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Injection molded parts. Image Credit: luchschenF/Shutterstock.com

Basics of Plastic Injection Molding

Explore the injection molding process and how it works.

Xomety X
By Team Xometry
April 29, 2022
 9 min read
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Plastic injection molding is a popular manufacturing technique in which thermoplastic pellets are converted into high volumes of complex parts. The injection molding process is suitable for a variety of plastic materials and is a vital aspect of modern life—phone cases, electronic housings, toys, and even automotive parts would not be possible without it. This article will break down the basics of injection molding, describe how injection molding works, and illustrate how it is different from 3D printing.

What Are the Basics of Plastic Injection Molding?

The basics of plastic injection molding process includes creating the product design, making a tooling a mold to fit the product design, melting the plastic resin pellets, and using pressure to inject the melted pellets into the mold.

See a breakdown of each step below:

1. Creating the Product Design

Designers (engineers, mold maker businesses, etc.) create a part (in the form of a CAD file or other transferrable format), following fundamental design guidelines specific to the injection molding process. Designers should try to include the following features in their designs to help increase the success of a plastic injection mold:

  • Bosses for threaded inserts/fasteners
  • Constant or near-constant wall thicknesses
  • Smooth transitions between variable wall thicknesses
  • Hollow cavities in thick sections
  • Rounded edges
  • Draft angles on vertical walls
  • Ribs for supports
  • Friction fits, snap-fit joints, and other non-fastener joining features
  • Living hinges

Additionally, designers should minimize the following features to reduce defects in their designs:

  • Non-uniform wall thicknesses or especially thin/thick walls
  • Vertical walls with no draft angles
  • Sudden geometrical changes (corners, holes, etc.)
  • Poorly designed ribbing
  • Undercuts/overhangs

2. Making a Tooling Mold to Fit the Product Design

Highly skilled machinists and toolmakers, using the product design, fabricate a tooling mold for the injection molding machine. A tooling mold (also known as simply a tool) is the heart and soul of the injection molding machine. They are carefully designed to contain the negative cavity for the product design and additional features such as sprues, runners, gates, vents, ejector systems, cooling channels, and moving components. Tooling molds are made out of specific grades of steel and aluminum that can withstand tens of thousands (and sometimes hundreds of thousands) of heating and cooling cycles, such as 6063 aluminum, P20 steel, H13 steel, and 420 stainless steel. The mold fabrication process takes upwards of 20 weeks to complete, including both fabrication and approval, making this step the most extended aspect of injection molding. It is also the most expensive part of injection molding, and once a tooling mold is fabricated, it cannot be drastically changed without incurring additional costs.

3. Melting the Plastic Resin Pellets

After operators obtain the finished mold, it is inserted into the injection molding machine, and the mold closes, starting the injection molding cycle.

Plastic granules are fed into the hopper and into the barrel. The reciprocating screw is drawn back, allowing materials to slip into the space between the screw and the barrel. The screw then plunges forward, forcing the material into the barrel and closer to the heater bands where it melts into molten plastic. The melting temperature is kept constant as per the material specifications so that no degradation occurs in the barrel or in the mold itself.

4. Using Pressure to Inject the Melted Pellets Into the Mold

The reciprocating screw forces this melted plastic through the nozzle, which is seated within a depression in the mold known as a mold sprue bushing. The moving platen pressure fits the mold and the nozzle together tightly, ensuring no plastic can escape. The melted plastic is pressurized by this process, causing it to enter all parts of the mold cavity and displacing cavity air out through the mold vents.

FREE Injection Molding Principles E-Book

Injection Molding Machine Components

The components of an injection molding machine include hopper, a barrel, a reciprocating screw, heater(s), movable platen, a nozzle, a mold, and a mold cavity.

