There are more factors involved in selecting a thermoplastic material and molder for micromolded components than for macro applications because as parts get smaller, the relative effects of manufacturing processes increase. To start, here is a brief refresher on the basics of thermoplastics.
Thermoplastics can be divided into two primary categories — amorphous and semicrystalline materials. Common amorphous materials include ABS and polycarbonate, while nylon, acetal, PBT, and PEEK are examples of semicrystalline materials. The majority of thermoplastic materials used in medical device and implantable components are semicrystalline.
All polymers contain a distribution of polymer chain lengths. Amorphous materials and semicrystalline materials in their molten state resemble cooked spaghetti noodles. The molecules are disorganized and positioned randomly relative to neighboring molecules. As material temperature rises in preparation for molding, the molecules become excited and move apart from each other. This increased spacing lets the polymer chains flow through the molding machine and eventually to the mold cavity where the heat is removed.
As the polymer cools, the major difference between amorphous and semicrystalline materials becomes evident. Chains in amorphous polymers maintain their random orientation and merely draw closer together. Semicrystalline materials, however, develop multiple crystallization sites in which the crystal structures grow as molecules continue to align in a tightly folded formation. (To continue the spaghetti analogy, think of these sites as the “meatballs.”)
The crystal structures continue to grow as long as free molecules have enough heat energy to move and “find” a crystal to join. A properly cooled semicrystalline material “freezes” into the correct mixture of crystalline and amorphous regions (the proper balance of spaghetti and meatballs). The amount of crystallization dictates physical properties such as long-term dimensional stability, chemical resistance, fatigue strength, and hardness of the finished molded part.
All other factors being equal, crystallization depends on the cooling rate. Cooling too quickly will result in a disproportionately amorphous material and the loss of potentially critical physical properties. When a material is not cooled properly, post-mold re-crystallization can continue for months, even years, after molding. For example, parts in an overly warm warehouse might later warp and shrink from continued crystallization.
Material degradation is a significant issue in micromolding applications. Degradation is the change in physical properties from the reduction of a polymer's average molecular weight during molding. Unfortunately, there is no consensus in the industry regarding how to measure degradation or how much is too much.
Generally, the molding industry attributes degradation primarily to residence time, the amount of time the material stays at processing temperatures. This is calculated by dividing the barrel capacity of the injection-molding press by shot weight and multiplying by cycle time. As parts become smaller and smaller, shot sizes dramatically decrease and residence times increase.
So when is the residence time too high? One material supplier recently recommended a 15-min maximum residence time for its semicrystalline PEEK. Follow-up conversations with other PEEK suppliers provided ranges of 4 to 6 hr. A range of 15 min to 6 hr is not reasonable. Overlooked is the fact that residence time is not the only determining factor in material degradation. In fact, mechanical stress and contamination must be considered as well.
Mechanical stress is induced in the material throughout the molding cycle. In a standard reciprocating-screw molding machine, material is heated to its molding temperature via heat conducted through the barrel. The screw also introduces shear heat by compressing and smearing the material against the barrel wall during plastication. This smearing action improves heat transfer into the melt and helps mix the material to homogenize melt temperature. The mechanical working of the material, however, also induces stress and contributes to molecular chain breakage.
The tortuous path the material travels through the barrel, check-ring assembly, press nozzle, runner system, gate, and mold cavity induces further stress. Increasingly smaller parts and expensive exotic materials (up to and even over $20,000/lb) have pressured companies to reduce material waste by thinning part walls and reducing the size of runners and gates. The shear stress imposed during material flow through these restrictions can be considerable, significantly contributing to degradation.
Contamination from the wrong amount of moisture in the material can also cause degradation. Nylon, for example, needs some moisture to lubricate it during molding. Over-drying can result in increased viscosity and oxidation while excessive moisture breaks molecular chains and inhibits proper crystal formation because water molecules bond with chemical groups within the polymer chain.
Degradation measurements vary from simple melt-flow analysis to complex gel permeation chromatography (GPC). These methods measure average molecular weight, but they don't indicate whether the material will perform as stated. Combining these analyses with mechanical testing such as dynamic mechanical analysis and chemical-compatibility testing provides a clearer indication of suitability for use.
Don't fall into the trap of using residence time as your only measure of degradation. This would be akin to rating a movie on its run time. You've got to evaluate what's happening to all the characters in the molding process as well as in the movie.
Selecting a molding partner
With other industry segments shrinking, many molders are jumping into the medical market. Desperation to survive drives some to chase business in which they have little or no experience. To provide the best product, a molder needs to be your partner, not just a supplier. A good molder is involved as early as possible in the product-development process, provides product-design assistance, and uses up-front design and process analysis. These attributes combined with sound procedures, a solid understanding of the science behind the molding process, knowledge of the benefits and limitations of their equipment, and open and honest communication with customers and suppliers will provide product designers the greatest opportunity for success.
Lastly, the molder must have a solid understanding of how and where the final product will be used to ensure the molding process produces parts with the correct properties. Factors such as assembly methods, sterilization, service temperature, exposure to cleaning or bonding chemicals, and exposure to UV all play a part in successful material selection and processing.
For more on micromolding, see Tech Spotlight, page 22.
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