Two of the major disadvantages to injection molding are the high mold costs and large required lead times. Mold is almost a project in and of itself and only one phase of the entire injection molding process.Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing), then you have to design and prototype a mold that can produce replicas of the part in volume. Lastly, and typically after extensive testing in both of the aforementioned stages, you get to injection mold a part. As you can imagine, all of the iteration required to get the mold correct prior to mass production requires both time and money. It is rare that you would prototype an injection molding mold. It does happen though, especially for parts that will be made in a multi-cavity mold. For example, let’s say we were going to injection mold a new shampoo bottle cap. That cap would likely have threads to attach it to the bottle, a living hinge, a snap closure, and potentially some overmolding too. A company may choose to make a single cavity mold of that part to make sure all of the features will mold as desired. Upon approval, they will make a new mold, that is capable of molding, for example, 16 caps at a time. They do the single cavity mold first so if there are any issues, they don’t have to pay and wait for it to be fixed 16 times for each cavity.
Because molds are typically made out of steel (a very hard material) or aluminum it can be difficult to make changes. If you want to add plastic to the part you can always make the mold cavity larger by cutting away steel or aluminum. But if you are trying to take away plastic you need to decrease the size of the mold cavity by adding aluminum or metal to it. This is extremely difficult and in many cases might mean needing to scrap the tool (or part of the tool) entirely and start over. In other cases you might be able to weld metal into the cavity that is undesired.
Injection molding necessitates uniform wall thickness. If you were to cut a cross-section of the Panasonic mold above you would notice that the wall thickness is approximately 2-3mm thick throughout. Keeping walls from being too thick is important to prevent inconsistencies in the cooling process resulting in defects like sink marks. A good rule of thumb is to keep walls less than or equal to 4mm thick. The thicker the walls the more material you will use, the longer the cycle time will be and the higher your cost per part will be. Conversely, if wall thickness is any thinner than 1mm or so you might experience trouble filling the mold (resulting in gaps or short shots). Designers can compensate for this potentiality by using a material with a higher melt flow index like Nylon which is often suitable for walls as thin as 0.5mm. Different manufacturing techniques like CNC don’t require uniform wall thickness at all.
Oftentimes large parts cannot be produced via injection molding as a single piece. This is due to the size limitations of injection mold machines and the molds themselves. For example of a large injection molded part consider the shopping carts at Target. Although the machinery exists to mold very large pieces (e.g. 1000 ton presses roughly the size of a train’s caboose), using it is very expensive. For this reason, objects that are larger than a typical injection molding machine’s capability are most often created in multiple pieces. CNC machines have similar limitations regarding product size while 3D printing has even more limitations. CNC is limited to the travel and size of the bed in the milling machine while large 3D printed parts often need to be printed in multiple pieces and then bonded together.
Large undercuts require experienced design to avoid and can often add costs to the project.
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 has a number of advantages. It minimises molding costs and is a highly repeatable way of producing plastic or elastomeric parts with high precision. Once set up, it can manufacture a large volume of parts per hour, from a wide range of different plastics plus other materials such as liquid silicone rubber.
However, like any manufacturing process, it does have some disadvantages, and if these prove significant to the success of your project, then you may be better off exploring alternative manufacturing technologies such as 3D printing or CNC machining.
I will take you through the main advantages of injection molding and explore when it may be better for you to consider an alternative production method.
What are the advantages of injection molding?
1. Efficient high production
Once you have developed the molds, the process is extremely fast with cycle times as short as 10 seconds. It is excellent for medium and high-volume production runs for anything from 10,000 parts to well over 100,000 depending on what molds you use.
Sometimes you can increase production by using a multi cavity or family mold, where several parts are produced from one press to further increase the manufacturing rate.
If you are outsourcing your molding then it’s also important to consider an efficient front-end design and order process – take a look at our on demand manufacturing.
2. Low cost per part
For high output production runs the cost per part is very low. Even for medium volumes – in the range of 10,000 to 25,000 parts, you can keep the costs down by using aluminum molds instead of steel.
3. Repeatability
You can manufacturer identical products over and over again. This is ideal when you need to have parts with high tolerances and reliability across high volumes.
4. Large material choice
There is a huge range of plastic materials that you can select from depending on what properties you need from your final part. And you are not limited to plastic, Protolabs also offers liquid silicone rubber molding.
You can even use fillers in the molding material, which adds greater strength to the completed part, and you have a huge range of colors to choose from as well. Talk to us about what you need your part to achieve – there are generally a number of different options.
5. Low waste
The injection molding process produces very little waste when compared to many other manufacturing processes. Even if there is any unused or waste plastic, you can recycle it for future use.
