Feature first published in the July/August 2021 issue of 3D ADEPT Mag. Roles, interviews and examples reflect that period.
Additive manufacturing can produce inserts for injection moulds, notably for design trials or selected production runs. Choosing between a metal and photopolymer insert depends on temperature, pressure, injected material and the required number of cycles.
The rise of 3D printing has had impacts not solely on the production of end-used parts as a direct manufacturing technology, but also as an indirect manufacturing technology. For example, Additive Manufacturing is sometimes used to create moulds for the purpose of urethane or silicone casting. In other circumstances, it is used to create tools such as jigs or fixtures that enhance the performance of machining cells. This ability to create custom tools for traditional manufacturing processes also extends into the realm of injection moulding, where 3D printing technology is used to create custom injection mold inserts. But when it comes to creating a custom mould insert, which technology is best? We spoke with a couple of digital manufacturing experts to learn more about two of the most popular 3D printing technologies used for this application, material jetting and laser powder bed fusion.
This concerns elements of the tooling, distinct from insert moulding in which an insert becomes part of the finished product. The comparison focuses on metal laser powder bed fusion and PolyJet-type photopolymer jetting.
The use of Additive Manufacturing technologies for the production of custom mould inserts
With the goal of addressing the needs of mass customization, AM companies have been exploring their manufacturing processes to produce the tooling used in injection moulding with varied levels of success.
“Theoretically, any AM technology can be used. One might immediately think of extrusion 3D printing which is probably the most widely used technology for prototypes but that’s probably because it is the technology people know the most. There are no reasons why SLS, other powder-bed processes like Electron Beam, or even binder jetting can’t be used. At the end of the day, it’s the results obtained, that matter the most”, Cullen Hilkene, CEO at 3Diligent told 3D ADEPT Media from the outset.
Several processes are used to make tooling, including SLA, DLP, SLS, metal laser powder bed fusion and PolyJet. This feature focuses on the latter two without claiming a market-wide ranking of their use.
That being said, no matter what AM technology is used, the manufacturing perspective reveals that “traditionally, the moulding with metal inserts is carried out with the aim of reinforcing the mechanical properties of the inserts, reducing the cycle time and obtaining moulded articles with a better aesthetic / dimensional quality.”
Furthermore, still according to ZARE’s experts, another manufacturing goal would consist in integrating an item with specific thermal properties, in a mould done with a different material.
Moreover, “another use, more critical, concerns the creation of particular and equidistant cooling channels from the surface. Such cooling channels allow a greater heat dissipation in the unit of time with the consequent reduction of the duration of the moulding cycle and a reduction of the deformations on the finished product, increasing the quality of the final product”, they said.
Brief description of DMLS/ Laser powder-bed fusion (LPBF).
LPBF means laser powder bed fusion. Here it refers to the metal variant, also called PBF-LB/M. DMLS is an EOS commercial designation; SLM is another commonly used term for selective melting of metal powder.
In metal laser powder bed fusion, the laser melts material in selected regions. Thermoplastics can also melt: the historical term “laser sintering” used for SLS does not mean they cannot melt.
In any case, for those who are not familiar with it, this manufacturing process enables the production of “net and near-net shape parts directly from a digital drawing file with dimensional tolerances of less than 0.1 mm” through an interaction between a high-energy laser and a metal powder feedstock.
The sub-0.1 mm tolerance cited here is not a guarantee for every part. Accuracy depends on the machine, material, geometry, orientation and any finish machining.
A powder layer is spread on the build platform, then the laser scans the regions corresponding to the part cross-section. Successive layers are processed in a controlled atmosphere. Layer thickness, power, laser count and spreading mechanism vary with equipment and qualified parameters.
Brief description of PolyJet/ Material Jetting
PolyJet deposits photopolymer droplets that are cured with UV light. It belongs to the material jetting family, whose other variants must not be conflated with this resin-based process.
Developed by Objet, PolyJet became associated with Stratasys following the companies’ merger in December 2012, rather than a 2011 acquisition.
Material jetting covers several materials and consolidation methods. The performance discussed here for photopolymer inserts does not automatically transfer to metal- or wax-based processes.
Material is supplied and conditioned to the machine specification to achieve consistent deposition. Temperatures depend on formulation and system; they are not a universal range.
As the X-axis carriage starts to move across the build plate, the print heads begin selectively jetting hundreds of tiny resin droplets. As the UV light sources follow the print heads’ trajectory and instantly cure the sprayed resin. Once an entire layer is completed, the build platform drops one layer in height and the process is repeated until the part is entirely formed.
