What Need to Consider When Choosing Liquid Silicone Overmolding?

What's silicone overmolding?

Silicone Overmolding is also known as Liquid Silicone Overmolding, rubber overmolding, lsr overmolding, insert overmolding, or 2 shot overmolded.

Liquid Silicone Overmolding is a process that liquid silicone rubber material overmolding onto another existing part usually a plastic, metal or silicone rubber part in liquid silicone overmolding(LSR injection molding ) process. 

Liquid Silicone Overmolding - LSR overmolding is widely used for medical silicone overmolding, silicone overmolded plastic, silicone overmolding glass, metal overmolding, silicone overmolding electronics, fabric overmolding, etc. For example, a silicone over-molding process may be used for providing a ear handle for a breathing mask that can make the bandage to go through. And to provide a flexible septum on a hard plastic surface for better sealing performance, and to encapsulate an electronics assembly for mounting or protection from the environment.

Liquid Silicone Overmolding note must be considered prior to developing a silicone over-molding bonding:

1. Liquid Silicone Overmolding is suitable for the substrate or not?

   Different substrates are resistant to different work temperature, which determines which overmolding method is most suitable.

   For example, the typical silicone curing temperature is from 220℉ to 350℉(100℃ to 160℃ ), but most of plastic is not resistant to high work temperature, otherwise the plastic will melt and deform, so that the burrs/flash will increase and the sealing will be not good and tight between silicone part and plastic part. So, when the substrate is plastic with low work temperature, liquid silicone overmolding is the most suitable method.

   Please check following table for some basic features of plastic materials:

   Besides silicone overmolding onto plastic, silicone overmolding glass, silicone overmolding electronics, metal silicone overmolding, ceramics silicone overmolding, silicone overmolding onto silicone rubber is also available.

  Such as cable overmolding, PCB overmolding, overmolded with circuit or chip, silicone overmolding onto USB cable, silicone overmolding to LED lights, overmolded cable assemblies, overmolded connector, overmolded cable connectors, overmolding metal inserts, aluminum silicone overmolding, etc.

   As the above information shows, we can know that for silicone overmolding, the most important thing need to consider is the temperature resistant for the parts that need to put into liquid silicone injection molding cavity to process. 

   When we choose suitable substrate for silicone overmolding, will there be any other questions during silicone overmolding process?

2. Liquid Silicone Overmolding case analysis:

    (1) Temperature of overmolding --- Liquid silicone overmolded plastic part

We customized a silicone face mask for one of customers. During silicone overmolding production, we found the silicone material cannot overmold the plastic part well(the liquid silicone cannot cover the plastic part perfectly as the 3D design shows ). So our engineers checked the temperature of the silicone injection molding, then found that the mold temperature is not high enough, so adjusted the mold temperature to be a little bit higher. The silicone material could cover the plastic part (PC) well, but we found that the PC part was melted under that higher temperature. For common PC material, the highest temperature resistant is 150℃. So we need to balance the liquid silicone injection molding temperature with the overmolded plastic part temperature resistant. After few times adjustment and test, we got a perfect molding temperature for plastic silicone overmolding production.

    Liquid Silicone Overmolding onto medical plastic mask part:

    (2) Note for liquid silicone overmolding process:

    First, keep the substrate part be clean and no dirt. Before liquid silicone overmolding, should clean and dry the substrate, or keep the substrate be clean when finish the substrate molding.

    Second, one of the most important notes for silicone overmolding: glue or primer.

    As you know, general silicone no adhesion and cannot be stuck on other materials, so before silicone overmolding, the substrates in other materials need to be brushed with specific glue, treating agent or primer and dry it. Otherwise, the substrate cannot be sealed well with silicone part.

    In addition, it's better to apply silicone overmolding without using demolding agent. Some glue/treating agent/primer may have a chemical reaction that would make the glue/primer ineffective. If demolding agent is necessary for some overmolding product, wait until have used demolding agent for half an hour before starting overmolding production.

    Liquid silicone mask part and plastic mask part stick together by glue:

Now we know all notes for applying silicone overmolding, when apply silicone overmolding correctly, we can see many benefits of this overmolding method.

Liquid Silicone Overmolding’s advantages:

· Liquid Silicone Overmolding can in combination with thermoplastic, metal, LSR components and various kinds of materials;

· Liquid silicone overmolding with higher production efficiency compared to labor assembly;

· LSR overmolding can achieve complicated structure and the parts with more stable design and quality;

· Silicone overmolding eliminates the risk and cost for secondary assembly;

· Greater design latitude in applications;

· Cost-effective solutions.