Dimensional Stability Issues In Nylons and Acetals
I’m sure you’ve all encountered at some point a part machined to a tight tolerance and passing QC being out of tolerance when removed from inventory some time later or when arriving at a customer. Some materials, like thermosets, are generally quite good while others, like thermoplastics tend to have more movement. This movement, or lack of stability, can most often be over-come if the right approach is taken. Let’s look at the four basic factors that have the most influence on nylons and acetals moving out of tolerance.
Residual Material Stress
Because of the processing temperatures and methods used to produce thermoplastics, there is always some degree of residual manufacturing stress that remains in the material. This stress is generally related to the surface of the material cooling faster than the core and a zone of tension that exists between the core and the surface. Once the surface is removed in machining the stress is relieved and the part moves. Most nylons and acetals are annealed, or stress relieved, after production to remove these residual stresses but some degree of stress generally remains, particularly in heavier section materials. This can often be improved by asking for heavy section material to be re-annealed: while this often adds cost and lead time to the raw material, it can save you time and money later on.
Machining Induced Stress
The simple act of machining nylons and acetals produces heat at the cutting point. Since nylons and acetals are poor conductors of heat, the heat remains at the surface of the material and causes the surface to expand. When the part is checked while still in the machine, or immediately after removal it looks fine, but later moves out of tolerance as it cools. It is critical to follow the manufacturer’s machining instructions when machining all plastics, this will reduce the amount of heat generated and subsequently lower the stress generated. When removing large amounts of material or producing parts with tight tolerances, it is best to first rough machine the parts leaving some 0.60” - .090” on all surfaces. Let the parts sit for 24 hours then finish machining. This allows any heat stress generated in rough machining to dissipate and allow the part to settle before being brought to final dimension. Use coolant for heavy work; coolants used in machining are not in contact with the part long enough to be absorbed and cause swelling.
Coefficient of Linear Thermal Expansion, or CLTE, is the rate of movement of a material as temperature changes. The CLTE for nylon and acetal is generally 5 x 10-5, or .00005 inches per inch of part size per degree F temperature change. So nylons and acetals subjected to the same temperature variations would move the same amount. A three inch diameter rod machined in an ambient temperature environment of 70°F would grow .012” if exposed to a temperature of 150F. (.00005 x 3 x (150- 70)). By the same token the same rod would shrink .012” if it were exposed to a temperature of minus 10°F. The amount any material moves as a result of temperature change is calculated the same way: CLTE x part dimension x difference between machining temperature and use or current environment temperature. Keeping finished parts in a controlled temperature environment prior to shipping or use will minimize movement. Always allow shipments of new parts to stabilize for 24 hours in their new environment before inspecting.
Moisture absorption will also affect a parts dimensions but in a different manner to thermal expansion. Moisture is absorbed into all exposed surfaces of a part and causes all surfaces to expand at a uniform rate relative to part thickness. Moisture absorption factors are generally published as percent weight change, i.e. nylons will absorb 5- 6% moisture by weight at saturation, and acetal will absorb 0.3%. This does not mean that nylon will grow 5- 6% and acetal will grow 0.3%, only that their weights will increase by those amounts. Moisture can only penetrate into the surface of a part. In nylons this penetration is no more than 1/2” deep. Penetration is less in acetal parts so acetals are generally regarded as having better dimensional stability than nylons. This advantage is restricted to moisture absorption only since CLTE is the same for both materials. Part growth is relative to part dimension and is summarized as follows;
| Part Dimension (inches)||.125||.188||.25 ||.378||.5||.75||1 and over|
|Nylon Part Growth (inches) ||.012||.017||.021 ||.026||.030||.032||.33|
|Acetal Part Growth (inches)||.0006||.0008||.0011||.0013||.0015||.0016||.0017|
As can be seen, the rate of growth diminishes as part dimension increases up to a maximum part dimension of 1”. While the 3” diameter rod example used above would apply to both nylon and acetal, a 3” nylon rod would grow an additional .033” at saturation where a 3” acetal rod would only grow .0017”. The rate of absorption can also change with temperature, parts placed in a steam environment will pick up moisture at more than double the rate of those simply immersed in ambient temperature water. Finished parts can be placed in poly bags while in inventory and waiting to be installed.
Minimize machining induced heat stress by following proper machining procedures. Never use tooling that has been used to machine metals without first regrinding and configuring the tool for plastics.
Keep finished parts in a stable temperature environment to minimize thermal change.
Keep finished parts in plastic bags to minimize moisture absorption.
Always allow newly received parts sufficient time to stabilize before inspection.
Never be afraid to call the manufacturer for additional advice, remember they want your application to succeed as much as you do.
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.
For more information, contact Cast Nylons Limited, 4300 Hamann Parkway, Willoughby, OH 44094, 800-543-3619, Fax: 440-269-2323, E-mail: email@example.com, Web: www.castnylon.com.
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