With technology advancements, machine manufacturers have developed 3D printers with multiple print heads in order to achieve multi-material printing. Depending on the machine used, multi-material 3D printers can enable dissolvable support material or multiple varieties or colours of functional material.
Other variants jet materials such as waxes or particle-loaded suspensions. The retained 3Diligent diagram in this article illustrates a metal-based variant, not the photopolymer PolyJet process described above.
“While it is important to distinguish between material jetting with metals and plastics, it should be noted that mould inserts are usually made out of metals”, Hilkene notes.
However, this does not mean that, one cannot produce a mould insert in plastic. As a matter of fact, the aforementioned AM technologies that can be used to produce mould inserts include FFF which processes plastic materials. Here is the thing, we need to understand what explains the move from traditional manufacturing processes to additive manufacturing processes; and thereafter the choice for one specific AM technology over another.
Surface finish depends on process, orientation and finishing operations. A metal insert may also be selected for thermal and mechanical properties; surface appearance alone cannot determine the choice.
Photopolymer inserts can facilitate trials and design changes. Their cycle life nevertheless depends on resin, geometry, temperature, pressure and the injected polymer. Stratasys guidance for injection moulding describes these operating constraints.
For the production of mould inserts, what will tip the balance in favour of LPBF or MJ?
In order to assess which technology best suits the production of mould inserts, we’ve compared these two technologies based on five main criteria: complexity of the geometry, post-processing, timing, production volume and costs.
The analysis draws on the 2021 interview with Cullen Hilkene, then CEO of 3Diligent, ZARE’s contributions and the cited technical documentation. It compares selection criteria rather than identical test results for the two processes.
Complexity of the geometry
It goes without saying that there are some products that are easy to manufacture using injection moulding because of their simple single cavity design and their size. However, it is no secret that AM is appealing because of its ability to produce complex geometries, especially complex cooling channels.
Hilkene highlights LPBF’s ability to produce cooling channels that follow the cavity geometry. Their design aims to distribute heat extraction more effectively. The gain depends on sizing, channel condition and thermal validation of the mould.
Post-processing
LPBF requires finishing appropriate to the tooling. The technology “does not always print as smooth as the other manufacturing processes, hence the need for a post-processing step at the end of the printing process”, 3Diligent’s CEO notes.
Photopolymer jetting can provide a fine surface, but supports must be removed and cleaning or finishing may still be needed. A good surface finish does not guarantee wear resistance or thermal-cycle life.
Time, costs and volume production
Lead time and cost must be compared for the same function and required service life. A photopolymer insert can suit a trial campaign, while a metal insert may better meet higher loading or cycle-count requirements.
The calculation should include design, printing, treatments, machining, support removal, inspection and any insert replacements. Post-processing is not invariably the largest cost item.
The required injection-moulding quantity is central. Photopolymer inserts mainly serve trials and selected short runs within their operating limits. Metal inserts, whether printed or machined, also require qualification for the intended production run.
Compare for the intended application
The table below provides a quick look at the items that have been assessed to compare LPBF and MJ for the production of mould inserts.
| Criterion | LPBF metal insert | Material-jetted photopolymer insert |
|---|---|---|
| Conventional alternative | Machined metal tooling | Machined tooling or another prototyping route |
| Geometry | Conformal channels and complex shapes within manufacturing constraints | Fine details within material and service limits |
| Post-processing | Depowdering, support removal, treatments and machining as required | Support removal, cleaning and finishing as required |
| Lead time | Calculate across the complete process route | Can suit rapid iteration; include preparation and trials |
| Cost | Assess manufacture, qualification and service life | Assess manufacture and possible replacements |
| Moulded quantity | Depends on alloy, design and mould qualification | Trials and selected short runs within validated limits |
Select and validate the tooling
We can’t say for sure that the challenges surrounding tool design within injection moulding industries will all be addressed anytime soon. However, one can testify to the various manufacturing processes that can now produce a tool having complex geometries, “with accurate dimensions, feeding of material into cavity, cooling channels and easy ejection of solidified part”.
Additive manufacturing can shorten some iterations and enable geometries that are difficult to machine. Acceptable lead times, dimensions and quantities must nevertheless be established with the supplier for each project; values from one case do not define a general limit.
Validation must cover both the moulded parts and insert performance over the required cycles. This comparison therefore replaces neither representative moulding trials nor tooling qualification.
This exclusive feature has first been published in the 2021 July/August Edition of 3D ADEPT Mag. Featured image: 3D Hubs
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