Engineering Plastics for Extreme Applications

I'd like to clarify a few points about the Dimensional Stability of Duratron PAI and the ability to hold tight tolerances on finished machined components. Duratron PAI, as with any thermoplastic material, has a tendency to move with heat, stress, and moisture. Typically, when a customer is experiencing dimensional movement, the reason is not one or the other, but a combination of heat, stress, or moisture. To ensure critical tolerances are met, each of these items must be controlled. Let's review how and why this happens.

Thermoplastic materials will grow or shrink based on ambient temperature variations. During machining, area-specific heat generation can also cause movement. Aggressive feeds, speeds, or improper tooling can all be reasons for heat generation during the machining process. This will cause the component to grow while on the machine, giving the machinist inaccurate final dimensions. After the component is completed and removed from the machine, it cools and shrinks back to its steady state.

All thermoplastics have some amount of internal stress. When machining intricate details or when removing a lot of material, the finished component will move as the material relieves those internal stresses. In addition, aggressive machining and heat generation can actually induce more stress into the material. A great solution is rough machining your component. This lets the material stress-relieve to a more stable state, allowing for closer tolerance finish machining without any further risk of movement.

One of the inherent limitations of poly-amide-imide-based materials is the affinity to absorb moisture. In critical components, this can result in out-of-tolerance dimensions. One can protect tight-tolerance machined parts from moisture absorption through environmental controls and proper packaging after machining.

Through my experience, I've identified some key procedures that must be followed to ensure the dimensional stability of such critical machined components. The recommended procedures to ensure dimensional stability are:

• Incoming material should be allowed to stabilize in the environment in which it'll be machined (typically 24 hours);

• rough machine component to within 0.020 inch to 0.030 inch on all surfaces;

• let component sit for up to 48 hours;

• finish machine;

• immediately store the component in a controlled environment;

• immediately seal the component in a bag with desiccant.

Annealing the products prior to leaving the production facility helps to reduce internal stresses that have built up in the material during production. Despite this step, as noted above, internal stresses may still be present, or even machined in. Note that I recommend a "rough machining" procedure for critical tolerance parts to stabilize material from any internal or machined-in stresses.

Essentially, the part will go where it wants to go, or stress-relieve itself. Then you can come back and lightly finish machining the part and not worry about movement. Rough machining eliminates the added heat cycle and the potential for error.

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— Jim Hebel is the manager of technical service and application development for Quadrant Engineering Plastic Products.