Inventors and Injection Molding

Part 3 of 4: An Inventor’s Guide to Plastic Selection

In Part 1 of this blog series, we discussed the importance of clearly defining your ideal customer, what price point will cause consumers to choose your new product over other solutions, and how much market share your new product can secure. Part 2 of this blog series explored the most popular plastic processing options available (injection molding, blow molding, etc.) and compared up-front and ongoing costs of each method. In Part 3 of this blog series, we are learning about selecting the best plastic material for product performance and cost.

In most cases, it will be the inventor’s responsibility to select a material and the specific material grade to be used in a product. Injection molding companies typically do not have the time to assist in this process, nor do they want to incur any risk or blame if the selected material does not meet the inventor’s needs. Therefore, the information presented below is meant as general advice to assist the inventor in selecting an appropriate material but does not represent material selection advice.

It is unlikely that there is only one material that will work for your product. Sometimes performance is key (mechanical and/or visual), while other products just need a low-cost material with limited performance requirements. Generally speaking, higher-performance materials have higher prices. Two broad categories of materials are commodity grades and engineering grades. As the name implies, commodity resins are produced in large quantities and have acceptable performance characteristics for a broad range of products. Engineering grades will have higher melting points, greater strength, etc., but will also cost more.

GreenLeaf’s blog post “Using The Right Thermoplastic Material For Your Project” will be a good starting point for helping you consider which materials might work well for your new product. This guide covers the most common commodity and engineering grades. To explore the whole spectrum of potential materials, UL Prospector has a description of both common and obscure material types. You will also be able to find technical data sheets for specific material grades.

Plastics News provides weekly resin pricing charts as well as more than 25 years of historical archives and pricing. With a four-week print and digital subscription that costs $15, you can get an overview of material prices. The prices provided are “industry averages,” but they give the inventor a sense of the price per pound. However, the “price per pound” mindset can be misleading when comparing different material types with different densities. Converting to the price per volume is always useful when thinking about the cost of desired material properties. For example, nylon has a specific gravity of about 1.2 while polypropylene (PP) has a specific gravity of 0.9. Assume nylon is $2.00 per pound and PP is $1.00 per pound. For a given part design, a nylon part might appear to have two times the material cost per part when compared to PP. Due to the difference in specific gravity (density), the nylon material cost per part is actual 2.67 times that of PP.

The time required for a molten plastic to solidify in the mold is called “cooling time,” and it varies based on the material selected, which can be a significant cost driver. Unfortunately, this critical piece of information is difficult to find since the variables of mold design and the injection molding process impact cooling time. For example, mold temperature and post-mold shrinkage are two variables that prevent a material manufacturer from advertising a cooling time per wall thickness. However, the chart below will give the inventor a general idea of cooling times for a few standard materials.

Once the inventor has performed his due diligence on material selection, a materials expert can be contacted. Most material distributors have technical advisors on staff that can help an inventor choose a material that balances the engineering needs of a new product with the material costs.

Needless to say, there are many performance and cost-related variables that go into selecting a material for a new part design. Be sure to stay tuned for the fourth and final installment of this inventor blog series where we’ll talk about economics.

Looking to find the right injection molder for your project? Contact GreenLeaf Industries today to get started!