Overview: How Injection Molding Works?

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Injection molding is a manufacturing process used to create objects from plastic and other materials. It works by melting down raw materials into a liquid which is then injected into a mold cavity. Once the material cools and solidifies, it takes on the shape of the mold. Understanding how injection molding works allows manufacturers to optimize the process, creating high quality products. This overview covers the steps and key components that make injection molding one of the most popular manufacturing processes today.

The Injection Molding Process Step-by-Step

The injection molding process can be broken down into the following steps:

1. Clamping

The clamping unit is what holds the mold closed and resists the pressure of the injection process. First, the two halves of the mold are brought together by the clamping unit and locked into place. Tie bars are used to align the mold halves and resist separation forces during injection.

2. Injection

With the mold clamped shut, the raw plastic material is melted in the injection unit. Common injection unit types include reciprocating screw and hydraulic ram configurations. The molten plastic, now a viscous liquid, is then injected into the mold cavity through the sprue. This is done at high pressure, typically between 8,000 – 30,000 psi depending on the material and part size.

3. Cooling

After being injected, the molten plastic must be cooled and solidified. Cooling is assisted by a cooling system that circulates water or another coolant through channels in the mold. The plastic shrinks during this cooling phase, allowing the part to solidify and maintain shape.

4. Ejection

Once adequately cooled, the mold opens and ejector pins push the finished plastic part out of the mold. The part then goes through any secondary processes like trimming or finishing operations. The empty mold closes and the clamping unit applies force to hold it shut as a new cycle starts.

5. Repeat

The injection molding process is cyclical. As one shot is cooling, another is injected to keep production continuous. Some processes like multi-shot injection molding have additional steps where different materials are injected into the cavity. But the core phases of clamping, injection, cooling, and ejection repeat for each part produced.

The Four Main Components of an Injection Molding Machine

There are four major components that make up a complete injection molding machine:

1. Injection Unit

This is where the raw plastic is melted and injected into the mold. The injection unit typically consists of a hopper, reciprocating screw or ram, barrel, and heating elements. Granular plastic is fed from the hopper into the heated barrel. The screw turns, melting the plastic which is then injected through the nozzle at the front.

2. Clamping Unit

Responsible for opening, closing, and holding the mold halves together during injection. This applies several tons of force which is determined by the projected area of the part and required clamping force per square inch. Hydraulic cylinders, power screws, and toggle clamps are common methods used.

3. Mold

The mold is composed of two halves that separate to eject the finished plastic part. It is CNC machined from tool steel or aluminum into the shape of the desired part. The mold also contains a sprue, runners, gates, and cooling channels. These components control the flow of plastic into the cavities during injection.

4. Control System

The brains of the operation, the control system regulates all functions including clamping pressure, injection speed and pressure, temperature, time, etc. This allows the process to be precisely tuned for the material being used and specifications of the part. Operators program the machines through touch screens and controller interfaces.

The Mold Design Process Explained

Molds are responsible for giving plastic parts their shape, so mold design is a critical step. Here are the key phases in mold design:

1. Mold Base Selection

Standard mold bases made from P20 steel or aluminum are selected that are suitable for the part size and shape. These contain pre-made cavities, sprues, water channels, ejector pins, etc. This simplifies the custom mold components that need to be made.

2. CAD Software Mold Design

With the mold base selected, the core and cavity are designed in CAD software. The part geometry is split into halves and the shutoff surfaces are defined. Components like ejector pins, gates, and cooling channels are laid out. 3D modeling gives a complete virtual representation of the mold tool.

3. CNC Machining

The CAD model is then used to CNC machine the core and cavity inserts. Harder tool steel or aluminum alloys are used which are able to withstand repeated injection molding cycles without wear. Precision machining is critical to achieve the mold’s desired shape and finish.

4. Mold Assembly

The machined cavity and core halves are assembled and inserted into the mold base along with components like ejector pins, bushings, and cooling tubes. The mold is then mounted into the injection molding machine and wiring for sensors and actuators is connected.

5. Mold Testing and Tuning

Initial test runs are conducted to ensure the mold produces full, complete parts. This may involve polishing spots, adjusting sealing alignment, or tweaking process parameters like injection pressure, clamp force, and temperature. Fine tuning ensures optimal mold performance.

Key Process Parameters That Influence Injection Molding

There are several important injection molding process parameters that determine the quality and precision of finished plastic parts:

  • Melt and Mold Temperature – The raw plastic must be heated to a melt temperature high enough to flow through thin wall sections. However, excess heat can degrade material. Mold temperature also impacts solidification with hotter molds slowing cooling.
  • Injection Pressure – Sufficient injection pressure must be applied to completely fill the mold cavity before freezing. Typical injection pressure ranges from 8,000 – 30,000 PSI depending on part size and material viscosity.
  • Fill Time – The fill time is how long the mold cavity takes to fill. Faster fill times are desirable to avoid early freezing but can cause jetting effects.
  • Packing Pressure – After initial injection, packing pressure is applied to pack more material into the mold to account for shrinkage as the plastic solidifies. Insufficient packing can lead to sink marks and voids.
  • Cooling Time – Cooling channels in the mold draw heat out from the hot plastic. Parts require sufficient cooling time before opening the mold to avoid deformation. Faster cooling times increase production rates.
  • Clamping Force – Clamping force needs to be higher than the total force exerted on the mold during injection and packing. This prevents mold halves from separating under pressure.

