A modem is a device for signal conversion between a computer and a telephone line. It consists of a modulator and a demodulator. Modulator is a device that modulates computer's digital signal (such as files) into an analog signal that can be transmitted on telephone line. At receiving end, demodulator converts analog signal into a digital signal that computer can receive. Data communication between computers can be realized through modems and telephone lines. External modem is placed outside chassis and connected to host through a serial communication port. This kind of modem is convenient and smart, easy to install, and flashing indicator light is convenient to monitor working condition of modem. In addition to data transmission, these modems also have fax and voice transmission functions. This article introduces key points of injection mold design for modem bottom case.
Product diagram of modem bottom shell is shown in Figure 1. Maximum size of product is 172.83 mm * 123.88 mm * 37.66 mm, average thickness of plastic part is 2.0 mm, material of plastic part is HIPS, shrinkage rate is 1.004, and plastic part weight is 62.25 Grams. Technical requirements for plastic parts are that there must be no defects such as peaks, dissatisfaction in injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc.

 

It can be seen from Figure 1 that plastic part is a flat rectangular shell with multiple long grooves on the top surface for heat dissipation. One side is also designed with a long groove for heat dissipation. In addition, there are multiple function holes on both sides, including USB interface, network cable interface, data input port, various light holes, etc. These three sides of plastic part all form an undercut button, which requires design of an overall large slider core. There is an undercut on inner side of two side walls of plastic part, which needs to be designed for lifter demoulding. Plastic parts are made of HIPS, which is precisely use of its excellent electromagnetic properties.
Size of plastic parts is large, and there are slider core pulls on three sides, so mold design cavity is ranked as 1 cavity. Glue feeding method of plastic parts is one point on the top of large nozzle plastic part. Front mold, back mold and sliding block are designed with cooling water (not shown in 3D diagram). Ejection of plastic parts is two lifters and multiple ejector pins. 3D drawing of mold is shown in Figure 2. Front mold core diagram is shown in Figure 3. Back mold core diagram is shown in Figure 4.
Modem bottom shell products do not allow slight thimble sharpening on plastic parts, that is, thimble cannot be completely flush with top of back mold core, thimble needs to be shortened by 0.1mm to avoid thimble sharpening and affecting product performance.
Three large sliders of mold are all simplified design methods, slider is large, slider seat is reduced, upper and lower structures of slider and slider seat are combined, which saves mold embryo space and reduces mold costs. Moreover, upper and lower combination facilitates design of water transport rubber ring at combined position. Combination of slider and slider seat is shown in Figure 5.

 

Figure 1 Product diagram of modem bottom shell

 

Figure 2 Modem bottom shell mold diagram

 

Figure 3 Front mold core diagram

 

Figure 4 Rear mold core diagram

 

Figure 5 Schematic diagram of slider
Key point of mold design of modem bottom shell is that front mold has many heat sinks, which need to be processed in front mold as a whole, and cannot be used as inserts. It must be processed in place by a high-speed CNC machine at one time, instead of manual trimming. Through injection molding production, it is shown that mold design and manufacturing fully meet requirements of mold style book, which can be used as a reference for similar mold design.

 

Shrinkage control of PP polypropylene modified material is an important aspect of polypropylene modification. Good shrinkage control is of great significance to promotion and use of polypropylene modified materials, and it is also an important aspect of ensuring product quality. Especially when using modified polypropylene to replace traditional engineering plastics, shrinkage rate is very important. Following is a brief description of impact on shrinkage from minerals, glass fiber, polyethylene, melting grease, etc.

Mineral additives used for polypropylene PP mainly include calcium carbonate, talc powder, mica powder and so on. Influence of various mineral additives on molding shrinkage rate of polypropylene. It can be seen that influence of mineral additives on molding shrinkage rate of PP modified materials is more obvious. Influence of mineral additives on shrinkage rate of polypropylene modified materials mainly has three aspects: one is that mineral filler itself does not shrink, and its addition reduces shrinkage rate of polypropylene modified materials from overall ratio; second, addition of mineral additives will inevitably affect crystallinity of polypropylene, thereby affecting shrinkage rate; third, after addition of fine mineral agents, it acts as a nucleating agent, changing structure of polypropylene, preventing formation of large spherulites, and also affecting molding shrinkage rate of polypropylene.

Glass fiber has the greatest impact on molding shrinkage of polypropylene PP modified materials. When content of glass fiber reaches more than 30%, molding shrinkage rate of polypropylene modified material decreases from 1.8 to 0.5, and surface-treated glass fiber has a greater impact on molding shrinkage rate than untreated glass fiber. Addition of glass fiber destroys crystallinity of polypropylene and affects shrinkage rate. More importantly, glass fiber limits crystal shrinkage of polypropylene.

Addition of polyethylene also affects molding shrinkage of polypropylene modified materials. Although polyethylene is also a kind of plastic with high crystallinity, molding shrinkage rate is also very large, but after being added to polypropylene, crystallinity of each other is destroyed to different degrees, which reduces overall molding shrinkage rate.

Molding shrinkage of polypropylene is affected by its crystallinity, and crystallinity is affected by its own molecular weight. When MI increases, molecular weight decreases, its crystallization speed increases, and molding shrinkage increases. When polypropylene MI changes, it also has a certain effect on molding shrinkage.