Injection Molding Residual Stress Prediction in Variable Wall Geometry
Polycarbonate parts with 2.0mm to 4.5mm wall steps generate 34 MPa thermal stresses; accurate 3D viscoelastic simulation prevents expensive re-cuts.
Hole Drilling
Meaning
Semi-destructive testing techniques measure the internal strain states of manufactured parts by selectively removing material. The practice of hole drilling involves machining a tiny cavity into the part to release local residual stresses. This strain relief is measured by adjacent gauges to calculate the original stress state.
Moulding Assessment
Injection molded plastic parts often harbor deep internal tensions from non-uniform cooling. Applying the hole drilling technique to these parts provides a depth profile of the molded-in stresses. This information helps molders adjust pack pressure and cooling time to prevent subsequent warpage.
Experimental Setup
The procedure requires bonding a special strain-gauge rosette to the surface of the plastic part before machining begins. A high-speed drill or a specialized milling cutter then creates a hole of precise diameter and incrementing depth. Automated data acquisition systems record the strain changes at each step, ensuring high resolution.
Using an air-turbine drill minimizes the risk of introducing new mechanical stresses during the test.
Resin Behavior
Amorphous polymers are generally easier to analyze with this method than highly crystalline resins. However, the localized heat from hole drilling can cause thermal expansion or stress relaxation in thermoplastic materials. To prevent this, the drilling feed rate must be carefully controlled, and coolants must be used when necessary.