Engineering Plastics

An in depth guide

metals4U stock PA6 and Acetal copolymer in extruded rod, tube, and sheet presentations.

Nylon 6 and Acetal are both engineering plastics that are categorised as 'thermoplastics'; these types of plastic materials are easily moulded and pliable when heated to a specified maximum temperature and solidify and harden when cool. Engineering plastics can offer some useful advantages over the use of metal in many applications. Plastics will not rust, they have excellent thermal and electrical insulation properties, and are very economical to purchase and use.

Nylon 6 and Acetal are lightweight materials that are easy to machine, mould, and fabricate, with the added advantage of strength and good resistance to creep.

Engineering plastics are suitable for the manufacture of plain bearings, bushes, gears, cams, rollers, seal rings, slide bearings, insulators, relay and transformer housing, pump components, valve bodies, and general applications in engineering, automotive, and catering sectors.

How engineering plastics are made

Nylon, also referred to as PA6, or Polycaprolactum, is produced from a monomer called Caprolactum; this is an organic compound which is a cyclic amide (a lactum) of caproic acid. Nylon 6 is the product of the polymerisation process of aminocaproic acid to produce a polymer. (A polymer is repeated chain of subunits joined by amide links) These nylon polymers are then heat processed to produce extruded or moulded profiles for use.

Acetal, POM C, or Polyoxymethylene Copolymer as it is also known, is produced by converting formaldehyde into trioxane by sulphuric acid, or acidic ion exchange resins, via acid catalysis. The trioxane is then purified by extraction or distillation to remove water and other impurities that contain hydrogen. Acetal is then processed by heat and pressure to be formed into injection, blow, and rotational moulded profiles or extrusions.

Properties

Excellent for machining, lightweight, and low friction qualities make engineering plastics a viable alternative to metal. Engineering plastics can be machined without the need for lubricant and coolants for most processes; this makes work handling and cleaning up easier.

Nylon is hygroscopic, which means that it is moisture sensitive. This factor needs to be considered when choosing the correct engineering plastic for your project.

Acetal is not hygroscopic; therefore, it can be used in applications where the absorption of liquid is not required or desired.

When thermoplastics become cold they are prone to becoming brittle. It is not recommended to store plastics in very cold environments or attempt to work with them in a chilled state.

Magnetic

Engineering plastics are not magnetic.

Corrosion resistance

Nylon 6 and Acetal have very good corrosion resistant properties to a wide range of chemicals.

Acetal has good resistance to most common solvents, esters, lubricants, aqueous alkali and acidic solutions between ph.5 through to ph.11, and ketones, however, it has low resistance to phenols, concentrated mineral alkalis and acids, and halogens.

Nylon 6 is highly resistant to fats, oils, ethers, esters, ketones, and hydrocarbons, but is not resistant to mineral acids and some organic acids, oxidising agents, and halogens.

Recycling

Thermoplastics can be heated to their melting point, cooled and reheated again without suffering degradation of their properties; this is particularly useful in terms of recyclability of Nylon 6 and acetal.

Nylon and Acetal can both be recycled by chemical and mechanical recycling processes without compromising their physicochemical integrity.

Mechanical recycling

The waste thermoplastic is cleaned and processed (by shredding) into pellets to be reused.

Chemical recycling

The plastics are chemically de-polymerised and broken down into their monomer components. The plastic is then re-polymerised and re-manufactured into stock profiles for re-use.

Biodegradable

Acetal and PA6 nylon are not biodegradable.

As there are many similarities and differences between Acetal and Nylon 6, it is important to select the type of plastic best suited to your chosen application. It is important to consider exposure to chemicals, behaviour with moisture and temperature, expansion and creep, and machinability.

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  Nylon 6  

PA6 is a member of the Polyamide family of engineering plastics. The PA identifies the Polyamide chemical structure and the 6 indicates the number of atoms in the lactam monomer in its chemical structure.

If the nylon is intended to be use outside, the black pigmented nylon is recommended as is has better UV stability than the natural colour presentations.

