Digital cameras are common small household appliances, and many people likely have them in their homes. A product image of a digital camera front cover is shown in Figure 1. Maximum dimensions are 118.87 mm x 63.79 mm x 35.42 mm, with an average plastic thickness of 1.61 mm. Plastic material is ABS+10GF, with a shrinkage rate of 1.0035 and a weight of 38.92 grams. Technical requirements for plastic parts include no defects such as burrs, undermolding, flow marks, pores, warpage, silver streaks, cold slugs, and spray marks.

 

Figure 1: Product Image of Digital Camera Front Cover

As high-end, everyday electronic products, digital cameras have very high requirements for their appearance. Another requirement for front cover shell was strength. Product needed to withstand a drop after assembly, so ABS with fiberglass was chosen for added strength. The overall shape of plastic part is a rectangular shell, requiring large sliders on three sides for core pulling. A threaded hole on the side of part requires fully automatic thread removal. Inner side of threaded hole forms an undercut, necessitating a lifter molding system. Difficulty in mold design lies in design of thread removal mechanism.

 

Figure 2: 2D mold diagram for a digital camera product.

Because thread removal mechanism and sliders on three sides occupy a significant amount of space, mold cavity layout is 1-in-1. Mold base is a Fudeba standard mold base, MDC FC 33 35 80100 120 H, which is a simplified fine-gate mold base. A replaceable insert is designed where bottom of sprue bushing meets mold plate, allowing for replacement in the event of wear. Material storage cavity on pull rod head is rounded to resist cracking and is particularly suitable for plastics containing glass fiber. Rack and pinion drive in thread removal mold is available in both speed-increasing and speed-decreasing modes. When using mold opening force to drive a rack, which in turn drives a gear for thread removal, if number of threads is large and rack length cannot exceed mold thickness, a speed-increasing transmission is necessary to fully remove thread core within limited mold opening stroke. When using electric and hydraulic motors for thread removal, motors and hydraulic motors operate at high speeds. To prevent damage to plastic part threads, a speed reduction mechanism is generally used to reduce thread core speed. Regardless of whether speed-increasing or speed-reducing mechanism is used, transmission should not exceed two stages.

 

In this mold, side threads interfere with slider core pulling mechanism, creating a design challenge. Spatial constraints limit design of thread removal mechanism. To this end, a rack-driven pinion threading mechanism was designed. Module of pinion and rack is 1.5, with both pinions having 16 teeth and large gear having 64 teeth. Speed ratio is 1:4.
Thread core passes through a large slider with a spring mounted on it. A pinion is mounted on outer end of thread core. Large gear and another pinion are coaxially mounted on a separate drive shaft. Pinion of thread core meshes with large gear, and rack meshes with pinion on drive shaft. When designing mold, attention must be paid to orientation of rack. This orientation is determined by rotation of thread shaft and is related to left-right rotation of thread.

 

After mold is opened and the parting surface opens, rack drives pinion to rotate drive shaft, which in turn drives large gear. Large gear meshes with pinion on thread core shaft, driving thread core. Under action of spring, thread core rotates and retracts, achieving thread extraction. Undercut on the inside of housing corresponding to threaded hole uses a two-stage bevel ejector, connected to bottom ejector pin via a T-slot.

 

Figure 6: Digital Camera Front Cover Mold
Three corners of plastic part are ejected using a sleeve, while remaining parts are ejected using flat ejectors and pins. Ejector plate is designed with a center support that also serves as a support head, providing precise guidance for ejection mechanism. Large sliders on three sides of plastic part all use inclined guide pins for core pulling. Left and right sliders are wider and driven by two inclined guide pins. Wear-resistant plates are designed on the bottom and inclined surfaces of sliders.

 

Figure 7: Digital Camera Front Cover Mold