3D printing is a relatively new process as compared to injection moulding. Both are quite different from each other. However, engineers have still managed to incorporate the technique of 3D printing into injection moulding to enjoy various benefits this hybrid can provide. Before getting into the details of 3D printing for injection moulding, the basics of injection moulding need to be discussed.
Injection moulding, as the name suggests, is a manufacturing process in which the raw material, in its molten form (usually plastic), is injected with high pressure into a mould. It then takes the specific shape of that mould and starts to cool. Once it has solidified, the manufactured part is ready to be taken out. This part is then required to go through the post-manufacturing process to get to its final shape.
Post-processing includes many different steps, such as machining and filing. These processes provide the required finish, properties, and tolerances of the manufactured part. For example, trimming removes the flashes –the extra layer of the molten raw material that flows out from the meeting point of the upper and lower parts of the mould and then solidifies –and heat treatment removes or minimizes the internal stresses from the manufactured part.
The Main Advantages of Injection Moulding
Although injection moulding offers various advantages, such as flexible designs, consistent products, and ease to manufacture complex parts in detail, two of its most important benefits are its low costs and short cycling time:
The process of injection moulding can be completely automated. It means that designated machines and robots manufacture the parts. These machines and robots do not require regular wages and incentives and can work faster to provide a consistent output. It reduces labour costs, along with many other benefits.
However, injection moulding requires a huge number of iterations to get the tool right before mass production. For this reason, the start-up costs are often very high. Although prototyping single cavity molds in the beginning can save a lot of correction costs and time, micro injection moulding is a better and more cost-efficient solution where applicable.
In micro injection moulding, relatively smaller parts are made at a time and hence, the tooling costs reduce to a great degree (approximately 40%). Furthermore, these lighter parts require smaller machines, which ultimately lead to lesser operating costs.
But since micro injection moulding involves creating smaller parts of a relatively larger piece separately, it is better suited for low-volume productions. The cost break-even point between micro moulding and conventional injection moulding is 200,000 pieces, meaning that micro moulding is the less expensive option out of the two for producing less than 200,000 parts.
Short Cycling Time:
Injection moulding has the major advantage of having very short times for a single manufacturing cycle, which includes the time for all the stages – from the insertion of the molten raw material to the ejection of the solidified part out of the mould. On average, a single injection moulding cycle can last from a mere 2 seconds to 120 seconds. And it leads to a very high production rate.
This technique works great when there is a requirement for high-volume production. However, micro injection moulding, having shorter barrels, gates and runners, provide much shorter cycle times even for multi-cavity moulds. It also allows for a quicker re-start of the machine for a different material, and that is another reason why micro injection moulding should be preferred for smaller-scale production.
Why using 3D Printing in Injection Moulding?
In the hybrid process of 3D printing and injection moulding, a 3D printer creates the mould/die to convert the molten raw material into the required shape. 3D printing is used here because of the many benefits it provides for the process of injection moulding. Some of these benefits are as follow:
Reduced Lead Time (Rapid Tooling):
3D printing is a much more accessible, readily available and easy process for moulds manufacturing as compared to the traditional methods. The traditional methods, such as machining and milling, take a large amount of time to bring the raw material, usually, aluminum or steel, into the required shape and lead to material waste.
With 3D printing, however, there is little to no material waste (only through the supports), and the turn-around time is much faster (approximately two weeks).
As soon as a 3D printer has finished printing the mould, it is almost ready to use. Hence, this method is perfect for making prototypes as design changes can be accommodated quickly, and saves time and resources.
Nowadays, 3D printers with multiple nozzles also exist. If the mould is required to be made from only one type of material (usually the case), those multiple nozzles can speed up the process further by working simultaneously.
One of the biggest reasons for production cost enhancement in injection moulding is the iterations needed to get the design and quality right. Even with soft tooling that uses the least expensive material (aluminum), the costs can go up to $25,000 or more.
3D printing is a highly cost-efficient method to get functional prototypes that can produce finished product samples. These moulds, made of plastic, perform the same way as metal moulds. However, plastic being cheaper and many degrees softer than the metals lead to lower prototyping costs than traditional moulding.
3D printing, although cuts down the initial costs and modification expenses to a great deal, is only suitable for the prototyping or manufacturing of a small number of parts, therefore, it is highly applicable in micro injection moulding.
When to use 3D Printed Moulds?
The process of 3D printing for injection moulding is highly advantageous, but not suitable for every kind of project. Depending upon the turn-around time required, the scale of production and investments, you can choose between traditional methods and 3D printing for injection moulding.
Here are a few scenarios where using 3D printers is the better alternative:
When in need of short cycles:
3D printing is suitable when a quick turn-around and shorter production cycles are the goals. The use of harder raw material in injection moulding leads to high cycle times. However, since 3D printing requires no mould production, merely a layer-by-layer manufacturing of the part using a 3D file and a softer material, hence it provides relatively shorter cycles.
