Injection molding is a manufacturing process for producing parts in large volumes. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession. Why would you use injection molding? That is what this article is all about. Injection Molding uses a plunger to force plastic (or metal) through a nozzle into a mould cavity, it then cools and hardens to the shape of the mould. Hey everyone, today we are going to be looking at injection molding and see why it’s such an important process within modern day manufacturing.

How Does Injection Molding Work?

The process of injection moulding consists of several steps that all work together to produce a high-quality part that has incredible accuracy:

1. Clamping

Securing the mold cavity and core halves within the injection molding machine. So, how is this done? The answer is that they are secured using a clamping unit within the injection molding machine that locks the mold in place so it can resist the pressures from molten plastic during injection.

2. Injection

Plastic granules or pellets are fed from a hopper (a funnel-shaped container) into the heated barrel of the machine. They are melted and homogenized in the barrel by the rotating action of an auger (a type of screw). The molten plastic is then injected into the mold through a nozzle, under pressure using a reciprocating screw or a plunger. After it fills the mold, the plastic is packed down until it completely fills all areas of the mold cavity or cavities.

3. Packing and Holding

After the mould is filled, pressure is kept constant (holding phase) for a period of time to compact the material in the mould to compensate for shrinkage as the plastic begins to cool. This step is important to obtain dimensional accuracy in the part and prevent sink marks or voids at locations requiring high packing.

4. Cooling

Once the plastic has filled the cavity, it must cool and solidify so that it takes the shape of the hollow cavity. There are typically cooling channels within the walls of the mold which circulate a liquid coolant to help remove heat from the part, speeding up its cooling rate. The amount of time required for a part to cool within a mold will depend on various factors such as wall thickness, material type and mold design but it’s also an important factor in determining how long each part will take to produce and how easily it can be ejected from the tool.

5. Ejection

After the part has cooled and solidified, the mould opens and an ejection system usually made up of ejector pins or air blasts pushes, or in some cases pulls, the part out of the mould. This step must be engineered carefully so that the part is not damaged and ejects successfully.

6. Repeat Cycle

After the required curing time, the mold opens and the part is ejected. The mold closes and the process is repeated. Production of parts in most rotational molding operations is relatively slow compared with some other plastics processes using different molding techniques.

Materials Used in Injection Molding

Several types of materials can be used in injection molding, and the choice is usually based on the product’s requirements. Thermoplastics and thermosetting polymers are the two most commonly used materials for injection molding.

Thermoplastics

They are the most common materials for injection molding because they are versatile, recyclable, and have easy processing. Some of the most common thermoplastics include:

• Polypropylene (PP): Lightweight, flexible, moisture and chemical resistant. It’s used in packaging, automotive parts and medical devices.
• Polyethylene (PE): High resistance to impact and durability. It can be found in containers, bottles or toys that are manufactured.
• Acrylonitrile Butadiene Styrene (ABS): Products made from ABS are tough, highly impact resistant and with good surface appearance. Used in making electronics housings or automotive parts.
• Polycarbonate (PC): This material is transparent, strong and heat resistant. It’s used in applications such as lenses, safety gear, and enclosures.
• Nylon (PA): Nylon is a strong and abrasion resistant plastic that’s used for gears, bearings, and mechanical parts.
• Polystyrene (PS): It’s cheap and easy to form. PS is used for disposable cutlery, CD cases, and food packaging.
• Polyvinyl Chloride (PVC): It’s most commonly used in plumbing fittings, but would also likely be high on the list for window profiles.

Thermosetting Plastics

These are the materials which on curing forms irreversible chemical bonds and hence they become heat resistant and dimensionally stable. E.g.

• Epoxy Resins: Epoxy resins have excellent adhesion, chemical resistance and dimensional stability. Therefore, they are used in a variety of applications such as aerospace and electronics.
• Phenolic Resins: Phenolic resins are known for their high heat resistance and insulation properties. Due to these characteristics, phenolics are best suited for electrical applications as well as automotive component manufacturing.
• Melamine Formaldehyde: Just like urea formaldehyde, melamine formaldehyde is also widely used to create a durable thermosetting plastic material. It possesses excellent heat- and fire-resistant properties and doesn’t conduct electricity either. This makes it extremely well-suited for electrical insulation applications such as outlet and switch covers.

Elastomers

Elastomers are rubber-like materials which are flexible and used for products needing elasticity:

• Thermoplastic Elastomers (TPE): A mix of rubber with thermoplastics, often used in seals, grips and components requiring soft feel.
• Silicone Rubber: Highly flexible and heat-resistant. Used in medical devices, bakeware, and gaskets.

Advanced and Specialized Materials

For high-performance applications, materials beyond standard plastics may be used:

• Filled Plastics: Thermoplastics with glass fibers or minerals to increase the strength and stiffness.
• High Performance Polymers: Like PEEK or PSU, for aerospace, medical, and high-temperature environments.
• Metal Injection Molding (MIM): Metal powders are combined with polymer binders to form small and complex metal parts.
• Ceramic Injection Molding (CIM): It is used in the production of high-precision ceramic components with very good wear and thermal resistance.

Mechanical performance, appearance, regulatory requirements, cost and environmental exposure are among the factors that determine the suitability of a given material.

Types of Injection Molding

There are different types of injection molding which are based on what you want to achieve with your product. There is flexibility with this part of the design process because you will likely not be the one actually injecting the plastic, but it is good to be familiar with each type based on your intended end product.

