Ultrasonic energy travels in a certain direction and can penetrate objects. If it encounters obstacles, it will produce echoes. Different obstacles will produce different echoes. People collect and display this echo on screen through instruments, which can be used to understand internal structure of object. Using this principle, people use ultrasound to diagnose and treat human diseases. There are many types of ultrasonic diagnostic equipment used in medical clinics, such as A-type, B-type, M-type, fan-shaped and Doppler ultrasound type. Type B is one of them, it is the most widely used and convenient one in clinical practice. Through B-ultrasound, various cross-sectional graphics of various organs in human body can be obtained relatively clearly. B-ultrasound is a non-surgical diagnostic test, which is painless, non-invasive, non-radioactive to subject, and can be tested with confidence. B-ultrasound can clearly display various cross-sectional images of various organs and surrounding organs. Since images are rich in solidity and are close to true structure of anatomy, application of ultrasound can be used for early diagnosis.
Rear shell of B-ultrasound display is shown in Figure 1. Maximum size of product is 339.58 mm * 286.53mm * 318.50 mm, average thickness of plastic part is 2.00 mm, material of plastic part is ABS, shrinkage rate is 1.005, and weight of plastic part is 1211.5 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc.

 

This is an early B-ultrasound monitor, which has now been replaced by LCD monitors. As can be seen from Figure 1, plastic structure is similar to an old-fashioned liquid crystal display. There are multiple heat dissipation holes on the top and one side of plastic part, hole diameter is ø3, and one side has an arc-shaped notch. There are two pillar positions on the top of plastic part, and there are more than one reinforcement ribs on the top edge. Big end of plastic part is designed with a full circle of spigots around opening edge for installation of front shell.
Structure and shape of plastic part are simple, difficulty of mold design is design of large sliding block on the side of plastic part to prevent plastic part from being ejected and deformed. Difficulty of mold processing is that depth of cavity is large, ø3.015 boss at the bottom of slider and cavity is difficult to process.
Mold design cavity ranks 1 cavity, and one side of large sliding block is designed on ground side. Mold base is a non-standard mold base 7075, thickness of A plate is 450, and thickness of B plate is 210. Mold is a large mold. Cavity is directly made on A board. This method of leaving original body is a typical method for large molds. A plate is made of NAK80, which is a pre-hardened steel with a hardness of 38～41 HRC. It has good polishing performance and etching performance. A section of positioning stop with a slope of 10 ゜ is designed on four sides of mold base, rear mold is raised, and front mold is recessed. During injection molding, due to large depth of cavity and large lateral force caused by injection pressure, positioning stop can effectively lock front and rear molds to prevent cavity from expanding. Exhaust groove on mold base is mainly for exhaust of guide sleeve. A horizontal exhaust groove is designed at the bottom of each guide sleeve, see mold design figure 2. For large molds, exhaust of guide sleeve is very important. If exhaust is not smooth, abnormal noises will occur when mold is opened.
A label position of 65mm*42mm is designed on the outer surface of top of plastic part with a sink of 1mm. This position will eventually be designed to stick a label. Gating system is a sprue-type direct gate, which is designed in the center of label position and cut off with electric shears after injection. Diameter of sprue sleeve is ø20, and diameter of small end opening is ø4.5mm. Taper of inner hole of main runner is 2.42゜. Due to oblique elastic slider of front mold on one side, sprue sleeve is eccentric by 40mm.
Design of large sliding block on one side of plastic part is design difficulty of this set of molds. For plastic parts such as turnover boxes and display rear shells, slider area is large and occupies entire side of plastic part. A large locking force is required to resist injection pressure of mold cavity. Therefore, in mold structure of this plastic part, large side slider must be designed as a front mold oblique elastic slider. For details, see mold design diagram 2. If it is designed as a rear mold slider, size of mold base needs to be increased, stroke of slider has to be increased a lot to release plastic part, which is a bad mold design.

 

Figure 1 Product picture of rear shell of B-ultrasound display
 

 

A T-slot guide with a width of 80 is designed in the middle of oblique elastic slider of front mold. Two ø30 springs are designed on the top of slider, and guide rod is used as spring guide. On parting surface, two sets of draw hooks are designed to assist spring to open mold at the moment of mold opening. Main function of draw hook is to overcome static friction force at the moment of mold opening. Draw hook is shown in section C-C. Positioning of front mold oblique elastic slider is limited by two limit blocks at both ends of slider.
Due to large depth dimension of plastic parts, tightening force on back mold is relatively large, ejection of plastic parts is carried out by using an L-shaped push block combined with a thimble. 4 L-shaped push blocks are respectively designed at 4 corners of plastic part, arrangement of L-shaped push blocks is shown in Figure 5. Thimble is arranged near stiffener on the top of plastic part. Each L-shaped push block uses two thimble as push rod, M8 screws are used to fix push rod and push block. In two screw posts, front mold is designed with inserts, rear mold is designed with thimble and collides with inserts.
Cooling system of mold is cooled by a ø10 cooling circuit, distance between water hole wall and plastic part is 34mm, front mold and large sliding block use straight water. Rear mold adopts a pond.
ø3.015 boss at the bottom of cavity and large slider is used to form multiple ø3 holes. These bosses need to use new tools to be processed in place on a high-speed CNC to ensure a good surface roughness, so as to avoid later manual polishing, to avoid deformation of boss shape and roundness, and affect appearance of plastic part.

 

 

 

 

Figure 2 Die diagram of back shell of B-ultrasound display

 

Figure 3 3D drawing of dynamic mold

 

Figure 4 Fixed mold 3D diagram

 

Figure 5 Shape and layout of L-shaped push block