As a leading contract manufacturer, one area of plastic injection molding where we often find the most distinctiveness in is resin selection. When selecting plastic resins you must keep in mind that each molded part is unique based on the project requirements. Beyond aesthetics, like color, a material’s properties, processability, and cost all play a key role in determining a resin’s suitability for an application. With thousands of grades to choose from, and new ones being developed to fill market niches, customers often find themselves needing help when navigating the resin selection process. Before we delve into how to choose plastic resins, first we'll tackle the most important question.
What is plastic resin?
Plastic resin is the main ingredient in creating molded plastic. It is created from the byproducts of oil refinement, when crude oil is distilled. The resulting chemicals are forced through a process that encourage them to make hydrocarbon chains called polymers. The process to get the resultant plastic resin can be altered in countless ways to create resins that have very specific properties for very specific situations.
Now that you have a basic idea of what a resin is, here are some of the key considerations to weigh when selecting plastic resins:
1. Mechanical Requirements
A part’s strength requirements need to be considered as resin classes have various tensile strength, tensile modulus, and elongation at break. Thermoplastic resins offer a variety of strength properties that can often be modified with fillers like glass or carbon fiber.
2. Chemical Compatibility
Many customers are concerned about how chemicals, including cleaning solvents, and process reagents, interact with their resin selection. In these cases its best to consult published testing data from resin manufacturers that show a material’s performance in each chemical. In some cases the data may not exist and testing with specific chemicals will be required.
3. Environmental Compatibility
Parts that will be exposed to extreme hot or cold conditions need to be made with resins that are rated accordingly. This means the long term service temperature and heat deflection temperature are critical performance metrics for your particular resin. Other environmental conditions to consider include exposure to high humidity and UV light, such as plastic parts used in medical devices.
4. Commodity vs. Performance
Whenever possible a quality injection molder will strive to pair customer applications with commodity resin grades, due to the inherent cost and availability advantages that come with higher volume raw material production. Some projects call for specific properties – strength, heat resistance, etc. – which is where performance resins excel.
5. Amorphous vs. Semi-crystalline
Selecting amorphous resins can often be advantageous as they can provide wider processing windows and improved dimensional control due to their random molecular structure. They can be transparent and are compatible with many adhesives. Semi-crystalline materials offer improved mechanical and thermal properties, but can be more difficult to process.
6. Material Shrinkage
The amount a resin shrinks during the molding process can have a huge impact on the ease of building a tool or developing a successful molding process. For this reason we try to select resins that have lower shrink rates whenever possible.
Once all of these factors are accounted for, the list of suitable resins is typically cut down to a manageable number to consider. It is here that a quality injection molder leverages its proficiency with particular materials, coupled with expertise in product design and development, to make the final determination of a grade. This leads to the development of a product with the most optimal material selected for product performance and moldability.
To meet the diverse project requirements presented by so many clients, we draw on our extensive knowledge and experience with plastic resins allowing us to stay competitive in an always changing market. When we choose materials for plastic injection molding, it is based on customer specifications, moldability of design, and cost. Choosing the right material early in the product development process, streamlines product launch by picking the correct path every time. This a value add in the process, for both our clients and our design process.
About Synectic Product Development and Mack Molding: Mack Molding is a leading custom plastics molder and contract manufacturer with an extensive portfolio of vertically integrated services, including: supply chain management, machining, sheet metal fabrication, molding, painting, assembly, testing and even fulfillment. Synectic Product Development is a full-scale, fixed-fee, product development company specializing in design, development, prototyping, and product launch of medical devices and consumer products. Synectic and Mack combined have over 130 years of knowledge and experience allowing us to stay competitive in an always changing market. Learn more about our services and see how we can bring your product to life.
-By Scott Rishell, Design Engineer, Mack Molding and Kathleen Murray, Marketing Manager, Synectic Engineering. Updated 1/28/20