What are the disadvantages of injection molding?

Car grilles contain plastics that are produced using injection moulding

What is Injection Moulding?

Injection moulding is the most common manufacturing process used to produce plastic parts. The process utilises a machine, raw material, and a mould of the product part being produced to consistently manufacture high volumes of identical objects at a fast rate.

Machines are categorised and rated based on their clamping pressure or tonnage. The higher the clamping pressure, the larger the machine, and the larger the parts that it is able to fabricate. Clamping pressures vary anywhere from 5 tons to 6000 tons. The clamping pressure required to give optimum results for a product part is determined by the projected area of the part in the mould. Using clamping pressures that are too high or too low for a certain part can lead to defects such as flashing, where molten material seeps out of the mould and solidifies, leading to an unwanted thin layer of material forming around the part.

The raw materials used are chosen based on the function and specifications of the final product part. However, each material also has various parameters that need to be considered within the process. Most polymers can be used in the process, including all thermoplastics (nylon, polystyrene, polyethylene) and some thermosetting plastics (epoxy, phenolic). Though being mainly used for the production of plastics, other materials may also be used with injection moulding.

The main components of the process are described below:

Injection Moulding Process

1. Moulding – A mould in the shape of the product part is designed using CAD, manufactured, and split into two halves.
2. Clamping – The two halves of the mould are pushed and held securely closed by the clamping unit of the machine. Larger machines will require a longer time than smaller machines to carry out this step.
3. Injecting – The raw materials, usually in the form of pellets, are melted by heat and pressure, then injected into the mould very quickly, filling the entire space within it. The build-up of pressure packs and holds the material together. The exact amount of material injected in the mould is referred to as the shot.
4. Cooling – The molten material within the mould begins to cool as it makes contact with the mould surfaces, solidifying into the shape of the desired part.
5. Ejecting – The clamping unit separates the two halves and the cooled, finished part is ejected from the mould via the ejection unit.

The production cycle is very short, usually lasting between 2 seconds and 2 minutes. Upon the completion of step 5, the cycle restarts at step 2, manufacturing a replica of the part.

Types of Injection Moulding

There are various types of the injection moulding process, ranging from:

• Gas-assisted injection moulding
• Liquid silicone rubber injection moulding
• Cube moulding
• Die casting
• Micro injection moulding
• Reaction injection moulding
• Metal injection moulding
• Thin-wall injection moulding

Uses of Injection Moulding

Injection moulding is the preferred production method for most mass-produced plastic products, due to its high output rate and consistency of quality. These products include but are not limited to:

• Automotive parts – dashboards, bumpers, grilles.
• Electronic components – electrical connectors, enclosures, protective sleeving.
• Medical devices – syringes, valves, dishes.
• Consumer plastics – mobile phone cases, bottle caps, toys.
• Furniture parts – seat cushions, chairs, seat covers.

Advantages

• Ability to use multiple materials simultaneously, including multiple types of plastic as well as multiple colours.
• Ability to design and manufacture intricate and complex geometrical shapes – as a result of the exceptionally high pressures that the moulds are applied with in the process.
• Fast production, leading to a high output rate and efficiency.
• Low labour costs due to the process being highly machine orientated.
• Repeatable process, leading to a consistent quality of products.
• Environmentally friendly process with little waste – scrap plastic is reground and re-used in the process.
• Produced parts need little work after the ejection since they have a more-or-less finished appearance.

Disadvantages

• High initial cost to set up the machinery properly.
• Not as cost effective if used to produce only a low volume of parts.
• Moulds need to be designed to a high standard, which takes skilled workers.

Considerations

There are various considerations to take into account before undertaking injection moulding, ranging from:

• Financial: The process can be costly to set up and operate, therefore it’s most suitable to those with an appropriate budget.
• Production Quantity: It’s important to take into account how many parts you wish to manufacture in order to decide whether injection moulding is the most economical production method for your product.
• Design Factors: Designing the mould itself is a key part of the process which can determine the quality and presence of defects during production. Simplifying the geometry and number of parts will lead to an easier process.
• Production Considerations: Simplifying assembly requirements, cycle times, tooling, design and the use of hot runner moulds cam all lead to cost and labour savings.

Injection moulding is just one of the processes carried out in plastic manufacturing plants. Once the plastic parts are manufactured, they are bonded together to make a product. As a result, bonding plastics is a process just as important as producing them.

Originally published on fastradius.com on March 23, 2022

Injection molding involves injecting molten plastic into carefully designed molds before cooling and ejecting the final part. It’s a highly repeatable process that enables companies to create high volumes of identical plastic parts with good tolerances at a low cost per part.

