Posted by sachin on | Comments Off on Insert Molding Best Practices for Critical Applications – Part 2
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.
Have a question about an insert molded part or assembly? Reach out to us, we’re happy to help.
Posted by PDI on | Comments Off on Insert Molding Best Practices for Critical Applications – Part 1
Insert molding is a type of injection molding method that allows a pre-formed metal component to be placed within the molded part. A great example is a battery connector – where the metal is required for conductivity, while plastic offers design flexibility. Designing insert molded parts and processes for more demanding applications is not without its challenges. This is especially true when considering the several variables involved such as material types, positioning of the insert, thermal considerations, and quality assurance. In this two-part blog series, we’ll review each of these considerations, and how to best optimize them for long lasting performance and reliability.
As with all injection molding, both part and mold design are critically important for part success. This is particularly true for mission critical components in demanding applications such as steel blades in medical instruments or reinforcement studs in aerospace components. Since failure can result in safety hazards or premature part failure, engineering becomes more complex than standard injection molding.
Metal Insert Selection
As you can imagine, selecting the right metal for the job is another crucial aspect. Ultimately, the material selection must balance several competing requirements that are specific to the application. While there are many to select from, below are a few of the more popular ones for insert molding:
Thermal Stability – metals must withstand the high temperatures of the injection molding process, which can range from 400-600° or higher. Brass and stainless steel both perform well here, but end-use factors come into play. When considering mechanical loads, environmental threats, and electrical/conductivity requirements, brass and stainless often fall short.
Electrical Requirements – for high current applications, copper alloys are often selected due to their excellent conductivity, despite their higher thermal expansion coefficients.
Weight Considerations – certain applications, particularly those in aerospace and defense, are weight-sensitive. The goal is to have the lightest components possible without sacrificing performance. Aluminum serves weight critical applications quite well, however galvanic compatibility must be carefully considered. This is particularly challenging when dissimilar metals may come into contact within the presence of moisture or certain chemicals. For instance, you wouldn’t want an aluminum insert in a brass housing. This is a recipe for failure, as the aluminum will surely corrode.
Surface Treatment
While surface treatment is often an aesthetic consideration, for insert molded parts, it’s very much a function of performance. Surface treatment must meet the stress demands that the part will be subjected to. If there is particular concern that the insert might loosen or pull out, knurling works well to prevent this. Knurling leaves a textured pattern of ridges grooves, or cross-hatching, giving the part gripping strength. Once the plastic cools around the knurled insert, it creates a “mechanical lock”, which prevents it from slipping out.
Chemical Compatibility
Another key factor for long term insert molded part reliability is making sure that your components are prepared to stand up to the chemicals they will be exposed to throughout their life. This goes beyond the obvious resin-metal interaction during molding. You must consider long-term exposure to operating fluids, cleaning agents, and other environmental contaminants. Then make material selections and insert placement decisions accordingly.
To Be Continued…
Material selection, surface treatment, and chemical compatibility are just three of several key considerations for demanding insert molding components. Check out the second part of this blog where we cover insert placement strategy, thermal considerations, and quality assurance.
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