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  • Writer's pictureLaFlesche

What is Injection Molding?

Injection molding is a manufacturing process that allows for parts to be produced in large volumes. It works by injecting molten materials into a mold (or ‘mould’ in the UK). It is typically used as a mass production process to manufacture thousands of identical items. Injection molding materials include metals, glasses, elastomers, confections, and, most commonly, thermoplastic and thermosetting polymers.


Injection molding equipment comes in a variety of sizes and capabilities.


How Does Injection Molding Work?


The first stage of injection molding is to create the mold itself. Most molds are made from metal, usually aluminum or steel, and precision machined to match the features of the product they are to produce.


A machined injection mold.

Once the mold has been created by the mold-maker, the material for the part is fed into a heated barrel and mixed using a helical shaped screw. Heating bands melt the material in the barrel and the molten metal or molten plastic material is then fed into the mold cavity with very high pressure, where it cools and hardens, matching the shape of the mold. The cooling time can be reduced through the use of cooling lines that circulate water or oil from an external temperature controller. Mold tools are mounted on plate molds (or ‘platens’), that open once the material has solidified so that ejector pins can eject the part from the mold.


Separate materials can be combined in one part in a type of injection molding called a two-shot mold. This technique can be used to add a soft touch to plastic products, add colours to a part, or produce items with different performance characteristics.


Molds can be made of single or multiple cavities. Multiple cavity molds can have identical parts in each cavity or can be unique to create parts of different geometries. Aluminium molds are not best suited to high volume production or parts with narrow dimensional tolerances since they have inferior mechanical properties and can be prone to wear, deformation, and damage due to the injection and clamping forces. High volume production, such as the manufacturing of automobiles, typically opts for steel molds that are more durable, but also more expensive than aluminium. For example, one car will typically be created from thousands of molds, and often just a single mold costs more than the car itself. But when producing millions of vehicles month over month, year over year, the cost benefit of steel molds makes them the logical solution by far.


The injection molding process requires careful design, including the shape and features of the part, the materials for the part and the mold, and the properties of the molding machine. As a result, there are various considerations that need to be taken into account when injection molding.



What Needs to be Considered for Injection Molding?


There are a number of considerations to bear in mind before undertaking injection molding:


1. Financial

The entry cost for injection molding can be considerable due to the tooling costs of the molds themselves. However, the price per unit is much lower than many other plastic manufacturing processes when looking to make higher volumes of any one product.


2. Production Quantity

It is important to determine how many parts you wish to manufacture so as to decide whether injection molding is the most cost-effective production method.


3. Design Factors

Minimizing the number of parts and simplifying the geometry of your items will make injection molding easier. In addition, the design of the mold tool is important to prevent defects during production.


4. Production Considerations

Minimizing the cycle time will aid production as will using machines with hot runner molds and well thought-out tooling. Such small changes and use of hot runner systems can equal production savings for your parts. There will also be cost savings from minimizing assembly requirements, especially if you are producing many thousands or even millions of parts.



How Can I Reduce Mold Costs?


Injection molding can require significant investment at first, but there are several ways in which you can reduce mold costs, including:

  • Eliminate undercuts

  • Remove unnecessary features

  • Use a core cavity approach

  • Reduce cosmetic finishes

  • Design parts that self-mate

  • Modify and re-use existing molds

  • Monitor DFM analysis

  • Use a multi cavity or family type of mold

  • Consider your part sizes

When to Use Injection Molding?


Injection molding is ideal for making higher volumes of widely used products, such as bottle tops, plastic mechanic parts, syringes, and more intricate medical devices. It is also commonly used for manufacturing larger items such as car body panels.


Once you are looking to make many thousands or millions of identical parts from a mold, injection molding is often the way to go.


Plastic stadium seats are made with injection molding.

What are the Different Types of Injection Molding?


There are many different injection molding process variations including:

  • Cube molding

  • Die casting

  • Gas-assisted injection molding

  • Liquid silicone rubber injection molding

  • Metal injection molding

  • Micro injection molding

  • Reaction injection molding

  • Thin-wall injection molding

  • Co-injection molding (two or more materials)

What Materials are Used in Injection Molding?


Injection molding can be performed with a variety of different materials including metals, glass, elastomers, confections and, most commonly, thermoplastic and thermosetting polymers. Materials can be combined to deliver different properties and effects for the finished parts.



What are the Advantages of Injection Molding?


The main advantage of injection molding is being able to scale up production to produce a large number of parts. Once the initial costs of the design and the molds have been covered, the price of manufacturing is very low. The cost of production drops as more parts are produced.


Injection molding also produces minimal wastage when compared to traditional manufacturing processes like CNC machining, which cuts away excess materials. Despite this, injection molding does produce some waste, mainly from the sprue, the runners, the gate locations, and any overflow material that leaks out of the part cavity (also called ‘flash’).


Another advantage is that the production of repeatedly identical parts allows for part reliability and consistency in high volume manufacturing.


Lastly, injection molding allows the grain of molecules to be lined up in the mold, specifically around part corners. This provides added strength to the finished part as compared to parts that are built by cutting and removing material.



What are the Disadvantages of Injection Molding?


