Injection molding is a manufacturing method commonly used to make plastic parts (metal parts are usually known as die casting). Injection molding works by injecting molten plastic into a mold to form parts with the shape of the mold.
Injection molded parts are almost everywhere around us. Look around for plastic parts around you, most of them are most likely injection molded.
Before 3D printing was introduced, injection molding remains as one of the best method to produce complex geometry as compared to other plastic manufacturing methods such as blow molding and extrusion.
With the emergence of 3D printing, it is no longer the case. In fact, 3D printing is capable of producing parts with even more complex geometry as compared to injection molding.
However, you will notice that 3D printed plastic parts are still not seen everywhere (in fact, you hardly see any), most of the plastic parts you see around you are still mostly injection molded.
Why is it so? Is it just because it is not the time yet? Will 3D printing eventually replace injection molding in the future?
Before we answer those questions, let us compare injection molding and 3D printing on various aspects.
1. Geometry Complexity
Injection molding could actually produce parts which have complex geometry, but 3D printing could produce parts which are far more complex than injection molding is capable of.
This geometry complexity indicates that parts could be design with minimal materials required while not compromising its function, which greatly reduces material consumption.
The cost per unit depends on the quantity of the parts. This is because the main cost of injection molding is on the mold itself. Therefore for injection molding, the cost per unit would decrease when the quantity of parts increases.
For low quantity parts, 3D printing would be cheaper than injection molding.
For high quantity parts, injection molding would be cheaper than 3D printing.
3. Lead Time
Lead time is the time required from the start to the completion of the production process. Similar to the cost, the lead time depends on the quantity since the making of the mold requires a lot of time.
For low quantity parts, 3D printing would be way faster than injection molding.
For high quantity parts, injection molding would be faster than 3D printing.
However, 3D printing could be faster than injection molding for high quantity parts by setting up more 3D printers. Since 3D printers are getting cheaper, the price of one injection molding machine could be equivalent to a lot of 3D printers. And with the number of 3D printers available, it could eventually be able to produce high quantity parts faster than injection molding.
Besides that, 3D printing is still a relatively new technology. This indicates there are a lot of room of improvement to further increase the speed of production using 3D printing, which will make 3D printing a lot faster than injection molding, even at high quantity.
4. Strength of Parts
Injection molded parts are isotropic, which means it has strength which is uniform in all direction (depends on the shape as well).
On the other hand, 3D printed parts are anisotropic, which means the part is strong in certain direction, and weaker on other direction.
This is because 3D printed parts are printed layer by layer, where the layer lines are the weakest link in the 3D printed parts. Since injection molded parts do not have these layer lines, its strength is uniform in all directions.
This does not mean that 3D printed parts are weak, it just means that you have to take note of the direction of load to gain the maximum strength out of 3D printed parts.
5. Design Changes
Changes in design is inevitable in most cases. Most products were changed a couple of times before settling with a final design.
In this case, 3D printing is going to be a better choice when the parts require lots of design changes. This is because of the long lead time and high cost for changing the mold in injection molding.
3D printing do not need to go through the hassle and allows the designer to change the design at ease without increase in lead time and cost.
In fact, the common application of 3D printing is rapid prototyping.
So will 3D printing replace injection molding?
3D printing sounds like a good choice overall as compared to injection molding, so will it replace injection molding? Not anytime soon.
There are still lots of parts out there which are made in high quantity and requires uniform strength of parts in all direction, which is something injection molding can do better than 3D printing. With the current technology of 3D printing, it is still very difficult to replace injection molding, at least not anytime soon.
Instead, 3D printing could be a great complement for injection molding. Parts which requires low quantity and constant design changes could be 3D printed. Once the design has been finalized, then companies can then confidently commit into injection molding, knowing that their designs have been repeatedly tested with the help of 3D printing.
However, 3D printing is still a relatively new technology and there are still lots of room for improvement in the process. Therefore, no one knows if 3D printing will ever evolve to a state where it could outperform injection molding for high quantity production.
So when to use which?
Since 3D printing will not replace injection molding at the moment, that means both will have their own application which suits them. So when do we use 3D printing? And when do we use injection molding?
If you are a designer who could create really complex stuffs but the manufacturing method always seem to be your limit, then use 3D printing.
If you need parts which are not too complex and requires a lot of them (like 10,000 of them), then use injection molding.
If you are designing some one-off projects and only some custom parts, then use 3D printing.
If you need parts which have uniform strength in all direction and do not want any uncertainty when it comes to the strength of the parts then use injection molding.
At the moment, 3D printing will not completely replace injection molding. In fact, both injection molding and 3D printing has their own suitable application and will work great side by side.
There are several factors such as geometry complexity, quantity, part strength and amount of design changes, which determine which method is suitable for your application.
Go for injection molding if you require parts of very high quantity and if your parts require uniform strength in all directions.
If geometry complexity is your game, and you are only looking to make one or few of the parts, then go for 3D printing.
Did we miss out any pros and cons of 3D printing and injection molding? Let us know in the comments below.