For low-volume production:
Injection moulding requires a high volume of production to bring costs to an acceptable level. Therefore, traditional methods of creating moulds get extremely expensive at lower production volumes.
Since 3D printing does not require large sums of money for launching and making modifications, it leads to faster production rates at a lower cost per part. That means that 3D printing provides an optimized solution for low volume production and should be opted in such cases.
For relatively small parts:
The process of 3D printing creates moulds with sharp details. This detailing is usually difficult to achieve using traditional methods. Even after post-processing, the details on the moulds are not as precise as the 3D printed ones. As a result, 3D printed moulds for injection moulding can manufacture parts with better precision and sharper details. And this makes them perfect for the manufacturing of small parts that require complex shapes and features.
Microinjection moulding, which is also a type of injection moulding using 3D printers, manufactures small and detailed parts. These parts can have their tolerances in a few micrometres only. Even though the micro moulding machines also inject the raw material into the mould with high precision for even distribution, the mould also plays an important role. If the mould does not have precise features and surfaces, the manufactured parts will not be able to achieve very low tolerances. It is the reason 3D printed moulds are being advantageously exploited in micro moulding.
3D printers, like the BMF MICROARCH S140 and the Pro2 Dual Extruder 3D Printer, can be used to produce moulds with extreme detailing for micro moulding.
How to Design 3D printed Moulds?
The designs for the moulds are usually made on computer software that creates a 3D computer-aided design (CAD) model of the mould. Plenty of CAD software is available in the market, such as FreeCAD and OpenSCAD, each having its own unique user interface and special features. The designer creates design for the moulds by keeping in mind the end-product (the manufactured part from injection moulding).
Main points for consideration:
- A Strong Base: Since the 3D printer does a layer-by-layer printing, these layers are required to stick to each other. And the first layer must stick to the print bed during the entire process. If it is unable to do so, the 3D print of the mould will become useless. To ensure that the first layer adheres to the bed firmly, perfectly level the print bed so that the 3D printer can spread the first layer evenly. Increase the thickness of the first layer to provide a stronger foundation for the whole structure and avoid damage to the mould.
- Overhangs: The overhang in 3D printing is the part of the 3D printed object that extends outwards and has no support below. Each type of 3D printers has a limit to the angle by which they can let the overhang extend. Usually, an overhang angle greater than 45 degrees is not recommended because it will cause the 3D printed object to bend downwards. A 3D printer works by stacking layers on top of each other. But since an overhang will not have much support from the layers below, it can start to bend due to its weight. For this reason, it is best to avoid overhangs as much as possible.
- Warps: The layering material used in 3D printing the moulds go through physical change when being deposited, such as melting, and then is cooled over another layer of itself. This cooling can cause the material to twist or shrink, also known as warping. Due to warping, the 3D printed mould has less strength and is prone to deformation. Warping can be avoided by:
- Keeping any sharp edges to a minimum because they act as the points of stress concentration which can easily crack or fracture,
- Avoiding large flat surfaces,
- Correcting machine calibration,
- Sufficient surface adhesion between the print bed and the part being printed.
- Wall Thickness: The properties of the mould also depend on the thickness of the printed wall. If the wall becomes too thin, it has a higher chance of failure as it is weaker, while if the wall thickness is increased too much, internal stresses can arise. If the wall thickness is not taken care of, the mould will not work like intended and can even deform once the molten raw material is injected. So, it is recommended to keep the wall thickness of at least 0.8 mm to avoid any complications with the 3D printed mould.
Tips & Tricks:
Here are a few tips to improve the design for the 3D printed mould:
- Avoid Supports: If the part to be printed has overhangs (with an angle greater than 45 degrees), supports are used to keep the overhangs from bending. But these supports can leave a mark when removed. So, it is best to design the 3D structure of the mould with the least amount of supports needed.
- Correct Orientation: Most printers can only vary the resolution of the structure along the z-axis because it depends on the nozzle’s width. So, try to keep the side of the mould that requires some specific property, like high strength and fine details, along the z-axis. This way, that specific property can be set as per the requirement, by varying the thickness of the layers.
- Tolerances: The tolerance of the 3D printed mould depends on the nozzle width and the raw material used. After deposition, the material will expand, and it’s crucial to take care of this. The raw material usually expands 1.2 times the nozzle’s width. So, keep this in mind when designing for specific dimensions and tolerances.
- Material: The material has many different characteristics that can affect the properties of a 3D printed mould, such as the thermal expansion coefficient, hardness, and flexibility. Choose the right ones so that the design can be printed as it is, without any complications.
3D printing in injection moulding has, no doubt, revolutionized the field by providing cost-efficient solutions and faster production rates. However, utilizing it as a complementary method instead of a competitive replacement to the traditional methods of injection moulding can provide highly optimized solutions in terms of both time and investment. For more details related to the subject, learn more here.
Interesting Related Article: “What You Need to Know Before Starting a Career In 3D Printing?“