Nylon 6 has good machinability and is often used in place of metal for bearings and bushes due to its lighter weight reducing wear on mating parts. PA6 is less noisy during operation because of its very good damping qualities and can reduce, or remove, the need for external lubrication due to its low coefficient of friction.

Nylon 6 is suitable for most general engineering uses and for food processing machinery components, seals & gaskets, rollers, wear pads and components, gears, wheels.

PA6 is suitable to be used in direct contact with foodstuffs, although its hygroscopic qualities should be countered into suitability for some food hygiene uses.

Chemical properties

(C6H11NO)n

Nylon 6 is formed by polymerisation of caprolactam. Caprolactam is a cyclic amide of caproic acid.

Technical properties of PA 6

Property Result Unit Test
Specific Gravity 1.14 g/cm3 ISO 1183
Water absorption 9 % ISO 62
Maximum service temp. Upper temp limit (no stronger mechanical stress involved)
- Short term 160 ° C
- Long term 85 ° C
Lower temp limit -40 ° C -
Tensile Strength at yield 76 N/mm2 ISO 527-2
Tensile strength at break - N/mm2 ISO 527-2
Elongation at break ≥50 % ISO 527-2
Impact Strength No break KJ/m2 ISO 179 / 1eU
Notch impact strength 5.5 KJ/m2 ISO 179 / 1eA
Ball indentation/ Rockwell hardness 150/M85 N/mm2 ISO 2039-1/-2
Flexural Strength - N/mm2 ISO 178
Modulus of elasticity 3250 N/mm2 ISO 527
Vicat-softening point VST/B/50 - °C ISO 306
Heat deflection temperature HDT/B - ° C ISO 75-2
Coefficient of linear thermal expansion 0.9 k-1* 10-4 ISO 11359
Thermal conductivity at 23° C 0.28 W/(m*K) DIN 52612
Surface resistivity - Ω IEC 6093
Dielectric constant at 1MHz 3.3 - IEC 60250
Dielectric loss factor at 1MHz 0.021 106 Hz IEC 60250
Dielectric strength 25 kV / mm IEC 60243-1
Comparative tracking index (CTI) 600 - IEC 60112
Food compliance + - FDA
Flammability HB (slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm) - UL 94

Machinability

PA6 has a machinability rating of good.

The melting point of engineering plastics is much lower than those of metals, therefore, when machining PA6 there are a few points that need to be remembered to ensure success and reduce wastage.

When PA6 becomes cold it will lose some of its toughness and will have less resistance to machining stresses; it is therefore not recommended to machine PA6 in a chilled condition.

Tools should be kept sharp and single flute end mills are recommended as these blades reduce the heat generated while working and provide a cleaner cut. High Speed Steel (HSS) are recommended over carbide as they are sharper, this provides better results.

PA6 can be machined without the need for lubricants and coolants, however, coolant may be necessary for drilling and 'parting off' operations. Air cooling is recommended while machining to reduce the risk of the Nylon 6 melting at the cut surface or suffering thermal expansion and deformation. The use of lubricant / coolant is best avoided for some projects as it will be absorbed during working which will disrupt measured tolerances. Once the lubricant has evaporated out of the plastic it will return to its original dimensions. Ethylene Glycol coolant can be used sparingly with good results but Isopropyl and Butyl Alcohol should not be used due to their flammability.

When using chemical coolants, it is advised to use appropriate PPE, including eye protection, and to work in a well-ventilated area observing standard safe working practices.

For best results, keep the rpm low and the feed rates high.

Burrs can be removed post machining with a sharp blade if necessary.

Formability

PA6 has excellent formability by extrusion and moulding techniques, this makes it well suited to injection moulding and 3D printing.

Extrusion

Before extrusion, PA6 must be dried for 3 hours at 60°C.

Use a single screw extruder with a polyolefin screw or barrier screw with equal feed, transition and metering with a compression ratio of 3 or 4:1.

