Differences Micro to Macro

My Dad always used to tell me that you can’t make chicken salad out of chicken shit!

Using the right tools correctly is the key to success in all walks of manufacturing. Effectively, you only get good stuff out if you put good stuff in, and nowhere is this more apt than in the field of mold flow analysis in the micro manufacturing world.

Picture the scene. A designer rubs his chin and muses on the situation he finds himself in now he has designed his micro part.

“What’s the big deal?” he thinks. “So the part is smaller than I’m used to. It’s just scaled down, right? I must be able to use the same software tools I already use in the macro world, and still get the results I need. Surely I can use the same Mold Flow Analysis techniques to get the fill pattern I need to make this smaller part.” Unfortunately not.

You hear it everywhere you go in the micro manufacturing world, the theme of tools that work in the macro world but that don’t work in the micro world. Often times, it is assumed that parts made from conventional moulding methods can be designed and tested with the same software and same modelling methods. “Isn’t it the same filling process going through a 0.020” (0.51 mm) gate as a 0.003” (0.076 mm) gate?” is the often heard refrain.

The answer is no, it isn’t.

Mold Flow Material Characterisation

Micro moulded components can be as small as dust specks and/or have features that are just as small. Because the tooling and initial parts can be costly, mold flow analysis provides an alternative to spending the tooling and prototype dollars up front, and provides a cost effective insurance policy that a particular design will fill with a particular material.

There are literally hundreds of thousands of polymeric materials. Within a mold flow database, the majority of these materials have previously been rheologically characterised. It is sometimes acceptable to use an existing material that is “close” to the properties of a material that hasn’t yet been characterised.

When critical tolerances are being sought, however, it is important to have the material pre-characterised to obtain the precise rheological coefficients of the particular material. With this information it is possible to predict a material’s flow, micro and sub-micron flatness, perpendicularity, and micro features; its ability to fill out, and its ability to thermally shrink to the specifications needed.

Importance of Mesh

Definition: A mesh is a mathematical representation of a solid model broken down into triangular nodes.

The difference between a part going through a 0.002–0.003” (0.0508–0.0762 mm) gate as opposed to a 0.020–0.030” (0.51–0.76 mm) gate is that it feels much more thermal energy from the shear heat when being pushed through such a small orifice.

In addition, the pressures needed to fill a small orifice part are on average 30,000–40,000 psi. For this reason, the mesh resolution needs to be altered, and the need to create many more data points within that area becomes critical to the analysis.

Similarly, when macro parts are scaled down to micro parts, the tolerances follow suit. That means that flatness and feature sizes are in the microns to tens of microns.

The same way an inspection is required to be an order of magnitude more precise than the specification measured, the mesh of a solid model being placed in a mold flow simulation requires submicron nodes when microns are needed in the part.

The shear heat going through this small gate is assisting in the flow of micro components by increasing the L:T (length to thickness) ratio. A part that is 0.005” (0.127 mm) thick and 0.500” (1.27 mm) long is not theoretically possible but is actually possible due to the shear factors that the material feels going through the small orifice. This is a very real example of the difference between the macro and the micro world.

There are three basic ways to complete a mold flow simulation. One is by fusion, one is midplane, and one is 3D analysis. The difference between these three and when to use them is critical.

Fusion analysis is a mirror image meshing technique that the model recognises when it is looking for its mirror opposite point across the model. This is particularly useful when surfaces (deflection, warpage, flatness, perpendicularity) are important.

Midplane analysis for creating a model is similar to a stick of gum in a rectangular box.

3D analysis uses a tetrahedral mesh that is useful for chunky, thicker parts.

In any mould simulation, a solid model is analysed for rogue elements (elements that have not been properly translated). These rogue elements must be manually manipulated and on average, there are 150–200 rogue elements per model. It is important to recognise these rogue elements and address them in the mesh because they can give false flow paths, which will produce inaccurate flow results. Remember the chicken salad!

The Importance of Knowledge

Another important factor in a mold flow or thermal simulation is processing knowledge and direct application knowledge concerning injection moulding and extrusion. It is important to know the practical experiences in these processing techniques, along with the mould and die design, to accurately depict proper mold flow. Knowledge of plastics engineering, mould design, gate location and size, and runner and sprue geometry is critical to proper analysis of the results of a mold flow simulation.

Processing conditions for resorbable polymers, for example, are extremely critical. Resorbable polymers are used for implantable applications and designed to “degrade” in vivo. For this reason, they are both ultra moisture and heat sensitive. Given this application and applying process-specific knowledge, qualified engineers will know that the gates will have to be properly sized so as not to place undue heat stress on the material entering the cavity. Also, it is important to understand and minimize the residence time in the injection barrel, nozzle, and hot runner so that additional heat is not placed on the material during processing. These practical experiences are then used in the application knowledge and analysis portion of a mold flow simulation to gain better perspective on part design as well as process design criteria.

Several factors produce chicken salad with mold flow simulation and its important to choose a mold flow simulation provider that has this application knowledge for micro components. Full knowledge of materials, rheological properties, and surface finish effects of flow are also important. Some eat chicken salad with celery, some with lettuce, some even with cheese, but the ingredients must be robust and combined well to result in the best chicken salad possible. Otherwise, it may leave a nasty taste in the mouth!

Good things may come in small packages, but great things come in micro ones.

Donna Bibber is a well-known micro manufacturing expert and the Technical Partner in microPEP, East Providence, RI. USA. microPEP provides a one-stop design and manufacturing solution for small and micro component applications. microPEP’s core competencies included micro stamping, micro cleanroom moulding with subsets of micro insert moulding, micro polymer optics moulding, micro moulded-In features and micro component plating. T. +1 774 230 3459, E.donna.bibber@micropep.com, W. www.microPEP.com.