What is Injection Molding? A Complete Beginner’s Guide to the Process

Injection molding is a common manufacturing process used to produce plastic parts on a mass scale. This beginner’s guide will teach you what injection molding is, how the injection molding process works, its key benefits, and more.

Whether you’re just curious or exploring manufacturing options for a new product, this article will cover everything you need to know to get started with injection molding. Let’s dive in!

What is Injection Molding?

Injection molding is a manufacturing technique for producing polycarbonate injection molding parts in large volumes. It works by injecting molten plastic material into a mold cavity where it cools and hardens into the final part.

The injection molding process consists of four main steps:

  1. Clamp the Mold – Clamp an open mold shut securely while injecting the molten plastic
  2. Inject the Plastic – Melt the raw plastic and inject it into the mold cavity
  3. Cool the Plastic – Cool the molten plastic to harden it inside the mold
  4. Eject the Part – Eject the finished part from the open mold

Repeat this cycle over and over. This allows manufacturers to rapidly produce thousands of identical plastic parts in a short time.

How Does Injection Molding Work?

Now that you know the basics of what low volume injection molding is, let’s look at how the injection molding process actually works.

1. Clamping

The injection molding machine consists of two main parts – the injection unit and the clamping unit.

The clamping unit holds the mold closed during the injection and cooling phases. A standard mold is made of two primary components – the core side and the cavity side. When closed together, these two halves create the mold cavity where the molten plastic will be injected.

Hydraulic, mechanical, or electric clamps are used to keep enough pressure on the mold to prevent the opening from the force of the injection. click for more.

2. Injection

After the mold is securely closed, the raw plastic material is injected into the mold cavity.

The injection unit is responsible for both melting the plastic and injecting it into the mold. The plastic starts out as plastic pellets that are gravity fed from a hopper into the injection barrel.

The barrel is heated using electrical heating bands to melt the plastic. A reciprocating screw pushes the melted plastic forward and injects it into the mold through the injection nozzle at high pressure.

3. Cooling

Once the cavity is filled, the molten plastic begins to cool and harden into the shape of the mold.

Cooling channels circulate coolant around the mold to speed up the cooling process. This cooling time can be a few seconds to a few minutes depending on the plastic part thickness.

4. Ejection

After adequate cooling time, the clamp opens the mold and uses ejector pins to push the finished plastic part out of the mold. The part will be cooled and solid by this point.

Removed parts are often briefly trimmed of any excess plastic flashed and transported via conveyor to the next station for additional post-processing if needed.

The mold then closes again and the injection molding cycle repeats for the next part.

The Benefits of Injection Molding

Now that you understand the basic injection molding process from start to finish, let’s discuss some of the key benefits that make it a popular manufacturing technique:

High Production Speeds

  • Injection molding is extremely fast compared to alternatives. Cycle times range from 15 seconds to 2 minutes.

High Repeatability

  • Produces identical plastic parts cycle after cycle. The process is highly repeatable.

Complex Geometries

  • Can produce complex plastic parts and shapes that would be impossible with other methods.

Material Flexibility

  • Works with both thermoplastic and thermoset plastics. A wide range of material options.

Labor Requirements

  • Automated process requires minimal labor once molds and settings are dialed in.

Lower Waste

  • Very little excess plastic wasted compared to processes like CNC machining.

Low Costs at High Volumes

  • Extremely economical way to produce thousands+ of plastic parts when amortized over production volume.

Injection molding offers low per part costs, excellent consistency, and great design flexibility – making it ideal for high-volume plastic manufacturing.

Common Types of Injection Molding

There are several different types of injection molding machines and processes that are better suited to certain types of plastic part production.

Here are some of the most common injection molding machine types:

  • Plastic injection molding – The standard method described above. Used for a wide range of thermoplastic materials.
  • Die casting – Used to injection mold metal parts like zinc and aluminum.
  • Insert molding – Places a separate pre-made object like a metal component inside the plastic during injection.
  • Gas-assisted injection molding – Injects an inert gas into the melted plastic to create hollow interior cavities.
  • Water-assisted injection molding – Similar to gas-assisted but injects water instead of gas.

Each process has its own benefits and best use cases depending on the application.

The Injection Molding Process Step-By-Step

Now that you have a general overview of injection molding, let’s walk through the full injection molding process step-by-step from plastic pellet to finished part.

