The Basics of Injection Molding – All You Need to Know

Injection molding, also known as injection molding, is a prevalent manufacturing technique utilized in industries such as aerospace, automotive, medical, and consumer products. This article provides answers to frequently asked questions about injection molding.

What is injection molding?

Injection molding is a method of producing parts by injecting material into a mold. Metals (for which the process is known as die-casting), glasses, elastomers, confections, and, most commonly, thermoplastic and thermosetting polymers can all be used in injection molding. The part’s material is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the cavity’s configuration. After a product is designed, usually by an industrial designer or engineer, molds are made from metal, usually steel or aluminum, and precision-machined to form the desired part’s features. 3D printing materials like photopolymers which do not melt during the injection molding of some lower-temperature thermoplastics can be used for some simple injection molds. Injection molding is widely used for producing a wide range of parts, from very small to very large. The ability to produce parts with varying geometrical shapes and sizes is determined by the type of machine used in the operation.

The history of injection molding

Injection molding equipment originates from metal die casting processes. For example, John Wesley Hyatt is credited with inventing a patent for the first plastic injection molding machine. This patent was granted in August 1877. However, the origins of injection molding date back to 1872 when Brothers Francis and John Downing invented and patented the bottle-making machine that produced embossed bottles from a continuous sheet of glass. This technology made it possible to create seamless bottles that could not be created with traditional blow-molding techniques.

How does plastic injection molding work?

Injection Molding takes plastic resin, heats it up, and forces it into a mold. The mold is usually made of steel or aluminum. The mold is made in two halves, called the A-side, and B-side. The A-side is attached to the injection unit on the injection machine. When the molten plastic comes out of the barrel of the machine, it fills the mold and then hardens.

Once the plastic has hardened in the mold, both sides are opened and the part is ejected. The process starts again when you close the mold back up and pressurize it to push more plastic in.

The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

What are the benefits of plastic injection molding?

#1 Very high production rates

One of the most appealing characteristics of plastic injection molding as opposed to other processes is its very high production rates. Depending on the complexity of the design, the size of the part being molded, and other factors, individual molds can produce hundreds or even thousands of finished parts per hour. This allows manufacturers to keep costs low, while still benefiting from a fast turnaround time for their products.

#2 High tolerance precision

With the use of this process, you can produce high tolerance precision parts. A mold is used to create the shape of your part. The molds are held to very close tolerances by design. For example, if you need to make a thousand identical parts, you can use the same mold and they will all be exactly the same size and shape.

#3 Low labor costs

Compared to other manufacturing processes, plastic injection molding is relatively labor-free once a mold has been manufactured. Many companies that offer injection molding services are able to produce large quantities at a very low price point because they do not need to hire much labor for this type of work. This allows them to remain competitive in today’s market and keep prices low for consumers.

#4 Environmentally friendly

When compared to other manufacturing processes like CNC machining, which creates a lot of waste by cutting away at raw material, plastic injection molding is a much more eco-friendly solution.

#5 Very durable

Another great benefit of plastic injection molding is the ability to create very durable products that do not scratch or break easily. You can also choose from different types of plastics based on your product’s needs, like an impact-resistant or heat-resistant plastic.

#6 Variety of plastics available for choices

There are a variety of plastic resin material options to choose from for use in the plastic injection molding process. Each material has its own unique properties; therefore, understanding the differences between them is crucial to ensure selecting the most suitable material for your intended application.

What are the typical materials for plastic injection molding?

Injection-molded parts can be made from a variety of thermoplastic materials including ABS, nylon, polycarbonate (PC), polyoxymethylene (POM), polypropylene (PP), polystyrene (PS), polyurethane (PUR), thermoplastic elastomers (TPE) and TPU.

