A Clear Guide for Manufacturers and Product Teams

Injection molding is one of the most efficient ways to produce plastic parts at scale. It combines material science, precise tooling, and repeatable machine control to create parts that remain consistent from one production run to the next. 

Here’s a clear look at how injection molding works and why the process continues to support everything from consumer products to infrastructure and industrial equipment.

What Is Injection Molding?

Injection molding forms molten plastic into a fixed shape using a mold, heat, pressure, and controlled cooling. Thermoplastic injection molding is widely used because it can produce high volumes of identical parts with tight dimensional accuracy. The mold can be made from steel, aluminum, epoxy, and even 3D-printed plastic.

Industries that rely on injection molding include automotive, materials handling, infrastructure, agriculture, furniture, and medical equipment.

What Is Plastic Injection Molding Best For?

• High-volume production with consistent results
• Tight dimensional tolerances
• Complex geometries and integrated features
• Automation and repeatable quality control
• Lower per-part cost at scale

 

The Injection Molding Process, Step by Step

1. Material Preparation

The process begins with resin pellets. Depending on the part’s performance requirements, these may be blended with additives, colorants, recycled content, or alternative fillers. Drying the resin removes moisture that could cause splay, bubbles, or inconsistent melt flow.

A strong materials program, including lab testing and formulation, helps ensure the resin behaves predictably once melted.

2. Melting & Plasticizing

The pellets move through a heated barrel, where a rotating screw mixes and melts them into a uniform molten material. Temperature, screw speed, and back pressure determine how the material flows and how consistently the part fills during injection.

3. Injection Into the Mold

Once the plastic is melted, the screw advances and propels molten material into the mold cavity. Pressure and injection speed must be carefully controlled so the cavity fills completely without defects like jetting, burning, or short shots.

Complex parts may use advanced methods such as multi-shot molding, co-injection, or insert molding.

4. Packing & Holding

After the cavity fills, the machine applies additional pressure to pack material into the cavity as it begins to cool. This compensates for shrinkage and helps prevent sink marks or voids in thicker areas of the part. Higher-performing molders excel here, as fine pressure control greatly impacts quality.

5. Cooling & Solidification

Cooling is often the longest part of the cycle. Internal cooling channels within the mold help draw heat out of the plastic. Uniform cooling maintains dimensional stability and reduces warpage.

Cycle times can be improved with the use of highly conductive mold materials or by utilizing conformal cooling channels that follow the natural shape of the part.

6. Mold Opening & Part Ejection

Once solid, the mold opens and the part is released using ejector pins, air assists, or other mechanisms. Features like undercuts may require lifters or side actions to remove the part safely.

Any trimming, testing, or assembly steps occur after ejection.

Key Components of an Injection Molding System

• Injection unit: Melts and pushes material into the mold.
• Clamping unit: Holds the mold closed with the required tonnage during injection.
• Mold: Contains the cavity and core that shape the part, along with runners, gates, vents, and cooling lines.
• Runner system:
  • Cold runners deliver melt but create scrap.
  • Hot runners keep plastic molten, reducing waste and improving cycle time.

 

Types of Injection Molding

• Multi-material / multi-shot molding: Combines multiple polymers in a single part.
• Co-injection: Places one material inside another (e.g., core and skin layers).
• Insert molding: Encapsulates metal or other components.
• Large-part molding: Produces oversized components requiring high clamping force.
• Thin-wall molding: Enables lightweighting, faster cycle times, and reduced material usage.
• Water injection: Creates hollow sections in thick-walled parts to save material and improve cooling efficiency.

 

Materials Used in Injection Molding

Material choice affects strength, weight, heat resistance, environmental durability, and recyclability. Common options include:

• Polypropylene
• Polyethylene
• ABS
• Nylon
• Polycarbonate
• Engineering-grade blends

 

Some manufacturers incorporate recycled plastics, bio-based fillers, or proprietary formulations validated in a materials lab.

Injection Molding vs Other Manufacturing Methods

• Vs. compression molding: Better for high volume production and complex parts with holes and other features.
• Vs. thermoforming: Offers stronger structural detail and repeatability and also better for high volume and parts requiring structural performance.
• Vs. CNC machining: More efficient for high-volume plastic parts

 

Injection molding excels when consistency, scale, and part complexity matter.

What Affects Part Quality?

• Resin composition
• Mold design (wall thickness, ribs, draft, gate placement)
• Cooling uniformity
• Press tonnage
• Processing parameters
• Tool maintenance

 

​​Custom Injection Molding With Cascade Engineering

Injection molding works best when material selection, tooling design, and machine capability are aligned from the start. Cascade Engineering supports custom injection molding programs by combining materials science, engineering support, and a wide range of press sizes to match real-world production demands.

Our teams work closely with manufacturers and product developers to evaluate part geometry, material behavior, and performance goals early in the process. This collaboration helps reduce downstream issues related to shrinkage, warpage, cycle time, and durability.

Our in-house materials lab plays a central role in this approach. By testing and formulating resins, including recycled and alternative materials, we help confirm that materials perform consistently under production conditions. That expertise is paired with a broad fleet of injection molding presses capable of producing both complex geometries and large-format parts.

Because many custom programs evolve over time, Cascade supports design refinement, tooling optimization, and process adjustments as production scales. The result is a manufacturing process built for repeatability, long-term performance, and predictable outcomes.

Ready to Talk About Your Injection Molding Project?

Whether you’re developing a new product or scaling an existing design, understanding how injection molding works is only the first step. The next is choosing a partner with the materials knowledge, press capacity, and engineering support to bring your part into production with confidence.

Connect with the Cascade Engineering team to discuss your application, timelines, and production goals. We’ll help you evaluate feasibility, material options, and manufacturing considerations before your project moves forward.

Frequently Asked Questions

How long does the injection molding process take?

Cycle times range from seconds to minutes depending on part size, material, cooling needs, and press tonnage.

How much does injection molding cost?

Tooling is typically the largest upfront cost. Part cost depends on material, cycle time, complexity, and production volume.

Is injection molding automated?

Yes. Many operations use automation for material handling, part removal, inspection, and secondary processes.

Can injection molding use recycled plastic?

Yes. Recycled resins and alternative fillers can be incorporated when properly tested and validated.

What’s different about molding large parts?

Large parts require higher clamping force, advanced cooling strategies, and specialized handling equipment.

What materials work best?

The answer depends on performance goals — stiffness, impact strength, heat resistance, or sustainability priorities.