To mold micro thin-wall devices, polymer selection, ultra-precision tooling, and knowledge of CFD (computational fluid dynamics) is critical. For micro molded devices to be small, compliant, comfortable, and/or flexible, they may require very wall thicknesses as thin as 0.002″ (50 µm). Also necessary is the analysis, tooling, processing, and handling of very thin-walled micro molded components. A growing number of medical and drug delivery devices require extremely thin walls that are consistent, pliable, and strong enough for either single-use or multiple uses over an extended period of time.
Thin-walled vessels always accompany a discussion about aspect ratio (how thin and for how long it’s thin). In this article, we will highlight some examples of products with up to 180:1 aspect ratios from wall thickness to length using several medical and implantable grade polymers.
The tiniest of parts in a completed assembly is most challenging but also usually the enabling component(s) of the entire device. Similarly, micro tooling created by micromachining is critical to these parts ever making it to validation. Many new advances in micromachining technology are enabling the design and manufacturing of micro molds, which then enable very thin wall injection molding in thermoplastic, silicone, and metal. This is leading to the worldwide advancement of less invasive, less painful, and navigable medical and drug delivery devices.
Examples of these applications include:
A few planets need to be aligned in order for thin-walled micro molding to be possible: a generous melt flow polymer (generally speaking >10-g/10-min melt flow); an extremely accurate core-to-cavity alignment (generally speaking <25% of tolerance); and highly balanced flow path (by mold design, part design, venting, etc.).
Most thin-walled micro molding applications exhibit a level of extreme manufacturing challenge. It is rare that they are manufacturable as initially designed or napkin-sketched, but often they are scaled-down versions of something similar on the market, just several magnitudes smaller or requiring an experimental material due to a drug-releasing agent or characteristic.
These devices may require pharmaceutical drugs directly compounded with or added to polymers, metals, membranes, and have working gears, levers, and drive mechanisms to make the device function repeatedly and with robust longevity. With these functions in mind, and the fact that these devices are so thin-walled, it is important that they are developed in such a way that it is expedient, robust, and tested to form, fit, and function to be able to work in extreme environments such as the human body or at exposed temperatures and pressures. As a result, thin-walled micro molded components require a solid process plan for success.
This thin-walled micro-molding plan may include some or all of the following critical considerations:
A process plan for thin-walled micro molded device or assembly must first consider if the thin walls will fill with a certain material, melt flow, and tooling strategy. A solid model is used to perform computational analysis to theoretically estimate these conditions and their design. Because micro molds and initial parts can be costly in their development cycle, micro mold flow analysis provides an insurance policy that a particular design will fill with a particular material. Micro molded components can be as small as dust specks or have features that are that small.
An assumption made all too often when comparing parts made by conventional molding methods to micro molding methods is that parts can be filled with the same software and same modeling methods. Flow analysis and modeling parts flowing through a 0.02″ (500-µm) gate is very different from parts flowing through a 0.003″ (75-µm) equivalent gate.
The difference between a part going through a 75-µm equivalent gate vs. a 500-µm equivalent gate is that it sees much more thermal energy from the shear heat pushing through that small orifice. As a result, the solid model mesh resolution is required to be extremely high to determine what is happening in the gate and thin-walled areas. Similar to this is using an inspection gage requiring one decimal point higher of resolution than the specification requires: The mesh of a solid model used in a mold flow simulation requires single-digit micron mesh when tens-of-microns mesh is required in the part.
Another important factor in a mold flow or thermal simulation is processing knowledge and direct application knowledge with injection molding and mold making. It is important to know the practical experiences in these processing techniques along with the mold and die design to accurately depict proper mold flow. Knowledge in plastics engineering, micro mold design, gate location and size, and runner and sprue geometry is critical to proper analysis of the results of a micro mold flow simulation.
Process-specific and material-specific knowledge help to draw from experience. Critical to many micro molded components, the gates will have to be properly sized so as not to place undue heat stress on the material entering the cavity. For heat-sensitive materials such as bioresorbable and biopharmaceutical polymers, the residence time in the injection barrel, nozzle, and hot runner is important to understand and to minimize such that additional heat is not placed on the material during processing. These factors are important for theoretical filling and starting out with a solid process plan is one step in the right direction. The true enabler, however, to creating a thin-walled micro molded component is ultra-precision molds and tooling.
With any micro molding technology (thermoplastic or silicone), the tooling is the most critical component to success. Because the parts and molds are so small, the tolerances get smaller and the tooling must still be made to 25% of part tolerance to provide a good processing window. With tolerances of ± 0.01 mm, the steel tolerances must be ± 0.003 mm to achieve a good window.
There are some nuances to consider with thin-walled micro molded parts whether they are thermoplastic or silicone injection molded.
Runner/sprue: The runner and/or sprue (if one exists), can be our friend or foe in micro-assembly. We could use it as part of an assembly aid to hold onto a part in the automated assembly or add special locating “jogs” in the runner aid in the positioning in an assembly nest.
Parting lines: Molded halves come together and form parting lines on molded parts on the order of ~10 µm. These parting lines need to be considered when they will be assembled to other parts, they can prevent proper fit if they are not “guided” or moved through the assembly process properly. These 10 microns can easily make or break your assembly and may need to be positioned in the assembly to avoid these features being stacked up against each other.
Draft: More is better but can be as small as 0.1° of taper. But this taper (inside or outside) of a molded part can be cumbersome to deal with. Having your micro part “ride” on a taper will provide an arbitrary or irregular surface with which to improperly position it for assembly to other parts. Ways around this are to eliminate draft on a small portion of the part being positioned, draft the assembly station/fixture with the matching draft angle, or add a feature to the part or runner that can be used and removed later on.
Gate location: It is critical to choose a gate location that will actually create uniform flow in a micro molded part. Without a uniform flow, the part may not fill the thin-walled cavities and do damage to the delicate pins and cavities. Thinking ahead as to how to remove this gate later on in the assembly process is important because if it has already been de-gated, the gate trim job may have left a divot or a proud protrusion that has to be rotated away from nesting, guide rails, or other parts.
Gate vestige: Most micro molded parts are kept on an edge gate. If so, they need to be de-gated properly to avoid issues with small “picks” of material causing damage to an artery or causing issues with automation and assembly. These small picks can be addressed in the mold design by placing a dimple in the wall thickness (if the walls are thick enough that is) so the vestige will “sit” below the surface of a guide or a mating component in the assembly.
Surface finish: Often overlooked, the surface finish of a molded part is important in “riding” or “guiding” features into other features. Some surface finishes in assembly are best served with vapor honing or roughening the surface to provide improved surface area for bonding, for example. Smoother surfaces in assembly can cause problems in ejection from the mold and a trade-off surface may be required in order to “steal from Peter to pay Paul.” Which one will be the lesser of the two evils is dependent on material selection.
This article was first published in the May 2013 edition of Manufacturing Engineering magazine. See examples of thin-walled injection molding devices here: Isometric Micro Molding Portfolio.