#1 Acrylonitrile Butadiene Styrene(ABS)

ABS is a commonly used plastic injection molding material with three main ingredients: acrylonitrile, butadiene, and styrene. Each of these monomers imparts specific properties and provides ABS terpolymer with a robust combination of features. ABS offers high strength, toughness, and resistance to impact and temperature. It is easily molded and gives a high-quality glossy surface finish. This plastic polymer does not have a specific melting point.

#2 Polycarbonate (PC)

A polycarbonate is a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonate has a high degree of stiffness and thermal resistance due to its molecular structure and a reasonably high viscosity when processed. Even so, polycarbonates can be molded and thermoformed with great ease, making them a popular choice for a wide range of products.

#3 Polyoxymethylene (POM)

Polyoxymethylene also known as polyacetal/acetal/polyformaldehyde/Delrin, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas. POM is characterized by its high strength, hardness, and rigidity to − 40°C.

#4 Polypropylene (PP)

Polypropylene also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and is partially crystalline and non-polar. PP is inexpensive and easy to access, and due to flexible consistency, PP is widely used for manufacturing storage containers, such as bottles and plastic boxes. Polypropylene has a strong resistance to fatigue and chemical corrosion, making it useful for most types of plastic storage containers, kitchenware, water bottles, and even insulation and piping systems.

#5 Nylon Plastic (PA)

Nylon plastic (PA) is a synthetic thermoplastic polymer commonly used in injection molding applications. It’s a versatile, durable, flexible material often used as a more affordable alternative to other materials like silk, rubber, and latex.

#6 Acrylic (PMMA)

Acrylics are a group of polymers prepared from acrylate monomers. These plastics are noted for their transparency, resistance to breakage, and elasticity. They are also commonly known as acrylate polymers or polyacrylates. Acrylate polymer is commonly used in cosmetics, such as nail polish, as an adhesive. The most common acrylic plastic is polymethyl methacrylate (PMMA).

#7 Polyethylene (PE)

Polyethylene thermoplastic materials are generally divided into multiple groups, based on density. These include low-density polyethylene (or LDPE), medium density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHMWPE or UHMW). In general, the higher the density, the higher the tensile and flexural strength, chemical and abrasion resistance, and surface hardness.

#8 High Impact Polystyrene (HIPS)

This cost-efficient material offers excellent machinability, dimensional stability, impact resistance, and aesthetic properties. It is highly customizable and can be glued, printed, bonded, and decorated with ease.

What are the surface finishes for injection molding?

It’s possible to give an injection-molded object a distinct look or feel by applying surface finishes. For more than just decorative reasons, surface finishes can also be used for practical objectives. In the case of sliding elements like plain bearings, the average surface roughness (Ra) can have a significant impact on the lifetime.

The mold itself can be polished to varying degrees, but injection molded products are rarely post-processed.

For this reason, it is necessary to use a draft angle that is more than 90 degrees when fabricating parts with rough surface finishes.

The Society of Plastics Industry (SPI)explains several standard finishing procedures that result in different part surface finishes.

Finish	Description	SPI standards*	Applications
Glossy finish	The mold is first smoothed and then polished with a diamond buff, resulting in a mirror-like finish.	A-1 A-2 A-3	Suitable for parts that require the smoothest surface finishfor cosmetic or functional purposes (Ra less than 0.10 μm). The A-1 finish is suitable for parts with mirror-like finish and lenses.
Semi-gloss finish	The mold is smoothed with fine-grit sandpaper, resulting in a fine surface finish.	B-1 B-2 B-3	Suitable for parts that require a good visual appearance, but not a high glossy look.
Matte finish	The mold is smoothed using fine stone powder, removing all machining marks.	C-1 C-2 C-3	Suitable for parts with low visual appearance requirements, but machining marks are not acceptable.
Textured finish	The mold is first smoothed with fine stone powder and then sandblasted, resulting in a textured surface.	D-1 D-2 D-3	Suitable for parts that require a satin or dull texturedsurface finish.
As-machined finish	The mold is finished at the machinist’s discretion. Tool marks will be visible.	–	Suitable for non-cosmetic parts, such as industrial or hidden components.

