Injection Molding Guide

The basics

What is a injection molding? How does it work and what is it used for?

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Injection Molding Guide

The basics

What is a injection molding? How does it work and what is it used for?

In this section, we answer these questions and show you common examples of injection molded parts to help you familiarize yourself with the basic mechanics and applications of the technology.

injection molding basic guide

What is injection molding?

Injection molding is a manufacturing technology for the mass-production of identical plastic parts with good tolerances. In Injection Molding, polymer granules are first melted and then injected under pressure into a mold, where the liquid plastic cools and solidifies. The materials used in Injection Molding are thermoplastic polymers that can be colored or filled with other additives.

Almost every plastic part around you was manufactured using injection molding: from car parts, to electronic enclosures, and to kitchen appliances.

Injection molding is so popular, because of the dramatically low cost per unit when manufacturing high volumes. Injection molding offers high repeatability and good design flexibility. The main restrictions on Injection Molding usually come down to economics, as high initial investment for the is required. Also, the turn-around time from design to production is slow (at least 4 weeks).

The injection molding process

IM 101 - Injection molding process

Injection molding is widely used today for both consumer products and engineering applications. Almost every plastic item around you was manufactured using injection molding. This is because the technology can produce identical parts at very high volumes (typically, 1,000 to 100,000+ units) at a very low cost per part (typically, at $1-5 per unit).

But compared to other technologies, the start-up costs of injection molding are relatively high, mainly because custom tooling is needed. A mold can cost anywhere between $3,000 and $100,000+, depending on its complexity, material (aluminum or steel) and accuracy (prototype, pilot-run or full-scale production mold).

All thermoplastic materials can be injection molded. Some types of silicone and other thermoset resins are also compatible with the injection molding process. The most commonly used materials in injection molding are:

  • Polypropylene (PP): ~38% of global production
  • ABS: ~27% of global production
  • Polyethylene (PE): ~15% of global production
  • Polystyrene (PS): ~8% of global production

Even if we take into account all other possible manufacturing technologies, injection molding with these four materials alone accounts for more than 40% of all plastic parts produced globally every year!

A brief history of Injection molding

IM101 US114945 Patents for Injection Moulding by John Wesley Hyatt Diagram

Plastics replace ivory

In 1869, John Wesley Hyatt invented celluloid, the first practical artificial plastic intended to replace ivory for the production of... billiard balls! Early injection molding machines used a barrel to heat up the plastic and a plunger to inject it to the mold.

IM101 history Reciprocal screw

A revolutionary invention

In the mid 1950s, the invention of the reciprocating screw single-handedly revolutionized the plastics industry. The reciprocating screw solved key issues with uneven heating of the plastic that previous systems faced, and opened up new horizons for the mass production of plastic parts.

LEGO Sustainable Bricks Plants

Injection molding today

Today, injection molding is a $300 billion market. 5+ million metric tons of plastic parts are produced with injection molding globally each year. Recently, the demand of biodegradable materials is increasing for environmental reasons.

Injection molding machines: how do they work?

An injection molding machine consists of 3 main parts: the injection unit, the mold - the heart of the whole process - and the clamping/ejector unit.

In this section, we examine the purpose of each of these systems and how their basic operation mechanics affect the end-result of the Injection molding process.

Watch a large injection molding machine in action while producing 72 bottle caps every 3 seconds in the video here:

The injection unit

IM101 - Injection molding schematic

The purpose of the injection unit is to melt the raw plastic and guide it into the mold. It consists of the hopper, the barrel, and the reciprocating screw.

Here is how the injection molding process works:

  • The polymer granules are first dried and placed in the hopper, where they are mixed with the coloring pigment or the other reinforcing additives.
  • The granules are fed into the barrel, where they are simultaneously heated, mixed and moved towards the mold by a variable pitch screw. The geometry of the screw and the barrel are optimized to help build up the pressure to the correct levels and melt the material.
  • The ram then moves forwards and the melted plastic is injected into the mold through the runner system, where it fills the whole cavity. As the material cools down, it re-solidifies and takes the shape of the mold.
  • Finally, the mold opens and the now solid part is pushed out by the ejector pins. The mold then closes and the process repeats.

