Machining Plastics: Case Study Ertalyte
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.
Ertalyte® PET-P: Case Study
In this study we are going to talk about Ertalyte materials and where we used them. This material has the wear resistance of Nylon with the added stability of an Acetal. Great for both wet and dry environments, high strength and rigidity for close tolerance parts. Excellent stain resistance with good wear resistance and excellent dimensional stability. Ertalyte has better resistance to acids than Acetal or nylon which makes an excellent candidate for food processing parts.
Ertalyte® PET-P: is unreinforced, semi-crystalline thermoplastic polyester based on polyethylene terephthalate (PET-P). It is manufactured from proprietary resin grades made by Quadrant. Only Quadrant can offer Ertalyte®. It is characterized as having the best dimensional stability coupled with excellent wear resistance, a low coefficient of friction, high strength, and resistance to moderately acidic solutions. Ertalyte®’s properties make it especially suitable for the manufacture of precision mechanical parts which are capable of sustaining high loads and enduring wear conditions. Ertalyte®’s continuous service temperature is 210°F (100°C) and its melting point is almost 150°F higher than Acetal. It retains significantly more of its original strength up to 180°F (85°C) than nylon or Acetal. In addition, Ertalyte® PET-P offers good chemical and abrasion resistance. Its low moisture absorption enables mechanical and electrical properties to remain virtually unaffected by moisture. Ertalyte® PET-P can be machined to precise detail on standard metal working equipment. Ertalyte® is FDA compliant in natural and black. Natural Ertalyte® is also USDA, 3A-Dairy and Canada AG compliant. Ertalyte® is an excellent candidate for parts used in the food processing and equipment industries.
Ertalyte® TX: is pale grey internally lubricated thermoplastic polyester providing enhanced wear and inertness over general purpose Nylon (PA) and Acetal (POM) products. Containing uniformly dispersed solid lubricant, Ertalyte® TX provides a lower wear rate and coefficient of friction than unmodified polyesters like PET or PBT and even internally lubricated materials like Delrin® AF blend. Ertalyte® TX excels under both high pressure and velocity conditions. It is also ideally suited for applications involving soft metal and plastic mating surfaces.
Common Applications for Ertalyte:
Food Equipment Components
Carousel, Filter Track, Locating Disk and ring
Rollers, Wheels without Bearings
Wear and Slide Pads
Thrust Washers and Journal Bearings
Ice Production Equipment
Transportation (Railroad Cars)
Note: Ertalyte tends to be rather notch and impact sensitive, all “internal” corners should have a minimum radius of (0.039"/1mm) and to avoid chipping the edges during turning, boring or milling, chamfered edges are advantageous, providing a smoother transition between the cutting tool and the plastics work. Important reminder that Ertalyte® and other polyesters have less resistance to hot water than Acetron® GP Acetal.
Because it is more rigid and offers greater thermal performance than nylon and Acetal Ertalyte machines differently. We will start with sawing, rip and combination blades with 0° tooth rake and 3° to 10° tooth set for general sawing in order to reduce frictional heat. Hollow ground circular saw blades without set with tungsten carbide tipped blades wear well and provide smooth cuts up to 3/4" thickness. Table saws are convenient for straight cuts and can be used to cut multiple thicknesses and thicker cross sections up to 4” with adequate horsepower. Saw blades should be selected based upon material thickness and surface finish desired. Fewer teeth per inch are typically recommended to generate less heat. Below is for band saw cutting based on the materials thickness.
Drilling Tip: Coolants are strongly suggested during drilling operations, especially with notch sensitive materials such as Ertalyte® PET-P, Duratron® PAI, Duratron® CU60 PBI, and glass or carbon reinforced products. The insulating characteristics of plastics require consideration during drilling operations, especially when hole depths are greater than twice the diameter.
Milling Tip: Sufficient fixturing allows fast table travel and high spindle speeds when end milling plastics. When face milling, use positive geometry cutter bodies. Climb milling is recommended over conventional milling. To ensure finished part flatness, always machine a plate flat to start. Do not force a plate flat with a vice or vacuum.
