In
rapid prototyping, CNC machining PC is favored for its strength and
transparency, but it is highly prone to stress deformation, leading to
scrap parts. Here’s a concise guide to prevent it:
1. Material Preparation (Most Critical Step)
2. CNC Machining Strategy
Sharp Tools: Use sharp, dedicated plastic-cutting tools (e.g., single or two-flute end mills) with high rake angles. This ensures a clean cut, minimizing heat and mechanical stress.
Cutting Parameters: Employ a high spindle speed with a moderate to high feed rate. This creates a "shearing" action rather than "grinding," preventing heat buildup. Use light depths of cut.
Cooling: Use compressed air
for cooling and chip evacuation. It effectively removes heat without
the risk of moisture absorption or chemical stress cracking that
coolants can sometimes cause.
3. Workholding
Avoid Excessive Clamping Force: Too much force will bend the part; it will spring back once released, ruining accuracy.
Use Distributed Force: Use multiple clamps with soft pads to distribute pressure evenly.
Ideal Solution: Vacuum Table: A vacuum chuck applies uniform, distributed force across the entire part base, virtually eliminating clamping stress.
4. Post-Processing
Post-Machining Annealing:
For critical tolerance parts, a second, shorter annealing cycle after
machining can relieve any new stresses introduced during cutting.
Control Humidity: Store finished parts in a low-humidity environment, as PC is hygroscopic (absorbs moisture) and can warp as it equilibrates.
Summary: The key to success is a systematic approach:
relieve inherent stress (pre-bake), machine correctly (sharp tools,
high RPM, air cool), clamp wisely (vacuum table), and relieve new stress
if needed (post-bake). This ensures accurate, functional prototypes and
minimizes scrap.