Design tips for injection molding parts

Designing parts for injection molding can be simple, but there are a few basic rules you should follow to avoid common pitfalls and make sure your part is made in the best way possible.

To design parts for injection molding, try to keep these rules in mind:

#1 Uniform wall thickness

Wall thickness should be no less than 4mm (0.16″) and no greater than 8mm (0.31″). Part walls thinner than 4mm (0.16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. Walls thicker than 8mm (0.31″) will require more material and longer cooling times, which can lead to warpage or increased cycle times.

A wall thickness between 1.2 mm and 3 mm is a safe value for most materials.

Below are the recommended wall thicknesses for some of the most commonly used injection molding materials.

Material	Recommended wall thickness [mm]	Recommended wall thickness [inches]
Polypropylene (PP)	0.8 – 3.8 mm	0.03” – 0.15”
ABS	1.2 – 3.5 mm	0.045” – 0.14”
Polyethylene (PE)	0.8 – 3.0 mm	0.03” – 0.12”
Polystyrene (PS)	1.0 – 4.0 mm	0.04” – 0.155”
Polyurethane (PUR)	2.0 – 20.0 mm	0.08” – 0.785”
Nylon (PA 6)	0.8 – 3.0 mm	0.03” – 0.12”
Polycarbonate (PC)	1.0 – 4.0 mm	0.04” – 0.16”
PC/ABS	1.2 – 3.5 mm	0.045” – 0.14”
POM (Delrin)	0.8 – 3.0 mm	0.03” – 0.12”
PEEK	1.0 – 3.0 mm	0.04” – 0.12”
Silicone	1.0 – 10.0 mm	0.04” – 0.40”

#2 Round all edges and corners

The uniform wall thickness limitation also applies to edges and corners: the transition must be as smooth as possible to ensure good material flow. For interior edges, use a radius of at least 0.5 x the wall thickness. For exterior edges, add a radius equal to the interior radius plus the wall thickness. This way you ensure that the thickness of the walls is constant everywhere (even at the corners). Wherever possible, round all corners to increase both the looks and the durability.

#3 Add draft angles
To make the ejection of the part from the mold easier, a draft angle must be added to all vertical walls. Walls without a draft angle will have drag marks on their surface, due to the high friction with the mold during ejection. A minimum draft angle of 2° is recommended. Larger draft angles (up to 5 °) should be used on taller features.

#4 Take care of the ribs

Ribs serve as structural features that help maintain the part’s overall stability. They are thin wall protrusions that extend perpendicularly from a wall or plane. Adding ribs rather than thicker walls will offer greater structural support.

#5 Avoid undercuts if possible

Injection molds are made up of two halves, which means they have parting lines and a “blindside” that cannot be reached by the injection nozzle or runner system.

#6 Transition from thick to thin

Parts will form better if plastic flows through features moving from greater to lesser wall thickness starting from the gate(s) (where the plastic first flows in to fill the part).

Besides, injection molding part design is not just creating an injection molded part that functions in its environment, but one that will delight your customer. It’s a given that you need to understand what it is you’re trying to design for manufacturability — but taking the step further, here are some suggestions on how to make your part stand out:

1. Think in terms of functional design

2. Make it easy to assemble

3. Make it easy to manufacture

4. Minimize the cost of manufacturing

5. Follow the design for manufacturability and assembly (DFMA) principles

Start Injection molding with SEANC

Nothing compares to injection molding as a mass-production process. It is an efficient way to produce large quantities of parts ranging from a few grams to several kilograms. The injection molding process requires an injection molding machine, raw plastic material, and a machined mold. As a manufacturing process, injection molding is one of the most versatile ways to produce plastic parts. It allows you to create complex shapes at low costs; however, it can be challenging to get your product off the ground with an injection molder without first investing in tooling.