More information on each of the injection molding components in the list below:

  • Hopper: the opening where plastic granules are fed into the machine.
  • Barrel: the outer housing of the injection molding machine, which contains the reciprocating screw and the plastic granules. The barrel is wrapped in several heater bands and is tipped with a heated nozzle.
  • Reciprocating screw: the corkscrew component that conveys and pressurizes the plastic material as it melts through the barrel.
  • Heaters: also known as heating bands, these components provide thermal energy to the plastic granules, turning them from a solid form to a liquid. form.
  • Movable Platen: The moving component connected to the mold core that applies pressure to keep both mold halves airtight and also releases the mold core when revealing the finished part.
  • Nozzle: the heated component that provides a standard outlet for molten plastic into the mold cavity, keeping both temperature and pressure as stable as possible.
  • Mold: the component or components that contain the mold cavity and additional supporting features like ejector pins, runner channels, cooling channels, vents, etc. At a minimum, molds are separated into two halves: the stationary side (closer to the barrel) and the mold core (on the moving platen).
  • Mold Cavity: the negative space that, when filled with molten plastic, will shape it into the desired final part plus supports, gates, runners, sprues, etc.

Below is an image of typical injection molding machine diagram:

Slide 1 of 1
injection molding machine
injection molding machine
injection molding machine

Injection molding machine.
Image Credit: Wikipedia

How Does Injection Molding Work?

Once the plastic has filled the mold including its sprues, runners, gates, etc., the mold is kept at a set temperature to allow uniform solidification of the material into the part shape. A holding pressure is maintained while cooling to both stop backflow into the barrel and reduce shrinking effects. At this point, more plastic granules are added to the hopper in expectation of the next cycle (or shot). When cooled, the platen opens and allows the finished part to be ejected, and the screw is drawn back once more, allowing material to enter the barrel and start the process over again.

The injection molding cycle works by this continuous process—closing the mold, feeding/heating the plastic granules, pressurizing them into the mold, cooling them into a solid part, ejecting the part, and closing the mold again. This system allows for the rapid production of plastic parts, and upwards of 10,000 plastic parts can be made in a workday depending upon the design, size, and material.

For more information, see our guide on the basic injection molding process.

The diagram below illustrates the four essential steps of the injection molding process:

Slide 1 of 1
processing cycle injection molding process
processing cycle injection molding process
processing cycle injection molding process

Processing cycle of the conventional injection molding process.
Image Credit: Processing cycle/ResearchGate.net

How to Choose the Best Plastic for Plastic Injection Molding?

Choosing the right plastic for plastic injection molding can be difficult—there are thousands of options in the market from which to choose, many of which will not work for a given goal. Luckily, an in-depth understanding of the desired material properties and intended application will help narrow the list of potential options into something more manageable. When considering the application, it is important to keep in mind the following questions:

  • Where will the part be used?
  • How long is its operational lifespan?
  • What stresses are involved in the application?
  • Does aesthetics play a role, or is the performance of paramount importance?
  • What are the budget constraints on the application?

Similarly, the questions below are useful when determining the desired material properties:

  • What are the mechanical and chemical characteristics needed from the plastic?
  • How does the plastic behave when heating and cooling (i.e., thermal expansion and shrinkage, melting temperature range, degradation temperature)?
  • What interactions does the plastic have with air, other plastics, chemicals, etc.?

Included below is a table of the common injection molding plastics, each with its own set of advantages and general industry applications:

MaterialGeneral Industry ApplicationAdvantages
Material

Polypropylene (PP)

General Industry Application

Commodity

Advantages

- Chemical resistant, impact resistant, heat resistant, sturdy

Material

High Density Polyethylene (HDPE)

General Industry Application

Commodity

Advantages

- Chemical resistant, impact resistant, cold resistant, and sturdy

Material

Polystyrene

General Industry Application

Commodity

Advantages

- Impact resistant, moisture resistant, flexible

Material

Polyethylene (PE)

General Industry Application

Commodity

Advantages

- Leach resistant, recyclable, flexible

Material

High Impact Polystyrene (HIPS)

General Industry Application

Commodity

Advantages

- Cheap, easily formed, colorful, customizable

Material

Polyvinyl Chloride (PVC)

General Industry Application

Commodity

Advantages

- Sturdy, impact resistant, flame resistant, insulative

Material

Acrylic (PMMA, Plexiglass, etc)

General Industry Application

Engineering

Advantages

- Impregnable (glass, fiberglass, etc.), heat resistant, fatigue resistant

Material

Acrylonitrile Butadiene Styrene (ABS)

