Research on Processing Technology of Moveable Mold Core of Outer Cover of Automobile Glove Box

Time:2020-08-04 09:07:08 / Popularity: / Source:

plastic parts 
(A) Glove box
plastic parts 
(B) Outer lid of glove box
Figure 1 Glove box and glove box cover
Interior of car glove box is used to store items. When glove box is closed, its appearance can be seen, as shown in Figure 1 (a). In order to make outer surface of glove box beautiful and comfortable, give users a good experience, an outer cover must be added to outer surface of glove box, as shown in FIG. 1 (b), outer cover and glove box are integrated by ultrasonic welding. Such a design scheme can be realized not only in process, but also in consideration of appearance requirements of product.

1 Structural analysis of plastic parts and movable mold core

plastic parts 
(A) Front
plastic parts 
(B) back
Figure 2 Movable mold core
External dimensions of glove box outer cover are relatively large, with a maximum length of 462.3mm and a maximum width of 303.1mm. Left and right sides of product are hinges, as shown in Figure 1 (b), which needs to be formed by side core pulling. Therefore, it is necessary to design slider mechanism to match movable mold core. Slider should be designed on movable mold side as much as possible. Grooves in lower left corner and lower right corner are installation positions of slider, as shown in Figure 2 (a). There are also 5 reverse buckles on inner surface of product, inclined top block mechanism needs to be designed. 5 groove positions shown in Figure 2 (a) are installation positions of inclined top block. In addition to complex structures such as sliders and inclined top blocks, movable mold core also has a structure that penetrates fixed mold core, a latent gate insert structure, a reinforcing rib exhaust insert structure, push rod holes, and cooling water channels and screw through holes.

2 Process analysis

Preparation of processing technology should be reasonable, and order of processing procedures should meet processing technology requirements. Proposed processing technology should reduce processing procedures as much as possible and facilitate clamping of workpiece. Processing efficiency of workpiece should be high to reduce manufacturing cost. The most important thing is to ensure precision of size of plastic parts and quality of molding, to provide reliable products and a good user experience for market.

01 Material properties of mold parts

Material of outer cover of glove box is polypropylene, and NAK80 is usually used as mold steel. NAK80 is a precipitation hardened pre-hardened steel with high hardness. Factory hardness is about 40HRC. Cutting resistance is greater than other steel types. It is processed with super-hard alloy cutting tools. When drilling, it is advisable to process at a low speed. The larger diameter of drilled hole, the lower cutting speed, otherwise drill bit may break. When machining on a milling machine, a cutting speed of less than 20m / min can be used to obtain a good machining surface. If feed is too large, tool life will be shortened.

02 Analysis of processing technology of movable mold core

EDM machining 
(A) Insertion surface, hook gate
EDM machining 
(B) Sloping top block
EDM machining 
(C) Cooling channel
Figure 3 Main characteristics of cut structure
Main surface of movable mold core mainly includes main parting surface, surface of molded plastic part, installation position of slider, installation position of inclined top block, insertion position, collision position, hook gate insert position, pusher hole position, shunt runner, screw countersunk holes, screw through holes, etc. Back of core mainly has cooling channels and cooling wells. Side of core mainly has process holes and threaded holes of cooling water channel, and main features are shown in Figure 3.
EDM machining 
(A) 2D picture
EDM machining 
(B) 3D picture
Figure 4 Distribution of waterways, push rod holes and threaded holes
Main parting surface of movable mold core and surface of molded plastic parts are complex curved surfaces, which need to be processed by CNC. After CNC processing is completed, front surface of movable mold core is irregular and difficult to clamp. Therefore, this type of core generally needs to process features such as water channels and push rod holes before arranging CNC machining procedures. Position accuracy of water channel and push rod hole is not high. In order to reduce manufacturing cost, ordinary machine tool is generally used for processing. Outer size of movable mold core is larger, size is 540mm * 340mm, so processing of water channel cannot be processed by ordinary milling machines, deep hole drilling is required. Push rod hole can be processed by an ordinary milling machine, which is more convenient to determine position. Distribution of water channel, push rod hole and threaded hole is shown in Figure 4.
Processing technology of water channel is: scoring → dotting → clamping → drilling → tapping.
Processing technology of push rod hole is: scribing → clamping → punching → drilling → reaming.
Machining process of front surface of movable mold core is: clamping → setting workpiece coordinates → offset region clearing model rough machining → offset region clearing model residual machining → contour semi-finishing → semi-finishing along angle clearing → machining channel of reference line → flat surface finishing plane → parallel finishing → contour finishing → automatic angle finishing → EDM machining → grinding and polishing. 