Cylinder temperatures will need to be set between 230°C and 260°C. The melt temperature for extrusion is 230°C to 290°C.

If bubbles are visible then the temperature is set too high, yet to prevent 'freeze-off' PA6 must be worked at above 220°C. Decreased wear and increased output can be achieved by preheating the Nylon 6 to 75°C or heating the feed section to 150°C.

Injection moulding

The melt temperature of PA6 for injection moulding is between 230°C and 280°C.

The optimum mould temperature for hot injection moulding is greater than 80°C; for structural parts where more crystallisation is necessary, the mould should be heated to between 80°C and 90°C. Higher mould temperatures are also recommended for the long flow length of thin walled components. Cold mould temperatures of between 20°C and 40°C are recommended for components with a wall thickness exceeding 3mm.

The pressure is recommended to be between 75MPa and 125MPa depending on the product design with a high injection speed.

Cutting

Sharp hand tools can successfully be used to cut rod, tube, and sheet. Using tools with few teeth will make the process easier and will produce less burring and heat.

Bandsaws, circular saws, and reciprocating saws can all be successfully used to cut PA6. The use of blades with widely spaced teeth are recommended to enable good chip removal.

Flame cutting techniques and traditional laser cutting are not recommended for PA6 as the low melting point of the plastic would result in melting and distortion at the cut edge.

CO2 laser cutters can be very successfully implemented for vector cutting, vector engraving, and raster engraving as the controlled melt at the cut edge ensures no fraying occurs; no deburring or post processing is necessary with this method of cutting.

When drilling PA6, a small amount of coolant may be necessary to reduce the risk of unwanted surface melting, however, it should be remembered that PA6 will absorb some of the liquid coolant which may affect dimensional tolerances once the piece has dried out. Conventional drilling emulsion is particularly suitable for use when cutting threads and deep boring.

Joining

PA6 can be successfully joined using welding, mechanical, and adhesive processes. Due to the hygroscopic properties of nylon, it is important to ensure the piece has been correctly dried out before attempting any bonding process; this can be done by holding in an oven at 60°C for several hours, or preferably overnight.

Mechanical

PA6 nylon can be joined using rivets, nails, self-tapping screws, thread fasteners, interference or press fittings, and snap fit techniques.

Welding

PA6 can be welded using a wide range of techniques for welding thermoplastics. For hot air and extrusion welding techniques, it is important to use a plastic welding rod of the same plastic type.

Adhesives

PA6 nylon can be successfully bonded using industrial adhesives. Prior to adhesive application the surfaces to be bonded should be provided with a 'key' by using emery paper, wire wool or grit blasting to improve the strength of the bond.

Epoxy and two-part polyurethane adhesives work well, as do structural acrylic adhesives such as methyl methacrylate.

Anaerobic adhesives are suitable for thread locking purposes but are not recommended for structural joint bonds.

UV cure adhesives are not recommended as PA6 blocks out UV light which would stop the adhesive being correctly or adequately cured, however, if the UV adhesive was being used through a clear substrate it could be used.

Annealing

After substantial machining, it may be necessary to relieve the machined-in stress or reduce the high stress state of extruded shapes by completing an anneal cycle. The benefit of completing an anneal cycle is that the risk of post work distortion and cracking is reduced, the physical properties are restored, and the stability of the finish part's dimensional integrity can be ensured.

To anneal

  • Place worked piece in an oven where the air can freely circulate, next, heat the oven at a maximum rate of 7°C per hour until a top temperature of 160°C is reached.
  • Hold the oven at 160°C for 30 minutes with an additional 15 minutes per 3mm of cross section thickness.
  • Cool oven to 65°C at a maximum cooling rate of 12°C per hour.
  • Turn off oven and allow to cool naturally, once the oven is back to room temperature the annealed pieces can be removed.

Hardening

Nylon cannot be heat treated to increase hardness, nor can it be work hardened, however, the toughness and impact strength can be increased by 'conditioning'.

