In this Insight Blog Series, we are transcribing handwritten notes taken over the years working with plastics, the notebook did not site all references so we cannot give credit to where it is due. Machinists took the time to write down knowledge gained and learned, we feel it should be put to use and hopefully help someone reading this to learn from it. All materials have inherent limitations that must be considered when designing parts. To make limitations clear, each material profiled in these engineering notes are dedicated to identifying these attributes. We hope these insights about material strengths and weakness simplifies your selection process.
Engineering Notes: Acetal
When considering Acetal material in your design or application, it is important to understand the different properties of each grade. There are 2 standard grades of Acetal that are available for machining applications. Homopolymer Acetal, known as Delrin® (Only by Dupont) and Copolymer Acetal. What's the difference: Homopolymers are plastics with molecular chains containing identical DNA repeating its sequence, Copolymers contain two different DNA sequences that repeat. These two grades of Acetal look the same, there chemistry is similar enough to be called Acetal; however, their properties are very different and if overlooked can cause costly part failures. Copolymer Acetal has a lower water absorption rate, performs better than Homopolymers in thermal, wear and chemical resistance. Homopolymers have a higher crystalline structure which gives the material a higher fatigue performance.
Additional Note: Centerline Porosity
Homopolymers are also known to be susceptible to develop centerline porosity when they begin to cool during the manufacturing process (Extruded) because the exterior portions solidify first, creating loss of volume in the interior. Centerline can typically be found as a small circle in the center of a part, as for sheets and rods, it will appear as a white stripe or band in the center or middle of the material.
What Causes Centerline Porosity
The root cause of centerline porosity in Acetal is due to gasses trying to escape the strict extruded “shell” that restrains volume change, causing voids or pockets to form.
Why Be Concerned
Centerline porosity can cause many undesirable effects in applications such as, weaken the part, provide a route for leakages of liquids and gasses, and permit bacterial growth in food processing and/or medical applications.
Copolymer is not as susceptible to develop center-line porosity; however, it is indeed important to keep it in mind that it does happen, and it is in your best interest to choose a different material if you have the option.
Material Notes: Acetron GP is Quadrant EPP's general purpose acetal and is the only porosity-free acetal product available today. Investments in process technology by Quadrant EPP now provide the performance and machinability of acetal without center core porosity. Their in-line photometric quality procedure assures every plate and rod is porosity-free as measured by Quadrant EPP's quick check dye penetrant test. For details of test methods, contact Quadrant EPP. Acetron GP natural is FDA, USDA, NSF, Canada AG and 3A-Dairy compliant.
Bearings, bushings, cams, pulleys
Pump and valve parts (Impellers, submersible, centrifugal, proportioning, reinforced diaphragms, jet wells, beverage valves)
Gears, spur, cluster, helical, bevel, worm
Jigs and fixtures
food process and other packaging machinery parts
Bread slicer motors
Door lock housings
Fuel system components (gas caps, floats, senders)
Heavy load bearing gears (window lifts, windshield wipers)
Safety system components (buckles, seatbelt hardware, spools, guide tracks, clock springs)
Gas and Water meter components
Fan and blower blades
Irrigation sprinkler components
Shoe base and pushing caps (Package equipment)
Clips, fasteners, buckles
Window and drapery hardware
Chair coasters, rollers
Appliance components (gears, knobs)
Garden chemical sprayers
Household water softeners
Paint mixing paddles, canisters
Scrubbing discs on power rug cleaners
Hot air: heater blowers, clothes dryers, automotive light sockets
35 mm slide projector parts
Garage door opener components
Ski bindings and straps
Roller-skate brake supports
Salt water: marine fittings, fishing reels
Garden hose nozzles
Toilet flush valves
Faucet valve cartridges
CNC Tool changers
Laser printer gear trains
Video tape drives
Telephone terminal strips
Relay components, buttons, and knobs
Audio and video tape cartridge platforms and wear parts
Electromechanical counter frames
Can Acetal Be Used in Abrasive Applications?
In general, acetyls do not perform as well in abrasive wear applications as Nylons. Compensation for moisture related growth generally allows Nylatron® nylons to be used for wet, abrasive applications. If your application requires dimensional consistency in an abrasive, high humidity or submerged environment, Ertalyte® PET–P will often offer improved performance.
All statements, technical information and recommendations contained in this publication are presented in good faith and are, as a rule, based upon tests and such tests are believed to be reliable and practical field experience. The reader, however, is cautioned, that Diversified Designs does not guarantee the accuracy or completeness of this information and it is the customer’s responsibility to determine the suitability of Diversified Designs products in any given application.