3 CNC machining technology

01 Positioning and clamping of movable mold core

EDM machining 
Figure 5 Movable mold core clamping
Moulds are generally processed in one piece, and it is necessary to develop multiple sets of molds unless order quantity of product is particularly large. To develop multiple sets of molds, generally one set of molds is first developed, after this set of molds is qualified, other molds are manufactured under same conditions. Fixtures used for mold parts processing generally use universal fixtures. Special fixtures will increase manufacturing cost and extend processing period. Common universal clips include flat-nose pliers, magnetic table and lock template. Shape of core of movable mold is large, ordinary flat-nose pliers and magnetic tables cannot be clamped. Therefore, lock template is used for clamping, as shown in Figure 5. Two backing plates in lock template assembly have same specifications and are independent of each other. Center of backing plate has a U-shaped groove, which can be adapted to processing of cores of different sizes, and is widely used in mold processing industry.

02 Parts processing and tool selection

Table 1 shows machining process and tool selection of parts.
CNC machining 

03 Programming

Model optimization
CNC machining 
Figure 6 Remove push rod hole and threaded hole features
In CNC programming, it is necessary to distinguish which processes are before CNC machining and which processes are after CNC machining. For example, processing of push rod holes, threaded holes and water channels is generally arranged before CNC machining; positions where CNC machining is not in place, such as electric discharge machining of reinforcement ribs and corner clearing, are generally arranged after CNC machining. Therefore, when programming, it is necessary to remove push rod hole, threaded hole and other features that do not require CNC machining (see Figure 6), so as not to spend a lot of time during programming to calculate tool path of these features. For ribs and angle-clearing positions that need to be discharged after CNC machining, to retain these features, CNC machining should try to remove residual margin to avoid excessive discharge machining. After CNC machining, there will be traces of slight depressions in position of ribs. These traces can be used as a reference for position during electric discharge machining, which can effectively reduce accidents caused by wrong discharge position during electric discharge machining.
Clear material roughing in offset area
CNC machining 
Figure 7 Rough machining simulation effect
Main goal of roughing is to pursue material removal rate per unit time, and to prepare geometric contour of workpiece for semi-finishing or finishing. In process of high-speed roughing, it cannot be limited to setting a larger value than traditional machining cutting speed, cutting depth and feed, but also need to pay attention to following points: ①Stable cutting conditions; ②Constant cutting load; ③Avoid rapid tool turning; ④Reasonable choice of tool path trajectory; ⑤Choice of cutting method; ⑥Choice of cutting amount at corner position; ⑦Rationalization of flat surface machining; ⑧Residual machining tool path should be safe, efficient and intelligent. Amount of roughing cutting is large, wear and damage of tool are serious. Therefore, rough machining of indexable rounded face milling cutter is generally used. If this type of tool is damaged during processing, it can stop rotating spindle, carbide blade can be indexed or replaced, which is widely used in mold processing industry. Current tool diameter is φ50mm, tip radius is R5mm, spindle speed is 1500r / min, feed speed is 2000mm / min, down-cutting step is 0.5mm, and rough machining simulation effect is shown in Figure 7.
Residual processing
CNC machining 
Figure 8 Residual processing simulation effect
Residual processing is process of automatically identifying and processing area based on remaining material remaining in previous processing process. Generally, following situations require residual machining: ①Pre-processing process uses a large-diameter tool for processing, and post-process requires a small-diameter tool for residual processing; ②Pre-roughing process uses a tool for side cutting, with a deep cut step at a time, then same diameter or small diameter tool is used for residual processing; ③Pre-processing is to use other machining such as wire cutting and roughing on a conventional milling machine, refer to residual amount of previous process for residual processing; ④When turning over clamping process, carry out residual processing on remaining part of previous clamping position processing. Movable mold core uses an indexable tip rounded face milling cutter with a diameter of ϕ50mm during roughing. Tool diameter is large, so a small diameter tool should be used for residual processing in subsequent process. Reference object can be previous tool path or residual model. Residual machining of reference tool path can only be tool path generated by area removal strategy or flat surface finishing strategy. Minimum tool diameter used for residual machining is ϕ4mm. Simulation effect after multiple residual machining is shown in Figure 8.
