the Machine
An injection molding machine has two main halves: the injection unit (which melts and delivers the plastic) and the clamping unit (which holds the mold). Understanding each component helps explain why the process is so repeatable.
Everything starts at the hopper, a funnel-shaped container mounted on top of the machine that holds raw plastic pellets. Gravity feeds these pellets down into the barrel, a long heated cylinder where the actual melting happens. Band heaters wrapped around the outside of the barrel maintain specific temperatures along its length so the plastic melts evenly. Inside the barrel, a reciprocating screw rotates to mix the material and push it forward. This screw does double duty: its rotation generates friction heat that helps melt the pellets, and then it acts like a plunger, driving forward to inject the molten plastic into the mold.
On the other side, the clamping unit opens and closes the mold and applies the tonnage needed to keep it shut during injection. Machines are often categorized by their clamping force, measured in tons. The required tonnage depends on the projected area of the part (its footprint as seen from above) multiplied by the average cavity pressure. A small electronics housing might need 50 tons. A large automotive panel could need 1,000 or more.
How Plastic Gets Into the Mold
Molten plastic doesn’t just flood into the cavity through one big opening. It travels through a carefully designed channel system: the sprue (the main entry point from the nozzle), runners (branching channels), and gates (the small openings where plastic actually enters the part cavity). The gate’s size, shape, and location directly affect how evenly the part fills and how it looks when finished.
Edge gates are the most common type. They sit along the parting line where the two mold halves meet and require the runner to be trimmed off after molding, leaving a small square or line-shaped mark. Tunnel gates (sometimes called submarine gates) taper to a small point below the parting line and can shear off automatically when the mold opens, saving a manual trimming step. Cashew gates work similarly but with a curved channel shape. For parts with large circular openings, diaphragm gates stretch across the opening like a membrane and get cut away afterward.
Production molds often use hot runner systems, where heated channels keep the plastic molten all the way to the gate. This eliminates the solidified runner that would otherwise need to be trimmed and recycled. Hot runners also allow multiple injection points on a single part, which helps complex shapes fill more evenly. The trade-off is higher mold cost upfront.
Materials and Temperatures
Most injection molded parts use thermoplastics, materials that can be melted, shaped, and re-melted without degrading. Each resin has its own processing window. Polypropylene, one of the most widely used plastics, melts between 200 and 280°C. ABS, popular for consumer electronics and toys, processes at 190 to 270°C. Nylon 6/6, chosen for its strength and heat resistance in automotive and mechanical parts, requires higher temperatures of 270 to 300°C.
The mold itself is kept at a much lower temperature to draw heat out of the part. Polypropylene molds typically run at 30 to 80°C, while nylon molds may be set between 40 and 90°C. The gap between melt temperature and mold temperature drives how quickly the part solidifies. Glass-fiber-reinforced versions of these plastics often need higher melt temperatures and tighter mold temperature ranges because the fibers change how the material flows and shrinks.
Common Defects and What Causes Them
Even with tight process control, several things can go wrong. Recognizing these defects helps explain why so many variables in the process matter.
- Sink marks are shallow depressions on the surface, usually over thick sections of a part. They happen when the interior of the plastic shrinks as it cools but there isn’t enough packing pressure or hold time to push extra material in to compensate.
- Flash is a thin film of plastic that leaks out along the mold’s parting line. It results from injection pressure that’s too high, material that’s too hot, or a clamping force that can’t hold the mold shut tightly enough.
- Short shots occur when the mold doesn’t fill completely, leaving a portion of the part missing. Slow injection speed, too little material, or poor venting (trapped air blocking the flow) are typical causes.
- Warping means the part bends or twists after ejection. Uneven cooling, insufficient cycle time, or poorly placed gates that create asymmetric flow patterns are usually responsible.
Most of these defects trace back to the relationship between temperature, pressure, and time. Adjusting one variable almost always affects the others, which is why dialing in a new mold can take hours of trial runs.
Hydraulic vs. Electric Machines
Traditional injection molding machines use hydraulic systems to generate clamping force and drive the screw. They’re powerful, capable of very high clamping tonnage, and cost-effective for large-scale production. The downside is high energy consumption, because hydraulic pumps run continuously whether the machine is actively injecting or just waiting for a part to cool.
Electric machines use servo motors for each axis of movement. They consume significantly less energy because the motors only draw power when actively moving. They’re also faster, quieter, and more precise, which makes them a strong choice for small, intricate parts where tight tolerances matter. Maintenance costs tend to be lower too, since there’s no hydraulic fluid to manage and fewer seals to replace. The trade-off is a higher purchase price and, for very large parts requiring extreme clamping force, hydraulic machines still hold an advantage.
Many modern facilities use hybrid machines that combine a hydraulic clamp with electric injection and metering, splitting the difference between raw power and energy efficiency.
Why Cycle Time Matters So Much
Since cooling accounts for 80 to 85 percent of the total cycle, even small improvements in cooling efficiency can dramatically increase output. A mold that cycles every 30 seconds produces twice as many parts per hour as one cycling at 60 seconds. That’s why mold designers spend considerable effort on cooling channel layout, placing channels as close to the cavity surface as possible without weakening the steel.
Conformal cooling channels, which follow the contour of the part rather than running in straight lines, have become more common as metal 3D printing makes them easier to produce. They pull heat away more uniformly, reducing both cycle time and the risk of warping from uneven cooling.