Machining Plastics: Case Study Acetal
In this Series of discussions, we will explore some past projects that involved some trouble shooting and some machining tips that were learned along the way. This might help you set your mind when certain problems arise in your next project.
Acetal: Case Study
In this continued study we are going to talk about Acetal, Acetal provides high strength and stiffness coupled with enhanced dimensional stability and ease of machining. As a semi-crystalline material, Acetal is also characterized by a low coefficient of friction and good wear properties especially in wet environments. Because Acetal absorbs minimal amounts of moisture, its physical properties remain constant in a variety of environments. Low moisture absorption results in excellent dimensional stability for close tolerance machined parts. In high moisture or submerged applications, Acetal bearings outperform nylon 4 to 1. Acetal is ideally suited for close tolerance mechanical parts and electrical insulators which require strength and stiffness. It also offers resistance to a wide range of chemicals including many solvents.
Acetron® GP: a general-purpose copolymer Acetal and is the only porosity-free Acetal product available today. Investments in process technology by Quadrant provide the performance and machinability of Acetal without center core porosity. This material undergoes a photometric quality procedure that assures every plate and rod is porosity-free as measured by a dye penetrate test making it the preferred Acetal for food contact and medical applications. Acetron® GP natural is FDA, USDA, NSF, Canada AG and 3A-Dairy compliant.
Delrin Acetal: Delrin, a Homopolymer Acetal, is also manufactured and stocked in rod and plate. It offers slightly higher mechanical properties than Acetron® GP Acetal, but may contain a low-density center, especially in larger cross-sections. Copolymer Acetal also offers better chemical resistance than Homopolymer Acetal. Delrin is ideal for small diameter, thin-walled bushings that benefit from the additional strength and rigidity of Homopolymer Acetal.
Delrin AF Blend: is a unique thermoplastic material for use in moving parts in which low friction and long wear life are important. It is a combination of PTFE fibers uniformly dispersed in Delrin Acetal resin. This combination offers better wear characteristics than unfilled Delrin. Delrin AF Blend, supplied as a 2:1 blend of Delrin AF100 and Delrin 150 resins, has excellent sliding/friction properties. Bearings made of Delrin AF Blend can operate at higher speeds while exhibiting reduced wear. These bearings are also essentially free of slip stick behavior because the static and dynamic coefficient of friction is closer than with most plastics. Delrin AF Blend retains 90% of the strength that is inherent in unmodified Delrin Acetal. Some properties are changed due to the addition of the softer PTFE fiber. The natural color of Delrin AF Blend is dark brown.
Delrin AF 100: Is an unblended Delrin AF and offers a slightly higher limiting PV and lower coefficient of friction due to additional PTFE content. This added PTFE typically decreases the wear capability and impact strength. Delrin AF 100 is available on a custom basis from Quadrant.
Common Applications for Acetal:
Bearings and Bushings
Structural Keels (Prosthetics)
Note: In general, Acetal does 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.
Case Study #: Reduce Weight; Reduce Assembly Cost (Various Acetals)
We are again talking about a machine assembly for a client; this machine is working well at the moment. This machine consists of bushings (Brass) several manifold halves that have both electrical and fluid running thru them that attach to other sub-assemblies. Stainless Steel isolator spring packs used for elevating certain portions of this machine from a drip tray. Lots of Bronze gear assemblies that mate to stainless steel pinion gears attached to several servo motors. Last sub assembly is a Cast Iron filter lift used to load, carry and lock 2 large filters into there position (no hoses used). The Cast Iron lift is guided by cast aluminum rollers on a track assembly that lower and align the filters to their final docking stations. The filters are made of stainless tube housing and stainless steel end caps (heavy stuff). We are not sure why some these material choices where made but hey it's working fine. Hopefully I explained this machine without giving too much away, we can not provide pictures or specifications to the various parts we made in order to meet the client's objectives.
Challenge: Replace heavy and costly steel components; reduce some of the assembly burden.
Solutions: We began with the eight (8) steel isolator packs that hold the assemblies from the drip tray but also reduce shock and cushion the moving parts from the housing. There is only one area of this drip tray that comes into contact with moisture but the machine uses all stainless steel springs through out the entire tray. First thing we did was get a weight measurement on the various sub assemblies that rested on these springs, we knew the rating on the current springs but we needed to get a sense of how stiff the mounting needed to be. Remember this machine is working fine; we don't want to change that. We decided to design four (4) structural keels (not like boats) we borrowed this design from prosthetic devices that we have seen in the past. We could have used nylon here but the keels are made from Delrin for its outstanding fatigue and impact resistance in this continuously loaded application. Eliminating the eight (8) steel isolator packs reduced the cost, availability concerns, weight and time to assemble.
Moving on to gear assemblies that attach to the servos, we kept the stainless steel gears on the servos but opted to replace the bronze gears with Acetron® GP to reduce rotating weight. This material maintains tight tolerances despite environmental or chemical exposure on this equipment. We also replaced the brass bushings with Acetal in the areas that are exposed to moisture due to its excellent stability and good wear resistance in those environments.
From there we reviewed the stainless steel manifold halves that attach to each sub assembly and are used to carry fluid and electrical current like a distribution block. This was rather neat; we chose to replace the stainless steel blocks with Acetron® GP. This manifold had several fluid channels with electronic actuated valve seats and o-ringed on its matting surface to seal from the tight tolerance machined holes that where press fitted with gold plated wire pins (like a computer pin cable). Acetron® GP is porosity free and is ideally suited for close tolerance mechanical parts and electrical insulators. Acetal absorbs minimal amounts of moisture; its physical properties remain constant in a variety of environments. Low moisture absorption results in excellent dimensional stability for close tolerance machined parts in high moisture or submerged applications.
We now get to the filter lift assembly area; this is the most complicated assembly due to its movement and staging of the filters. We kept the cast iron filter lift as is; client is fully invested in the casting process and does not want to discard the sand molds. We did however replace the cast aluminum guide rollers with Acetron® GP to keep the filter lift operating smoothly and reliably in its track system, this also reduced staging noise. We also replaced two (2) Dairy nickel scraper blades that attach to the underside of the filter lift with Acetron® GP plate. The blades keep their shape and are free of pores that could entrap particles and prevent complete track cleaning. Acetron®’s low stress levels also assure blade flatness to maximize cleaning efficiency.
Last area that made the customer's day is the large stainless steel filter caps. After reviewing the specifications on the filters it was concluded that these could be replaced with Delrin Acetal. It offers slightly higher mechanical properties than Acetron® GP Acetal, but contains a low-density center, especially in larger cross sections. Copolymer Acetal also offers better chemical resistance than Homopolymer Acetal. The stainless steel filter housing was kept and is fairly light. The weight savings and time and cost of machining were greatly reduced, this improved overall shipping and handling costs. The filter lifting system also benefitted from the reduced carriage weight, less wear and improvement in staging speed was realized.
Final Note: Although many of the components here could have benefited from the use of Nylatron® nylons, these applications are not exposed to abrasive, high humidity or submerged environments. Acetal is also less costly and providing the low rotation speeds, customer should experience a long service life.
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