Injection molding has numerous applications in a wide variety of industries — from the automotive industry to the medical industry — but it isn’t the best choice for every project. Before you decide whether or not to injection mold your part, carefully consider the injection molding pros and cons.

The Advantages of Injection Molding

Many manufacturers turn to injection molding because:

1. It allows for complex geometries with tight tolerances.

Injection molding allows for large volumes of uniform, complex parts. However, you must pay attention to vent and gate placements, weld lines, corner transitions, wall thickness, rib and boss design, and more to ease ejection and achieve precise parts.

With injection molding, you can easily achieve repeatable part tolerances of ± 0.500 mm (0.020’’). In some cases, you can even produce parts with tolerances of ± 0.125 mm (0.005’’), giving you parts that are accurate enough for most applications and comparable to 3D-printed or CNC machined parts.

2. It’s compatible with a wide range of materials and colors.

Today, there are over 25,000 engineered materials that are compatible with injection molding, including thermoplastics, thermosets, resins, and silicones. With all of these options, you’ll be able to find one that offers the right balance of physical, mechanical, and chemical properties. Commonly used materials include acrylonitrile butadiene styrene (ABS), polyethylene (PE), polystyrene (PS), and polypropylene (PP). You can also use a mixture of materials to produce a part with the strength, impact resistance, or stiffness you need. For example, you might add glass fibers to your thermoplastic to create a strength-enhancing composite.

You also have a variety of options when it comes to colors. Consider using masterbatches, pre-colored resins, liquid colorants, or salt and pepper blends to achieve your desired color.

3. It’s very efficient.

While it can take several minutes — or even hours — to 3D print or CNC machine a single part, most injection molding cycles only last 10 to 60 seconds. Even if you have a complex geometry that takes around 120 seconds to mold, you can include several smaller parts in one larger mold. This helps maximize efficiency and gets the most out of each mold, allowing you to create hundreds of identical parts an hour at a low cost.

4. It offers high repeatability and reliability.

One of the main benefits of plastic injection molding is its high repeatability. Once you’ve created your mold, you can produce thousands of parts before needing to maintain your tooling. An aluminum mold will generally last between 5,000 and 10,000 cycles, and a full-scale steel production mold can last for over 100,000 cycles. Plus, since injection molding uses the same mold for each part, you’ll have identical products.

5. You can reuse material.

Though injection molding generates less post-production material waste than many other manufacturing processes, it still creates excess scraps. However, you can easily regrind, melt, and reuse any sprues, runners, or other leftover plastic parts to save on material and reduce material waste.

The Disadvantages of Injection Molding

There are plenty of advantages of plastic injection molding, but it’s not without its drawbacks. Some disadvantages include:

1. Start-up costs are high.

Since custom tooling must be created for each injection molded part, initial start-up costs are high and this isn’t economical for low-volume production runs. Tooling for a simple design and a small production run can cost between $2,000 and $5,000, but tooling for large, complex molds ready for full-scale production can cost several times that. Although you can reuse these molds again and again and save on tooling costs down the line, it’s worth considering how much molds cost upfront. An injection molding manufacturing partner can help you maximize your budget and refine your mold design so you can produce the best possible part for the best price.

2. Initial lead times are long.

A CNC machined part can be delivered within 5-10 days, and industrially 3D printed parts often have lead times of 3-5 days. However, injection molding has a longer lead time. It often takes 5-7 weeks to manufacture tooling and 2-4 weeks to produce and ship parts.

In part, this long lead time can be attributed to the complexity of the molds themselves. In addition to containing the negative of the part, these molds have complex runner and water cooling systems to facilitate material flow and faster cooling. It can take months of design and testing before the final mold is ready for production, and any design changes can further increase turnaround times.

With the help of a manufacturing partner’s expertise, you can avoid falling into common mold pitfalls that might set you back weeks and thousands of dollars. They can also help accelerate the design, testing, and production phases.

3. Design changes are expensive.

With 3D printing, you can simply upload a file and print a new part whenever you make a design change, but that’s not the case with injection molding. If you make a design change, you’ll likely need to create a new mold from scratch, which means pouring more time and money into your project.

To avoid costly design changes and ease the demolding process, avoid undercuts and sharp edges, ensure wall thicknesses are uniform, and add draft angles. If you need some guidance, an experienced manufacturing partner can offer expert design advice.

Injection Molding With SyBridge

Between its efficiency and high repeatability, injection molding has plenty to offer. However, it’s all too easy to make an expensive mistake that sets production back weeks. That’s where a trusted manufacturing partner can help.

When you work with SyBridge, our team of experts can guide you through the entire production process and answer any questions you have about injection molding’s advantages and disadvantages. Ready to get started? Contact us today.