Over Molding

What is Overmolding : Types, Benefits, Application

Injection molding is very popular method for plastic part manufacturing. Overmolding is a type of injection molding process. where two or more plastics or plastic with thermoplastic elstomer are molded together. This helps in achieving more complex geometries. Overmolding also helps in eliminating the assembly operation.

What is Overmolding ?

Overmolding is an injection molding process of creating one single part using two or more  materials. During overmolding, additional material layer are molded over and around the initially molded part.

Overmolding Process Types

Overmolding process can be classified into three categories. 1) Insert Molding, 2) Two Shot Molding, 3) Co injection Molding

Insert Molding

Insert molding is low cost and most commonly used overmolding process. Example of insert molding is, metal screw with plastic knob.

Step-1 : Pre-molded or standard insert is placed inside injection mold. For example screw is placed inside mold. 

Step-2Secondary material (usually thermoplastic or elastomers) is injected inside molding tool.

Single shot Injection Molding machines can be used for insert molding. Insert mold tooling costs is very less compared to other overmolding process.

Two-Shot (Multi-Shot) Molding

Two-Shot molding process is also known as Multi-Material or 2K molding.

Multi Material molding requires special injection molding machine consisting of two or more barrels and tool rotating arrangement. Multiple barrels allows multiple materials to be injected into the same mold in one cycle. 

Parts manufactured with multi shot molding can also manufactured with insert molding and vice-versa. But the advantage of multi shot molding is superior part quality in reduce cycle time and cost.

Multi shot molding can not be done on conventional injection molding machines. They require special multi-shot or two-shot molding machines.

Step-1 : Molten plastic is injected into injection mold using first barrel. After that core and cavity are Seprated.

Step-2 ; Injection tool is rotated and bring in position of second barrel.

Step-3 : Tool is closed and molten plastic is injected into injection mold using next barrel. After material is cooled, part is ejected out of mold.

Co-Injection Molding

Co-Injection molding involves injection of two dissimilar materials into one mold. This process is very expensive and difficult to control. During co-injection, both materials becomes in a fully molten state. Therefore high bonding strength between materials is achieved. 

Co-injection molding is done in three steps :

Step 1 : As illustrated in image, Skin or outer material is partially injected into mold.
Step 2 : Core material is injected until cavity is nearly filled, which displaces the skin material outside.
Step 3 : Skin material is injected to purge the core material from the sprue.

Advantages of Overmolding Process

Overmolding process requires additional investment. But it offers numerous advantages.

  • Improved Product Quality and Performance

Overmolding ensures proper alignment and rigid assembly.  Therefore product quality and performance is improved.

  • Reduced Secondary Operation

Overmolding eliminates secondary assembly operation. 

  • Eliminates Fasteners and Adhesive’

Toothbrush rubber grip is overmolded with plastic handle. This eliminate the need of adhesive.

  • Reduced Cost

Overmolding eliminates use of secondary operation that helps in reducing labor cost, time and fasteners or adhesive. Therefore overall cost is reduced.

  • Improve product ergonomics and User Experience

Overmolding is used to provide rubber grips to tools, tooth brushes and other handheld products. Rubber grips helps in improving product ergonomics. 

Over molding is also used to manufacture parts in two or more colors. Multiple colors helps in improving product aesthetics.

    • Enhanced Design Flexibility

    Complex assembly design and manufacturing of multi material parts is feasible with overmolding. For example car headlights are manufactured using overmolding process.

    Applications of Overmoulding

    • Multi Color / Finish Parts

    Car taillights are manufactured in multi-materials. Clear and Red tinted glass. This color and material combination can be achieved using overmolding different. 

    • Multi Material Parts

    In some of products such as smart watches. Top screen is manufactured in clear PMMA and remaining housing is manufactured in ABS or PC to achieve required finish.  

    • Soft Grip over Parts

    Over molding is used to add TPU or silicon soft grip over metal or injection molded plastic parts.

    • Over molding Gaskets on Plastics

    Complex gaskets can be overmolded over plastic parts using insert molding.

    Conclusion:

    To sum up, Overmolding is a type of injection molding process where two or more plastic parts or plastic with thermoplastic elstomer are molded together. It offers various advantages compared to conventional injection molding. High cost of Overmolding tool investment and machines can amortized with lower part cost. 

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