Electric shaver battery cover product is shown in Figure 1. Maximum external dimensions of product are 50.67 mm * 47.62 mm * 12.00 mm, average thickness of plastic hard glue is 2.00 mm, base material of hard plastic is ABS, shrinkage rate is 1.005, weight of hard plastic is 4.38 grams. Soft rubber is TPE. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product map of electric shaver battery cover
As can be seen from Figure 1, shape and structure of plastic parts are relatively simple. As a two-color mold product, plastic parts generally need to meet decorative appearance, feel comfortable or meet waterproof function. Two-color injection molding has advantages of high product precision, stable quality, beautiful appearance, and reducing impact of fuel injection on the environment, which increases added value of product. It is usually used in the fields of automobiles, decorations, medical equipment, computers, daily necessities and hand tools. Two-color injection molding is a molding method that uses an injection molding machine with two or more injection molding systems to inject plastics of different varieties or colors into same product at the same time or successively to form injection molded parts with different colors or textures. Electric shaver battery cover is designed with two-color products mainly for waterproof sealing. As product and mold design of two-color mold, following points need to be paid attention to;

 

Figure 2 3D mold design
1. Two materials of two-color plastic part must have good mutual fusion. Hard plastic and soft rubber have a structure that can be well combined. Hard plastic is partially made into rough lines or connection buckles that facilitate attachment of soft rubber, etc. In order to make two plastics stick more tightly, stickiness between materials and roughness of mold surface should be considered.
2. As far as possible, soft rubber can be fed from front mold, so that soft rubber of front and rear molds can be connected together. Connection part should be able to ensure effective flow of soft rubber. Do not make connection part too thick to save soft rubber material.
3. Mold design should ensure that injection molding can have a high quality. Because of any defective products, it is more difficult to separate and recycle two plastics.
4. When designing mold, first distinguish which two-color injection machine customer uses. Master first-hand information on injection molding machines. Two-color injection molding machines in Japan, my country and Taiwan all have two sets of parallel injection systems. Most of two-color molds in Europe are based on ARBURG injection molding machines. One injection system is horizontal and the other is vertical. This set of molds introduces Asian two-color molds.
5. When designing mold, it is necessary to inject hard rubber in the first injection, and soft rubber is generally placed in second injection. Because soft rubber is easily deformed. Mold is designed so that second soft glue can cover first hard glue entry point as much as possible. In order to facilitate precise mold closing after turntable is rotated, first nozzle material should be taken out in time, most of which are taken out by manipulator or manually. Nozzle of three-plate mold should preferably be designed to automatically release mold. Special attention should be paid to whether release action of soft glue nozzle is reliable.
6. Two mold bases of Asian two-color are exactly same. When ordering mold bases, you need to pay attention to machining accuracy, which is convenient for exchange and clamping.
7. Shapes of two front mold cavities are different, and one product is molded respectively. Then two shapes of back mold are exactly same. Since rear mold has a soft rubber position, when soft glue is injected, ejector plate retreats, leaving a space for soft rubber to enter glue. When injection molding ABS, long guide post is used to bounce thimble plate through seesaw.
8. After front and rear molds of mold are rotated 180º from center, they must be matched. During design, mold rotation action must be checked.
9. When designing CAVITY for second injection, in order to prevent CAVITY from inserting (or scratching) glue position of product that has been molded for the first time, a part of void can be designed. However, strength of each sealing position must be carefully considered, that is, in injection molding, is there a possibility that plastic will be deformed under a large injection pressure, which may lead to possibility of a sharp edge in second injection molding?
10. During injection molding, product size of the first injection molding can be slightly larger, so that it can be pressed more tightly with another CAVITY during second molding to achieve effect of sealing.
11. Note that during second injection, will flow of plastic impel first molded product to deform its glue position? If this is possible, there must be a way to improve it.
12. Before A and B plates are closed, pay attention to whether front mold Slider or Lifter will reset and crush product? In this way, we must find a way to make A and B plates to close mold first, and then SLIDER or LIFET of front mold can be reset.
13. Water transport arrangement of two CAVITY and CORE should be as sufficient as possible, should be balanced and same. All water-carrying joints come out from side and connect to water-carrying seat connection in the middle of code formwork
14. Pay attention to positioning of front and rear molds; inclination drop of all insertion and breaking surfaces should be as large as possible, and should be more than 0.1mm.

 

Mold design uses two sets of identical mold bases, brand is Fudaba, specification is DCI 2030, and reference angle is to be offset by guide post. (European two-color or Asian two-color of a mold base cannot make reference angle guide post offset. Attention must be paid to mold design.)
A very important issue is that there is no unified rule for setting of shrinkage rate of two-color mold, and key issue is whether to set shrinkage rate for material for second injection. Materials and glue levels depend on structure of the entire product. If material for second injection is soft rubber, and plastic part itself is very small, such as this set of molds, soft rubber can usually have same shrinkage rate as hard rubber.
Shrinkage rate of soft rubber of two-color mold needs to be determined according to specific structure of plastic part, and it is usually necessary to accumulate experience in practice.

 

 

 

 

 

Figure 4 Electric shaver battery cover mold diagram