6. High detail
The process involves injecting molten plastic into the mold under very high pressure. This presses the plastic hard up against the molds allowing complex and intricate shapes plus a lot of detail.
7. Little or no post processing
Generally, you will need very little post production as the parts usually have good aesthetics post production. We can produce the tooling with a special finish which will show immediately on the injection-molded part. You can even have your logo or text engraved on it.
What are the disadvantages of injection molding?
When should you consider alternative manufacturing methods to injection molding?
1. Initial cost
If you are committing to steel mold tools for high production volumes (100,000+) then it can take a great deal of time and machining to produce and this can be a significant capital cost. If you need lower volumes of parts then this will affect your cost per part price and it may be worth exploring other options such as using aluminum molds instead.
2. Initial lead times
It can take up to 12 weeks to produce steel tooling. If you need to get production running before this then take a look at our on-demand production; by using aluminum molds and digitizing our front-end process we can ship anything from 25 to 10,000+ parts in 15 working days or less from the point that you first upload your CAD. Sometimes parts shipped in as little as one day.
For very low production runs there are other technologies such as 3D printing or CNC machining which could provide a more cost-effective answer more quickly.
3. Design limitations
You will need to consider certain design elements, such as:
Using draft and radii to help ejection of the parts
Avoiding undercuts and sharp edges
Controlling wall thicknesses
You also need to consider where to place your gates, ejectors and cooling lines if aesthetics are important. And remember it’s also hard to change the design of a mold – you can remove part of the mold, or add plastic to your final part, but not the other way round.
For smaller production runs, 3D printing allows you to design virtually any shape or geometry that you require. As a rapidly developing technology there is an increasing number of plastic and other materials, even metal, that we can produce.
4. Small part runs are not always cost effective
Most people think that they should only turn to injection molding for orders of 100,000+ parts; but there are other options when you can use this process for smaller production runs.
Using more affordable aluminium molds and quick turnaround times (as fast as a day) our on-demand service helps keep your per part cost down. It is ideal for anything between 10,000 and 25,000 parts, but can help control costs for production runs even smaller than this.
For low production runs measured in the hundreds or less then it is also worth exploring other manufacturing technologies such as 3D printing and CNC machining. With these technologies the cost per part will be lower and you will not face the initial set up costs or lead times involved in producing molds.
Injection molding is an excellent manufacturing technology for producing mid- to high-volumes of parts in plastic and liquid silicone rubber. It is evolving over time so it is worth exploring new ideas which could help you reduce your part cost for smaller production runs.
It’s also worth investigating whether other technologies such as CNC or 3D printing could provide a better option for small production runs or even custom-built parts.
One of the few disadvantages for injection moulding is that there may be a high initial tooling and machinery cost. If you want to produce parts at a low volume then small runs of parts can be less cost effective.
It’s important to note that there are a few design restrictions for plastic injection moulding, however by working alongside our experienced design team you should be able to overcome initial issues.
Disadvantages of injection moulding
1) High tooling costs and long set up lead times. Up-front costs are high due to the design, testing, and tooling required. There is the initial design and prototyping (probably via CNC or 3D printing), then the design of a prototype mould tool to produce replicas of the part in volume. Lastly, and only after extensive testing during both stages, you can finally injection mould a part.
2) Part design restrictions. Plastic parts must be designed with injection moulding consideration and must follow the basic rules of injection moulding, for example:
Avoid under cuts and sharp edges as much as possible
Use uniform wall thicknesses to prevent inconsistencies in the cooling process resulting in defects like sink marks.
Draft angles are encouraged for better de-moulding.
Don’t forget, because tools are typically made from steel or aluminium, it can be difficult to make design changes. If you need to add plastic to the part, you can make the tool cavity larger by cutting away steel or aluminium. But in order to take away plastic, you need to decrease the size of the tool cavity by adding aluminium or metal to it. This is extremely difficult and in many cases might mean scrapping the tool (or part of it) and starting over.
Also, the weight and size of the part will determine the tool size and necessary press size. The larger the part, the more difficult and expensive it will be.
3) Small runs of parts can be costly. Due to the complexity of tooling, and the necessity to rid the machine of all previous material before the next product can be made, the setup time can be quite lengthy. Therefore small runs of parts have traditionally always been thought of as too expensive to injection mould.
One of the few disadvantages for injection moulding is that there may be a high initial tooling and machinery cost.
If you want to produce parts at a low volume then small runs of parts can be less cost effective.
It’s important to note that there are a few design restrictions for plastic injection moulding, however by working alongside the experienced design team you should be able to overcome initial issues.
The process of injection moulding means there are some restriction with regards to part design. You might need to make a few changes to your parts so they’re able to be produced for you or you may have to decide on a different manufacturing technique.