2K mold design requirements and design considerations

 ① Determination of shrinkage rate. If the same material (but different colors) is used for the two injection moldings, and the product has strict requirements on the external dimensions, it is best to set the shrinkage rate twice, but the plastic parts after the first injection molding should be considered as The cavity of the second injection molding will have an impact on the second injection molding; if the plastic parts of the second injection molding account for a small proportion of the entire product, it is not necessary to set the shrinkage rate for the second injection molding product; if The material for the second injection is silicon, rubber or relatively soft materials, and usually no shrinkage rate can be set.

Generally speaking, the shrinkage rate of the two-color mold depends on the plastic for the first injection, and the plastic for the second injection and the plastic for the first injection should have the same shrinkage rate. For example, the plastic for the first injection is ABS, and the plastic for the second injection is TPE. The shrinkage rate of ABS is usually 0.5%, and the shrinkage rate of TPE is usually 1.8%. When designing a two-color injection mold, all of them should be selected.

0.5%, because the first injection of plastic has already supported the outline of the product, and the second injection of plastic will not shrink more.

As shown in Figure 20-17, it is a small mouse wheel with POM material in the middle and SAN material on the outside. The POM material in the middle is injected for the first time, and the SAN material in the surface layer is injected for the second time. Normally, the shrinkage rate of POM material is 2.0 %, the shrinkage rate of SAN material is 0.6%, but in this two-color injection mold, the shrinkage rate of the outer dimension of the SAN material is 0.6%, and the inner dimension must be 2.0%.

C. The product injected for the first time should choose a latent gate to ensure that the product and the flow channel can be automatically cut off. When the latent gate cannot be used, a three-plate mold or hot runner can be considered. The disadvantage of the three-plate mold is that the pressure loss is large and the runner has a lot of condensate.

① Preload. This is a technical term used on two-color molds. Because the first product needs to be pressed into the mold cavity for the second time, this will have a very sensitive impact on the shape of the logo. Generally speaking, the height of the logo is 0.02~ larger than the theoretical value

0.05mm, if it is too small, the melt from the second injection will come in, making the outline of the text unclear; if the preload is too large, the line of 丨ogo will be too wide.

② Draft slope. The selection of the demoulding slope of the first injection part of the two-color mold is relatively flexible, and the angle should be as large as possible. The demoulding slope of the logo can even be enlarged to 15°, making the chassis bigger, so that it is not easy to be deformed by the second injection melt.

③ In the first injection molding, the size of the product can be slightly larger, so that it can be pressed more tightly with another cavity in the second molding to achieve the effect of sealing.

To make the mold base of the two-color mold, the middle dimensions must be exactly the same, and the positions of the guide post holes must also be the same. It is required that the movable and fixed molds of the two sets of mold bases can be freely interchanged, and the total height of the two sets of mold bases must also be the same, and the two sets of fixed molds and the two sets of movable molds must be equal in height, which is important when purchasing the mold bases. Special instructions are required.

② After the movable mold of the two-color mold is rotated by 180°, the four guide pillars of the movable mold must match the guide sleeves of the fixed mold (generally, the standard mold base of the commonly used injection mold has a misplaced guide pillar and guide sleeve, and the two-color mold’s The positions of the four guide posts and guide sleeves of the formwork are the same, and there is no misalignment).

③ The movable and fixed molds of the mold must be hard molds, and the heat treatment hardness is 48~52HRC.

④ For the two plastic parts formed in the two-color mold, they must have the same size reference to ensure that the two sets of molds of the two-color mold will not have inconsistent references during manufacture.

⑤ The design parameters of the mold are consistent with the parameter requirements of the two-color injection molding machine, such as the maximum mold thickness, minimum mold thickness, etc. The total thickness of the fixed mold panel plus A plate cannot be less than 170mm. It is necessary to check whether the parameters of the injection molding machine, such as the maximum mold thickness, the minimum mold thickness, and the distance between the ejector pin holes, meet the requirements.

⑥ Pay attention to the distance between the K. ◦. hole and the top stick hole. Generally, the minimum distance is 210mm. For large molds, the number of ejector holes must be appropriately increased. And, because the ejector stick attached to the injection molding machine itself is not long enough, the mold must be designed to lengthen the ejector bar, and the ejector bar is about 150mm longer than the bottom plate of the mold base. Two positioning rings must be designed on the bottom plate of the moving mold.

⑦ Generally, the hard rubber part of the product is injected first, and then the soft rubber part of the product is injected. Because the soft rubber is easy to deform, pay attention to the different types of the two molding plastics, and the shrinkage rate is also different.

⑧ The nozzle of the three-plate mold should preferably be designed in a form that can be automatically demoulded. Pay special attention to whether the demoulding action of the soft nozzle is feasible.

⑨ When designing, it is necessary to consider whether the flow of plastic will impel the first-formed product during the second injection molding, causing deformation and displacement. If there is this possibility, you must find a way to avoid it. During injection molding, the product size of the first injection molding can be slightly larger, so that it can be pressed more tightly with another dynamic mold during the second molding, so as to achieve the effect of sealing and avoid overflow.

⑩ When designing the fixed mold for the second injection, in order to prevent the fixed mold from inserting and damaging the glue position of the product that has been formed for the first time, a part of the void can be designed. However, the strength of each sealing position must be carefully considered, that is, whether the plastic is deformed under high injection pressure during injection molding, resulting in the possibility of waste edges in the second injection molding.

⑪The two shapes of the fixed mold are different, forming a product respectively, while the two shapes of the movable mold are exactly the same.

⑫ Pay attention to the manufacturing accuracy of the parting surface of the mold to prevent waste edges.

⑬Pay attention to the positioning of the moving and fixed molds, and the slope drop of all penetration and collision surfaces should be as large as possible, and should be above 0.1mm.

⑭ During injection molding, the size of the first injection molded plastic part can be slightly larger, so that it can be pressed more tightly with another cavity during the second injection molding to achieve the effect of sealing.

⑮Before the fixed mold A and the movable mold B are closed, pay attention to whether the fixed mold lateral core pull or the inclined roof will reset first and crush the product. If there is such a possibility, it must be ensured that the A and B plates are closed first. After that, the lateral core pulling or inclined top of the fixed mold can be reset.

⑯The arrangement of the cooling channels of the two cavities and cores should be as sufficient as possible, balanced and identical.

⑰In order to make the two kinds of plastic two-color injection molding have special butyl PU and "stick" tighter, the "stickiness" between the materials and the roughness of the mold surface should be considered. The smoother the mold surface, the tighter the double shot plastic will "stick".