36 experience of two-color injection mold design and dual shot injection molding

1. Basic principles of two-color injection mold design:
(1) 1 time for hard plastic and 2 times for soft plastic;
(2) 1 time for transparency and 2 times for non-transparency;
(3) 1 time for plastic with high molding temperature and 2 times plastic with low molding temperature;
Above is basic principle of making a dual shot injection molding, otherwise mold will be undone; in addition, when sealing glue, try to use sealant instead of inserting sealant, even if it is recommended that customer modify product as much as possible by using sealant;

2. Guide bush guide bushing must be symmetrical top and bottom, left and right. Front and rear modes are symmetrical.

3. Back mold should be rotated 180 degrees, and front mold does not move.

4. Spacing of products must be based on spacing of nozzles of plastic injection molding machine. Distance between nozzles of some dual shot injection molding machines in foreign countries is adjustable, some are not adjustable, and domestic ones are not adjustable.

5. Two separate ejector systems, two for top stick. Two products of rear die are same, thimbles are same, and rotation relationship is not made into a translational relationship.
6. Ejector plate can only be reset with a spring. It is not possible to force reset by screw because rear die has to be rotated.

7. Side lock must be on four sides of center of mold, front and rear molds are symmetrical. Otherwise, when rear mold is rotated 180, it will not be aligned with front mold.

8. If distance between glue points and distance between nozzles of plastic injection molding machine are different, top stick holes should be made into a waist type because spacing of top pins of plastic injection molding machine is not adjustable. Note that most of injection nozzles of domestic dual shot injection molding machines are not adjustable.

9. Pay attention to direction of parallel nozzle of plastic injection molding machine provided by customer. Whether it is X-axis or Y-axis to determine layout of product.

10. Direction of water in and out must be on upper and lower sides, and each circulating water must be on same surface. Water in should not be on the upper side and water out should be on the lower side. Because rear mold should be rotated 180 degrees, it should be noted that size of mold blank cannot exceed height of water outlet of plastic injection molding machine, otherwise water cannot be transported.

11. First injection of product should be placed on non-operating side, because product is rotated 180 degrees for second injection after the first injection, it is just right to operating side, which is convenient for taking product.

12. Pattern position of export mold should be on operating side and the non-operating side, not on upper and lower side, because their products should be fully automatic.

13. Note on parting surface: parting surface of rear mold parting surface should be back part after combination of two products; front part parting surface should be taken from a single product, it is not advisable to take combined product parting surface.

14. Tolerance of front and rear flanges is minus 0.05mm, gap tolerance of two flanges is plus or minus 0.02mm, gap between top stick and top stick hole is 0.1mm, center tolerance of guide bushes of front and rear die guide sleeves is plus or minus 0.01, tolerance of four sides and depth of mold frame should be increased. Otherwise, when rear mold rotates 180 degrees, front end is generated due to inconsistency of height. Frame depth tolerance is minus 0.02 mm.

15. If mold base has been processed in molding supplier and when molding supplier wants to process nozzle and top stick hole, it should take number based on the center of pitch of four guide pillar guide sleeves. Otherwise deviation is too much, and it is easy to jam die. When ordering mold base, it is necessary to indicate that it is a dual shot injection molding base. Four guide pillar guide sleeves and frame are symmetrical, and rear mold can be matched with front mold after being rotated by 180 degrees.

16. If it is two-color rotation of rear mold, it is much simpler. Two front cores are same, after hard plastic is injected, rotate 180 degrees (note that product can not be dropped when product is turned, gate can be automatically separated). After turning to other side, soft plastic is injected. Injection of hard plastic does not need to be placed on thimble. It only needs to be placed on soft plastic core. In addition, it should be noted that if soft rubber is completely entangled with hard rubber, it is only necessary to shrink hard plastic. If contours are connected, both hard rubber and softener should be shrunk.

17. If it is a dual shot injection molding of a mold, it is a straight barrel, a 90-degree barrel shot, no need to rotate, only a core, soft and hard glue separation method by row seal gum.

18. Cavity’s two shapes are different, forming one product separately. Two shapes of core are same.

19. After front and back molds are rotated 180° from center, they must be matched. This check must be done during design.

20. Pay attention to position of thimble hole, small mold is 210mm. Large molds must properly increase number of jack holes. Moreover, since thimble attached to plastic injection molding machine is not long enough, it is necessary to design a long ejector pin in our mold, and thimble grows about 150 mm from bottom of mold base.