Threading and Tapping Tip: Threading should be done by single point using a carbide insert and taking four to five 0.001” passes at the end. Coolant usage is suggested. For tapping, use the specified drill with a two flute coated tap. Remember to keep the tap clean of chip build-up. Use of a coolant during tapping is also suggested. Use of a coated tap will create radii at the root of the threads resulting in a stronger and tougher thread which is less prone to cracking from over-torquing.
Turning Tip: Turning operations require inserts with positive geometries and ground peripheries. Ground peripheries and polished top surfaces generally reduce material build-up on the insert, improving the attainable surface finish. A fine grained C-2 carbide is generally best for turning operations.
Finale Note: Proper feed rates are critical to ensure reduced heat generation, tolerance control and good surface finish. Machining speeds can be increased above those listed as long as recommended feed rates are maintained.
Post Machining Annealing: In our experience and parts we have produced this step was avoidable with a roughing in machine stage and leaving parts unclamped for 1 to 2 days. This worked great for critical tolerance work. Here is some information on the annealing process that we feel is important to at least mention.
1. WHEN SHOULD PARTS BE ANNEALED AFTER MACHINING?
Experience has shown that very few machined plastic parts require annealing after machining to meet dimensional or performance requirements. Most Quadrant stock shapes that we used are annealed using a proprietary stress relieving cycle to minimize internal stresses that may result from the manufacturing process. This assures that the material will remain dimensionally stable during and after machining. However, machined-in stress can result in poor tolerance control and premature part failure. To prevent machined-in stress, it is important to identify the causes. Machined-in stress is created by:
Using dull or improperly designed tooling
Excessive heat generated from inappropriate speeds and feed rates
Machining away large volumes of material (usually from one side of the stock shape)
ROUGHING - RECOMMENDED ANNEALING TECHNIQUE: A simple rough machining step is usually sufficient, and preferred over oven annealing, on almost all jobs to achieve even the most critical tolerances on final dimensions. When machining with a mill, rough the parts leaving 0.030” to 0.060” per side. For rounds, rough the diameter oversized by 0.125”. Once rough machined, let the parts sit for 24 to 48 hours to stabilize; then finish machine. The more material you need to machine away, the more material you should leave on during roughing. Also, balanced machining on both sides of the shape centerline should be followed during roughing to help prevent warpage.
2. WHEN SHOULD PARTS BE OVEN ANNEALED?
Post machine oven annealing may provide additional performance benefits in a few rare cases. Also, oven annealing is used by some to achieve extremely tight tolerances. If using an oven be sure to follow annealing guidelines below, as shortcuts will actually increase part stress and create more problems holding required dimensions. Oven annealing may be used for the following rare cases.
Tighter Tolerance Capability: For extremely close tolerance parts, sometimes rough machining alone is not sufficient. Close tolerance parts requiring precision flatness and non-symmetrical contour sometimes require intermediate oven annealing between machining operations.
Case Study: Material Replacement for Food Production Process
Customer was looking for a material to replace several steel and plastic parts in their food and beverage production process equipment. Customer stressed, it needs to operate under elevated temperatures, and constantly exposed to moisture. There cannot be any impact on the materials’ mechanical and electrical qualities under machine operation. The material must be approved for food contact under standard regulations.
Additionally the new material should hardly ever break and needs to be tested specifically on strength and low stresses. Customer is at a point now for the need to reduce costs of materials and productivity deficits caused by equipment maintenance and too frequently breakdowns of steel to steel equipment parts.
Customer's sensitive business requires high performance materials for process equipment parts which can withstand various chemicals over a long life-time. Currently customer has to replace critical parts too frequently, and at considerable expense. Customer is looking for a solution to this situation with a high quality and safe material which allows them to save costs.
Challenge: Replace costly steel parts, replace failing plastic parts, reduce frequent maintenance calls and meet all standard food regulations
Solutions: We began this project with the obvious requirement that really narrows down material selection for Food and Beverage equipment (FDA, 3A-Dairy, CFIA) compliant materials and then narrowed down the selection even further with chemical resistance to customer's list of chemical contact usages and hot water/steam. We narrowed the list down to Acetal, HDPE, Nylon, Peek and UHMW-PE.