1. Design the Plastic Part

You first design the plastic part you want to create. Use CAD software to model the part virtually. The CAD file defines the final plastic part geometry.

2. Build the Injection Mold Tool

Custom-build a mold tool to match the plastic part design. Use CNC machining to create the mold tool including the core/cavity shapes. Make the mold tool from tool steel and consider including multiple cavities to increase production throughput.

3. Choose the Plastic Material

Determine what plastic material to use based on application requirements, available materials, part geometry, budget and other factors. Common materials to choose from include ABS, nylon, polyethylene, polypropylene, acrylic, PVC and many more.

4. Set Up the Injection Molding Machine

Load the raw plastic pellets into the injection molding machine hopper. Set the injection pressure, melt temperature, mold temperature, injection speed and all other process parameters.

5. Inject the Molten Plastic

Heat the plastic pellets until molten and inject into the closed mold tool at high pressure, filling all mold cavities.

6. Cool the Plastic Parts

Allow enough time for the hot plastic parts to cool down and solidify completely in the mold tool.

7. Eject the Parts

Open the mold tool and use ejector pins to push out the finished plastic parts.

8. Perform Secondary Operations

Carry out any other needed part post-processing steps like trimming, drilling, painting or assembly.

9. Quality Control

Inspect sample parts to confirm they meet all specifications before full production. Tweak any molding parameters until quality is optimized.

Once the injection molding process is fully dialed in, mass production can begin, often making thousands of plastic parts per day.

Injection Molding Tolerances

A key characteristic of injection molding is the tight tolerances that can be held in mass production.

Tolerance refers to the deviation between the design dimensions and the actual plastic part dimensions.

Typical injection molding tolerances range from:

  • ± 0.005 inches (± 0.13mm) to ± 0.010 inches (± 0.25mm) for smaller plastic parts and features.
  • ± 0.010 inches (± 0.25mm) to ± 0.030 inches (± 0.76mm) for larger plastic parts and overall dimensions.

The actual tolerance that can be achieved depends on the plastic material used, part geometry, and mold complexity. Injection molded plastic parts can be engineered to extremely precise dimensional tolerances.

Design Considerations for Injection Molding

When designing a part for injection molding, there are some important design considerations to keep in mind:

  • Draft angles – Include a slight draft angle (1-5 degrees) on vertical surfaces to make the part easier to eject from the mold.
  • Uniform wall thickness – Uniform plastic thickness helps the part cool evenly and prevent defects like warpage.
  • Radius corners – Sharp corners are weak points in injection molded plastics. Radiused corners improve strength.
  • Text and logos – Any text or graphics on the part should be raised rather than engraved into the mold (a reverse relief).
  • Ribs and gussets – Adding ribs or gussets helps strengthen plastic parts.
  • Sink marks – Allow slightly thicker plastic in areas prone to sink marks to minimize their appearance.

With some basic design for manufacturing (DFM) practices, injection molded parts can be optimized for quality, appearance, strength and function.

Example Plastic Parts Made by Injection Molding

Injection molding produces a huge array of everyday plastic parts and products. Here are some examples:

  • Storage containers and lids
  • Toys
  • Combs and hairbrushes
  • Auto dashboards and trims
  • Cell phone cases
  • Household appliances
  • Electrical switch covers
  • Bottle caps
  • Sports equipment like football helmets
  • Medical devices and instruments

If it’s made of plastic, there’s a good chance it was injection molded. The next time you pick up a plastic product, you’ll know the manufacturing process behind it!


We’ve covered a lot of ground explaining what injection molding is, how it works, why it’s beneficial, and more. Here are the key takeaways:

  • Injection molding is used to rapidly produce plastic parts by injecting molten plastic into a mold cavity where it cools and hardens.
  • The four main steps are clamping, injection, cooling, and ejection.
  • Benefits include fast production, repeatability, complex geometries, low labor, and low costs at high volumes.
  • Types include plastic injection molding, die casting, insert molding, gas-assisted, and water-assisted injection molding.
  • Typical injection molding tolerances range from ± 0.005 inch to ± 0.030 inch.
  • Design considerations include draft angles, uniform wall thickness, radii, and ribs/gussets.

Now that you know the basics, you can decide if injection molding is the right manufacturing process for your next plastic product design!

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