Conditioning is a process where the nylon is subject to two basic processes; held at temperature or treated with moisture, (relative humidity 50% RH to total immersion) to alter the mechanical properties. This process enables absorption of water into dry polyamide parts to change the properties and dimensions, if the moisture absorption does not take place until the part used in the final service condition the changes may have an adverse effect in many applications. Additionally, dimensions, rigidity, and strength of most engineering components are designated with narrow tolerances, therefore, if the piece is conditioned during manufacture, the final component will directly correlate to the specifications determined for when the piece is in its equilibrium state in the planned final climatic condition.

The effect of conditioning is important to consider when processing nylon in applications such as moulding and welding as to how it may alter the end-use performance such as mechanical and dimensional tolerances, and surface appearance. It is important to remember that while the higher humidity will toughen PA6 and increase the impact strength, the tensile strength will be lessened.

Corrosion resistance

PA6 has excellent corrosion resistance to hydrocarbons, fats, oils, ketones, fuels and esters but poor resistance to strong acids, halogens and oxidising agents.

How supplied

Extruded round rod is available in black and natural colour in a wide a range of diameters. Round rod is supplied oversized to allow for machining to listed dimensions.

Extruded sheet is available in black and natural colour in a range of thicknesses.

Round tube is available in natural colour in a range of diameters. This is supplied with an undersized internal diameter and an oversized external diameter to allow for finishing to the listed sizing.

All Nylon 6 products are supplied in an unfinished presentation.

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Acetal Polyoxymethylene Copolymer

Acetal or POMC has low moisture absorption, high mechanical strength, and dimensional stability which makes it well suited for use in the manufacture of seals, bushings, pump and valve parts, bearings, rollers, electrical components, gears, and components for use in marine applications.

Acetal has a low coefficient of friction and high abrasion resistance which leads it to behave as self-lubricating, however, lubricating a load bearing acetal component will significantly lengthen its service life. Both dry lubricants such as moly or graphite and any commercially available 'wet' lubricants, such as Teflon based products, are suitable.

Acetal poses no environmental hazards, however, as with all good workshop practices, inhalation of dust and accidental combustion fumes should be avoided.

Acetal is suitable for direct contact with foodstuffs.

For outside use, black pigmented Acetal has excellent UV stability and natural coloured POM C presentations are rated as very good.

Chemical properties

(CH2O)n

Acetal is formed from polymerising anhydrous formaldehyde to form an Oxymethylene chain.

Technical properties of Acetal

Property Result Unit Test
Specific Gravity 1.41 g/cm3 ISO 1183
Water absorption 0.8 % ISO 62
Maximum service temp. Upper temp limit (no stronger mechanical stress involved)
- Short term 140 ° C -
- Long term 105 ° C
Lower temp limit -40 ° C -
Tensile Strength at yield 63 N/mm2 ISO 527-2
Tensile strength at break 63 N/mm2 ISO 527-2
Elongation at break 31 % ISO 527-2
Impact Strength 220 KJ/m2 ISO 179 / 1eU
Notch impact strength 8 KJ/m2 ISO 179 / 1eA
Ball indentation/ Rockwell hardness 140/M84 N/mm2 ISO 2039-1/-2
Flexural modulus of elasticity 2500 N/mm2
Tensile modulus of elasticity 2600 N/mm2 ISO 527
Vicat-softening point VST/B/50 150 °C ISO 306
Heat deflection temperature HDT/B 96 ° C ISO 75-2
Coefficient of linear thermal expansion 1.1 k-1* 10-4 ISO 11359
Thermal conductivity at 23° C 0.31 W/(m*K) DIN 52612
Surface resistivity 1013 Ω IEC 6093
Dielectric constant at 1MHz 3.8 - IEC 60250
Dielectric dissipation factor at 1MHz 0.008 106 Hz IEC 60250
Dielectric strength 20 kV / mm IEC 60243-1
Comparative tracking index (CTI) 600 - IEC 60112
Food compliance + - FDA
Flammability HB (slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm) - UL 94

Machinability

Acetal has excellent machinability; it has good chip forming properties that ensure worked pieces have a smooth surface when finished. Close tolerances can be achieved when machining due to acetal's low moisture absorption and good dimensional stability; this makes it particularly suited to the manufacture of parts that require maintenance of tight tolerances while performing complex machining applications.

