Injection moulding is the technology used to mass-produce end-use parts and products. Moulds are at the heart of this process, however their development is often complex, highly expensive and time intensive. Moulds made from tool steel are used for mass production, lasting for millions of cycles. When just tens of thousands of injection moulded parts are needed, aluminium is an option. These moulds are less expensive, and faster to produce (2 – 6 weeks) than steel tools, the lead time for which is months. But for a few hundred parts, is aluminium still the most convenient solution? It is with these issues in mind that manufacturers have begun to embrace the use of 3D printed moulds to produce parts directly or, following a more recent trend, to make moulds that will be used to create functional IM prototypes.
PolyJet technology is an exclusive method of 3D printing additive-based production patented by Stratasys. Its operating principle positions successive layers of liquid photopolymer, which is then cured with IV light. The printing apparatus with 16 micron layers and precision to 0.1 millimetres produces smooth surfaces, thin walls and complex geometries. Polyjet technology has already been used make moulds for plastic injection moulding precision prototypes from the same material that is specified for use in the final product. These precision prototypes give manufacturers the ability to create realistic, finished-product examples that can then be used to gather true-to-life data. PolyJet injection moulds are not intended to be replacements for steel or aluminium tools. Rather, they are intended to fill the gap between soft tool moulds and 3D printed prototypes. Table 1 shows the areas where PolyJet printing is used for prototype production methods.
*Although FDM and laser-sintered processes use thermoplastics to create prototypes, the mechanical properties will not match those of an actual injection moulded part because a) the processes used to create the prototypes will be different, and b) the materials used to create FDM and laser-sintered prototypes are not generally the same as those materials used to injection mould final parts
Advantages of PolyJet technology
Compared to traditional moulds, PolyJet technology offers numerous advantages. First and foremost, the initial cost is relatively low. However, PolyJet moulds are best suited for runs ranging up to 100 parts depending on the type of thermoplastic used and mould complexity. Another favourable feature, as mentioned above, is that building a PolyJet mould is relatively quick. A mould can be built within a few hours as compared to days or weeks to create traditional moulds. This means that when design changes are required, a new iteration of the mould can be created in-house at minimal cost. Another beneficial aspect is greater design freedom. Complex geometries, thin walls and fine details can easily be programmed into the mould design. What’s more, these moulds cost no more to make than simpler moulds. Last but not least, moulds created in Digital ABS material can be precisely built in 30 micron layers, with accuracy as high as 0.1 mm. These production features create a smooth surface finish so post-processing is not needed in most cases. It is also useful to take a look at the following cost benefit analysis to understand how the use of injection moulding with a PolyJet mould compares to injection moulding with an aluminium mould (table 2). As can be seen in the analysis, the cost to produce the 3D moulds was significant, the cost for each mould being generally 40-70% lower, and the time savings were significant.
Proper material selection is critical for success when injection moulding using PolyJet moulds. Digital ABS is the best choice for printing IM moulds; it combines strength and toughness together with high thermal resistance. Other PolyJet materials like FullCure 720 and Vero also perform well as IM moulds, although when used to create parts with complex geometries, moulds made from these materials will have shorter lives. In general, the best materials for creating injection moulded parts are those that have reasonable moulding temperatures (< 300 °C) and good flow behaviour. This means that ideal candidates are: polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), thermoplastic elastomer (TPE), polyamide (PA), polyoxymethylene (POM) or acetal, polycarbonate-ABS blend (PC-ABS), glass-filled polypropylene or glass-filled resin (G). Plastics requiring processing temperatures of 250 °C and higher, or those that have high viscosity at their processing temperature will shorten the life of the mould and, in some cases, the quality of the finished part. Figure 1 outlines the relative number of parts that are typically produced using the different tooling methods.
Best practice guidelines
Injection mould design, an art in itself, requires years of experience and a profound understanding of the injection moulding process. Although the design considerations for creating and using a PolyJet mould are fundamentally the same as traditionally crafted moulds, there are some variations. Let’s take a closer look at the best practices suggested by Stratasys.
Designing the mould
In the design stage, draft angles should be increased as much as the part design allows. This will facilitate ejection and reduce stress on the tool as the part is ejected. Gate size at the point of injection should be increased to reduce shear stress. Moreover, the gate should be located so that the melt entering the cavity will not impinge on small/thin features in the mould. Lastly, tunnel gates and point gates should be avoided. Instead, gates that reduce shear such as a sprue gate or edge gate should be used.
Printing the mould
To maximise the opportunities created by PolyJet 3D printing, glossy mode is recommended to ensure smoothness, and to orient the part in Objet Studio software to maximise smoothness. The mould, on the other hand, should be oriented so that the flow of polymer is in the same direction as the print lines. A key benefit of PolyJet moulds is that they can be designed, built and used within hours. Most will require little or no post-processing work. However, further finishing may be needed if:
• Inserts are being fitted onto a base;
• Extra smoothing of surfaces is needed;
• The mould will be fitted to an ejection system.
Occasionally, light sanding of surfaces transverse to the mould opening is recommended. For example, prior to using a mould with a tall core, some light smoothing can facilitate part removal.
Stand-alone moulds – those that are not constrained to a base frame – can be mounted directly onto standard or steel machine back-plates using screws or double-sided tape. For example, figure 2 inserts are fitted into a base mould using bolts. With any chosen mounting option, it is critical to avoid direct contact between the nozzle and the printed mould by using standard sprue bushing. An alternative option would be to centre the mould’s runner with the sprue located on a regular steel plate.
Injection moulding process
The injection moulding procedure also requires special attention. The manufacturer has provided best practice procedures for using the PolyJet mould for the first time.
• Start with a short shot and a slow injection speed. The fill time can be as high as the melt does not freeze off as it enters the mould.
• Increase shot size until the cavity is 90-95% full.
• In the holding process, use 50- 80% of actual injection pressure and adjust the holding time as needed to avoid sink marks.
• Apply normal calculated clamping force value (injection pressure x projected part area) as initial value.
• PolyJet moulds have low thermal conductivity so they will require extended cooling times. For small or thin parts (wall thickness of 1 mm or less), start with a 30 second cooling time and adjust as needed. For larger parts (wall thickness of 2 mm or more), start with 90 seconds and adjust accordingly. The cooling time will vary depending on the type of plastic resin used.
• Minimum cooling is recommended to avoid too much shrinkage of the part on the printed cores. Extensive cooling may stress the mould when the part is being ejected and cause it to fail.
• After each moulding cycle, it is critical to allow the mould’s surface to cool by applying pressurized air: This will preserve part quality and mould life. Alternatively, automated mould cooling fixtures may be used.
A promising technique
The use of PolyJet 3D printed moulds allows manufacturers the ability to take functional testing to a new level, by creating product prototypes from the same IM process and materials that will be used to create the final product. With this technology, companies can generate superior performance data and validate certification confidence before starting production.