Double shot injection molding can make two different characteristics of resin and color into a single two-color product, which can reduce assembly and post-processing of molded products, save cost of dissolution and printing, increase aesthetic appearance of product, and enhance grade and added value of product. Below I will introduce difference between advantages of double shot injection molding, molding principle, structural form, technology and overmolding.

 

Double shot injection molding not only has function of anti-slip and friction, but flexible resin material makes it more ergonomic and feels better. Quality of double shot injection molding product is high, product deformation is easy to control, molding cycle is short and output is high, loss can be 7% lower than that of overmolding, and manufacturing cost of product can be 20%-30% lower than overmolding.

Double shot injection molding usually refers to molding of two colors or different kinds of plastics. Since two sets of injection units and nozzles of double shot injection molding machine are separately separated, color of finished products is mostly two-color and is not mixed. The biggest difference between double shot injection molding machine and general injection moulding machine is design of active template of injection unit. In general, double shot injection molding machine has two separate sets of injection units, and general injection moulding machine has only one injection units. Configuration of two sets of injection units has various forms depending on design of each manufacturer. As for design of movable template, rotation mechanism must be provided. Commonly used designs include adding a turntable or a rotating shaft mechanism to provide a 180° reciprocating rotation function to cause mold to alternately move. Other special double shot injection molding do not require a turntable or a rotating shaft mechanism, but are alternately or horizontally rotated by mold.

1. Double shot injection molding machine consists of two sets of plastic injection moulding devices with same structure and specifications. Nozzles are required to have a special structure according to production method, or two sets of molding dies having same structure that can be rotated and transposed. When plasticizing injection, process parameters such as melt temperature, injection pressure and injection melt amount in two sets of plasticizing injection devices are required to be same, and fluctuation of process parameters in two sets of devices should be minimized.
2. Compared with ordinary plastic injection molding products, two shot molding plastic products should adopt higher parameter value for melt temperature and injection pressure at the time of injection. Main reason is that mold flow path in double shot injection molding is relatively long, structure is relatively complicated, and flow resistance of injection melt is large.
3. Two shot molding plastic products should use raw materials with good thermal stability and low melt viscosity to avoid decomposition due to high residence temperature in flow channel due to high melt temperature. Plastics used in many applications are polyolefin resins, polystyrene and ABS materials.
4. When two shot molding plastic products are injection-molded, in order to make melt of two different colors can be well welded in mold during molding and to ensure molding quality of plastic injection molding products, higher melt temperatures, mold temperatures, injection pressures, and injection rates should be used.

 

1 "Punch shift" mold has two concave molds and one convex mold. When the first barrel is shot, punch is closed with the first die. Complete injection of the first plastic. After the first plastic is solidified, concave and convex molds are separated, semi-finished product formed of the first plastic stays in punch and moves with punch to a position aligned with second die. After injection moulding machine is closed, second barrel is injected with second plastic. After second plastic solidifies, injection moulding machine opens mold and takes out complete product.
2 Double shot injection molding in the form of "Punch Rotation" has two female molds and a convex mold composed of two back-to-back combinations. This punch is rotatable. Two-color injection moulding machines are equipped with two barrels, and second barrel is preferably designed in vertical direction of movement direction of injection moulding machine. Two different plastics can be injected simultaneously. When two plastics injected are solidified, injection moulding machine opens and automatically ejects complete product. After article is removed, "rotatable punch" is rotated through 180 degrees. Injection moulding machine is closed for next cycle.
Since the first and second barrels of "Punch shift" mode are injected in turn, the first and second barrels of "Punch Rotation" mode are simultaneously injected. Therefore, for same product, efficiency produced by "Punch Rotation" mode is twice that produced by "Punch shift" mode.

Double shot injection molding machine is equipped with two barrels. Mold is single-cavity and has common cavity required for the first plastic ("hard plastic") and second plastic ("soft"). Die and punch required for this technology are single and fundamentally different from the first two production modes. After the first cartridge is injected with the first plastic, the first plastic is partially solidified or solidified. Power cylinder drives isolation blade to retreat to profile of punch. At this time, second barrel is injected with second plastic. After second plastic is completely solidified, injection moulding machine is opened and complete product is taken out. Injection moulding machine is then turned off for next cycle. Obvious advantage of this technology is that it simplifies mold and reduces need for injection moulding machine workspace (a smaller double shot injection molding machine can be used to reduce investment and reduce energy consumption). Moreover, production can be doubled more than "encapsulation method" previously employed, and defective product resulting from re-injection of "hard plastic" which has undergone shrinkage and deformation into cavity is fundamentally avoided.

 

Two kinds of plastic materials are injection-molded on same injection moulding machine, and molded in two parts, but product only molds once. Generally, this molding process is also called double shot injection molding, usually completed by a set of molds, and requires a special double shot injection molding machine. Two shot molding can make two different characteristics of resin and color into a single two-color product, which can reduce assembly and post-processing of plastic injection molding products, save cost of dissolution and printing, increase aesthetic appearance of product, and enhance grade and added value of product. Not only does it have function of anti-slip and friction, flexible resin material makes it more ergonomic and feels better. Double shot injection molding product has high quality stability, easy product control, short molding cycle and high output, and loss can be 7% lower than that of overmolding. Manufacturing cost of product can be 20%-30% lower than that of overmolding.

 

Two plastic materials are not necessarily injection molded on same injection moulding machine, and are molded in two parts; product is taken out from a set of molds, it is placed in another set of molds for second injection moulding. Therefore in general, this molding process is usually completed by two sets of molds without need for a special double shot injection molding machine. Its’ mold has same mold structure as single-shot mold, mainly by adjusting flow rate of injection parameters to control bonding point of seed material. Overmolding is mainly soft plastic rubberize hard plastic. Soft plastic is usually made of artificial rubber, TPU, TPR, etc. Hard plastic can be ABS, PC, PP, etc.