By Zachi Fizik, ZF Consulting
The majority of medical devices utilize plastic components. When designing these devices, it is vital to know how to calculate the risks, what questions to ask, and when. Even a great component idea can be undermined by gaps in knowledge surrounding plastic materials, from engineering and design considerations to optimal procedures for turning that idea into a reliable plastic part that can be consistently produced, affordably and with high quality.
Failure to heed these considerations can result in project cost overruns, device malfunction, and user risk. Thus, designers must understand the limits of each material and technology to fully harness innovation and creativity. For example, a particular plastic may be easier or less costly to manufacture, but has material attributes making it less-than-ideal for your product (e.g., heat resistance, chemical resistance). Designers must consider the development process downstream from CAD software outputs.
Good plastic design is based on a three-legged foundation: materials, the injection process, and the mold itself.
Plastic parts for use in a medical device must be considered in the context of both functionality and manufacturability. Plastics used for these parts are polymers divided into three basic families: thermoplastic, thermoset, and rubbers (also considered a thermoset). As medical devices are primarily based on thermoplastic materials, this article focuses on them.
Thermoplastics can flow under pressure, heat, and shear, as well as in injection molds (tooling). The material is built of cross-linked monomeric chains that form a resin with distinct characteristics, and is divided into two main categories: amorphous and crystalline. These thermoplastic categories differ in their polymer chain order, resulting in different processing behaviors, shrinkage rates and mechanisms, and mechanical and environmental characteristics.
Commonly used amorphous thermoplastics include polystyrene (PS), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polymethyl methacrylate (PMMA, also called plexiglass), polysulfone (PSU), and polyphenylsulfone (PPSU). Crystalline thermoplastics include polypropylene (PP), cyclic olefin copolymer (COC), polyoxymethylene (POM), polyamide (PA), polybutylene terephthalate (PBT), polyphthalamide (PPA), and liquid-crystal polymers (LCP).
Further, special attention should be given to the thermoplastic elastomers (TPE) because of their growing use in medical devices. TPEs are rubber-like, injection-processed thermoplastics (as opposed to rubbers processed by compression). They are usually based on thermoplastics like styrene (TPE-S), polyolefins (TPE-O), polyurethanes (TPE-U) and volcanized (TPE-V), each of which bears unique characteristics. TPEs are over-molded, double-molded, or co-molded with rigid materials.
Consider, too, bioplastics, whose definition is not always clear. Generally, it is fair to characterize bioplastics as materials either fully based on biomass or mixed with fossil-based resins, with a low or zero carbon footprint, in addition to being biodegradable and/or compostable.
Finally, the resin must be commercial grade. Although resins are referred to by their scientific name (e.g., POM), they might have a commonly known name, like Acetal, in addition to their commercial name. Some commercial names have become generic. The market has hundreds of grades to offer, produced by different manufacturers (compounders) under different commercial names. Each brand name shares the basic characteristics of the base resin, but each is tailored by the compounder to certain specifications. Note that, when submitting a medical device regulatory approval, the material grade must be specified and cannot be changed later without an expensive engineering change order (ECO).
A comprehensive resin selection procedure will take into account mechanical, electrical, environmental, and visual requirements. The process also will consider regulatory compliance, mold and injection manufacturability, and operational restraints, such as cost, logistical challenges (e.g., supply chain), and minimum order quantity (MOQ).
Injection machines come in different sizes and feature varied clamping forces and injection unit capacities. In the injection molding process, material is melted and pushed (injected) into a mold under extreme conditions, then cooled to a solid and expected to exhibit specific characteristics. However, the process is not as straightforward as it sounds.
Because the injection can change material properties at different temperatures and under different shear conditions, it is critical to monitor the process and ensure it does not modify the selected material’s intended properties. A combined great design, mold, and machine still can be compromised by a negligent injection setting or process. Further, part failure can include not only observable defects, but by those “hiding” within the material.
Injection molding begins with plastic in the form of solid pellets. Most materials should be dried before injection, too. Failure to do so may reduce the resulting part’s strength and rigidity, as well as negatively affect its aesthetics.
The pellet material is processed in the injection unit, where a combination of heat and shear gradually melt the material until it is ready to be injected into the mold. This process is called metering and it is done with a special screw that also rams the melted material into the mold. Then, the heat is extracted to create a solid part, which is ejected out of the mold.
But producing a single high-quality part is not enough. Molders aim for a repeatable, fast, and efficient procedure, which ultimately defines the part’s cost. The machine’s hourly rate and the part cycle time are the basis for the injection cost which, added to the material cost, equals the part cost. To this end, machine settings must be optimized.
The most significant settings are screw position and rotating speed (metering), injection speed and pressure, holding pressure, cooling time, melt temperature, and mold temperature. Finding the best combination of these parameters requires skilled personnel — a good molder is defined not only by its machines, but by its set-up technicians and engineers.
A mold is heavy-duty machine! It can be based on a hydraulic, pneumatic, or electric system. It works under loads measured in tons, in temperature and mechanical stress cycles, and in contact with water, oils, and (occasionally) plastic material that can attack the steel. Molds can be as small as 100x100 mm and can reach weights of over 15 tons.
As an injection molder’s main asset the mold can be both costly and time-consuming to create, with some molds surpassing $100,000 and taking from three to six months to develop. That cost and timeline are determined by mold specifications: number of cavities, complexity of the parts it will produce, accuracy, and branding. The mold’s life span is measured in its number of mold shots (presses), which could span from a few thousand up to millions.
Before the mold is designed, it is recommended to hold a pre-design phase (also called a design for manufacture, or DFM, phase) wherein the mold maker reviews the part’s design, confirms the feasibility of building the mold, requests technical modifications, describes the functionality and layout of the mold, and suggests gate, ejection, and parting line for the designer to approve.
In addition to gate type and location, mold complexity is gauged by the number of features in a part’s design that cannot be released from the mold without incorporating a moving element into the mold (under cuts). Mold components such as slides and angled lifters are used to release these under cuts.
It is recommended that molds be built from standard components, the use of which will reduce the mold’s initial cost and its maintenance expenses. Using a standard mold base is common and the standard is, in most cases, defined by the mold maker based on the mold specifications, availability, and cost.
It is important to understand that, no matter what is designed and how much one is willing to pay, the mold is composed of machined steel parts subject to their own limitations. Accordingly, the process of reaching a final approved product is sometimes based on trial-and-error testing procedure commonly called “T1” to “Tf.” The part produced in T1 may not achieve the intended final dimensions because of mistakes in the mold or deliberate mistakes in the design allowing for calibration (i.e., steel safe design), as well as other part design modifications. Ultimately, success is based on making the right mistakes and correcting them at the right time.
Finally, designers must consider compatibility between the mold and the injection machine. The interfaces are affected by the machine size, as well as its mounting onto machine platens and the injection and ejection systems.
It has been said that plastic material has a life of its own. However, if you wish to be innovative and creative with plastic, do not work against its physics. Work to understand the importance of the three basics — materials, the injection process, and the mold itself — to successfully harness the best qualities of the plastic in your design.
About The Author:
Zachi Fizik specializes in all aspects of plastics engineering with particular expertise in injection molding tooling and supply chain strategy. He has over three decades of experience supporting both startups and well-known brands in the medical, military, telecom, and consumer industries throughout the entire plastics lifecycle, from the early design phase through the engineering phases and into mass production. He holds a BSc. in mechanical engineering from the Technion Israel Institute of Technology. Recently he served as consultant to several promising innovative Israeli medical start-ups whose novel devices reached regulatory approvals. You can reach him at email@example.com and connect with him on LinkedIn.