Plastics are incredibly versatile. There are thousands of types available to satisfy a myriad of medical applications. Yet, certain plastics present special challenges during the injection molding process that — if not properly addressed — can impact product quality and ultimately preclude medical devices from meeting exacting specifications. The medical device or component cannot — and must not — fail. To do otherwise increases the risk of recall, warranty claims, and litigation. In worst-case scenarios, failure may prove lethal.
Ideally, when developing devices that will be made of plastics and thus require molding of parts, an experienced, technical injection molder should be included in the project concept and design phase as well as throughout production. Their expertise in plastics, part design, and engineering is invaluable in avoiding delays, added costs, and potential dangers of designs that aren't well suited for the injection molding process.
Involving an experienced injection molder early on in the design process helps safeguard against adverse outcomes. So, when is early too early? Never. The sooner a molder's specially trained engineers are integrated with a medical OEM's design and engineering teams to develop an injection molded solution for a critical-use medical part or component, the more accurate the consistency and performance of the final application.
Injection molders can identify and correct design flaws that can lead to failure like faulty gate types and locations, insufficient draft, trapped steel, or knit lines. Knowing what to look for (and what to avoid) promotes plastic part consistency, which is pivotal in keeping failure rates low.
While improper manufacturing may lead to medical device malfunction, not all defects are unanticipated since devices with moving parts will wear and degrade over a lifetime of use. The consistency offered by injection molders is crucial in these circumstances, too. Since the injection molding process for making the original and replacement plastic parts is consistent, the components are identical and can be substituted with minimal risk.
Experienced plastics engineers bring specific insights about end-use into the design phase. Beyond resin strength and durability, they can speak to available material characteristics that match, and could enhance, application performance. Specific to medical devices, there may be a need for antimicrobial or chemically resistant components, or resins that can withstand high temperatures or repeated impact.
Likewise, a molder's experience with stress loads and stress points will inform and possibly modify the design well before it goes to production, preventing costly re-work and end-use defect or failure.
Early examination of key functionality factors and the environments and expectations surrounding a medical application's usage helps lead to desired outcomes: quality design and high performance.
Process and Analysis Working Together
The key to helping prevent defects is partnering with an injection molder that uses design for manufacturability (DfM) to apply comprehensive moldflow analysis and extensive plastics engineering experience during the design phase to identify any potential issues and refine the production strategy.
Using moldflow simulation and analysis during the design phase provides multiple benefits: higher quality parts, optimized cycle times and fewer tooling trials. The process uses a sophisticated computer program to analyze and predict the flow and cooling of plastic during all phases of the injection molding process.
Since there are different levels of analytical software available, selecting the right one for a project is as important as interpreting the data for successful results.
The most basic level of moldflow analysis includes a simulation of the fill and packing phases. This helps optimize gate location, gate sizes, runner balance, and process conditions.
More advanced simulations compare runner systems and mold temperatures and can even predict fiber orientation. They provide critical information for predictions regarding more complex parts and processes.
Complex injection molders entrusted with producing plastic components for medical and other critical-use applications play a pivotal role in ensuring that the device performs as designed. Partnering with one that specializes in DfM provides an added degree of confidence.
Identifying and Correcting Defects in the Injection Molding Process
A complex injection molder's experience dictates how well they understand what could potentially go wrong in the molding process and, as importantly, how to prevent missteps that could otherwise compromise medical device quality, consistency and performance.
Focusing on five critical areas helps prevent or correct potential defects: residual stress, warping, sink marks, short shots, and flash.
Residual Stress. Residual stress results from internal forces that accumulate while a part is cooling in the mold. Residual stress often fluctuates due to a tooling or injection molding machine change. The fluctuations are imperceptible visually and, despite dimensions and all other factors being the same, residual stress could cause a part to perform poorly in the field.
Solution: Residual stress is anticipated. A molder compensates for it by qualifying a part using the same production process and equipment time after time. If a different molder is subsequently used, that molder is responsible for re-qualifying the part and ensuring process and equipment consistency to prevent residual stress-related defects. Optimizing the process to determine a tolerable amount of residual stress is one of the primary reasons to retain a molder that practices scientific molding.