As with all engineering plastics, Acetal will lose some of its toughness when it is cold, this loss of toughness leads to less resistance to machining stresses. It is recommended to not attempt to machine acetal when the product is in a chilled state.

When machining acetal, long, string like chips are sometimes formed which can wrap around the tool shank or stick to the surface of the workpiece which can slow down machining time and increases the risk of burrs forming. To prevent this, it is important to use sharp tools and pay attention to coolant delivery to ensure the waste is flushed away from the cutting zone. Acetal can be machined without the need for lubricant due to its low friction properties. When 'parting off', the use of coolant will ensure an improved surface finish and reduced frictional heat.

When drilling acetal, a small amount of coolant may be used to reduce the risk of melting at the surface edge. Acetal does not absorb much moisture so there is less risk of affecting the dimensional tolerances from coolant absorption after the piece has been drilled; this makes conventional drilling emulsion suitable for use when cutting threads and deep boring processes.

As acetal does not absorb moisture in the same quantity as PA6 engineering plastic, the use of lubricants and flood coolants is less of a concern regarding post working dimensional tolerance issues.

HSS tools with single flute end mills are recommended over carbide tools as they are tougher and retain their sharpness; these properties will give better results when used on acetal.

Formability

Copolymer acetal has very good formability through moulding and extrusion techniques.

Injection moulding

The melt temperature of Acetal when injection moulding is between 182°C and 199°C with an optimum mould surface temperature between 82°C to 121°C.

The pressure is recommended to be between 100-138 MPa for first stage injection and 55-103MPa for the hold (2nd) injection stage (depending on the design) with a medium to fast injection speed.

The finished work should then be dried in a dehumidifier at 82°C for 3 hours.

Acetal should never be processed at temperatures exceeding 238°C due to formaldehyde fumes that may be given off. If overheating occurs, it is recommended to immediately reduce the cylinder temperature and purge the remainder of the overheated acetal into water- keep back from the nozzle and hopper to avoid inhaling fumes and ventilate the area to disperse any lingering exhaust.

Extrusion

For optimal success, use a metering screw with a L/D ratio of 20:1 to preferably 24:1 with the feed and metering zone being 35% each of the total screw length and the transition zone around 30%.

The melt temperature for extrusion is 180°C to 210°C with a compression ratio of 3:1 and cylinder temperatures will need to be set between 180°C and 205°C.

Acetal is not affected by contact with zinc, copper, nickel, iron, bronze or brass during the extrusion process which ensures freedom of choice over the most cost effective or suitable die to use for the given design. The die and head temperatures should be between 190°C and 230°C; the best results are achieved by setting the die temperature to between 10°C and 38°C higher than the head temperature.

Cutting

Sharp hand tools with few teeth can successfully cut rod and sheet; some engineers recommend applying a thin layer of wax to the blade prior to cutting to ease cutting.

Bandsaws, circular saws, and reciprocating saws can all be successfully used to cut acetal. Using blades with widely spaced teeth is recommended to enable good chip removal and reduce the amount of 'grab' between the blade and the plastic.

Flame cutting techniques and traditional laser cutting are not recommended for acetal as the low melting point may result in distortion at the cut edge.

CO2 laser cutting acetal can be very successful to vector cut, raster engrave, and vector engrave as no fraying occurs on the cut edge. This process also has the advantage of not requiring any post processing or deburring which cuts down on production and manufacture times.

Joining

Acetal can be joined by mechanical techniques and bonded by welding and industrial adhesives.

Mechanical

Acetal components can be joined by nailing, riveting, self-tapping screws, thread fastening, interference or press fits, and snap fit techniques.