General Industry Application

Engineering

Advantages

- Sturdy, temperature resistant, colorful, chemically safe

Material

Polycarbonate (PC)

General Industry Application

Engineering

Advantages

- Impact resistant, optically clear, temperature resistant, dimensionally stable

Material

Nylon (PA)

General Industry Application

Engineering

Advantages

- Impregnable (glass, fiberglass, etc.), heat resistant, fatigue resistant

Material

Polyurethane (TPU)

General Industry Application

Engineering

Advantages

- Cold resistant, abrasion resistant, sturdy, good tensile strength

Material

Polyetherimide (PEI)

General Industry Application

Performance

Advantages

- High strength, high rigidity, dimensionally stable, heat resistant

Material

Polyether Ether Ketone (PEEK)

General Industry Application

Performance

Advantages

- Heat resistant, flame retardant, high strength, dimensionally stable

Material

Polyphenylene Sulfide (PPS)

General Industry Application

Performance

Advantages

- Excellent overall resistances, flame retardant, harsh environment resistant

Does Product Design Affect the Type of Plastic to Be Used in Plastic Injection Molding?

Yes, material selection absolutely depends on the product design, both in the application and in complexity. For example, designs calling for bendable sections/living hinges will require a strong yet flexible plastic like polypropylene. Similarly, structural parts that must be resistant to chemicals, abrasions, etc. should prioritize plastics like PEEK, Nylon, and others within these families.  

Is 3D Printing the Same as Plastic Injection Molding?

No, 3D printing is a separate set of processes from injection molding. With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part. With injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once. Both processes are considered to be "Additive Manufacturing".

In 3D Printing, Supports for overhangs and voids are also printed with the part, as well as additional features to reduce warping and other defects. 3D printing is a lengthy process, taking at minimum several hours, and can take a few days depending upon the size and complexity of the printed object. 3D printed parts are commonly produced out of thermoplastics, but other possible materials include thermosets, impregnated plastics, ceramic powder, and metal/alloy powder.

In Plastic Injection Molding, plastic injection molds come out with suitable surface quality and are essentially ready for application (with only sprues, runners, gates, and minor defects to manage). Thermoplastics are generally the preferred material used in injection molding, though newer injection molding processes can accept thermosets, impregnated materials, some metal powders, and some types of ceramics.

What Is the Difference Between 3D Printing and Injection Molding?

With 3D printing, the material is deposited in a layer-by-layer process guided by a 3D model of the desired part, while with injection molding, molten plastic is injected into a mold cavity under high pressure, creating a part all at once.

Below is a list of comparisons between 3D Printing and Injection molding using key metrics:

ParameterInjection Molding3D printing
Parameter

Materials

Injection Molding

Thermoplastics, some specialty materials in newer machines

3D printing

Thermoplastics, thermosets, metals, ceramic, impregnated materials

Parameter

Cost

Injection Molding

High initial investment cost, low cost per-part

3D printing

Medium to high initial investment cost, medium to high cost per part

Parameter

Production volume

Injection Molding

1000s-10,000s parts per workday

3D printing

1-10 parts per workday

Parameter

Lead times

Injection Molding

Slow initial lead times for mold creation, fast production lead times

3D printing

Fast overall lead times

Parameter

Part size

Injection Molding

Highly variable (both very small and large parts possible)

3D printing

Restricted by build space, parts are generally no bigger than a cubic meter

Parameter

Waste

Injection Molding

Low/no waste, and waste can be reground back into stock

3D printing

High waste output

Parameter

Design changes

Injection Molding

Difficult, changes are slow

3D printing

Very easy, rapid changes possible

Summary

This article presented the basics of injection molding to explain how it works, the materials that can be used, and how the process differs from other techniques such as 3D printing. We hope this article helped readers understand this manufacturing process and how it can add immense value to manufacturing capacity, given a few key considerations.

Xometry offers a full range of injection molding capabilities to help with your production needs. Visit our website to explore the full range of our capabilities or to request a free, no-obligation quote.

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information. 

Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.