Scientific molding takes the injection molding process to a much higher level of precision — making it ideal for complex, critical-use plastic parts and products like those used throughout the medical industry. More companies are turning to scientific molding for their critical-use complex parts and devices, or precision-molded plastic parts, because the standard molding procedures they have used in the past simply cannot meet their current product specifications.
Scientific molding relies on sensors and sophisticated software that monitor each phase of the injection molding process. Engineers can control and make real-time adjustments to critical variables like temperature, flow rate, fill rate, and cooling temperature to maintain a precise, repeatable production process.
The sensor data allows the engineering team to completely understand, down to the molecular level, what's happening during each stage of the injection molding process. Any variations in key parameters such as viscosity, moisture content, etc., can be detected and corrected in seconds, ensuring consistent quality.
Because all the data is recorded, engineers can easily replicate the manufacturing process as needed — even when production is transferred from one machine to another — saving a tremendous amount of setup time. This is also essential for validating the process and meeting any regulatory requirements.
By using their scientific molding skills, material science knowledge, and ability to solve issues, specialized engineers design robust and repeatable processes that consistently deliver top-quality parts with high tolerances. This approach results in much higher repeatability and at least 10 times greater process control compared to traditional molding methods — reducing costs to the OEM by conserving time and materials.
Warping. Warping is an unplanned and unwanted bend, curve, or other geometry in a molded part. Warping appears either during prototyping or full-scale production, introducing potentially significant increases in project time and cost.
Solution: Temperature and timing are critical to prevent warping. Resin temperatures should be carefully monitored and maintained both prior to and during the injection process. This attention alleviates overheating, compromised solidification properties, and risk of improper cooling at the end of the process.
Likewise, the timing surrounding the release of a part from a mold must be precise to prevent warping. If ejected too soon, the part does not appropriately restrain residual stress and is not properly solidified, so the plastic reverts to its natural state which causes warpage.
Sink Marks. Sink marks are visual and measurable depressions or craters that appear on a plastic part. Sink marks are not only aesthetically displeasing, they can be detrimental to component functionality and multi-part assembly.
Solution: Preventing sink marks can range from minor fixes, like cleaning a dirty mold, to more complex solutions that start in the design phase — such as proper gate location, wall thickness, and other potential design challenges revealed through moldflow analysis. Incorrect machine pressure could also be the culprit. Checking and calibrating the machine may resolve sink mark issues, but that could also mean deviating from the scientific molding process, creating the need for re-qualification.
Short Shots. Short shots occur when insufficient plastic is injected into a mold. Short shots form incomplete molded parts that can be misshapen, the wrong size, or incapable of holding correct dimensions — making them unusable.
Solution: Short shots are avoided or resolved by maintaining the ideal pressure rate to fill out the part, as determined during scientific molding. As in other aspects of production, moldflow analysis can direct engineers to make part design and gating changes that will yield a robust molding process.
Flash. Flash is liquid resin that escapes from a mold parting line and solidifies on the outside of a part. Flash can be extremely sharp, making general handling dangerous. It can also easily damage packaging or cut through O-rings and seals.
Solution: Tooling issues and flash go hand in hand. Therefore, expert tool design is fundamental and largely dependent upon molder engineering expertise and experience. Tooling developed based on a solid design concept, detailed specs, and advanced knowledge of scientific molding practices alleviates the tool wear issues that generally lead to resin leaks and flash during production. In addition, it is crucial that a complex injection molder correctly calculate tonnage in order to use the right size injection molding machine to prevent flash.
Collaboration All through the Molding Process
Undoubtedly, critical-use medical applications require the repeatable quality, high-performance and flawless manufacturability that only an experienced complex injection molder can provide. By taking a step back from production and bringing the injection molder in during the design phase, medical OEMs have the opportunity to leverage knowledge that will protect their business and everyone who engages with the plastic medical parts and devices they provide.
This article was written by Ray Dorow, Quality Manager, Kaysun Corporation, Manitowoc, WI. For more information, visit here.