Donna Bibber, vice president, Isometric Micro Molding
It is well known that the micromould and micromachining are key to the realisation of micromoulded components and microautomated assemblies. Both capabilities require tooling and fixtures manufactured to extreme precision and positional accuracy in order to produce micro core pins, micro cavity geometry and microfluidic surfaces. In Part 1: Tooling—the enabler, published in the February issue of CMM1, I began to address the risk-mitigating criteria in micromould design, focusing on runner style and gate style. This second instalment details other such criteria in relation to gate diameter, parting lines, ejection and cooling.
Mould design criteria
Gate diameter
Figure 1 shows gate diameters of 0.15–0.33 mm (150–330 μm), and figure 2 shows a pin gate and edge gate. Whenever possible, and when wall thicknesses allow, a pin or tunnel (sub-gate) is selected so that the part does not require a secondary operation to remove it from the gate. This is because additional handling increases the risk of unwanted particulate and damage.
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Figure 1
Gate sizes used in micromoulding
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Figure 2i
Micromolding pin (2i) and edge gate Styles (2ii)
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Figure 2ii
Micromolding pin (2i) and edge gate Styles (2ii)
Selection of the proper gate size in micromoulding is similar to conventional moulding; the gate diameter is typically 70 percent of the wall thickness and the location of the gate is still required on the thickest section of the part. For example, a micromoulded part with a wall thickness of 0.1 mm (100 μm) may be able to have a 0.15 mm (150 μm) diameter gate without the gate pulling the material and leaving a void. This unorthodox increase in gate diameter may be the difference between being 99 and 100 percent full. Not all polymer materials can be subjected to larger gates without their creating void damage, so testing this ahead of building a mould is good practice.
Injecting polymers through these tiny gate diameters demands significantly higher injection pressures than seen in conventional moulding; 40,000–50,000 psi of injection pressure at 0.01 seconds of fill time are commonplace. As a result of the polymer being subjected to this high injection pressure and speed, gas builds up in the mould cavity and requires very precise mould vents.
As can be seen in figures 3 and 4, the sharp point of the 0.16 mm (160 μm) tall microneedles represents the bottom of the mould cavity and is the last place to fill. In the absence of proper venting, the polymer gas is trapped and collects at the end of fill, resulting in rounded microneedles. This rounding effect would render the microneedles useless because they would not be sharp enough to pierce the skin and could cause the patient pain. On the other hand, vents that are too large have excessive flash. It is therefore necessary to create extremely accurate vents between 0.003 and 0.005 mm (3–5 μm) to achieve a balance of crisp and sharp features without excessive flash.
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Figure 3
Micromoulded needles
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Figure 4
Micromould vents
Parting lines
The locations of parting lines are selected based on a balance of part design and micromould design simplicity. Figure 5 shows a pupil expander that required a very tight parting line. This device holds the very delicate tissue of the eye open to expose the pupil for a surgeon; if the parting line shut-off has too much of a gap between the mating halves (>0.005 mm (>5 μm)), it could tear the tissue.
Figure 5
Micromoulded pupil expander
Parting line precision is often one of the most critical features of the mould, and the machining method needs to be evaluated during the design phase to ensure that the proper fits can be achieved. For example, if a parting line requires sinker electrical discharge machining (EDM) instead of grinding due to geometry constraints, this needs to be reviewed by the manufacturing team to ensure that the required accuracy is achievable given the specifications and chosen plastic material for the moulded part. Regardless of the parting line machining method (machining, grounding, wire EDM or sinker EDM), specialised micromachining equipment is required in order to create shut-offs that can withstand extremely high injection pressures and not flash. If the machining process cannot hold micron-level accuracy, the risk of moulding a defective product increases significantly. This is especially true with resins that flash easily.
Parting line maintenance is much more important in micromoulding than it is in conventional moulding. Bearing in mind that flash can often occur if there are gaps in the tool steel of >0.005 mm (>5 μm), even microscopic gate residue left by the de-gating process can result in flash or other part defects over time. There are critical-use medical device products where microscopic defects of this type, although not visible to the naked eye, are not allowed. For this reason, it is necessary to document and implement a very stringent maintenance regime that covers cleaning, inspection and proper care of the parting line as well as pulling the mould for cleaning more frequently than is required in conventional moulding.
Ejection
The ejection of microparts is more complex than conventional-sized parts due to the size of the surfaces available to eject on. Ejector pins and blades as small as 0.2 mm (200 μm) are common and, similar to vents and parting lines, require 0.001–0.002 mm (1–2 μm) fit clearance to avoid flash. Also, due to the fragility of many microparts, it is not uncommon to implement insert/core floats to retract steel, which alleviates the force required for small ejector pins to eject without breaking and/or deforming the part. Surface finish of the mould cavities and cores can be rough or highly polished, depending on the polymer being moulded. This marriage of surface area and ejection force is designed in the mould to prevent part warpage on ejection.
