Knowing the material your parts are going to be manufactured in early on in your development process can save you time, money and a lot of frustration. You should work closely with your manufacturer during design, so they can help you identify potential material issues before any parts are actually molded. Because some part geometries inherently work better with certain resins, your manufacturer can help guide you toward the appropriate material options.
Issues that can result from selecting an incorrect material:
- High fill pressure
- Poor cosmetic finish
- Shorting or burning
Uniform Wall Thickness
With injection molding, we talk a lot about how uniform wall thickness helps improve mold fill versus thin features that can restrict the material and create a number of the aforementioned issues. Having connected walls that are too thick and too thin can affect how a part cools, thus creating sink and warp. Furthermore, the same issues can arise if your entire part is too thick or thin.
This is why the appropriate rib-to-wall thickness ratio must be followed. The appropriate thickness for a rib that is extruded from another surface is approximately half the thickness of the adjacent surface. This is the optimal part design to provide strength while at the same time reducing your chances for significant warp or sink. Learn more on uniform wall thickness in plastic parts on our website.
Manufacturers have resin charts that can be used as guidelines when designing parts. That said, you can design a part below the recommended wall thicknesses for a particular resin, but you just have to be extremely cautious in doing so. Actual part size is a large factor — how long, tall and wide is your part? The more surface area you have, the greater the wall thickness you may need to fill your part.
Proto Labs’ Recommended Wall Thickness by Resin Type for Injection Molding
|ABS||0.045 – 0.140|
|Acetal||0.030 – 0.120|
|Acrylic||0.025 – 0.500|
|Liquid crystal polymer||0.030 – 0.120|
|Long-fiber reinforced plastics||0.075 – 1.000|
|Nylon||0.030 – 0.115|
|Polycarbonate||0.040 – 0.150|
|Polyester||0.025 – 0.125|
|Polyethylene||0.030 – 0.200|
|Polyphenylene sulfide||0.020 – 0.180|
|Polypropylene||0.025 – 0.150|
|Polystyrene||0.035 – 0.150|
|Polyurethane||0.080 – 0.750|
Note that we also have a helpful resin guide online that shows material strength, impact resistance and temperature along with moldability characteristics.
Where to Start
A good rule of thumb is to begin your parts design with a wall thickness no less than 0.040 in. for most parts you can hold in your hand. Depending on the material and how large your part is, you may need to begin adjusting the thicknesses to allow proper mold fill. Generally speaking, thermoplastic materials like PP, HDPE and K-Resin fill thin part geometries very well in comparison to resins like PC (Lexan 940) and most glass-filled materials that have greater resistance when filling thin features that ultimately increases molding pressure and part thickness.
Breaking the Rules
Thermoset materials like liquid silicone rubber (LSR) are much more receptive to thick and thin sections on the part, which can allowing you to break some of the general molding rules and guidelines. Molded part thicknesses greater than 1 in. that need to be as thin as 0.010 in. are no problem since LSR flows through a mold like water and sets up quickly, which helps eliminate sink or voids.
Finding a Balancing
Like many areas of injection molding design, do your homework beforehand to see how different part thicknesses affect the overall performance of your parts and products. You don’t want to design part that are too heavy, too brittle or cosmetically inferior. Work with your manufacturer to balance out the part geometry to ensure your part functions well with minimal revisions.
For more information on materials or injection molding, please visit our website at protolabs.com or contact one of our customer service engineers at firstname.lastname@example.org or 877.479.3680 with additional questions on resin selection.