Why Cast Nylon?

With all the different choices available, why would anyone choose cast nylon over the literally hundreds of other thermoplastics in today’s market? There are a number of reasons that come to mind that relate to performance, price and availability.

Cast nylons are used in a wide range of applications, from a simple equipment support pad, to a sophisticated drum in a machine that sorts thousands of coins a minute, to a sheave on a crane that can lift 1,000 tons, to a component used to eviscerate chickens! There is perhaps no other process that can produce parts weighing from one pound to over eight hundred pounds in tooling that, relatively speaking, costs peanuts.

So what makes this material suitable for so many applications? A quick look at the material’s basic properties compared to other common materials shows why. It has excellent tensile and compressive strength, good wear properties and is an excellent electrical insulator. It can also be modified to improve just about any individual property; plasticizers improve impact strength, lubricants reduce coefficient of friction, anti-oxidizers improve operating temperature range, glass fibers improve rigidity, other additives allow the use of colored nylons in direct food contact applications.

Let’s compare cast nylon with a couple of the other more common materials.

Extruded nylons are predominantly made from type 6/6 nylon and have basic physical properties very similar to those of cast nylon. Performance and size availability however are another matter. Cast nylon has lower residual stress levels, more uniform crystallinity and thus better dimensional stability than extruded. Extruded products are generally more attractive in the smaller size ranges where extrusion rates are highest. In sizes over 3” for rod and 1” for plate, cast nylons become more attractive due to the slow extrusion rates in producing extruded products.

Acetal – much has been said about the dimensional stability of acetal based on its low rate of moisture absorption, which is a fact but only relevant if the part is either submerged or subjected to considerable moisture exposure. Acetal comes in two forms, homopolymer and copolymer, properties shown are for homopolymer which are the higher than those of copolymer. Acetal and cast nylon have basically the same coefficient of thermal expansion however, which means that if temperature changes then both materials will change by the same amount. This means then that the only difference would be in the amount of expansion resulting from moisture absorption and if sufficient clearance is designed into a cast nylon part to accommodate this expansion, a cast nylon part should outperform an acetal part because of cast nylon’s superior bearing properties.

Both materials are FDA/USDA listed for direct food contact. While both materials machine easily, acetals release formaldehyde during machining and adequate ventilation must be provided to protect the operator from exposure.

UHMWPE is available in much wider size ranges than either extruded nylon or acetal and also costs much less. It has much lower moisture absorption than acetal and has better chemical resistance than either acetal or nylon and also has excellent wet abrasion resistance. Its downfall is in its physical strength; it has very low tensile and compressive strength and can not operate at temperatures beyond 160ºF. Bottom line however is that if UHMWPE will work for your application, you may as well use it because you probably won’t find anything cheaper to do the job.

Basic Physical Property Comparison
Property	Units	Cast Type 6 Nylon	Extruded Type 6/6 Nylon	Acetal (Homopolymer)	UHMWPE
Tensile Strength	psi	11,500	11,500	11,000	5,700
Compressive Strength	psi	15,000	12,500	16,000	4,000
Flexural Strength	psi	16,500	15,000	13,000	- - -
Hardness	Rockwell R	120	115	122	67
Notched IZOD Impact Strength	ft.lb/in	0.8	0.6	1.0	No Break
Maximum Continuous Service Temperature	Degree F	230	210	180	160
Coefficient of Thermal Expansion	in/in/DegF	0.00005	0.000055	0.000047	0.00011
Moisture Absorption of Saturation	%	5.5	7	0.9	0.01

When evaluating materials for an application, never make a decision based solely on the materials physical properties since these are intended to show how the materials compare when tested under the same lab conditions. If an application applies a compressive force of 3,500 psi then any of the materials discussed can be used, as the temperature of the application increases the material choices diminish. Always look at what the application is and how the material will perform in the application. If in doubt, contact the manufacturer for assistance.

Another advantage of cast nylon is that, with the exception of glass-filled, you can make any formulation in the same tool, so if you want to switch from a standard nylon to an internally lubricated nylon to enhance performance, just change the formulation by adding the desired lubricant. One of the major features of cast nylon is that you can make custom cast parts that are very close to the desired final part and require minimal, if any, cleanup machining to finish. This can save hundreds of dollars in material, labor and tool costs.

The nylon casting industry has come a long way in the past 30 years. There was a time when glass-filled cast nylon was not possible because we did not know how to keep the glass in suspension during the reaction process; today it is a standard product. While the extrusion and molding industries are often ahead of the casting industry in terms of specialty products because they do not have to deal with the idiosyncrasies of the reaction process, they are limited in terms of size and flexibility and tooling costs are astronomical compared to those of the casting industry. The casting industry is catching up with new formulations which, with existing size capabilities, make it a formidable opponent. As we discover new applications that require new additives to improve material performance, we must also develop means of introducing the additives in a manner that does not interfere with the reaction process and thus allow us to maintain the properties of this unique material.

Anyone can buy a molding machine or an extruder and a truck load of resin and be in business as a manufacturer of extruded or molded products. It takes imagination, patience and ingenuity to be a manufacturer of cast nylon.

Written by Michael Worthington, Chief Engineer for Cast Nylons, Limited, manufacturers of Nycast® rod, sheet, tubular bars, discs rings, custom castings and Nymetal®, Nyloil®, Nycast® 12 and Nycast® RX and GX solid lube-filled materials.