Common Plastic Materials Used in Injection Molding

Many different plastic materials are used in injection molding. The most common include:

  • Polypropylene (PP) – Versatile, low cost polymer used for packaging, appliances, automotive parts, and consumer products. Easy to mold with good chemical resistance.
  • Acrylonitrile Butadiene Styrene (ABS) – Amorphous thermoplastic often used for automotive trim components, electronic enclosures, and business machine housings.
  • Polyamide (Nylon) – Engineering-grade thermoplastic valued for its wear resistance, low friction, and chemical stability. Used for automotive fuel system components and appliance housings.
  • Polycarbonate (PC) – Extremely impact and temperature resistant material. Machinable grades are popular for medical equipment. Also used for transparent, shatter-resistant components.
  • Polyethylene (PE) – General purpose plastic that is resistant to chemicals and moisture. Often used to produce bottles, containers, liners, and other packaging.
  • Polyoxymethylene (POM) – Also known as acetal, it has high stiffness, low friction, and excellent dimensional stability. Used for small precision gears, pulleys, cams, and bearings.

There are many other plastic materials that can be injection molded including PVC, PET, PMMA, PBT, etc. Material selection depends on factors like cost, performance needs, production quantities, and regulatory compliance.

The Benefits of Injection Molding for Manufacturing

Here are some of the major advantages that make injection molding a popular mass production method:

  • High production rates – Once in full operation, injection molding produces parts at a very efficient rate. Automated molding machines can make hundreds of thousands of parts per year.
  • Repeatability and consistency – Molds repeatedly produce identical plastic parts cycle after cycle. Automation results in very consistent output quality.
  • Low labor costs – Heavily automated equipment reduces direct labor costs. Operators mainly load/unload parts and monitor production.
  • Material versatility – Many different plastic materials, from commodity resins to highly engineered polymers, can be injection molded.
  • Complex geometries – Molds can produce plastic parts with complex 3D shapes, internal features, thin walls, and tight tolerances.
  • Low waste – High raw material usage efficiency results in minimal waste plastic. The sprues and runners can also be reground and recycled.
  • Scalability – Injection molding production can be easily scaled up or down to meet changing market demands. Additional machines and molds can be added.

For short run prototype work, CNC machining and 3D printing have advantages. But for large production volumes, injection molding produces plastic parts with unbeatable consistency, efficiency, and cost-effectiveness.

Common Defects in Injection Molded Parts

While generally a reliable process, defects can occur with injection molding. Here are some of the most common:

  • Sink marks – Caused by insufficient packing pressure. Thin areas shrink more than thick areas resulting in visible depressions.
  • Warpage – Non-uniform cooling through the part thickness causes warpage as some sections shrink quicker or slower. Proper cooling channel design reduces warpage.
  • Burn marks – Discoloration and degradation from overheating the plastic material during injection. Adjusting process temperatures prevents burning.
  • Jetting – Fast injection speeds can erode molded surfaces and trap air bubbles. Slower fills and higher clamping pressures avoid jetting.
  • Flashing – Molten plastic leaks between the mold halves, leaving thin fins or ridges on the part. Improving mold alignment and increasing clamping force fixes flashing.
  • Short shots – When the molded part is incomplete due to early freezing before the mold cavity fills. Solutions include increasing injection pressure, temperature, and fill time.

With experience and process monitoring, potential defects can be minimized through proper injection molding machine setup and mold design optimization.

Frequently Asked Questions About Injection Molding

Here are answers to some frequently asked questions about the injection molding process and mold design:

What types of molds are used in injection molding?

There are three main mold types:

  • Two-plate molds contain a cavity and core plate for straightforward designs.
  • Three-plate molds add an ejector plate and simplify part ejection.
  • Stack molds inject multiple cavities in one machine cycle for high-volume production.

How accurate is injection molding?

Modern injection molding machines can produce plastic parts with accuracies between 0.005-0.02mm (+/- 0.0002 to 0.001 inches) depending on the configuration and setup. Proper maintenance of molds is critical to tolerances.

How many parts can you make from one mold?

Mold life varies widely by design complexity, material used, and run parameters. Short runs may yield 10,000+ parts whereas large production molds can make hundreds of thousands or millions of parts over years of operation.

What finishes are possible with injection molded parts?

As-molded finish quality depends on the condition of the mold tooling. Polished molds can produce glossy plastic parts. Textures and cosmetic features can also be machined into the mold.

How much does injection molding cost?

Injection molding costs depend on many factors like part size, complexity, material used, production volumes, labor rates, etc. Typical per-part costs range from $0.50 to $5+ for long production runs, but can be $15 – $100+ for short prototype runs.


Injection molding is able to produce precise, complex plastic parts at high production rates and relatively low costs. By melting plastic in an injection unit and injecting it into closed molds, intricate products can be formed with consistency and repeatability. The cyclic process involves clamping, injection, cooling and ejection stages. Crucial components include the mold tooling, injection unit, and clamping system. Numerous parameters and design considerations influence the quality of molded parts. When executed properly, injection molding provides an extremely efficient mass production process for plastic components. The versatility to mold diverse materials makes it a standard in manufacturing worldwide.