November 08, 2016 | Volume 12 Issue 42 |
Manufacturing Center
Product Spotlight
Modern Applications News
Metalworking Ideas For
Today's Job Shops
Tooling and Production
Strategies for large
metalworking plants
Achieve consistent walls and features with slight adjustments to geometry and materials.
By Gus Breiland, Customer Service Engineering Manager, Proto Labs
Take a look at an injection-molded toothbrush holder, a plastic gas can, or the silverware tray in the kitchen cabinet. Notice how the walls of these household items are all relatively uniform? This is one of the fundamental rules of plastic injection molding, and ignoring it can lead to sink, warp, and inaccurate or non-functional parts.
Yet the functional requirements of consumer, medical, aerospace, and industrial products often leave designers little consideration for the material flow and fill properties of plastic, both of which are at least partially determined by wall thickness.
To achieve uniform wall thickness, let's start with the basics:
Choosing materials
One of the biggest considerations with wall thickness is which material to use for your project. With literally hundreds of materials to choose from, deciding on the right one can be challenging. You can view available resins grouped by family with recommended wall thickness ranges along with detailed information on material properties, tensile and impact strength, and maximum operating temperatures online. More information can also be found at our materials page.
Recommended wall thickness by resin.
Start by looking at which attributes are most important to the finished product:
While all of these factors are being weighed, refer back to the website's section on wall thickness. Obviously, materials only make good candidates if they can be molded to the dimensions and geometry needed for your project, while still meeting their engineering requirements. Once you're close to selecting a material, give one of our customer service engineers a call. They can either advise you directly or put you in touch with an expert at one of our material suppliers.
For example, Nylon 6/6 flows well, is good for thin-walled parts, and has excellent impact resistance, but you might have rejected it because of its average strength and lack of resistance to heat. Adding glass-fiber filler to the resin not only makes the nylon much stronger, but far more heat resistant. Glass also reduces the chance of sink in thick sections, but may lead to warp in thin areas, depending on material flow during the molding process.
In some cases, you might be directed to a completely different material family:
Again, there are hundreds of materials and thousands of ways to adjust, blend, or fine-tune them to produce the desired results.
Clever tweaks
Even if the right combination of material attributes can't be found, don't despair. Some clever edits to part geometry go a long way to alleviate internal stress and potential weakness produced by less-than-optimal wall thickness. Parts shaped like dumbbells or sewing bobbins are perfect candidates for coring, which eliminates large cross-sections of material similar to removing wedge-shaped slices of an apple, but the strong core is left in place. This is a great way to avoid sink, reduce material usage, and make parts lighter but just as strong (possibly stronger). And parts like box lids that have tall, thin walls can be reinforced with gussets, so long as the relative wall thickness of the supporting material follows the 40- to 60-percent rules mentioned previously. This also eliminates the chance of shadowing, which occurs when one section of the part cools down faster than others.
Design for manufacturability
Once you receive your part quote, be sure to review the accompanying design for manufacturability (DFM) analysis, which provides feedback to improve the moldability of your part. Overly thick or thin areas will be color-coded based on nominal wall thickness, along with recommendations on changes to draft angles. Parting lines, ejector and gate locations, undercuts, side actions, and the need for hand-loaded inserts are displayed as well. If deemed necessary, a flow analysis can be performed to analyze pressure points around gate areas and to identify potential knit lines. As always, feel free to contact us at 877.479.3680 or customerservice@protolabs.com if questions or concerns emerge.
Published November 2016