The whole process can be repeated very fast: the cycle takes approximately 30 to 90 seconds depending on the size of the part.

After the part is ejected, it is dispensed on a conveyor belt or in a holding container. Usually, injection molded parts are ready to use right away and require little to no post-processing.

Manufacturing the mold

The mold is like the negative of a photograph: its geometry and surface texture is directly transferred onto the injection molded part.

It usually makes up the largest portion of the start-up costs in injection molding: the cost of a typical mold starts at approximately $2,000-5,000 for a simple geometry and relatively small production runs (1,000 to 10,000 units) and can go upwards to $100,000 for molds optimized for full-scale production (100,000 units or more).

This is due to the high level of expertise required to design and manufacture a high-quality mold that can produce accurately thousands (or hundreds of thousands) of parts.

Molds are usually CNC machined out of aluminum or tool steel and then finished to the required standard. Apart from the negative of the part, they also have other features, like the runner system that facilitates the flow of the material into the mold, and internal water cooling channels that aid and speed up the cooling of the part.

Learn more about CNC machining. Read the complete engineering guide

Recent advances in 3D printing materials have enabled the manufacturing of molds suitable for low-run injection molding (100 parts or less) at a fraction of the cost. Such small volumes were economically unviable in the past, due to the very high cost of traditional mold making.

An industrial mold design for producing a tens of thousands of parts number of plastic parts

An industrial mold design for producing a tens of thousands of parts number of plastic parts. The cavity is show on the left and the core on the right.

The anatomy of the mold

IM 101 - Schematic of a mold for Injection molding

The simplest mold is the straight-pull mold. It consist of 2 halves: the cavity (the front side) and the core (the back side).

In most cases, straight-pull molds are preferred, as they are simple to design and manufacture, keeping the total cost relatively low. There are some design restrictions though: the part must have a 2.D geometry on each side and no overhangs (i.e. areas that are not supported from below).

If more complex geometries are required, then retractable side-action cores or other inserts are required.

Side-action cores are moving elements that enter the mold from the top or the bottom and are used to manufacture parts with overhangs (for example, a cavity or a hole). Side-actions should be used sparingly though, as the cost increases rapidly.

Interesting fact: About 50% of the typical injection molding cycle is dedicated to cooling and solidification. Minimizing the thickness of a design is key to speed up this step and cuts costs.

The 2 sides of the mold: A side & B side

Injection molded parts have two sides: the A side, which faces the cavity (front half of the mold) and the B side, which faces the core (back half of the mold). These two sides usually serve different purposes:

  • The A side usually has better visual appearance and is often called the cosmetic side. The faces on the A side will be smooth or will have a textured according to your design specifications.
  • The B side usually contains the hidden (but very important) structural elements of the part (the bosses, ribs, snap-fits and so on). For this reason it is called the functional side. The B side will often have a rougher finish and visible marks from the ejector pins.
IM101 machines runner system

Injecting material into the mold: The runner system

The runner system is the channel that guides the melted plastic into the cavity of the mold. It controls the strong{flow and pressure} with which the liquid plastic is injected into the cavity and it is removed after ejection (it snaps off). The runner system usually consists of 3 main sections:

  • The sprue is the main channel in which all the melted plastic initially flows through as it enters the mold.
  • The runner spreads the melted plastic along the face where the two halves of the mold meet and connects the spur to the gates. There may be one or more runners, guiding the material towards one or multiple parts. The runner system is cut off from the part after ejection. This is the only material waste in injection molding, 15-30% of which can be recycled and reused.
  • The gate (is the entry point of the material into the cavity of the mold. Its geometry and location is important, as it determines the flow of the plastic.