Reference line finishing machining movable mold core runner
CNC machining 
Figure 9 Reference line finishing runner
Reference line finishing is a finishing strategy for projecting a certain area of model according to user-defined reference line. Looking down on this path strategy, tool center will always fall on reference line. Circular runner system has a single shape, center line of runner can be used as a reference line, so it is more suitable to use reference line finishing strategy. Tool type used is a φ6mm ball cutter, spindle speed is 5500r / min, feed speed is 1400mm / min, undercut step is 0.1mm. Simulation results of reference line machining runner are shown in Fig. 9.
Finishing of flat surface
movable mold core 
Fig. 10 Simulation effect of flat surface finishing
Flat surface finishing strategies mainly include offset flat surface finishing strategies and parallel flat surface finishing strategies. These two finishing strategies are mainly applicable to finishing of light planes, which is main way of processing planes. They are widely used in model processing and easy to use. It can process all planes of model at one time, which improves processing efficiency. Following points should be paid attention to when finishing a flat surface: ①Radial margin left when finishing a plane must be greater than side margin left during roughing, and tool diameter should also be larger than the smallest tool used during roughing ; ②Plane tolerance is set to ± 0.01mm to avoid possible distortion of model during data conversion. If plane tolerance is set to 0, plane with errors may not be processed; ③Fine holes should be ignored when processing a flat surface, otherwise tool path will produce a machining trajectory around hole edge at hole position. Tool path has too many corners and is not smooth, which will affect surface processing quality of workpiece; ④Allow tool to feed on the outside of flat surface or increase tool path from outside to flat surface, so that tool starts to cut from outside of flat surface of model, which can reduce wear of tool. Tool type used is a φ13R0.8mm tool tip rounded end mill, spindle speed is 6000r/min, feed speed is 1200mm / min, undercut step is 0.05mm, and it is processed in two layers, which are finished on a flat surface. Simulation effect is shown in Figure 10.
Contour finishing
movable mold core 
Figure 11 Semi-finishing effect of contour height
Contour finishing is a machining strategy that cuts along shape of model at a certain Z-axis undercutting step, and is suitable for machining steep or vertical surfaces. When selecting a high-precision machining strategy, you need to check "range", "extra blank" and "machining to a flat area" 3 options in order to optimize tool path. Tool used is a ϕ8R0.5mm tool fillet end mill,  spindle speed is 6500r/min, feed speed is 1400mm/min, undercut step distance is 0.1mm, and simulation effect after processing with a constant high-precision machining strategy is shown in figure 11.
Parallel finishing
movable mold core 
Figure 12 Simulation effect after parallel finishing
Parallel finishing is a finishing strategy that projects a set of equidistant parallel trajectories down Z axis onto model to generate a tool path. Tool used is a ϕ6R5mm ball cutter, spindle speed is 7500r/min, feed speed is 1200mm min, step distance is 0.08mm, and simulation effect after parallel finishing strategy is shown in Figure 12.
Clear corner processing
movable mold core 
Fig. 13 Simulation effect after angle-finishing and finishing
Angle-finishing finishing pointer performs local processing on the corners of model and positions where large-diameter tool cannot be machined. Stitching tool path is generated in steep area according to angle of boundary angle, machining strategy along angle-cutting tool path is generated in shallow area. Tool used is a ϕ6mm flat-bottom knife with a spindle speed of 7500r/min and a feed rate of 1200mm / min. Simulation effect after angle-finishing finishing machining strategy is shown in Figure 13.
Tool safety inspection
After tool path is generated, in order to verify correctness and rationality of tool path, tool path can be simulated. Tool path inspection can ensure safety of machining process and no over-cutting, which can minimize probability of errors. Through collision inspection, you can also check whether used tool holders, fixtures and other components interfere with model to ensure that processing process is safe and reliable.

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