The development of moulds for injection moulding processes is often painstaking, highly expensive and time-intensive. However, 3D printing is helping manufacturers make significant savings, both in costs and time.

Injection moulding (IM) — injecting plastic material into a mould cavity where it cools and hardens to the configuration of the cavity — is best used to mass-produce highly accurate, often complex, three-dimensional (3D) end-use parts and products. Hard-tooling moulds are usually made from tool steel with a CNC milling machine or via electrical discharge machining (EDM). When used in mass production, they can last for millions of cycles, but they cost hundreds of thousands of dollars. What’s more, lead times to produce these moulds are often measured in months rather than weeks or days.

When tens of thousands of injection moulded parts are needed, soft-tooling is an option. Made in aluminium, these moulds are less expensive (typically $2500-$25,000) and faster to produce (two to six weeks).

Unfortunately, the cost and time of tooling moulds is often compounded by factors like design mistakes that require the mould be remade correctly or the need to create multiple iterations before the final part design and quality are achieved. It is with these issues in mind that manufacturers have begun to embrace the use of 3D printed moulds to create functional IM prototypes.

Polyjet: The modern alternative

PolyJet technology is an exclusive method of 3D printing offered by Objet 3D printers from Stratasys that gives companies the ability to build injection moulds in-house, quickly and easily. PolyJet printing creates 3D objects by positioning successive layers of liquid photopolymer into desired configurations. The plastic is then cured (solidified) with UV light. Once fully cured, moulds can immediately be placed into IM equipment and used to create 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, performance data.

PolyJet injection moulds are not intended to be replacements for soft or hard tools used in mid- and high-volume production. Rather, they are intended to fill the gap between soft tool moulds and 3D printed prototypes.

Key points related to PolyJet moulds:

  • The initial cost of creating a PolyJet mould 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. As a result, the cost per part is medium.
  • 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.
  • In cases where design changes are required, a new iteration of the mould can be created in-house at minimal cost. This, combined with the speed of PolyJet 3D printing, allows designers and engineers greater design freedom.
  • Moulds created in Digital ABS material can be precisely built in 30 micron layers, with accuracy as high as 0.1mm. These production features create a smooth surface finish so post-processing is not needed in most cases.
  • 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.
  • No pre-programming is needed to create PolyJet moulds. Also, once the CAD design files are loaded, the 3D printing process can run without manual intervention.
  • The manufacturing time to injection mould a part using a PolyJet mould is relatively low, though not as low as conventional moulding.

Material selection

Proper material selection is important 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 temperature resistance. Other PolyJet materials like rigid FullCure 720 and Vero also perform well as IM moulds. However, when used to create parts with complex geometries, moulds made from these materials will have shorter lives than those made with Digital ABS.

The best materials for creating injection moulded parts are those that have reasonable moulding temperatures (< 300 degrees Celsius) and good flow behaviour. Ideal candidates are polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), thermoplastic elastomer (TPE), polyamide (PA), polyoxymethylene or acetal (POM), polycarbonate-ABS blend (PC-ABS), or glass-filled polypropylene or glass-filled resin (G)

Plastics requiring processing temperatures of 250 degrees Celsius 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.

It is also useful to take a look at how the use of injection moulding with a PolyJet mould compares with injection moulding with an aluminium mould. The time savings can be highly significant, ranging between a few days and several weeks. Additionally, the cost to produce the moulds is generally 40% -70% cheaper.

Field testing

Along with Nypro Healthcare, a global manufacturer of precision plastic products for the healthcare and packaging industries based in Bray, Ireland, Stratasys conducted a series of tests to assess the performance of rapid prototyped cores and cavities with critical features that included gears, interlocking legs, ratchets and catch features.

During one of the many tests conducted, sample ABS parts were injection moulded into a single PolyJet mould made from Digital ABS. Parameters such as maximum pressure, cushion, and core and cavity temperatures were tracked.

Upon completion of the tests, the mould was deemed to be stable as indicated by a constant injection pressure and cushion, and that by using the recommended procedure for mould cooling, the temperature in the core and cavity did not exceed 58 degrees Celsius. What’s more, the quality of the injection-moulded prototypes was deemed by Nypro to be “good.”

Nypro offered the following analysis of the tests: “It can be concluded that the injection moulding trials were very successful. The process of printing cores and cavities can be considered an advantage in terms of time, initial functionality evaluations and reduced tooling cost.”

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. Tool designers should consider the following changes when creating a PolyJet mould as opposed to a conventional steel mould design.

  1. Designing the mould
  • Increase draft angles as much as the part design allows. This will facilitate ejection and reduce stress on the tool as the part is ejected.
  • Increase gate size to reduce shear stress.
  • The gate should be located so that the melt entering the cavity will not impinge on small/thin features in the mould.
  • Avoid using tunnel gates and point gates. Instead, use gates that reduce shear such as a sprue gate or edge gate.
  1. Printing the mould

To maximise the opportunities created by PolyJet 3D printing, the following guidelines are recommended:

  • Print in glossy mode to ensure smoothness.
  • Orient the part in Objet Studio software so that the glossy surfaces are maximised.
  • Orient the mould so that the flow of polymer is in the same direction as the print lines.
  1. Finishing the mould

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:

  • The mould will be fitted to an ejection system. To ensure a tight fit between the ejector pins and the ejector pin holes, program the holes into the STL file but reduce their diameter by 0.2mm-0.3mm. Then, when the mould is cured, ream the holes to the exact final size.
  • Inserts are being fitted onto a base.
  • Extra smoothing of surfaces is needed.

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.

  1. Mounting

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. Figure 8 mould inserts are fitted onto 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.

  1. Injection moulding process

When using the PolyJet mould for the first time, the best practice procedure is:

  • Start with a short shot and a slow injection speed. The fill time can be 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 1mm or less), start with a 30-second cooling time and adjust as needed. For larger parts (wall thickness of 2mm 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 pressurised air. This will preserve part quality and mould life. Alternatively, automated mould cooling fixtures may be used.

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.

PolyJet moulds are unique in that they perform in the same way as metal moulds but are much cheaper, easier and faster to make. With PolyJet technology, manufacturers can produce prototypes at speeds and costs far below traditional methods. As a result, 3D printing allows manufacturers to easily evaluate the performance, fit and quality of potential products before mass production starts.

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