Note: We create a material list on every single component that will be replaced throughout the machine or application. There are several more columns of criteria that we use, but this should give you an idea of how to break down a material selection list:
This is not a complete list for this machine or to the criteria list. We also take material cost, machinability ratings and raw stock size availability into consideration when creating this chart. Some of the limitations we often find is there is a better choice, but material isn't available in the required thickness or it is far more expensive than its original material. If cost becomes a factor we try to evaluate if the extra cost of the material will be a better upfront factor versus the current equipment failure and frequent maintenance costs with the original material.
Note: This does require a maintenance history evaluation to help determine this decision.
Let's move on to the first machine replacement parts. In this machine assembly there are several cold water aluminum manifolds that distribute several chemical agents that have been causing staining; scaling and chemical build up on the inside bore channels. These parts are also exposed to a chlorine base wash down after production run downs. Our first choice was TiVAR® UHMW-PE due to its chemical resistance to Acids (pH 1-3), Alkalis (pH 9-14) and Chlorine (Aqueous). PEEK-1000 also has these properties but is many times more expensive than the UHMW-PE. Ertalyte® made this list due to its resistance to chlorine, but it lacks chemical resistance from acids, alkalis that are required to flow inside the manifolds and was not used in the Manifold-C. We did however use Ertalyte® to replace several piston and valves on a liquid filling meter manifold (Not Listed) that was originally made from stainless steel. Customer did experiment with Acetal and Nylon in the past with some success, Acetal wears quicker than Nylon and Nylon lacks the dimensional stability of Acetal. Ertalyte® is the better of the two materials combined and easy to clean making it a great choice for this section of the machine.
Next item was the aluminum forming die plates; these must be constantly sanitized with clean in place chemicals during wash downs. We used Ertalyte® over Acetron® GP due its ability to meet stringent tolerances, wear from constant sliding and clamping and great stain and chemical resistance. Acetron® GP is fine but we felt the ability to wear like Nylon would last longer from the constant sliding motion.
Down from the forming die plates are the filter tracks made from Nylon. This area had dimensional stability issues due to heat and is exposed to diluted hydrochloric acids. We where initially thinking of using UHMW-PE for these tracks to replace the Nylon version, we opted however to use Ertalyte® TX for its better strength properties in higher temperatures than UHMW-PE. We also needed a material more dimensionally stable than the current Nylon tracks, it is believed that the heat was softening the Nylon and causing locating issues. There where several points throughout the track that had been dented and caused the carriage erratic motion.
Another problem area was the stainless-steel valve shafts that experience high pressures and moving velocity conditions. Both manifold and valve shafts were made with stainless steel materials, the shafts however where constantly harder and cause excessive wear to the manifold housings. This created unacceptable and frequent maintenance to this area of the machine. Important factor here is the tight clearance between the shafts and the housing so wear and dimensional stability is critical in this application. We decided on using PEEK-1000 for the housing and Ertalyte® TX for the valve shafts. There were several stainless-steel manifold housings that had no wear and we opted to simply replace the valve shafts and use the current housing material as a test subject. We found that Ertalyte® TX worked great with the metal to plastic mating surfaces there for keeping that arrangement until the housing wears out.
Final parts are the aluminum Manifold-H which is used to transport hot fluid to various parts of the machine. These manifolds also contain built in electronic actuated shut-off valves mounted to the outside of the block on various ports and one temperature sensor. Lots going on here, we did have Ertalyte® listed here but we discovered that Acetron® GP is better suited to handle hot water or fluids; we didn't use Nylon due its water absorption rate and thermal expansion which might cause problems to the shut-off valves.
Another part worth mentioning that reduced costs where the stainless-steel roller shafts. We replaced these shafts with Ertalyte® TX due to their high PV values. There was also a Product Diverter Arms made of Aluminum and Hydex PBT, we again opted to Ertalyte® TX for its dimensional stability this part had to remain flat.
