Types of Injection Molding (and How to Choose the Right One)
Key Takeaways
- The main types of injection molding are conventional (high-pressure) molding, structural foam, structural web, gas-assist, and insert and overmolding, and they differ in how much pressure they use, how they fill the mold, and which parts they suit.
- Conventional high-pressure molding injects into a steel mold under high pressure for fast cycle times, fine detail, tight tolerances, and a smooth glossy finish, which makes it the standard choice for smaller, thin-walled parts like consumer electronics housings and precision components.
- Structural foam mixes nitrogen gas or a foaming agent into the melt and injects at low pressure so the gas expands to fill the mold, producing a solid outer skin over a lightweight foamed core with no sink marks on thick walls, often on lower-cost aluminum tooling.
- Structural web is a low-pressure process closely related to structural foam that produces a firmer, smoother surface, which makes it a fit for large panels and covers where an improved surface finish is required.
- Gas-assist injects the plastic and then follows it with pressurized nitrogen that hollows out the thickest sections, leaving clean hollow channels instead of a foamed core, which cuts weight and material on parts with heavy sections, handles, or thick ribs and can provide a surface finish similar to conventional injection molding.
- Insert molding and overmolding combine materials in a single step, with overmolding molding one material over another such as a soft grip over a rigid handle, and insert molding forming the plastic around a premade component like a threaded metal insert.
- Choosing a process comes down to five questions: how big and heavy the part is, how thick the walls are, what finish it needs, the production volume and tooling budget, and whether multiple materials or inserts are involved.
- 20/20 Custom Molded Plastics runs structural foam, structural web, gas-assist, and high-pressure injection molding across three North American plants, with multi-nozzle capability for large geometries and for running two materials or colors at once.
Injection molding is not one process but a family of them. They all melt plastic and force it into a mold, but they differ in how much pressure they use, how they fill the mold, and which parts they suit. The main types are conventional (high-pressure) molding, structural foam, structural web, gas-assist, and insert and overmolding. Which one fits your part comes down to its size, wall thickness, finish, and how many you need.
Here is a plain rundown of each, and a simple way to narrow the choice. We run several of these at 20/20 Custom Molded Plastics, so this is the same map we use when we help a customer match a part to a process.
Conventional (high-pressure) injection molding
This is the standard process most people mean by “injection molding.” Plastic is injected into a steel mold under high pressure and packed tight while it cools. It gives fast cycle times, fine detail, tight tolerances, and a smooth, glossy finish, which makes it the go-to for smaller, thin-walled parts like consumer electronics housings and precision components. The limit shows up with size: large or thick parts need enormous clamp tonnage and costly hardened steel tooling, and thick sections tend to sink and warp. For more on that line, see our post on low-pressure vs. high-pressure injection molding.
Structural foam (low-pressure) molding
Structural foam is the workhorse for large, rigid parts. A small amount of nitrogen gas or a foaming agent is mixed into the melt, and the plastic is injected at low pressure so the gas expands to fill the mold. The result is a part with a solid outer skin over a lightweight, foamed core, stiff for its weight, with no sink marks on thick walls, and it can often use lower-cost aluminum tooling. It suits pallets, bins, enclosures, and large housings. For the full picture, see What Is Structural Foam Molding? and our structural foam capabilities.
Structural web molding
Structural web is a close cousin of structural foam, also low-pressure and aimed at large parts. It produces a firmer, smoother surface, which makes it a good fit for big panels and covers where an improved surface finish is required. It shares structural foam’s strengths on large, rigid geometries.
Gas-assist injection molding
Gas-assist injects the plastic, then follows it with pressurized nitrogen that hollows out the thickest sections of the part. Unlike structural foam, which fills the core with foam, gas-assist leaves clean hollow channels. That makes it well suited to parts with heavy sections, handles, or thick ribs, where it cuts weight and material and holds the part flat as it cools, and requires a good surface finish.
Insert molding and overmolding
These two processes combine materials or parts in one step. Overmolding molds one material over another, such as a soft grip over a rigid handle. Insert molding places a premade component, like a threaded metal insert, into the mold so the plastic forms around it and locks it in. Both cut down on later assembly and make stronger, integrated parts.
How to choose
A few questions usually settle it:
- How big and heavy is the part? Small, detailed parts fit conventional high-pressure molding. Large, heavy parts point to structural foam, structural web, or gas-assist.
- How thick are the walls? Thin, even walls suit conventional molding. Thick sections without sink marks call for a low-pressure process.
- What finish does it need? A glossy, cosmetic face favors conventional molding. An industrial finish is fine for the low-pressure family.
- How many are you making, and what is the tooling budget? High volumes of a small part justify steel tooling. Large parts often run more affordably on aluminum tooling with a low-pressure process.
- Are there multiple materials or inserts? That points to overmolding or insert molding.
What 20/20 runs
Our focus is the large-part end of that list. We run structural foam, structural web, and gas-assist on the low-pressure side, along with high-pressure injection molding, and we offer multi-nozzle capability for large geometries and running two materials or colors at once. Our presses go up to 8,000 tons, one of the largest of its kind, with a full range of smaller machines across three North American plants. Because we run more than one process, we can match your part to the right one instead of forcing it into the only one we have.
Send us your part design and our engineers will review the geometry, recommend the process that fits, and put together a molding quote. Contact 20/20 Custom Molded Plastics to get started.