This is the second installment of a 2-part blog on Insert Molding for Critical Applications. If you missed the first half, you can find it here.
Previously, we discussed this importance of material selection, surface treatment, and chemical compatibility for demanding insert molding components. Today, we will cover the other critical factors including insert placement strategy, thermal considerations, and quality assurance. Let’s dive in!
Insert Placement Strategy
As with other considerations, insert placement is also determined by the demands of the application. (In this context, we’re not talking where the insert will be placed, where talking how.) Generally speaking, consumer applications are more forgiving, and may tolerate ±0.010 inch variation. Conversely, medical devices or aerospace components often require ±0.002 inch or tighter. Volume becomes a factor as well. For low-volume, high-consequence parts, nest fixtures with positive mechanical locators will get the job done. However, high-volume production typically requires vision-guided robotic placement with real-time verification.
Thermal Considerations
Thermal dynamics present yet another challenge to insert molding. The fundamental issue is that you have to manage heat transfer between a metal mass with thermal conductivity 1,000 times greater than plastic and a polymer melt that’s degrading every second above processing temperature.
The secret to resolving this dilemma is setting the right preheat temperature. Insufficient preheat will cause premature freezing and incomplete encapsulation. Conversely, excessive preheat can degrade thermally sensitive resins or cause flash. The target range will change based on the material itself.
Independent control circuits for mold temperature zoning around inserts will go a long way too. To prevent cold spots and flow hesitation, we’ve found it beneficial to heat the area immediately surrounding the insert, making it 20°F-40°F warmer than general cavity temperature.
One final thermal consideration for critical insert molded parts is the cool down phase. Parts left to cool in ambient air tend to develop residual tensile stresses at the plastic-metal interface, leading to cracks. Controlled slow cooling or post-mold annealing can reduce residual stress by a significant amount, dramatically improving long-term reliability.
Quality Assurance
Quality assurance for critical insert-molded components requires more than just your usual go/no-go dimensional checks. If delamination under thermal shock is a challenge, ultrasonic C-scan inspection may be used. When electrical continuity is critical, electrical testing becomes mandatory, even though it drags the production process out.
For the most critical applications, establish protocols that will best simulate end-use conditions. These include accelerated thermal cycling, vibration testing, and destructive pull-out testing. In regulated industries, traceability linking insert lot numbers, resin lots, process parameters, and inspection results to each serialized part enables root cause analysis when failures occur and demonstrates due diligence in qualification and control.
Final Thoughts
As you can see, insert molding for demanding parts requires a lot of additional design and production considerations when compared to conventional components. All deserve equal attention. Getting any one of these wrong will have a significant impact on the insert molded component’s ability to perform its intended function reliability over time. However, now that you know the key considerations, insert molded part success is well within reach.
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