See Figure 1 for light guide column products of medical therapeutic instruments. Maximum dimensions of product are 56.14 mm * 32.80 mm * 19.60 mm; average thickness of plastic part is 2.00 mm, material of plastic part is PMMA, shrinkage rate is 1.004, and weight of plastic part is 1.74 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilled injection molding, flow lines, pores, warping deformation, silver streaks, cold material, and jetting lines, and meet ROSH environmental protection requirements.

 

Figure 1 Product picture of light guide column for medical therapeutic equipment
Light guide column uses transparent materials to guide light emitted by LED light source of PCB board to outer surface of product to play role of guiding light for information, instructions, and flash lights. Since it is located on appearance surface, it also has a decorative effect. Appearance parts need to meet requirements for function and appearance at the same time. Top of lamp post is full of glue, which fully meets design requirements of plastic parts. Material of light guide column generally adopts transparent materials such as PMMA, PC or GPPS.
Surface treatment of light guide column is usually mirror polished or etched. Also, in order to avoid seeing internal components, surface is embossed, dyed, or made into a serrated surface to avoid light transmission, and at the same time to avoid scratches. All tops are work surfaces and must not have any defects, air bubbles etc. inside. There are two common ways to assemble light guide column: compression type and hot melt type. Hot-melt method is mainly used for larger light guide columns. It has strong strength, good reliability and is not easy to fall off; disadvantage is that process is complicated and production efficiency is low.

①Material of light guide column is transparent material, generally choose PC, PMMA and GPPS and other materials;
②Light guide column is generally installed on upper shell by thermal melting. Diameter of thermal melting column is generally 1.00~1.20mm and unilateral gap of thermal melting hole is 0.05~0.10mm.
③ Unilateral fit gap between light guide column and surface shell hole is 0.10~0.15mm.
④ Gap between light guide column and surface of light guide is about 0.50mm.
⑤ Light incident surface of light guide column adopts a smooth surface, reflective surface adopts a smooth surface, and light exit surface can adopt a textured surface;
It can be seen from Figure 1 that size of plastic part is small, structure is simple, and there is no complicated mold structure. Difficulty in mold design lies in choice of mold opening direction and mold exhaust. Specifically, gating system design is critical. Fluidity of PMMA is very poor, and plastic part is a closed straight rod structure. Therefore, exhaust insert of front mold must be designed, and tangent point is selected on the side of rod of light guide column of plastic part. In order to facilitate plastic flow and filling, a submerged gate needs to be designed at the bottom of each vertical pole. Design a thimble at the bottom of each rod, grind out exhaust, reduce injection pressure, and prevent plastic parts from sticking in the front mold.

 

Figure 2 3D drawing of mold

 

Figure 3 Die parting surface diagram
Light guide column is a common light-transmitting element in modern electronic products, most of which are made of PMMA and PC materials, and a few of them are made of GPPS materials. Top of light guide column is an appearance part and must not have any defects. Due to structural characteristics of plastic part, it is easy to trap air. Figure 4 is a common inlay method for this type of plastic part. An integral insert is designed on the top of plastic part. Each A thimble is added to bottom of column, and exhaust is designed at the end of runner.
Mold venting plays an important role in the overall performance of mold. If mold does not have good exhaust, it is impossible to produce products with stable quality. A good mold should have good airtightness and can ensure free discharge of gas instead of highly compressed gas. Selection of position and size of exhaust groove mainly relied on empirical judgment in the past. At present, mold flow analysis software can be used to determine final filling position of mold and design a good exhaust by referring to CAE analysis results for position of exhaust groove. In the design of export molds, an important concept is to actively exhaust, rather than wait for exhaust to be supplemented according to sample after trial mold is completed. Another design method for active venting is to evacuate cavity before injection molding.

 

Figure 4 Design points of light guide column exhaust
Because depth of exhaust groove is small, enough air can pass through. Exhaust depth depends on fluidity of plastic used in finished product. The better fluidity, the shallower depth; otherwise, the deeper depth; depth of exhaust groove is generally between 0.015mm and 0.05mm.
Design of exhaust grooves should not be too many, too many exhaust grooves will reduce contact area of parting surface. Insufficient contact area of parting surface will cause partial collapse of parting surface, which in turn will reduce depth of exhaust groove, resulting in poor mold exhaust. The thinner product, the farther away from gate, opening of the vent groove is particularly important.
One of the most common misconceptions is that too much exhaust leads to flaking. In fact, it is just opposite. Unreasonable exhaust requires excessive pressure. When gas is allowed to escape, rubber will come out with gas, and the only cause of exhaust Excessive cloaking is caused by depth of air groove is too deep. Therefore, when vent groove is properly designed and processed, vent groove will not cause front of injection molded part to appear.