1. Traditional Injection Molding

This is the most common type of molding used in the thermoplastic industry due to its relative low cost and ability to produce high volume. It can be used for both simple, easy to mold parts and also complex, intricate designs with precise tolerances.

2. Over molding

Over molding is a process of molding one material over another to form single component with the multiple materials. It is widely used for making soft-touch grips on tools or toothbrushes by molding soft material over rigid material, thus increases product functionality, ergonomics and aesthetics.

3. Insert Molding

Insert molding is a process that molds metal or other substance parts into a plastic part. The process creates a single multi-component part by partially molding the insert, and then plastic is injected and allowed to solidify, forming the desired shape around the insert.

4. Gas-Assisted Injection Molding

This process uses nitrogen gas during the injection to form hollow areas of the part. Nitrogen gas applies outward pressure on molten plastic against the mold wall, thus reducing material content, weight and warpage. Gas-assisted injection molding is most useful for making large, solid articles like handles or monitors covers.

5. Thin-Wall Injection Molding

This is used for the production of parts with thin walls. The wall thickness is generally less than 1mm. It helps to produce light weight parts as material consumption is reduced. Cycle time is also reduced due to faster cooling of the molten material in contact with mold surface. Thin-wall injection molding applications include food packaging, mobile phone casings and medical housing etc.

6. Liquid Silicone Rubber (LSR) Molding

LSR molding is ideal for manufacturing soft, flexible, durable and heat-resistant parts. The liquid silicone rubber injection molding process uses a machine to mix the silicone rubber with catalyst and additives. This two-component mixing ratio creates cross-linking that cures the material over time by adapting new shape from the mold cavity.

7. Micro Injection Molding

Micro injection molding is meant for producing very small parts with extremely tight tolerances, generally less than 0.5 gram in weight. This is essential for industries like electronics, medical devices, and micro-mechanics where miniaturization and accuracy are paramount.

Advantages of Injection Molding

Injection molding has emerged as a dominant method in the manufacturing of plastic products for many industries. Its efficiency, cost-effectiveness and precision have made it an ideal choice for mass production of identical plastic parts at a high rate. In this post we will discuss more about the advantages of injection molding.

1. High Efficiency

Automation is an inherent part of injection molding; therefore, the production speed of the process is usually very high. With injection molding you can produce a huge number of parts within a short period.

2. Low Cost per Part

Due to its repeatable and consistent tolerances plus high productivity, injection molding proves to be extremely economical when utilized in mass production.

3. Design Flexibility

The flexibility offered by this process considerably simplifies designing various complex features such as slots, cavities and screw threads among others that too in one pass with perfect accuracy.

4. Reduced Labor Costs

With automation being built into the plastic injection mold tool design itself, it becomes capable enough to provide ready-to-use molded components continually without any manual intervention which ultimately reduces operational cost.

5. Environmentally Friendly (less material wasted)

Customized gating and runner systems help conserve costly resin thereby minimizing wastage which not only saves money but also makes the process environment-friendly.

6. Low Waste and Material Efficiency

Injection molding is highly material-efficient, especially with thermoplastics, which can be reground and reused.

• Gating and runner systems can be optimized to minimize scrap.
• Excess material is often recyclable, reducing environmental impact and costs.

➡️ This aligns with sustainable manufacturing practices and contributes to lower total material consumption.

7. High Strength and Durability

• Parts can be engineered to offer excellent mechanical performance by:
• Using structural design features such as gussets, ribs, and cross-sections
• Selecting materials with high tensile strength or impact resistance
• Adding reinforcing fibers (glass, carbon)

➡️ These techniques make injection molded components ideal for load-bearing or high-stress applications such as automotive interiors or industrial equipment.

8. High Efficiency and Fast Production

• One of the most significant benefits of injection molding is its ability to produce an enormous quantity of parts in a relatively short period of time.
• After the mold is created and everything has been set up, the time it takes to produce parts in seconds is as follows: 30-60 seconds part complexity/number of parts being manufactured per cycle.
• Ideal for high volume production runs, which saves on time and cost per unit.

➡️ This is why plastic injection molding is perfect for so many industries including automotive, consumer goods, and packaging.

Conclusion

Injection molding is arguably the most popular and versatile manufacturing process currently in use in the plastics industry. With its ability to manufacture high precision and extreme complex parts at an efficiency and consistency unmatched by any other process, it is no wonder the method has been chosen as the primary means of mass production by more than half of all plastic manufacturers for almost every industry from automotive to aerospace, healthcare to consumer goods.

Utilizing these capabilities (high production speed, excellent repeatability, flexible design and material) of injection molding enables businesses to meet increasing customer demands with no drop-off in quality possible. Most notably however, perhaps due in part to its perfect unit economics allowing for highly cost-effective scale-up, this can be early during product development or continuing through final market launch.

The latest advancements in injection molding technology, along with smart automation, new advanced materials and sustainable practices are enabling manufacturers to create stronger, lighter and environmentally friendly parts that were not possible before without sacrificing performance.

By knowing the constraints of what injection molding can offer and its high upfront investment in tooling and mold design helps businesses better strategize their production plans for a higher return on investment. Right mold design, process optimization and quality control should be in place to eliminate common defects and make sure every part produced meets the required exact specifications.