Different gates types are suitable for different applications. There are 4 types of gates used in injection molding:

  • Edge gates inject material at the parting line of the two halves of the mold and are the most common gate type. The runner system has to be removed manually later, leaving a small imperfection at the injection point.
  • Tunnel gates inject material below the parting line. The runner system snaps off as the part is ejected from the mold, eliminating the need for manual removal. This makes this type of gate ideal for very large volumes.
  • Post gates inject the material from the backside of the cavity, hiding the small imperfection left from breaking the other gate types. These gates are used for parts that require excellent visual appearance.
  • Hot tips are directly connected to the spur and inject plastic from the top side of the part. No material is wasted this way on the runner system making them ideal for large scale production, but a dimple will be visible at the injection point.
IM101 hinge packers lego

The vestige

At the point where the runner system connected with the part, a small imperfection is usually visible, called the vestige.

If the presence of the vestige is not desirable for aesthetic purposes, then in can also be "hidden" in the functional B-side of the part.

The clamping and ejection system

On the far side of an injection molding machine is the clamping system. The clamping system has a dual purpose: it keeps the 2 parts of the mold tightly shut during injection and it pushes the part out of the mold after it opens.

After the part is ejected, it falls onto a conveyor belt or a bucket for storage and the cycle starts over again.

Alignment of the different moving parts of the mold is never perfect though. This causes the creation of 2 common imperfections that are visible on almost every injection molded part:

  • Parting lines which are visible on the side of a part where the 2 halves of the mold meet. They are caused by tiny misalignments and the slightly rounded edges of the mold.
  • Ejector (or witness) marks which are visible on the hidden B-side of the part. They are created because the ejector pins are slightly protruding above or indented below the surface of the mold.

The image below shows the mold used to manufacture both sides of the casing for a remote controller. Quick quiz: try to locate the core (A-side), the cavity (B-side), the runner system, the ejector pins, the side-action core and the air vents on this mold.

IM 101 - Example mold with visible parting line, ejector pins, runner system, cavity, core and side-action core

IM 101 - Example mold with visible parting line, ejector pins, runner system, cavity, core and side-action core

Benefits and limitations of injection molding

Injection molding is an established manufacturing technology with a long history, but it's constantly being refined and improved with new technological advancements.

Below is a quick rundown of the key advantages and disadvantages of injection molding to help you understand whether it's the right solution for your application.

Benefits of injection molding

Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost.

The recommended minimum production volume for injection molding is 500 units. At this point economies of scale start to kick-in and the relatively high initial costs of tooling have a less prominent effect on the unit price.

Almost every thermoplastic material (and some thermosets and silicones) can be injection molded. This gives a very wide range of available materials with diverse physical properties to design with.

Parts produced with injection molding have very good physical properties. Their properties can be tailored by using additives (for example, glass fibres) or by mixing together different pellets (for instance, PC/ABS blends) to achieve the desired level of strength, stiffness or impact resistance.

The typical injection molding cycle lasts 15 to 60 seconds, depending on the size of the part and the complexity of the mold. In comparison, CNC machining or 3D printing might require minutes to hours in order to produce the same geometry. Also, a single mold can accomodate multiple parts, further increasing the production capabillities of this manufacturing process.

This means that hundreds (or even thousands) of identical parts can be produced every single hour.

The injection molding process is highly repeatable and the produced parts are essentially identical. Of course, some wear occurs to the mold over time, but a typical pilot-run aluminum mold will last 5,000 to 10,000 cycles, while full-scale production molds from tool steel can stand 100,000+ cycles.

Typically, injection molding will produce parts with tolerances of ± 0.500 mm (0.020’’). Tighter tolerances down to ± 0.125 mm (0.005’’) are also feasible in certain circumstances. This level of accuracy is enough for most applications and comparable to both CNC machining and 3D printing.