21. Two positioning rings must be designed on the back plate.

22. Total thickness of front mold panel plus A board shall not be less than 170 mm. Please carefully check other reference data of this type of plastic injection molding machine, for example, large die thickness, small die thickness, top bar hole spacing, etc.

23. Depth of front side SPRUE should not exceed 65mm. Interval from top of upper side (large nozzle) SPRUE to center of mold is not less than 150 mm.

24. When designing second plastic injection molding of CAVITY, in order to prevent CAVITY from scratching glue position of the first time plastic molded products, a partial avoidance can be designed. However, it is necessary to carefully consider strength of each sealant position, that is, in plastic injection molding, whether there will be deformation of plastic under large injection pressure, which may lead to possibility of front-end in second injection.

25. When plastic injection molding, the first plastic molded products can be slightly larger in size so that it can be pressed tighter with another CAVITY during second molding to reach sealant.

26. Before clamping A and B plates, be aware that front die Slider or Lifter will reset and crush product first? In this case, it is necessary to think of method of clamping A and B boards first, and then SLIDER or LIFET of front mold can be reset.

27. Water transport layout of two CAVITY and CORE is as full as possible and balanced.

28. 99% of cases are that hard plastic part is injected first, then soft plastic part. Because soft plastic is easy to deform.

29. Pay attention to whether plastic activity will impulsify the first plastic molded products in second plastic injection molding to deform glue position. Assuming this is possible, we must think of ways to improve.

30. Dual shot injection molding must be carefully selected for gate location. One time plastic molded products selects latent plastic injection molding so that product and flow path can be automatically cut off. Three-plate molds or hot runner molds can be considered when latent feed cannot be used. If material is a point gate, do a wave, to avoid hitting secondary material due to the first entry residue.

31. Dual shot injection molding often use rotary injection molds. Punch/die at two positions of rotary injection mold requires uniform size and precision, and works well with die/emboss. When ejection mechanism on dual shot injection molding machine cannot be utilized, a hydraulic ejection and release mechanism must be provided on rotary table.

32. Dual shot injection molding usually uses same plastic of different colors, or two different plastic raw materials. At this time, it is necessary to consider interface effect, shrinkage difference, processing parameters and other aspects of two materials. Dual shot injection molding products are generally made of hard plastics such as ABS, PC, TPE soft plastics. Due to cost or application, it is necessary to fully consider two materials that may be used without good adhesion and fusion, mold treatment at joint (generally there will be a phenomenon of embossing or need to make a mold sealing groove), thickness of material and so on! In order to make two plastics “stick” tighter, think about “stickiness” between materials and roughness of outer surface of mold. Dual shot injection molding has a special TPU material; the smoother mold appearance, the more tightly they stick.

33. In general, shrinkage rate of a dual shot injection molding depends on primary material. Because primary material has already supported contours of plastic molded products, secondary material does not shrink more. As for how to determine primary and secondary materials, there are many factors to consider, such as fluidity of raw materials, shape of plastic molded products and so on.

34. Pay attention to positioning of front and rear molds; all insertion and slope of broken surface should be as large as possible, and should be 0.1mm or more.

35. ABS/PC, ABS/PC+ABS, ABS/PMMA dual shot injection molding, it is necessary to first injection high temperature PC, PC+ABS or PMMA. In case of a transparent face shell mold, most of flip mold structures are used.

36. Transparent large dual shot injection molding requires attention to structural design:
A. Reservation of gate location must be agreed in advance with molding supplier;
B. Thickness of material is recommended to be transparent 0.8mm or more, non-transparent 0.7mm or more, non-transparent as far as possible to use light color, when there is LED light, pay attention to shading;
C. Width of parting surface of transparent part and hole are the same, width is recommended to be 0.5mm, surface is smooth, avoiding side view, far-reaching, unsightly, and minimize hole;
D. Thickness of non-transparent parts is limited, and structure of upper ribs is as close as possible between 0.5 and 0.6 mm to avoid shrinkage;
E. For present time, it is not recommended to do dual shot injection molding with main lens. Cost of mold and product is too high. There is considerable demand for equipment and technical level of molding supplier. If you do, you need to give a detailed solution for entire process for evaluation, intermediate links need to be reviewed;
F. For large-area two-color injection molded parts, two key tests are drop and thermal shock. In both tests, transparent and non-transparent parts are easily disengaged. Hot and cold shock -40 -65, time 48 hours.
G. Structure on two-color parts should be as simple as possible, and complex parts should be made above matching parts. Width of ribs of non-transparent parts should be 0.5-0.6mm as much as possible to avoid shrinkage.

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