Rapid Tooling für den Dreikomponenten-Spritzguss (3C IM)
Ziel: Geometrie- und Materialqualifikation
UnionTech Pilot 250
Injection Moulding (IM) machine:
Wittmann Battenfeld HM 800, 80 t clamp force
3 different types of Polycarbonate for a variation of additives in the 3-component part
2000 bars injection pressure, 300 °C mass temperature, 80 °C mould temperature
IM cycles per tool/ component:
Why Injection Moulding?
You know that we love 3D printing, we really do. And this is not a contest because Additive Manufacturing and Injection Moulding are processing techniques that complement one another. While 3D printing can be used for individual parts, prototyping and tooling, Injection Moulding is predestined for serial production and parts that require a lower anisotropy or smoother surfaces compared to printed parts.
In our case, the part is used in an optical application, meaning that one requirement was to partially have translucent properties.
In technologies such as Fused Filament Fabrication (FFF) it can be difficult to achieve transparency. This is due to the strands that are placed next to one another and that can disturb the transmission.
In the resin-based technologies such as Stereolithography (SLA) or Digital Light Processing (DLP), transparent parts are feasible due to the technologies’ higher resolution. However, for the application we needed to combine different materials which is nearly impossible in the vat technologies. This leads us to the next question.
Why multiple components?
Multiple-component Injection Moulding has been around for decades. It is used when different properties are combined in one part. For example, you’ll find this in toothbrushes: A hard material delivers the necessary stability during the brushing motion. By surrounding the hard plastic with a soft material, the haptic becomes more comfortable for the user.
In our optical application, it was crucial that there would not be any light scattering. Therefore, the translucent part had to be surrounded by an opaque component.
In multiple component Injection Moulding, the miscibility of the different polymers is of major importance. The polymers have to contain similar functional groups in their chemical composition in order to provide a good enough adhesion. Otherwise, the two materials might just fall apart and would have to be interlocked mechanically – which leads to undercuts that complicate Injection Moulding.
For our application, the component we used in the first step was Poly carbonate (PC). PC is a plastic that provides excellent mechanical properties such as hardness and strength, as well as a resistance towards chemicals and heat. This polymer was used for the sheath which provides our part with stability. The inner disc on the other hand is the part that is essential in the optical application. There, we needed a PC with an improved refractive index. In between the two different, almost transparent PC materials, a PC with an incorporated light blocker was placed.
Why printed tools?
The Additive Manufacturing of Injection Moulding tools has different advantages that can prevent you from making bad investments:
Don’t prototype the part, prototype the tool instead. 3D printing prototypes commonly doesn’t give you any idea of the functionality of their moulded counterparts. This is mainly due to effects such as more pronounced anisotropy and different surface properties.
Use the original materials. 3D printing plastics often contain additives to make them processable.
Thoroughly investigate the feasibility of your application. Quickly adjust your geometries such as wall thicknesses before manufacturing the final expensive steel tools.
Additionally, for our application:
Do the otherwise impossible. You might think “Why not use an aluminum tool?” and you’re right: They are not as expensive as steel tools and could definitely be used for prototyping.
BUT: For the thin-walled PC parts, the lower thermal conductivity of the plastic tool was a huge advantage because it gave us better control over the Injection Moulding process. In a metal or alloy tool, the PC – being a flow-resistant material – would have cooled down way to quickly, counteracting a successful moulding.
What did we do?
For the tools, we used Covestro’s Somos® Perfom Reflect.
It is processed in SLA and, hence, delivers great surface qualities. Especially for the manufacturing of translucent parts, the surface of the moulds needs to be as smooth as possible.
It has a high stiffness, mechanical strength and heat deflection. It is the perfect solution for Injection Moulding because it withstands the clamp forces as well as the heat of the polymer melts.
The tools were printed on a Pilot 250 by UnionTech (SLA). The layer resolution was 100 µm and the printing of each tool set took around 30 hours.
The 3C parts were processed in 3 steps: First, the sheath was processed, then the opaque component was added and then the transparent disc was added.
The injection moulding process was done with a Wittmann Battenfeld HM 800 using a clamp force of 80 t, a mould temperature of 80 °C, a mass temperature of 300 °C and an injection pressure of 2000 bar.
Of course, plastic tools are not forever, their mechanical strength cannot be compared to tools made from metal (alloys). Here, the tools were exposed to more than 200 cycles while still being (mostly) intact. Which is a lot!
What are the costs?
The cost of tooling is a complex issue in which a variety of factors determines the final costs. However, we are going to give you rough values by comparing tool printing and conventional tooling, e. g. via milling.
It was already mentioned that metallic tools would not have worked for the injection moulding of this exact geometry with the selected thermoplastic materials.
But let’s just imagine you have to make this decision for a tool of the same size:
You have an idea for you tool and start designing. If the external tool maker of your choice is quick, they will give you a delivery time of 4 weeks. For a tool (two halves) of around the same size as shown the costs can vary from 2500 up to 6000 EUR per tool, depending on the supplier.
Why does this take so long?
If you choose an external supplier, your order will be processed according to their production planning. Milling also involves a lot of steps: After the CAD, the model must be adjusted in CAM, prepared for milling and the latter takes up quite some time as well, due to rearranging the blanks throughout the manufacturing. Afterwards, a post-processing of the tool surfaces can be necessary. And you haven’t even started your injection moulding yet.
Inhouse 3D printing
In 3D printing, the beforementioned numerous milling steps are omitted. If you have access to an IM machine as well as a 3D printer, you can start your tool fabrication right after the modelling in CAD and will have your moulded three-component part after just 5 days.
Your design for your 3 tools is ready and you start printing. One print job takes 30 hours. The printed moulds need a post-processing in a heated UV-chamber. While the previous mould is further cross-linked, the printer is free for the next tool, so the continuous printing of the 3 moulds takes 90 hours in total.
For a tool (two halves) the costs are under 1000 EUR per tool.
Then of course the IM machine needs to be equipped with the respective thermoplastic component and the tools. The machine needs to be heated and this whole procedure needs to be repeated for every new tool and component. The IM process naturally takes some time.
However, with good planning you will receive your three-component part within 1 week from manufacturing your tools yourself via 3D printing.
How can I achieve this?
To get functional parts from tools that survive a lot of IM cycles, you need to know:
How to design for Additive Manufacturing AND Injection Moulding
How to adjust the 3D printing process as well as the post-processing of the printed tools to make them more durable
How to arrange a multiple component IM process with printed tools and different materials, temperatures, forces etc.