It is important that the part is ejected directly from the mould into a very precise robot end-of-arm tool (EOAT). This allows the datum structure of the parts to remain intact for any additional automated assembly integrated directly at the micromoulding machine. If the parts are packaged in bulk, the datum structure must be re-established again later on. Having ejection-to-automation at the moulding machine keeps parts in the least number of setups, thus reducing handling, particulate, the possibility of datum structure errors and the risk of damage during shipment.
Cooling
As in conventional moulding, temperature control can be critical in micromoulding. Although the techniques are the same, the ability to achieve temperature control whether applying cooling or heating close to the moulded part can be challenging in micromoulding. For example, it is commonplace to have direct water cooling in a 4 mm diameter core, but it is significantly more difficult in a 1 mm diameter core. As with other aspects of the micromould building process, cooling circuits require micromachining techniques, including micro EDM drilling and/or high-speed micro CNC machining.
When heating is required close to the cavity steel, cal-rod heaters as small as 2 mm in diameter are implemented to provide concentrated heat where needed. One cannot assume that if the mould component (core pin or cavity geometry) is too small to accommodate conventional cooling circuits, that the moulding process will be okay without them. There is an inaccurate assumption that because the shot-sizes are so small, the heat input is also very small, making it negligible to the process. However, because the mass of individual tool sub-inserts is also relatively small, they can heat up very quickly. In addition, due to clearance being present between these sub-inserts and the larger cooled tool steel inserts that they are retained in, heat is not efficiently dissipated. This can cause part deformation, size issues and sticking in either side of the mould.
Differences between micromoulding and conventional moulding
It is a misconception in the moulding industry that micromoulding is the same as conventional moulding, just on a micro scale. However, as shown in figures 6–8, micromoulding is very different in terms of precision, tolerances and processing. The barrier to entry for truly micro-sized polymer components is in the mould itself. Regardless of the challenge to tight (single micron) tolerances, the micromould must be built to 20 percent of tolerance to allow the remaining 80 percent of tolerance to be distributed across gauge R&R, material lot-to-lot variation, and material drying and micromoulding processes. All of these inputs must contribute to a process that is capable of 1.33 Cpk or higher. Figure 6 shows a histogram of a micromoulded part with 0.008 mm (8 μm) tolerance. In order to achieve 1.33 Cpk, each input must be risk mitigated to produce nearly 0.001 (1 μm) of variation for the total to add up to 0.008 mm (8 μm). Literally seeking out single microns in each area is extreme but statistically necessary for a successful micromoulding project.
Figure 7 shows the importance of tooling and precision in micromoulding relative to conventional moulding. Only 30 percent of conventional moulders have in-house mould making, and in contrast, 100 percent of micromoulders have in-house mould making. Having micromould building in-house is paramount to micron-level tolerance plastic parts, because the design datums, communications relating to the customer’s requirements and inspection datums are all included in the design history record. With tooling as the enabling technology, a large gap would exist if this was outsourced to a third party and the highest risk to the device could not otherwise be mitigated.
Figure 8 shows the trifecta value proposition of micromoulding, which encompasses high complexity, extreme precision and micro features. The perfect, or bullseye, fit for this value proposition are microfluidic devices and microautomated assemblies, as these are larger in size (1–2 cm) but still require single micron features. Both the building of the micromould and inspection of these micron features require a micromoulder as this is where the risk is highest.
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Figure 6
Micron tolerances at 1.33Cpk
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Figure 7
In-house micromould making: barrier to entry1
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Figure 8
Micromoulding value proposition: barrier to entry 2
Should micromould building be outsourced?
Risk mitigation has developed significantly in recent years and although medical and drug delivery device OEMs have spent many years reducing their supply chains, they find themselves adding micromoulding and microautomation suppliers as niche suppliers. The tooling barrier to entry risks highlighted in figure 7 are too high to ignore, and the ability to measure fractions of a micron are necessary to build and validate micromoulded components and microautomated assemblies. Outsourcing the tooling or the inspection methods creates technology gaps and corresponding risks in design history files of medical and drug delivery device OEMs.
And so concludes my summarisation of micromould tooling as one of the highest risk enabling technologies in micromoulding and microautomation. The tiny core pins, surface finish, ejection, parting lines and gate design all contribute to a successful medical or drug delivery device.
References
1Bibber, D. (2019). How to approach today’s micromoulding challenges, Part 2: Tooling—the enabler. CMM; volume 12, issue 1. Available at: bit.ly/2WfbYb1