The growing use of nano and micro medical components prompts this look at best practices at Dynomax
By Peter Szewczyk
A critical factor for engineering of nano tooling begins with a breakdown of tooling components and the materials required. Materials used range from S-7 tool steel, for its overall good shock-resistance and high-hardness attainability, to H-13 tool steel, for its high-temperature strength and resistance to softening, to O6 oil-hardened tool steel, for its good “anti-frictional” properties and excellent metal-on-metal wear resistance.
Although high-strength materials are used, micro and nano-scale tooling components are still very fragile. Engineers must be aware of critical components that are susceptible to damage and allow for provisions to repair the tool in case of damage during molding production runs.
Our engineers incorporate the use of Computer Aided Analysis (CAA) software, commonly referred to as moldflow analysis, to run flow simulations on micro and nano-molded parts to verify the design and processability of the tool. At this point, data is observed to determine proper filling of the cavity, locate potential air traps, and ensure that weld/knit lines won’t form in critical areas, potentially reducing the structural integrity by as much as 80% of the nominal strength of the part.
Moldflow allows analysis of values such as shear rate, shear stress, and residual stress, which are highly important factors in the processing of a quality thin-walled part.
Shear rate is generally described as the rate of flow between layers. It is affected by the injection flow speed, and as speed increases, so does the shear rate. This creates shear heating—the ability to heat material without external temperature—and lowers the viscosity. Material suppliers will have a maximum level in their specifications, and if the maximum is exceeded, material degradation and other problems may occur. According to typical melt profiles, the shear rate is the highest where there is the greatest travel between layers of flow, typically between the molding wall and the melted material, or between stationary frozen-off material and melted material.
Shear stress is a measure of force that is generated by the polymer molecules flowing next to each other. Since shear rate is the highest at mold wall to melt or solidified material to melt, the shear stress will be the highest in these locations. Moldflow analysis ensures a uniform distribution of shear stress and that the force values are not too high.
Residual stress is basically a molded-in stress due to processing parameters such as flow or temperature that is “frozen” in a molded part. This relates to shear stress as it is one of the sources that can cause residual stresses. Other factors can be anything from temperature to processing conditions in general. Residual forces observed in moldflow analysis can be a cause of warping, shrinking and other similar problems.
Engineers use a set of process parameters in their simulations to minimize residual stress. Residual stresses in molding cause deformation as the melted material gets solidified too quickly and then tries to relax to its previous size. This is usually characterized by injecting the material too fast, followed by a rapid cooling cycle.
Nano and micro-scale molding can be categorized as precision high-speed injection molding since injection speeds can be ten times as fast as conventional injection molding. High shear rates, pressure, and heat must be used to counteract the restrictive flow path of thin walls, thereby allowing cavities to fill before the material solidifies.
A Narrow Window
These extreme conditions bring many processing issues, which in turn, gives a very narrow processing window where even the slightest deviation can cause a bad part.
With the increased shear rate, additional heat can be applied to the melted material without increasing mold temperature. However, care must be exercised so as not to exceed material specification for shear rate, as that can lead to material degradation during flow. As shear rates increase so do the stresses within a material. If the shear rates reach a value above the critical shear stress point, the primary bonds in the material can be jeopardized.
For example, if too high of a shear stress rate is reached with a material like Liquid Crystal Polymer (LCP) that has 30% glass filler (reinforcement fibers), the molecular chains can tear. This material tends to have high heat resistance, high chemical resistance and high mechanical strength.
In general, for glass-filled resins, the minimum fiber length must be assumed in order to retain the material’s structural properties. This means that if a resin has a glass filler with fibers 0.003″ long and the mold part has a 0.003″ wall thickness, serious problems could occur.
With LCP-molded nano parts, a commonly encountered problem is cracking or crazing. This is simply defined as a fracture in the molded part, yet it cannot be so easily remedied. By increasing the injection pressure and fill rate, internal stresses can be created that exceed the tensile strength of the material and cause the part to crack as it continues to cool after ejection.
Another factor contributing to the cracking of thin-walled components is initiated by the ejection force as it extracts the part from the tool. Elements such as undercuts, rough surface conditions, and/or no draft may cause this. In precision injection molding of nano parts, tight tolerances are of the norm—sometimes requiring features to be held within ±0.00025″ (0.0064 mm), which virtually eliminates potential to draft cavity features.
To overcome this, it is important to ensure that surface roughness is as smooth as possible. This begins with the manufacturing phase of tooling known as Electrical Discharge Machining (EDM). By utilizing high-density carbon electrodes with grain sizes of less than 1 µ, we believe we achieve the best possible surface finishes and produce near sharp corners—essential requirements for certain tooling. It is also critical to polish all molded surfaces in order to reduce the coefficient of friction during part extraction and to enhance molded part appearance.
Additionally, to overcome problematic elements such as deformation and cracking common with nano molded parts, we employ special processing equipment on our injection molding machines. In order to provide precise and better control of the material during melt and injection phases, a unique 8-mm injection screw unit is used. This ensures a uniform composition of the melted material before injection, along with minimal shot volumes and significantly shortened dwell times.
Material Selection and Process Monitoring
Proper material selection also becomes a big factor in precision injection molding for medical components. In certain cases, there are several different grades of a polymer available that can all be suitable for the customer’s requirements. Although these various grades may be similar, they can have very different melt flows in relation to one another; therefore, it is the processing engineer’s responsibility to determine the specific grade of material that will result in proper filling of the cavities, so that the molded part meets and exceeds customer specifications. Certain materials that may include abrasive fillers such as LCP, due to its glass content, can cause tooling wear. If the proper tool steels aren’t used, such materials can cause wear early in the life cycle of the tool. Given these factors, it is essential to inspect all precision tooling routinely to avoid problems that may arise down the road.
During production runs on injection-molded components, it is important to ensure a 100% quality-conforming product at all times. If there is a defect with a molded part during a production run, it is important to identify all molded parts that were affected and isolate them as “failure.” It can be catastrophic if a failed part is allowed in an assembled medical end product.
In addition to all the latest moldflow software, sometimes manufacturers may need to use process monitoring software to correlate theoretical data to experimental data. Specifically this means incorporating cavity pressure/temperature transducers for monitoring and evaluation.
Cavity pressure is widely regarded as one of the most important processing variables that can ultimately be characteristic of part integrity and quality. Slight changes in temperature and pressure, along with variations in composition of raw material, can result in scrapped/defected parts due to the narrow processing window for precision-molded nano components. To better understand what is happening inside the cavity of the tool, manufacturers integrate cavity pressure sensors that can log data on filling, packing, and holding phases, creating a cavity pressure profile curve. By analyzing this data, on the fly changes can be efficiently made to address processing issues or optimize cycles.
There are many instances in precision injection molding where duplicating processing parameters and injection molding machines can yield differences in the quality of a molded part. By actively monitoring cavity pressure profiles, a baseline can be set to theoretically duplicate part quality for future production runs and ensure an even pressure profile.
Automation can also be incorporated by interfacing robotics to scrap parts that process monitoring software deems defective. This can help manufacturers eliminate the need for manual part sorting. Processing parameters can also be automated with real-time control of certain machine parameters. This dynamic control relies on feedback from the transducer, and then interprets the data. By monitoring the cavity pressure, and generating and evaluating profile curves, the controller can then send signals to the injection molding machine to optimize processing parameters.
By taking advantage of the different cutting-edge technology resources available, manufacturers are able to produce quality precision parts exceeding customer requirements. From engineering and moldflow analysis, to precision manufacturing and specialized processing techniques, they contribute to ongoing advancements in the growing world of nano components. ME
Published Date : 10/30/2013