There are thousands of Plastic resins available for injection molding projects. Selecting the right injection molding resin will help ensure that the look, fit, and performance of the finished product matches project requirements.
Here are a few tips that will help you choose the correct resin, and also some traps to avoid along the way:
Tip: Many resins will offer a texture & finish that will be durable and visually appealing. Texture has multiple advantages including being used to make undercuts and hide parting lines, improving grip if your product needs it, improved paint adhesion, and can also allow gases to escape the mold during the injection process. The surface finish needs to be decided very early on when choosing your resin to allow for proper design in the mold.
Trap: Don’t leave your surface finish as an after-thought. The texture and finish can affect the plastic resin flow through the mold, could affect the shrink rate, and could even affect the material you choose. Decide early on what the finish needs to be. If you’re not 100% positive, discuss it with your mold designer. They can offer suggestions that may be flexible with the material chosen and allow you to fine tune the choice later in the process.
Tip: You need to understand the intention of the components and the combined product. Are they for abrasion resistance, high-impact applications, holding a liquid whether hot or cold, or anything else? Does it need to be flexible or rigid? Does it need to be heat resistant or chemical resistant? Choosing your resin based on the characteristics required will save development time.
Trap: Don’t forget to list all the criteria of the product requirements. Temperature variations, abrasion, and flexibility are just the tip of the iceberg. Forgetting something that the product needs to do may end up trapping you with a resin that can’t meet your customer’s expectations.
Tip: If your parts need inserts for fastening component parts together or bosses molded in for sonic welding, be sure to select a resin that can accommodate the stress of additional fastening or sonic welding.
Trap: Don’t select a resin that won’t hold up to the end-customer abuse. If you have additional fastening with threaded inserts or the parts are to be welded together, having a durable resin is going to be key to the success of the product.
Tip: Be sure to select a resin that will flow properly through the mold. Have your mold analyzed through the design process and have certainty that the resin you choose will fill the cavity correctly during injection. Sea-Lect has in-house design that will design the mold for your part, or we can analyze a pre-existing mold for optimization. This will prove out the mold theoretically and be a highly accurate representation of how the mold will operate in production.
Trap: Don’t skip this step or guess that the mold is correct to save time or money. You will be trapped with a design that may not work at the injection molding stage, and it may be costly to make mold corrections to accommodate a resin that won’t flow well with the mold design.
Tip: Consider if there will be any special appearance items on the product. Beyond a texture, does it need a color match to something else? Does it need embossing, pad printing, or hot stamping with a logo or visual design? You will need to choose a resin that will do well with the secondary process and absorb the color you need for your project.
Trap: Don’t select a resin that is not going to accept a secondary process that your end-customer expects on your product. Labelling, printing, and color matching doesn’t work well with all resins. Define what the product needs in the beginning, not as a secondary thought after mold design and resin selection.
Tip: Outline what regulatory requirements are needed for your product, and how it will drive your resin decision. If you plan to export your product, it may need to meet the export requirements for REACH or what the destination location requires. It may have to be food safe or safe for children, and in a foreign location that can mean different guidelines to follow.
Trap: Don’t expect that your product meet all requirements regardless of destination. While you may not know every regulation and requirement your product should meet in the beginning, knowing where it is going to can help the process of discovering what it should meet. Knowing the where and how initially may help you keep from getting trapped with a product that can’t be distributed globally.
Tip: Know what your recycling options and requirements are going to be for the resin, whether that is with defective product during molding or assembly at the end customer. Some resins aren’t recyclable while others are. It may make a difference in the product and molding cost that can save a few dollars over the course of production.
Trap: There are two basic types of resins, a thermoset and a thermoplastic. The primary physical difference is that thermoplastics can be re-melted back into a liquid, whereas thermoset plastics always remain in a permanent solid state. A thermoplastic can be melted and reshaped, which means you can reuse or recycle it much easier. That may save it from being discarded with a hefty cost. Don’t get trapped with the wrong basic type that may cost you in the long run.
Tip: If you choose a thermoplastic, you may have more options than you realize. They can be broken down into three basic groups; commodity, engineered, or high-performance resins. Typically commodity resins are the lowest cost options, while the high-performance are at the top of the scale for cost. Engineered resins reside in the middle and offer a balance between availability and cost, plus can increase strength and chemical resistance.
Trap: If need you need a higher performing resin, don’t get trapped in the high-performance category of resins. There may be an additive that can be mixed with a commodity resin to meet the performance requirements without the cost. Anti-microbial, anti-static, more flexibility, and flame retardant are all properties that can be enhanced through additives.
Tip: Work with your mold maker to determine the shrinkage needed when choosing your resin. There are two categories of resins; amorphous and semi-crystalline. Amorphous shrink less when cooled and will hold a tighter tolerance in size. They are also more brittle and usually lack chemical resistance properties. Semi-crystalline on the other hand shrink more, are better for abrasion and chemical resistance and are less brittle.
Trap: The shrinkage and other characteristics need to be determined on the front end. If you leave it to later in the process, you may find yourself trapped into a resin unless you can afford to change the mold to suit the correct resin.
As mentioned, there are thousands of resins out there. Amorphous, semi-crystalline, commodity, high-performing, it can be hard to determine what you need. Don’t feel overwhelmed. We are here to help. There are several online databases that you can peruse to provide a starting point. Three options to look through are: UL Prospector (formerly IDES), MAT Web, and The British Plastics Federation. All provide specification data and descriptions for resin selection.
If you need more advice, or are ready to start your next project, give us a call at (425) 339-0288
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.