A key strength of injection molding is it can produce finished products that need little to no extra finishing. The surfaces of the mold can be polished to a very high degree to create mirror-like parts. Or they can be bead blasted to create textured surfaces. The SPI standards dictate the level of finishing that can be achieved.

Limitations of injection molding

The main economic restriction of injection molding the the high cost of tooling. Since a custom mold has to be made for each geometry, the start-up costs are very high. These are mainly related to the design and manufacturing of the mold which typically costs between $5,000 and $100,000. For this reason, injection molding is only economically viable for productions larger than 500 units.

After a mold is manufactured, it's very expensive to modify. Design changes usually require the creation of a new mold from scratch. For this reason, correctly designing a part for injection molding is very important.

In Part 2, we list the most important design considerations to keep in mind while designing for injection molding. It Part 5, we'll also see how you can mitigate the risk by creating physical prototypes of your parts.

The typical turnaround for injection molding varies between 6-10 weeks. 4-6 weeks to manufacture the mold, plus 2-4 more weeks for production and shipping. If design changes are required (something quite common) the turnaround time increases accordingly.

In comparison, parts made in a desktop 3D printer can be ready for delivery overnight, while industrial 3D printing systems have a typical lead time of 3-5 days. CNC machined parts are typically delivered within 10 days or as fast as 5 days.

Examples of products made with injection molding

If you look around you right now, you'll see at least a few products that were manufactured with injection molding. You're probably looking at one right now actually: the casing of the device you are using to read this guide.

To recognize them, look out for these 3 things: a parting line, witness marks on the hidden side and a relatively uniform wall thickness throughout the part.

We've collected some examples of products commonly manufacturing with injection molding to help get a better understanding of what can be achieved with this manufacturing process.

Lego bricks

Lego bricks are one of the most recognizable examples of injection molded parts. They're manufactured using molds, like the one in the picture, which produced 120 million lego bricks (that's 15 million cycles) before it was taken out of commission.

The material used for Lego bricks is ABS because of its high impact resistance and excellent moldabillity. Every single brick has been designed to perfection, achieving tolerances down to 10 micro meters (or a tenth of a human hair).

This is partly achieved by using the best design practices, which we'll examine in the next section (uniform wall thickness, draft angles, ribs, embossed text etc.).

A retired Lego brick mold

Bottle caps

Many plastic packaging products are injection molded. In fact, packaging is the largest market for injection molding.

For example, bottle caps are injection molded from Polypropylene. Polypropylene (PP) has excellent chemical resistance and is suitable to come in contact with food products.

On bottle caps, you can also see all the common unavoidable injection molding imperfections (parting line, ejector marks etc.) and common design features (ribs, stripping undercuts etc.).

IM101 Bottle caps

Model airplanes

Model airplanes are another common example of injection molded parts. The material used here is mostly Polystyrene (PS), for its low cost and ease of molding.

What's interesting with model airplane kits is that they come with the runner system still attached. So, you can see the path the melted plastic followed to fill the empty mold.

IM101 model airplanes

Car parts

Almost every plastic component in the interior of a car was injection molded. The 3 most common injection molding materials used in the automotive industry are Polypropylene (PP) for non-critical parts, PVC for its good weather resistance and ABS for its high impact strength.

More than half of the plastic parts of a car are made from one of these materials, including the bumpers, the interior body parts and the dashboards.

IM101 automotive interior

Consumer electronics

The enclosures of almost every mass-produced consumer electronic device was injection molded. ABS and polystyrene (PS) are prefered here for their excellent impact resistance and good electrical insulation.

IM101 Apple Lightning connect

Medical devices

Many sterilizable and biocompatible materials are available for injection molding.

Medical grade silicone is one of the more popular materials in the medical industry. Silicone is a thermoset though, so special machinery and process control are required, increasing the cost.

For applications with less strict requirements other materials, like ABS, polypropylene (PP) and Polyethylene (PE), are more common.

IM101 medical syringe