While injection molding has its advantages, there are also a number of disadvantages with the process. Up-front costs can be high for injection molding, particularly with regard to tooling. Before you can produce any parts, a prototype part needs to be created. Once this has been completed, a prototype mold tool needs to be created and tested. This all takes time and money to complete and can be a costly process.


Injection molding is also not ideal for producing single parts that are large in size. This is because of the size limitations of injection mold machines and the mold tools. Items that are too large for an injection molding machine’s capability need to be created as multiple parts and joined together later.


The final disadvantage is that large undercuts require experienced design to avoid problems and can add even more expense to your project.



Injection Molding FAQs:


Is Injection Molding Environmentally Friendly?


Injection molding is becoming more environmentally friendly as innovations are being made in eco-friendly plastics, and as machinery becomes more efficient and materials such as thermosetting polymers are capable of withstanding extreme temperatures and conditions.


Options to include recycling and eco-friendly plastics are increasing rapidly.

Although there is some material waste with injection molding, it is less than with many other manufacturing processes. Most productions allow for re-use of the waste material since plastic can be remelted indefinitely. Of course, the exact materials used also have a bearing on the environment in relation to how long they last, if they can come from recycled materials, and how they are disposed of. There are also considerations around the carbon footprint of the lifetime of the products that are created, including during manufacture as well as transportation of products.


Advancements in modern injection molding machinery mean that they now use up to 50% less energy when compared to ten years ago. With such rapid advancements being made in the injection molding industry, the process is becoming more environmentally friendly every year.



How Much Does Injection Molding Cost?


Molding costs correlate with the number of cavities in a mold. Fewer cavities require less tooling work, which lowers the manufacturing costs to create an injection mold. The complexity of a part’s design also impact the cost, including factors such as surface finishing, tolerance, threads, detailing, and the number of undercuts. Added details such as these will increase the cost, as they require more tooling.


The most cost-effective type of injection molding is rubber injection molding, which produces a high yield of durable products. In addition, consistent vulcanisation processes with precise temperature controls can lower costs by reducing waste material.



What Plastics are used in Injection Molding?


With over 85,000 commercial plastic material options available from 45 polymer families, there are a wealth of different plastics that can be used for injection molding. Of these, the polymers can be broadly placed into two groups; thermosets and thermoplastics.


The most common types of plastic used are high-density polyethylene (HDPE) and low-density polyethylene (LDPE). Polyethylene offers a number of advantages including high ductility levels, good tensile strength, strong impact resistance, resistance to moisture absorption, and recyclability.


Polyethylene pellets.

Other commonly used injection molded plastics include:


1. Acrylonitrile Butadiene Styrene (ABS)

This tough, impact resistant plastic is widely used across industry. With good resistance to acids and bases, ABS also offers low shrinkage rates and high dimensional stability.


2. Polycarbonate (PC)

This strong, impact resistant plastic has low shrinkage and good dimensional stability. A transparent plastic that is available in different optically clear grades, PC can provide a high cosmetic finish and good heat resistance.


3. Aliphatic Polyamides (PPA)

There are many different types of PPA (or nylons), each of which has its own advantages. Generally speaking, nylons offer high strength and temperature resistance as well as being chemically resistant, apart from against strong acids and bases. Some nylons are abrasion resistant and offer good hardness and stiffness with good impact strength.


4. Polyoxymethylene (POM)

Commonly known as acetal, this plastic has high hardness, stiffness, strength and toughness. It also has good lubricity and is resistant to hydrocarbons and organic solvents. Good elasticity and slipperiness also provide advantages for some applications.


5. Polymethyl Methacrylate (PMMA)

PMMA, also known as acrylic, provides good optical properties, high gloss, and scratch resistance. It also offers low shrinkage and less sink for geometries with thin and thick sections.


6. Polypropylene (PP)

This inexpensive resin material delivers high impact resistance in certain grades but can be brittle in cold temperatures (in the case of propylene homopolymer.) Copolymers offer greater resistance to impact while PP is also wear resistant, flexible, and can provide very high elongation, as well as being resistant to acids and bases.


7. Polybutylene Terephthalate (PBT)

Good electrical properties makes PBT ideal for power components as well as automotive applications. The strength ranges from moderate to high depending on glass fill, with unfilled grades being tough and flexible. PBT also shows fuels, oils, fats, and many solvents, and doesn’t absorb flavours.


8. Polyphenylsulfone (PPSU)

A dimensionally stable material with high toughness, temperature, and heat resistance, PPSU is also resistant to radiation sterilisation, alkalis, and weak acids.


9. Polyether Ether Ketone (PEEK)

This high temperature, high performance resin provides heat resistance and flame retardancy, excellent strength, dimensional stability, and good chemical resistance.


10. Polyetherimide (PEI)

PEI (or Ultem) offers high temperature resistance and flame retardancy, along with excellent strength, dimensional stability, and chemical resistance.



Conclusion

Injection molding has a great many applications for manufacturing, particularly for producing high volume parts. While the tooling and molding can be a hurdle at first, the cost of production once this is completed is low. Providing the ability to produce near-identical parts, injection molding is a great solution for used for a variety of parts and materials.


With several hundred new plastic compounds being developed each year and modern mold-making techniques evolving rapidly, injection molding is certain to continue increase over the coming decades. That said, we understand the whole process can still feel daunting.


That’s why our team of dedicated specialists are standing by to help you every step of the way.






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