ANNEALING OF THERMOPLASTICS – WHAT IS IT, WHY DO IT?
Thermoplastic stock shapes and parts are produced by several methods – all create some level of stress in the material, which needs to be reduced for several reasons. The process most of these shapes undergo is called “annealing”.
The process is straightforward but isn’t “easy”. You heat the material to a temperature below its glass transition point, hold it there for time that is dependent on the material and thickness, then cool it. Both the heating and cooling must be done slowly, or the material will be thermally shocked which can cause, instability (rather than stability).
In plastics, extruded and injection molded shapes have the highest process-induced internal stress and require extensive annealing. Before cast nylon was available in FDA natural, companies would use extruded 101 (6/6) nylon rod up to 6” in diameter, and it wasn’t unusual even for thoroughly annealed 6” rod to explode every so often when contacted by a tool.
Compression molding is usually used to produce materials that cannot be melted – most often PTFE (“Teflon®”) based material, and UHMW-PE. These materials are “fused” under pressure and heat, similar to powder metallurgy. Despite this lower stress process, thick sections still require some extra attention to relieve stress.
Other materials are cast – several nylons, urethanes are examples. You pour the different base materials, and they react to become that material in the mold. With nylon, again, heavier sections require special handling. And, with cast nylon, secondary annealing also bakes off the residual unreacted monomer, which attracts excess moisture, and makes the nylon ma harder.
Occasionally, when machining parts that have unique designs – deep pockets, large cross-section differences – it’s appropriate to “rough” machine the part, reanneal it, then finish machine to print.
Is “slow-cooling” of cast nylon really “annealing”? – yes and no. Yes, in that the product is placed in an insulated box which cools the material more slowly = lower stress. No, in that there is no heat introduced to further align the molecules & really reduce the stress. Mills that slow cool usually find that the physical properties will indeed fall within the stated ranges – but there is no such range for internal stress, only the machinist will find that out.
Are there different methods of annealing plastics? Yes.
- OIL – Originally, engineering plastic stock shapes were annealed in an oil bath, which remains the most effective way to anneal. However, it’s expensive to maintain, and you have to machine away the oil stained surface before you can sell the material. Also, the EPA and OSHA weren’t fans & chased that method to Europe, where it is used sparingly.
- AIR – Domestic mills then turned to using “air” annealing, often with a nitrogen atmosphere to prevent surface oxidation (“browning”), is the primary method today. It is used both “free state” and with added pressure (on plate only).
- “IN-LINE” – in the 1990’s, technology was developed to anneal extruded materials “in line” immediately after extrusion, when residual heat in the material aids the process. Heaters in-line with the extruder add calories, the material continues through a slow-cooling zone. This process is extremely line speed & temperature sensitive, and is not as effective as air annealing, but it is faster and less expensive.
Here at WS Hampshire, we qualify all our source materials to insure you get parts that meet your requirements. Call us to discuss your needs – you’re in the right place!
Tom Connelly is a self f proclaimed “Street Engineer” with over 40 years in the plastics industry.
Acetal, an extremely popular engineering thermoplastic also known as POM (polyoxymethylene), comes in two basic types: homopolymer (POM-H) and copolymer (POM-C). While either one will work in over 95% of the applications, it’s important to know the differences.
Historically, homopolymer (widely known by its DuPont tradename “Delrin®) was the preferred type in North America; in Europe, it was copolymer. The difference was with sourcing – here, we “grew up” with DuPont, in Europe they had Hoechst-Celanese and BASF.
Copolymer traditionally had cost and processing advantages over homopolymer, advances in extrusion technology in the early 1990’s led to North America’s transition to copolymer.
So, what are the actual differences in the materials?
