Dimensional stability and the ability to maintain tight tolerances on finished machined parts. Often, when a customer is experiencing dimensional movement, the cause is not one or the other, but a combination of heat, pressure, or humidity. These items must be controlled to ensure tight tolerances are achieved. Let's review how and why this happens.
Thermoplastic materials will grow or shrink based on changes in ambient temperature. Area-specific heat generation during processing may also cause movement. Aggressive feed, speed, or incorrect processing can be the cause of heat generation during processing. This will cause the assembly to grow on the machine giving the machinist inaccurate final dimensions. After the component is finished and removed from the machine, it cools and shrinks back to a stable state.
All thermoplastics have a certain amount of internal stress. When machining intricate details or removing large amounts of material, the finished material will move as the material is relieved. In addition, aggressive machining and heat generation may cause more stress to enter the material. A good solution is to rough machine your components. This allows the material stresses to be released to a more stable state, allowing closer tolerance finishing without the risk of further movement.
One of the inherent limitations of polyamide-imide-based materials is the affinity to absorb moisture. In critical parts, this can lead to out-of-tolerance dimensions. Tight tolerance machined parts can be protected from moisture absorption by environmental control and proper packaging after machining.
Based on my experience, I have identified some key steps that must be followed to ensure the dimensional stability of these critical machined parts. The recommended steps to ensure dimensional stability are
The incoming material should be allowed to stabilize in the environment in which it is machined (typically 24 hours); with excellent machinability and dimensional stability.
Rough set components between 0.020" and 0.030" on all surfaces;
Allow components up to 48 hours;
Immediately store components in a controlled environment;
Immediately seal components in bags with desiccant.
Annealing the product before leaving the production facility helps reduce internal stresses that accumulate in the material during production. Nevertheless, as mentioned above, internal stresses remain and can even be machined. Please note that I recommend a "roughing" procedure for critical tolerance parts to stabilize the internal or process stresses in the material.
Essentially, the part will go where it goes or relieve itself of the stress. Then you can come back and gently machine the part without worrying about movement. Roughing eliminates the possibility of heating cycles and errors.
There is no danger in molding engineering plastics as long as the general safety rules for molding operations are followed. However, like other plastics, when heated to very high temperatures, engineering plastics can produce decomposing substances. If ventilation is inadequate, these decomposing substances can be stored and can lead to dangerous health conditions. Therefore, proper ventilation is highly recommended.
Do not allow the resin temperature to exceed 360°C to prevent thermal decomposition, gas emission, and pressure buildup inside the cylinder.
When the mold is interrupted for an extended period, the material in the gas cylinder must be completely removed and the temperature reduced to below 280°C.
Allow at least enough time for the cylinder to heat up before feeding or starting the screw and wait until the temperature reaches 300°C.
The operator must wear goggles (especially when cleaning). Gloves are also worn when handling hot dies. When the molding operation is interrupted, withdraw the injection unit to prevent the material in the nozzle section from curing through contact with the mold.
Handling natural color grades
When molding natural color grades, please note the following precautions.
When raw material pellets or molded products, etc. are exposed to direct sunlight, fluorescent lights, mercury vapor lamps, or high temperatures, they will discolor in a relatively short period. Therefore, such environments should be avoided.
Also, to reduce discoloration during molding, pre-drying conditions should be kept constant, considering the volume balance of the material retained in the molding machine
, the variation of the injection volume of the molding machine and the number of molds cavities during molding must be avoided for everything.
The screw shapes that can be used are a low shear-type or standard type. High shear-type screws generate more heat, which sometimes leads to discoloration.
When using nitride-treated screws, cylinders, and nozzle parts, gray discoloration tends to occur, so a chrome finish is recommended.
Occasionally, this occurs in particles that include black spots called "black spots". This is caused by the discoloration of PPS during our manufacturing process. Our company makes every effort to avoid this as much as possible by improving the process, but the spots cannot be eliminated at this time. Please take this into consideration in your use. In addition, depending on the conditions, it should be noted that speckling may also occur during the molding process. In addition, high-resolution colored products may discolor like natural color grades.