B. Over mould (double injection)：Two different plastic materials are not necessarily mould on one moulding machine, two step forming is adopted; Product is taken out from one set of mould, then put into another set of mould for second step forming. Thus this kind of moulding process is always formed by two sets of moulds, and no special double color moulding machine is required.

Overview of double injection mould (over mould) and double color mould:
A,During design of double mould, position of hard plastic part shall be considered. Reliable sealing gel shall be applied and back draft hole shall be made on the rubber part to prevent drawing deformation.
B,Double color mould is becoming increasingly prevailed in the market nowadays. This technique can make more beautiful product appearance, and is convenient for color changing without painting, however it also delivers high cost and high technical requirements.
1. Shapes of the two cavities are different for forming of one kind of product separately. But the shapes of cores for them are completely the same.
2. The cavity and core moulds must be able to engaged with each other after having rotated for a angle of 180°. In design stage this check must be conducted.
3. Pay attention to the position of ejector pin hole, and the minimum distance is 210mm. For larger mould, the quantity of ejector pin holes shall be increased accordingly. Besides, since thimbles attached to the moulding machine are not long enough, thus lengthened thimbles must be designed for our moulds. The thimbles shall extend from the mould base bottom plate by about 150mm.
Two position rings must be designed on the rear mould base plate.
4. The overall thickness of the front mould panel plus A plate shall be not less than 170mm. Please carefully check other reference data of this type of moulding machine, e.g. maximum molarity thickness, minimum molarity thickness, ejector stick hole distance etc.
5. Water spout of three-plate mould shall be designed for automatic knockout type. Pay special attention to whether the knockout of flexible-flue water spout is reliable.
6. The depth of the front SPRUE shall not exceed 65mm. The distance from the top of the upper（big water spout）SPRUE to mould center shall be not less than 150mm.
7. During the design of cavity for the second step moulding, in order to avoid any damage by the cavity to the formed gel mould in the first step, part of vacuity can be designed. However, strength of each sealing gel position must be considered sincerely, i.e.: During moulding, is it possible to happen plastic rubber deformation under big moulding pressure thus leading to series defects?
8. During moulding, product dimensions after the first-time moulding can be a little bit larger, thus to press tightly with another cavity during the second-step moulding to result in good plastic sealing effect.
9. Pay attention to whether any impact to the first-step formed product and thus lead to its displacement can be resulted owing to flow of plastic during the second-step moulding？ If there is this possibility, measures must be taken to improve it.
10. Before closing of A and B plates, measures must be taken to close A, B plates before the slider or LIFET of the front mould return to its original position, otherwise, the product will be damaged.
11. Water circulation for both cavities and cores shall be sufficient, uniform.
12. In 99% cases, hard plastic part of the product is mould at first, then the soft plastic part because soft plastic is easy to be deformed.
13. In order to make these two kinds of plastic materials “bond” with each other more tight, “viscosity” between materials and surface roughness of mould shall be considered.
Two-color moulding has special TPU；While the more smooth of mould surface is, the more tight they will “stick to” each other.
14. Keep an eye on positioning of the front and rear moulds; Obliqueness of all insert and slope surfaces shall be large enough (more than 0.1mm).