Penetration is less in acetal parts so acetals are generally regarded as having better dimensional stability than nylons. In nylons this penetration is no more than 1/2” deep. Moisture can only penetrate into the surface of a part. 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. nylons will absorb 5- 6% moisture by weight at saturation, and acetal will absorb 0.3%. Moisture absorption factors are generally published as percent weight change, i.e. 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 will also affect a parts dimensions but in a different manner to thermal expansion. Always allow shipments of new parts to stabilize for 24 hours in their new environment before inspecting. Keeping finished parts in a controlled temperature environment prior to shipping or use will minimize movement. 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. ![]() 012” if it were exposed to a temperature of minus 10☏. By the same token the same rod would shrink. 012” if exposed to a temperature of 150F. A three inch diameter rod machined in an ambient temperature environment of 70☏ would grow. So nylons and acetals subjected to the same temperature variations would move the same amount. 00005 inches per inch of part size per degree F temperature change. The CLTE for nylon and acetal is generally 5 x 10-5, or. Use coolant for heavy work coolants used in machining are not in contact with the part long enough to be absorbed and cause swelling.Ĭoefficient of Linear Thermal Expansion, or CLTE, is the rate of movement of a material as temperature changes. This allows any heat stress generated in rough machining to dissipate and allow the part to settle before being brought to final dimension. Let the parts sit for 24 hours then finish machining. 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”. 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 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. Since nylons and acetals are poor conductors of heat, the heat remains at the surface of the material and causes the surface to expand. The simple act of machining nylons and acetals produces heat at the cutting point. 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. 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. ![]() Once the surface is removed in machining the stress is relieved and the part moves. 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. Let’s look at the four basic factors that have the most influence on nylons and acetals moving out of tolerance.īecause 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 movement, or lack of stability, can most often be over-come if the right approach is taken. Some materials, like thermosets, are generally quite good while others, like thermoplastics tend to have more movement. 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. The Plastics Distributor and Fabricator - Your Industry Magazineĭimensional Stability Issues In Nylons and Acetals
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |