Liquid Silicone Overmolding
Liquid Silicone Overmolding also known as Silicone Overmolding, Rubber Overmoulding and LSR Overmolding, which mainly includes two steps wherein a completed plastic, metal (Aluminum, Steel, Brass), glass or electronic parts (PCB circuit board) is placed/inserted into the mold cavity of a Liquid Silicone Injection mold, after which the Liquid Silicone Rubber Matertail is over-molded directly onto the part. In order to ensure a stronger bonding, to add bonding adhesive or glue on the plastic/metal parts before overmolding is necessary.
Considering mold cost, many projects choose compression silicone molding/rubber molding or silicone transfer molding as molding method for overmolding part. While for many medical parts that require high material safety demands, strict tolerance, and the overmolded parts with low temperature resistance, liquid silicone overmolding is the best choice.
What need to consider when choose Liquid Silicone Overmolding? What will affect the quality of liquid silicone overmolded parts?
Adding glue on metal or plastic substrates to make sure liquid silicone rubber can not be overmolded with substrates tightly. With the glue/ primer, metal and plastic parts will not be easily taken apart from the elastomer and have stronger bonding.
Checking the temperature resistant property of the parts that need to put into liquid silicone injection molding cavity to process. The typical silicone curing temperature is from 220o F to 350o F (100° C to 160°C), for some plasict parts, the temperature assistant is lower than the silicone curing temperature,which will be melted in the mold. Let's compare the temperature resistance between different plastic parts.
Checking tolerance requirement and working environment of the part. If the quality requirement is strict, and need high safety material (medical grade), liquid silicone overmolding is the only choice. Because Liquid Silicone Injection Molding plant is clean. And liquid silicone rubber is a two-part platinum-cured silicone, which is a high-purity and non-toxic materials. LSR is an inorganic nature, which is clean, plasticizer-free, tasteless, and can resist oxidation and some chemical materials like acids and alkali. Liquid silicone rubber also resists to bacteria growth.
Liquid Silicone Overmolding Common Problems Analysis:
The most common problem for liquid silicone overmolding plastic part is LSR Injection Mold temperature control problem, air trap issue and short shot problem.
As the plastic temperature resistant is low, and the working temperature of liquid silicone molding is about 130-160°C, so it's difficult to control the mold temperature to coordinate with plastic part, especially PC. If the mold temperature is too low, the product will need very long curing time, and for some large product, the mold temperature is too low, which can cause the part uncured, and even short shot. So the liquid silicone rubber material can not overmold/cover the plastic part completely and without strong bonding.
If the overmolding design is unreasonable, which will also cause short shot issue, and some burrs and rubber flashing occur on the surface of the plastic part, which affects not only appearance, but also the function of the part.
Comparsion among Liquid Injection Molding, Silicone Compression Molding and Silicone Transfer Molding:
Liquid Silicone Overmolding's Advantages:
Liquid Silicone Overmolding process can meet high tolerance and pass strict quality control, making it suitable for use in any application where precision is required. With small burrs and good parting line.
By Liquid silicone overmolding, silicone directly onto a metal or plastic substrate, we can save the manually assemble cost and time. And the curing time is shorter than compression molding. Liquid Silicone Overmolding can dramatically reduce your manufacturing costs and turnaround time, allowing you to get your product to market more economically.
Liquid Silicone Overmolding's Disadvantages: High mold cost and unit cost.
Silicone Transfering Molding's Advantages:
It's preciser than Compression molding, the material pot can be vulcanized evenly and quickly. The metal part can be overmolded well and have strong bonding.
Transfering molding's cost is lower than liquid silicone injection molding.
Rubber Transfer Molding's Disadvantage: Material waste
Silicone Compression Overmolding's Advantages:
It's the most economical way to save project budget. Suitable for many projects that do not have high tolerance and material safety requirement.
It's convenient to produce different colors and hardness parts.
Silicone Overmolding's Disadvantages: rubber burrs and parting line will be larger after many shots. Overmolding parts will not have good bonding because of the tightness of positioning pin installation. Mold life is short and with lower production efficiency.