As key tool for forming products, structural design of mold has become determinant of product development cycle. Designer of mold structure generally adopts design scheme of mesh oil groove to increase its lubricity, as shown in FIG. 1.

 

Figure 1 Features of mesh oil tank

This solution is suitable for manual operation of low-cost milling cutters with ball cutters, but in environment of automated production, CNC machining centers in machining industry have long been popular. Its inefficient milling characteristics are no longer suitable for processing of mold parts. New solutions must be adopted to promote efficient production, shorten processing cycle of mold parts and improve market competitiveness of enterprises.

 

Figure 2 Status of milling of oil grooves

Regardless of whether it is a net-shaped oil groove or an O-shaped oil groove, CNC machining center uses a ball cutter for multi-layer milling when milling. As shown in Figure 2, programming and machining efficiency are low. CAM software programming for processing oil grooves (take PowerMILL as an example) needs to generate centerline of oil groove characteristics first, and then use "reference line finishing strategy" to select a ϕ1mm ball cutter to compile program. Its tool path trajectory is composed of multiple layers, resulting in a long total length of tool path trajectory, and because of its smaller tool, cutting feed speed Vf and cutting depth ap are both smaller, resulting in inefficient machining.

 

Figure 3 O-shaped oil groove characteristics

According to ISO13399 tool design standard and characteristics of rotating body, combined with actual function of oil groove, oil groove is designed to be circular, as shown in Figure 3, and its outer circle size conforms to diameter of tool to facilitate subsequent tool design and efficient cutting machining.
O-shaped oil tank design technical requirements: ① oil tank depth 0.50mm, width 1.0 ~ 2.0mm; ② oil tank and workpiece outline boundary ≥ 1mm to prevent oil leakage and sharp angles; ③ oil tank can be tangent with 2 "○"; ④ Oil groove design cannot extend to molding area on the surface of part, otherwise formed plastic parts will be stained by extruded dirty oil; ⑤ Surface roughness of oil groove surface Ra≤1.6μm.
Type of O-shaped oil groove is as follows.

 

Figure 4 O-shaped oil groove arrangement

1. When part width L≥10mm, use a circle "○" and two circles are tangent, as shown in Figure 4, there are six specifications: D6mm × d4mm, D8mm × d6mm, D10mm × d8mm, D12mm × d10mm, D17mm × d13mm, D21mm × d17mm.

 

Figure 5 Multi-row oil tank arrangement line spacing

2. When oil tank surface needs multiple rows in width direction, when D <16mm, distance between two rows is W = 1mm; when D≥16mm, interval between two rows is W = 2mm, as shown in Figure 5.

 

Figure 6 Rapid prototyping oil groove cutter

After standardizing above design schemes, according to standardized specifications and characteristics of O-shaped oil groove, "oil groove knife" with its contour can be designed, as shown in Figure 6 (number in figure is protection point of patent content). Tool design methods that rely on experience and experimentation, according to basic rules and experience of tool wear, tool breakage, and tool durability, tool material selection and preliminary selection of tool geometric parameters are selected for one or several targets. Optimal tool parameters are selected by analyzing test results to determine tool shape.

 

Fig. 7 Cutting movement principle of oil groove cutter

Cutting principle: Cutting tool rotates clockwise at high speed to generate cutting force. At the same time, Z axis of CNC machining center performs cutting motion at an appropriate feed speed. As shown in Figure 7, until Z axis depth set by NC program, cutting is stopped, spindle is retracted to a safe height, and quickly moves to coordinate point of next oil tank, so cycle is completed until oil tank cutting process is completed.

 

(A). Test model

 

(B) Tool wear after milling

Figure 8 Test model and tool wear after milling

Final test and verification for design and manufacturing effect of tool, after a total of 1050 oil grooves were milled (210 / times, 5 times in total), cutting edge of tool is only abrasion of coating. As shown in Figure 8, which shows that tool design, manufacturing and cutting parameters are reasonable, tool life is long, tool use cost is low, and efficiency is high. Test conditions and results are shown in Table 1.

 

Figure 9 Parameter setting of "Rapid Prototyping" program for oil tank
Use CAM software programming to select “drilling” strategy. Program parameter settings are shown in Figure 9. NC program generated when milling oil grooves on mold parts can achieve rapid formation of oil grooves, and O-shaped oil groove can be processed and formed in 3s.

 

Figure 10 Plug-in window of oil tank programming template

In order to achieve same efficiency and level for all technical staff of team to formulate oil tank program output, a comprehensive program such as optimal safety parameters, cutting parameters, and program settings can be made into a macro program template and fixed as [oil tank] processing programming template, which can realize automatic programming of oil tank processing. Programmer only needs to click [oil tank] programming template to automatically compile same efficient oil tank processing program. Whenever conditions are available, automated programming should be used as much as possible, which not only has higher efficiency, but also can avoid mistakes caused by clerical or calculation errors. Oil tank programming template plug-in window is shown in Figure 10.
Oil tank macro program is as follows (excerpt):
reset localvars
STRING $ str1 = ''
STRING $ wkps1 = ''
STRING $ wkps2 = ''
STRING $ bound1 = ''
IF entity_exists (Workplane) {
$ wkps1 = entity ('Workplane', ''). name
}
DEACTIVATE PATTERN
UNDRAW PATTERN ALL
IF entity_exists (Pattern, '') {
EDIT PATTERN; DESELECT ALL
}
...
PROCESS TPLEADS
EDIT PRATE '50'
RESET TOOLPATH FEEDRATE
EDIT TOOLPATH; REAPPLYFROMGUI
EDIT TOOLPATH; CALCULATE
}
NC programming is one of the most effective links in current CAD / CAPP / CAM system. It plays an important role in achieving design and processing automation, improving processing accuracy and processing quality, and shortening product development cycles. By design optimization, machining tool design, and programming scheme optimization, oil tank realizes efficient processing, standardization, and automatic oil tank programming of oil tank characteristics, avoiding repetitive work, and playing efficient role of CAM software.