Welding

Acetal can be successfully welded using a variety of standard thermoplastic welding techniques. When using techniques that require the use of plastic welding rods, it is important to ensure the same type of plastic rod is used as the part to be joined.

Adhesive

When bonding acetal to acetal, welding techniques would be the most usual option. When bonding acetal to other materials, including metal, the most commonly used technique is industrial adhesive application. Before applying the adhesive, it is important to provide the surfaces to be bonded with a 'key' by using grit blasting, emery paper or wire wool to improve the strength of the bond.

Cyanoacrylate, used with the appropriate primer, gives the best bond strength for structural joins. Threaded parts can be successfully bonded by using an anaerobic thread locker with activator.

Annealing

After substantial machining or when using extruded shapes, it may be necessary to relieve the stress by completing an anneal cycle. The benefits of annealing are that it reduces the risk of post work distortion and cracking, restores the physical properties, and ensures stability of the finish part's dimensional integrity.

To anneal

  • Place worked piece in an oven where the air can freely circulate, next, heat the oven at a maximum rate of 7°C per hour until 152°C is reached.
  • Hold the oven at 148°C for 30 minutes with an additional 15 minutes per 3mm of cross section thickness.
  • Cool oven to 65°C at a maximum cooling rate of 12°C per hour.
  • Turn off oven and allow to cool to room temperature, then remove the annealed pieces.

Hardening

Acetal can not be hardened or conditioned.

Corrosion resistance

Acetal has good corrosion resistance to solvents, lubricants, esters, ketones and solutions of alkali and acidic solutions between ph5 and ph11. Acetal is not resistant to phenols, formic acid, halogens, and concentrated mineral alkalis and acids.

How supplied

Extruded round rod is available in a range of diameters in black and natural colour. This is supplied oversized to allow for machining back to listed dimensions.

Extruded sheet in a range of thicknesses is available in black and natural colour.

Round tube in a range of diameters is available in natural colour. This is supplied with an oversized external diameter and undersized internal diameter to allow for finishing to the listed sizing.

All our Acetal products are supplied in an unfinished state.

Welding techniques and suitability for thermoplastics

Technique Welding PA6 Nylon to PA6. Must be dried thoroughly before welding at 60°C for minimum 3 hrs or overnight. Welding Acetal to Acetal
Ultrasonic. Mostly performed at between 20-40 KHz. Suitable Suitable
Ultrasonic Close weld Good Good
Ultrasonic Distant weld Fair Fair
Ultrasonic Inserting Good Good
Ultrasonic Spot welding Fair Fair
Ultrasonic staking Fair Fair
Speed Tip Welding Suitable Suitable
Hot plate Suitable Suitable
  Suitable Suitable
Laser welding. Particularly suitable for long runs due to the precision of heat and high speed. Suitable Suitable
Friction stir welding. No consumables required as the 'welding rod' does not melt, the rod melts the parent plastic. Suitable Suitable
Dielectric welding (radio frequency welding) uses electromagnetic energy to bond the thermoplastic surfaces, no consumables or adhesives are required to create a hermetic, cohesive bond. Suitable Suitable
Microwave Welding. Area of workpieces to be joined are pressed together and exposed to microwaves within the 2-10GHz range. Suitable Suitable
Spin Welding / Rotational Friction Welding. Suitable Suitable
Hot air -easy and economic- can be used to repair cracks in thermoplastics as well as for joins. Must use welding rods of same thermoplastic type as being welding. Different nozzles for tack, weld and repair. Use speed welding or pendulum welding techniques. Suitable with the appropriate PA 6 welding rod. Suitable with the appropriate Acetal welding rod.
Linear Friction Welding / Vibration. High Frequency 190-240 Hz and motion of 0.07-2mm Low Frequency 100Hz and motion of 2-4mm Suitable Suitable
Extrusion Welding. Use welding rod of same material. Particularly well suited to large, single pass welds of thermoplastic over 6mm thick. Suitable with the appropriate PA 6 welding rod. Suitable with the appropriate Acetal welding rod.
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