Functionally, either will work satisfactorily in most applications. Both are FDA compliant, machine very well and have similar properties, though POM-C has better chemical properties. Homopolymer’s more basic structure gives it higher physical properties, making it the correct choice for such applications as gears and “keels” (structural support) for artificial feet
|Tensile strength PSI||11,000||9,500|
|Flex Strength PSI||13,000||12,000|
|Compressive Strength PSI||15,000||13,500|
|Heat Deflection Temp (⁰F) 264 PSI||250||220|
Click to expand
Despite POM-H’s advantage in physical properties, its disadvantages are significant:
- Centerline porosity (low density area due to the way the more crystalline POM-H cools) – this often requires buying oversize material to prevent porous surface exposure in final form)
- Formaldehyde outgassing – POM-H’s backbone is anhydrous formaldehyde, and the odor generated during machining & occasionally in service is very noticeable and can be an irritant
- Size Limitation – due to the difficulty in processing POM-H, it cannot be made in very large cross-sections rod or plate, and not in tubes at all
- Cost – the resin is 10%-15% higher in cost than POM-C, and costs more to make shapes
Copolymer’s primary advantages:
- Uniform density, eliminating the centerline porosity issue (in food service, porosity = mold)
- Better steam / Hydrolysis resistance – up to 180F
- Less formaldehyde outgassing – most of the formaldehyde is transformed into trioxane.
- Less inherent stress – takes less pressure to process
- Size Availability – rod up to 24” diameter, plate up to 10” thick, and extruded tubing; it can be made by extrusion and compression molding
- Medical Applications – certain POM-C resins have USP approval for medical uses such as orthopedic trial implants
- Colors – an extension of medical needs, POM-C is readily available in standard & custom colors
NOTE: POM-H stock shapes are also available in modified versions, including three PTFE (“Teflon®”) filled grades, glass filled, UV stabilized and several different viscosity grades.
Have questions? We can help! The experts at WS HAMPSHIRE are ready to assist you to find the right material for your requirements! Call us – you’re in the right place!
Tom Connelly is a self proclaimed “Street Engineer” with 40+ years in the plastics industry.
Polyetheretherketone (PEEK) is an engineering thermoplastic with an unusual combination of properties- high mechanical strength, fatigue and creep resistance, excellent chemical resistance, and the ability to perform in high temperatures. It is so robust and does so many things well, it is the most utilitarian ‘Swiss Army Knife’ in the high performance material thermoplastic group.
First synthesized in 1978 by Victrex, it quickly became an excellent alternative to imidized materials (VESPEL® PI, Torlon® PAI). Offering lower cost and superior chemical and hydrolysis (steam) resistance, PEEK is an ideal material for seals, bearings and insulators in oil/gas/chemical processing and other high temperature/pressure applications. PEEK offers higher strength than fluoropolymers (Teflon®) at room and elevated temperatures.
There was only one resin source at the time, so pricing was high and general industry acceptance was slow. In recent years, new sources have introduced PEEK based materials, reducing the price and accelerating application development, making it attractive to more markets.
Processing techniques have also increased with shapes now being available via injection molding, extrusion, and compression molding.
Unfilled PEEK has the highest toughness and elongation of all the PEEK materials, and by itself has good bearing & wear properties. The most popular filled grades are 30% glass filled, 30% carbon fiber filled and a bearing grade with a carbon/graphite/PTFE additive package.
The addition of fiber reinforcement gives PEEK dimensional stability approaching that of metals.
Here’s a brief comparison of the properties of extruded PEEK variant shapes (at 73⁰F):
|Unfilled PEEK||30% Glass Filled PEEK||30% Carbon Filled PEEK||Bearing Grade PEEK|
Heat Deflection Temperature ⁰F
Coefficient of Thermal Expansion
Dynamic Coefficient of Friction
|Relative Wear Rate||375|
PEEK products are found in highly critical applications from High-Performance Liquid Chromatography columns to food applications as unfilled PEEK is FDA and USDA compliant. It is one of the few plastics compatible with ultra-high vacuum applications, which makes it suitable for aerospace applications ultra-pure specialty grade PEEK is considered an advanced biomaterial used in medical implants, used with a high-resolution magnetic resonance imaging (MRI), and for creating a partial replacement skull in neurosurgical applications.
Have a need for high performance at high temperatures? Call the experts at WS HAMPSHIRE to walk you through the candidate material selection process. You’re in the right place!
Tom Connelly is a self proclaimed “Street Engineer’ with over 40 years in the plastics industry.