How do you select which plastic material is right for your application? Do you have confidence in those decisions? And once you select a material, how do you optimize your part for that specific material?
Injection molded materials have chemical and mechanical properties such as stiffness and resistance to acids. Some materials are more susceptible than others to issues like sink, flash, and warp. It’s up to engineers and designers to sort through the books, charts, and online references to select the right material – and have confidence in that decision. However, too often we don’t have enough experience with all the materials available to make a well-informed decision.
That’s why we developed Plastic Sample Kits. We combined the best parts of typical plastic design aids, added features that specifically highlight the difference between plastics, and molded the parts in 20 different materials for comparison.
We wanted firsthand experience with these plastics to gain intuition and a practical understanding of the differences between these materials so that we could finally have confidence in our material selection and molded part design.
Fig 1. 20 Material Base Set available at PlasticSampleKits.com
In order to provide the most value for someone comparing these samples, we orchestrated several features into the design including stiffness test, living hinge test, six surface finishes, Pantone colors, heat-stake bosses, thread-forming bosses, front size test, different rib thicknesses, shrink comparisons, as well as draft angles and textures to check for drag marks. The samples can either be stacked for easy access and viewing or left in their included organizer shipping box.
Plastic Sample Kits include a booklet filled with information and guidance for each feature on the samples and, most importantly, information about each material, what its attributes are, and what its typical use is. Each material sheet also includes the Pantone color of the samples and the specific manufacturer and grade of plastic used.
The booklet also includes a plastic performance reference guide to aid in material attribute selection. This coincides with the colors of the samples. For example, suppose you want to compare materials that have good wear resistance and moderate strength. Simply compare all the pucks that are shades of green, which are all engineering grade semi-crystalline materials like POM (Acetal / Delrin), PA (Nylon), and PBT.
Fig. 2 Pages inside the Plastic Sample Kit booklet, including the Plastic Performance Guide
In order to design a product that compares a material’s moldability, we needed to look at several key factors for comparison: sink, warp, flowability, and susceptibility to knit lines. To do this, and to make sure we had the geometry dialed in, we used SOLIDWORKS Plastics Simulation from GoEngineer. This was the fun part! Normally we design parts to avoid defects, but in this instance, we got to design features that forced defects!
While designing the knit line test features, we used SOLIDWORKS Plastics to ensure these three holes in a row would force the plastic flow fronts to merge creating visible knit lines in some materials.
Fig. 3 Animation of fill showing flow front merging around holes, SOLIDWORKS Plastics from GoEngineer
Fig. 4 Knit lines shown between in-line holes, SOLIDWORKS Plastics from GoEngineer
For example, in the ASA material samples shown below, knit lines are clearly visible between the three holes. But as you’ll see in the kits, not all materials ended up with visible knit lines.
Fig. 5 Knit lines visible in ASA plastic sample part
We also included a flowability feature which demonstrates how each plastic may flow into thin areas. These features show how susceptible a material is to flash and allows you to see how LEDs might shine through different thicknesses of the materials if you’re designing hidden light indicators. SOLIDWORKS Plastics easily predicted that most materials would have difficulty filling all the way down to the thinnest 0.05mm “flow hex” on these parts.
Fig. 6 SOLIDWORKS Plastics predicts short shot on molded parts
Fig. 7 Flowability differs between molded materials and appears as short shots in the PA66 and PA66+33GF samples
Properly designed ribs should be somewhere between 40% and 80% of your nominal wall thickness to reduce sink – depending on material. But what materials are less susceptible to this sink, and at exactly what thickness ratio? How bad is the sink?
Some sink may be okay, especially on non-cosmetic parts. What would it look like if you made the ribs 100% of your nominal wall thickness, again, across 20 different material options?
Fig. 8 Severity of sink marks shown across a range of rib thickness, and on screw bosses for ABS in SOLIDWORKS Plastics from GoEngineer
Fig. 9 Sink marks shown across a range of rib thickness on a POM (Acetal, Delrin®) sample part[/caption]
In order to demonstrate a material’s susceptibility to the “corner effect” we included features with five bends, where one set received radii that created uniform wall thickness and the other was left with sharp corners. SOLIDWORKS Plastics shows the yellow areas at the thicker corners will shrink creating a warped and displaced end. The booklet included with the Plastic Sample Kits further details how to avoid this phenomenon in part design.
Fig. 10 Shrink and warp at thick sharp corners creates more warp when compared to radiused corners, a.k.a. the corner effect
Fig. 11 Corner effect shown on a PA+33%GF sample part, bottom edge of left feature (with corners) has warped more than the same feature with radiused corners
For years we searched for a solution to aid us in our material conversations and decision making. With nothing suitable on the market, we took it into our own hands and are pleased to offer Plastic Sample Kits – the design aid that lets you flex, twist, hold, and compare 20 different plastics. Get a Plastic Sample Kit by visiting our website and use promotional code GOENGINEER for a discount.
Plastic Sample Kits was founded in 2019 by two Mechanical Engineers who wanted a great reference tool for comparing materials, both for material selection and for discussing molded plastics with their teams and customers. Plastic Sample Kits' featured product is a 20 Material Sample Kit of injection molded plastics, common blends, and glass filled grades for comparison. Each sample is molded into the same shape, which is a design aid filled with features that allow for observations about mold-ability, material properties, and design optimization for that material. The kits also come with a booklet containing application information about each material and part features, as well as a material performance guide.
About the Authors
Stefan McClelland has been using SOLIDWORKS professionally since 2014 where he designed connector overmolds and mold tools for a variety of plastics. He holds bachelor’s degrees in Mechanical Engineering and Music Performance from UC Irvine and a Master’s in Mechanical Engineering from the University of Sydney. Stefan enjoys making music and plays the upright bass, guitar, and is learning to play the ukulele.
Brian Eastman has been an avid SOLIDWORKS user since 2005 and is a CSWE. Brian received his bachelor’s degree in Mechanical Engineering from the University of California, Irvine and has worked predominately in product development for medical device and laboratory automation. Since 2018, Brian has been a product engineering consultant for companies in the Bay Area, specializing in designs for injection molding such as oral care devices and diagnostics consumables.
If you have any questions or comments, please fill out the form below and a SOLIDWORKS Plastics expert from GoEngineer will get back to you soon.
GoEngineer delivers software, technology and expertise that enable companies to unlock design innovation and deliver better products faster. With more than 35 years' experience and tens of thousands of customers in high tech, medical, machine design, energy and other industries, GoEngineer provides best-in-class design solutions from SOLIDWORKS CAD, Stratasys 3D printing, Creaform & Artec 3D scanning, CAMWorks, PLM, Altair, and more
Get the latest articles delivered daily to your inbox, unsubscribe at any time.