working principle of the over molding and product design key points

- May 23, 2018-

Working principle of the over molding and product design key points


    Many product OEM manufacturers choose to assemble two different molded components together with glue, screws, ultrasonic or click fit, but this takes time and cost and may lead to un-satisfied results. Fortunately, the plastic injection over molding process offers another solution.



    Over molding is usually the use of mechanical or chemical bonds (usually both) to permanently bond the two parts together. This avoids the hassle of assembly, simplifies product design, and can improve the characteristics of many injection molded parts. Injection over molding process is currently widely used in various industries, such as medical device, electronic equipment housing, automotive plastic interiors, food, toys and other aspects.


    It works by placing a pre-molded part the substrate part back into the mold and injecting the second plastic around the original part. The secondary process requires a pair of molds - one for the substrate and the other for the completed over molded product. It also requires a person to manipulate the machine, load the substrate components and unload the finished product.


  Over molding products should consider several important design considerations below :


Material Bonding:  

   A strong bond between the two materials is critical for over molding. If the two materials cannot be bonded, the overmolded product still cannot be effectively bonded together. For example, the bonding effect between TPE material and PP.PE is very good. The bonding between TPU and ABS is very good, but ordinary TPU is difficult to bond with PP.


Material characteristics:  

  Substrate and over molded material should be physically, chemically and thermally compatible and should not be mutually exclusive. Generally two kinds of materials are improved from the same base material, and the effect of two materials will be better. For example, PP and TPE material modified by PP have very good material compatibility characteristics. Molding principles: The principles of normal injection molding apply equally to over molding. There are additional considerations that need attention, such as the strength of the base material required to be thick enough to prevent the product from burning or melting when doing the over molded works. The softening temperature of the base material is recommended to be greater than 10 to 30 degrees Celsius above the over molding material to avoid product defects.




Sealing position: 

   Between the base part and the rubberized part, in order to better control the rubber over molding effect and appearance, it is recommended to use a step or a flat large bevel sealant; if there is no sealant position or the slope is not enough, the plastic cover process is easy to leak material and have very bad flash issues.


Product thickness: 

  In principle, regardless of product design, the thickness of the product must be designed as evenly as possible, not too thin or too thick, too thin wall thickness will cause some problems such as short shot and air trap, and it is easy to shrink or fail to fill enough material For the thickness of the corresponding products for various materials, please refer to Asia Billion's paper on product design thickness guidelines.

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