What injection molding is and how a cycle works
Injection molding is a process where plastic pellets are melted and pushed under pressure into a closed metal mold. The plastic cools and solidifies into the shape of the cavity, the mold opens, and ejector pins push the part out. A machine has two main sections: an injection unit that melts and pushes the material, and a clamping unit that holds the mold halves shut.
A typical cycle follows the same order every time: the mold closes, material is injected, extra material is packed in to offset shrinkage, the part cools, the mold opens, and the part is ejected. Cooling usually takes the largest share of the cycle, which is why wall thickness has such a strong effect on cost.
When injection molding makes sense
Injection molding is best for high volumes. The mold is the expensive part, but once it exists each part is cheap and consistent. It is commonly used for packaging, toys, electrical housings, medical items and automotive components.
The usual disadvantages are:
- High upfront tooling cost. Molds are machined from steel or aluminum and take time to build.
- Design changes are costly. Changing a finished mold is much harder than changing a 3D print file.
- Design constraints. Parts need draft, reasonably uniform walls, and a way to release from the mold.
For a handful of parts, 3D printing is almost always more practical (see rapid prototyping vs 3D printing for how the two fit together). Molding starts to pay off when you need many identical parts and the design is stable.
Design tips that save the most trouble
Most molding problems are cheaper to fix on the screen than in steel.
- Keep walls uniform. Thick sections cool slowly and are prone to sink marks and voids; abrupt changes in thickness can cause warping. Where thickness has to change, transition gradually.
- Add draft angles. Walls parallel to the mold opening direction need a slight taper so the part releases without scraping. Textured surfaces generally need more draft than smooth ones. Ask your molder for their recommendation.
- Design ribs thoughtfully. Ribs add stiffness without thick walls, but a rib that is too thick at its base can show up as a sink mark on the opposite face. Keeping ribs noticeably thinner than the wall they attach to is a common guideline.
- Round the corners. Sharp internal corners concentrate stress and can disturb plastic flow. Generous radii help both strength and fill.
- Avoid undercuts where you can. Undercuts require side actions or lifters in the mold, which add cost and maintenance. Sometimes a small redesign removes them.
- Think about gate location. The gate is where plastic enters the cavity and it leaves a small mark. Placement affects fill, weld lines and appearance.
- Plan for features like living hinges, threads and text early. They are possible, but each has material and tooling considerations. Discuss them with the toolmaker.
Key process parameters
When people ask about the key parameters of injection molding, they are usually referring to the following settings:
- Melt temperature. Too low can cause short shots; too high can degrade the material or cause flash.
- Mold temperature. Affects surface finish, shrinkage and cycle time.
- Injection speed and pressure. Control how quickly and forcefully the cavity fills.
- Hold (packing) pressure and time. Compensate for shrinkage as the part cools; too little leaves sink marks or voids, too much can cause flash.
- Cooling time. Needs to be long enough for the part to hold its shape on ejection.
- Clamp force. Must be sufficient to keep the mold closed against injection pressure.
Always start from the material supplier’s data sheet recommendations, then adjust one variable at a time and record the results.
Choosing a material
Pick the material based on what the part has to do: strength, impact resistance, heat, chemical exposure, appearance and cost. Common choices include polypropylene, ABS, polycarbonate, nylon and polyethylene. If you already know plastics from 3D printing, many of the same names apply, but molding grades differ from filament, so check the data sheet for the specific grade. Glass-filled grades are stiffer but can warp differently and affect surface finish.
Surface finish
Finish is set by how the mold cavity is prepared. Polished cavities give glossy parts, while textured or bead-blasted cavities give matte surfaces and can hide minor flow marks. A rougher finish will usually need more draft. Heavy polish costs more, so specify only what you actually need.
Common defects and what usually causes them
| Defect | What it looks like | Common causes |
|---|---|---|
| Flash | Thin extra plastic at the parting line | Worn mold, low clamp force, excess pressure or temperature |
| Short shot | Part is incomplete | Low melt temperature, low pressure, blocked vents or gates |
| Sink marks | Dimples over thick areas | Thick sections, low hold pressure |
| Warping | Part bends after ejecting | Uneven cooling, uneven wall thickness |
| Weld lines | Visible lines where flows meet | Low temperature, gate placement |
| Burn marks | Dark or brown spots | Trapped air, poor venting, excessive speed |
Flash is one of the most common complaints, and it is covered in detail in our guide to injection molding flash removal.
What a T1 sample is
A T1 sample is the first set of parts produced from a new mold, often at the first trial run. It lets you check dimensions, fit, appearance and defects before approving production. Expect to find things to adjust. Typical follow-ups include process tuning and small mold modifications, after which a second trial (T2) may be run. Inspecting T1 parts carefully and giving clear feedback is one of the best ways to save time and money.
Ways to reduce cost
- Remove undercuts and unnecessary features or textures.
- Simplify the part so the mold is simpler.
- Keep walls thin and uniform to shorten cooling time.
- Use a standard, readily available material.
- Consider multi-cavity molds only for high volumes, since they raise tooling cost.
- Get quotes at several volumes so you can see where the per-part price levels off.
Related processes
Injection molding is one of several ways to shape plastics and composites in a mold. If you are curious about a related approach that uses heat and pressure in a mold, see how forged carbon is made. For surface finishing of molded or printed parts, see our guide to sand blaster media types.