Low pressure / temp plastic molding

Quote from: Niklas on January 22, , 01:30:10 am

Penetration of water through plastics could be an issue long term, especially for low power analog electronics like sensors.
Select a wire insulation material with good adhesion to Macromelt.
Avoid overmolding of connectors. You will be surprised how good penetration the molten plastic has, even inside crimped terminals.
Do not assume that the overmolding will provide a firm mechanical support if used above 70'C. There is a reason why the name ends with -melt.
Check chemical compatibility if the overmolded sensor is going to be submerged into other liquids than water.

I've had Macromelt underwater for more than 24 months continuously at about 2 meters, and as yet it has not failed. I won't say it won't, but I'd be quite surprised.

Overmoulding connectors is quite doable provided you do it properly and DESIGN for it. Please see the example above.

The plastic is a polyamide. It has good chemical resistance to most common substances, other than acids which will degrade it.

It all depends on the application, but sensors are typically low power and their output signal can be affected by currents in the microamps range. For instance, moisture acts as a resistor in parallel with an NTC or a capacitive sensor. We are currently working on a new project where a high-performance engineering plastic is considered as an option for the housing material instead of stainless steel. One of the main concerns is the long-term reliability due to water absorption and permeability. Expected service lifetime of the product is more than 10 years, submerged in a water-based liquid, operating between -40'C and +85'C, 24/7.

Not all PCB connectors, actually very few, are made for either potting or overmolding. Consider a standard wire-to-board connector with a pin header inserted into a connector housing with crimped terminals, assembled horizontally with complete overmolding/potting. You will then have three entry points: where the wire enters, through the hole for the latch and where the pin is inserted. The result is most likely a connector where even the volume between the pin and the terminal is filled with potting/overmolding material. There is also another possible entry point for liquid intrusion, which is where the wires from the connector exit the overmolding. If the adhesion between overmolding material and the wire jacket is not complete, also considering thermal expansion and possible annealing, then capillary force can cause a leakage.

Take the same kind of connector, but with vertical assembly and only the pin header overmolded to a point well above the plastic part that holds the pins together. Here you need excellent adhesion to the metal pins to avoid leakage due to capillary force. If the connector is shrouded, then the adhesion demand also applies to the plastic material. As we are talking about complete overmolding, we can assume no pumping effect from an enclosed gas volume (ideal gas law).

Our customers have a long list of chemicals that our product should withstand. Even if the product is used with a water-based liquid, it should still withstand some abuse by a clumsy end user, without any significant impact. It all depends on the application, but it is very easy to just focus on the main purpose and forget about the small details.

If you want to learn more, please visit our website Low-Pressure Overmoulding VS Traditional Injection.

The final consideration before finalizing the design is the environmental context in which the overmoulded product will be utilized.

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If the part is to be positioned outdoors, it might be exposed to regular UV radiation. In this case, specific polyamides that can withstand such conditions should be considered.

If the overmoulded part is to be used in stressful conditions, careful consideration needs to be given to the overmould tool design. For example, an overmoulding part meant for vibration damping will have a thick wall. This requires more material, which in turn increases cost as well as time because there will be a longer cycle time needed to produce each unit.

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While discussing low-pressure and low-temperature plastic molding, it's important to understand common challenges and best practices. For example, when molding components that will be exposed to harsh chemicals, it is vital to verify the chemical compatibility of both the molding material and the substances it will encounter. This step ensures longevity and reliability of the final product.

An often overlooked aspect is the method of connector assembly in the overmolding process. As noted earlier, choosing between vertical and horizontal assembly can significantly impact the overall performance and leakage resistance of the mold. Ensuring strong adhesion between the overmolding material and metallic parts of the connectors is crucial to prevent capillary action that can lead to leakage.

Another consideration is the potential influence of thermal expansion. If thermal expansion coefficients of the adjacent materials differ significantly, it may lead to mechanical stress and potential failure over time. Therefore, designing for thermal stability and accommodating various expansion rates can enhance the product's durability in varying temperatures.

Frequently Asked Questions (FAQs)

1. What materials are commonly used in low-pressure molding?
   Materials like polyamides and specific engineering plastics are frequently chosen due to their favorable properties, such as chemical resistance and mechanical stability.
2. How does low-pressure molding differ from traditional injection molding?
   The primary distinction lies in the pressure and temperature used during the process. Low-pressure molding employs lower temperatures and pressures, making it suitable for sensitive electronic components and creating less stress on the materials.
3. Can low-pressure molding be used for outdoor applications?
   Yes, but selecting the right material that can withstand UV radiation and other environmental stressors is crucial. Polyamides with UV stabilizers are often used for such applications.