Down from the Diverter Arms where Carriage carts or trolleys as the customer called it. The carriages are guided on a tapered track; each carriage had six (6) Permaglide bearing sets (PTFE coated steel bearing material). The main issue here is the noise that was created when the Teflon coating would wear away from the bearings causing a steel to steel condition and although there was not enough heat and pressure to cause bearing failures. Operators would have to often use noise canceling equipment (ear plugs, head phones) when in close proximity. Each carriage also had four (4) metal alloy scraper or wiper blades in front and behind the carriage wheels. These wipers where made from wear resistant material; there function was to keep the track clean and clear. These wipers would frequently damage the track and add to the already noisy bearing condition. Final parts to these carriages are two (2) tipping bearings used to allow the carriage to tilt and release its cargo. These bearings where also made from Permaglide and experienced no considerable wear but was also replaced to reduce weight and cost. We suggested using Ertalyte® TX to replace the six (6) steel wheel bearings and two (2) tipping bearings for its better wear resistance, longer life and no noise. We also replaced the steel wipers with Ertalyte® TX to remove the metal-to-metal contact from the track; we are still capable of maintaining the tight tolerances required for keeping the track clean.
To wrap this up let's talk briefly about how we select the ideal material or materials in situations like this complex machinery:
Step 1: What is the primary function of the part? Is it "structural" (start by choosing amorphous materials. Is it a "bearing and wear" (start by choosing semi-crystalline materials).
Step 2: What is the maximum "No Load" continuous service temperature in air? What the continuous use temperature? (Consider the material's heat deflection temperature) review the materials that offer the ideal temperature resistance to your application.
Step 3: What is the pressure or stress of the application required at temperature? Calculate or estimate the needed working pressure or working stress required for the application (a material's max working stress is estimated using compressive strength, Dynamic Modulus Analysis (DMA) and creep data along a chosen safety factor.
Step 4: What chemicals will be encountered during service or cleaning? Are strong acids (pH 1-3), strong alkalis (pH 9-14), Chlorine (Aqueous) and hot water or steam going to be present in your application? (As a general rule semi-crystalline materials offer improved chemical resistance than amorphous materials).
Step 5: If this is for bearing and wear applications, what is the LPV? LPV is the Limiting Pressure Velocity required by your application? Limiting PV = P (psi) x V (FPM) =? Consider materials with a LPV higher than calculated and also choose a material with the lowest wear factor (k) to ensure optimum wear performance.
Step 6: Is there Compliance (NSF, FDA, 3A Dairy, etc.) required? If yes use FDA compliant materials, and medical applications consider biocompatibility tested LSG (Life Science Grade) materials. Note: Care and research should be emphasized if there are compliance issues, no mater how well your part performs in its application. You will fail if your material certs does not contain proper agency specifications.
Step 7: Is dimensional stability important? If yes consider materials with a lower CLTE (Coefficient of Linear Thermal Expansion) and materials with improved stability versus temperature.
Step 8: Is toughness or impact critical in use? Consider the Izod impact (a higher value means improved toughness) Also design your parts with radius corners (R=0.039/1mm) to prevent tearing and localized stress areas.
Step 9: What size stock shape is required for machining? Rod, Sheet, Tube and Casted net shaped stock. Note: Sometimes the required material choice isn't available in certain plate thicknesses, we have many times opted to use rod for square thick parts due to this limitation. If the parts are a high production item we also use near shape casted stock to reduce excessive material removal.
Step 10: What materials meet performance requirements and offer the best value? We know cost is everything but try not to sacrifice a high performing long lasting part over a cheaper material with shorter life and frequent maintenance requirements. Sometimes the higher upfront costs save you money in the long-term repair costs. This however is not something easily known, cost analysis is often required and without a maintenance history a simple material swap isn't apparent, and parts are often redesigned and tested again in the same material.
Step 11: Is this a generic or name brand material? Remember to use proper name call-outs on specifications to ensure quality and consistency in your material. If using a certain brand or company specific material like Acetron® GP or Ertalyte® and even Nylatron®. Do not simply specify on your part drawings Acetal, PET-P and Nylon, there are differences between the general standard materials. In this same breath however, there are companies out there simply rebranding standard materials with fancy names. Also request material specifications and compare the name brand to a standard generic material to ensure there not identical.
Well, there you have it; this list of questions is a great way to get started when selecting a plastic material for your applications. It is not limited to this list, nor does it answer every question, it does however get you started. This is also an insight on where we used this great material called Ertalyte®, hopefully this will also help you in your next application.
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