【Abstract】After fully evaluating function of back cover of blood glucose meter, we have carefully discussed with plastic parts developers, redesigned plastic parts, and adopted a different structure from original mold, which improves life of mold and production efficiency, improved strength and use experience of plastic parts, verified rationality and superiority of new mold structure, and at the same time brought better economic benefits to company.

Figure 1 shows back cover of blood glucose meter. After plastic part mold was produced on the machine, it was found that plastic part stuck to lifter when mold was ejected, could not fall automatically, and manipulator could not take it out. Mold could only be demolded manually, which increased labor costs. Shortly after mass production, ejector rod of mold cannot be returned. Mold was repaired by removing machine, which includes rematching mold with lifter, lubricating ejector rod, increasing ejector spring, and other conventional improvement measures, mold still cannot be mass-produced. What’s more serious is that two products cannot be fastened after assembly of produced plastic parts and mutual parts. In product drop test, two plastic parts were not fastened tightly and loosened frequently, which caused great hidden dangers to safety of use of plastic parts.

 

Figure 1 Back cover of blood glucose meter

To solve problem, we must first understand cause of problem. After repeated research and verification of plastic parts and mold structure, it is found that mold has following problems:
(1) Due to fact that there are many undercuts in plastic parts (a total of 4 undercuts, one undercut and a full slanted top), it is unreasonable for mold to adopt a sloping top structure with 1 mold and 8 cavities. Because there are too many lifters (32 lifters in total), a slight irregularity in ejection and return process will cause lifer to burn and die. This kind of structure requires mass production is unrealistic.
(2) Because plastic part adopts inclined ejector structure, when plastic part is ejected from mold, due to imbalance of ejection, lifter pulls plastic part and causes side deformation. In addition, plastic part has a small fastening position, which causes situation that plastic part and mutual parts cannot be fastened. Moreover, when mold is opened to eject plastic part, plastic part cannot be automatically dropped due to lifter hanging on plastic part, it is difficult for manipulator to grasp and pick up plastic part, automatic production cannot be realized, which greatly reduces production efficiency.
(3) Since plastic part is glued in by submerged ejector rod, after plastic part is ejected, residual gate must be manually trimmed, which greatly increases production cost, reduces production efficiency, is difficult to trim and level. When plastic part is molded, flow of melt is sprayed from gate to surface of plastic part, which can easily cause flow lines and air lines on the surface of plastic part, which will adversely affect surface of plastic part, it is also difficult to adjust and set parameters during molding.

 

Figure 2 shows change of advancing glue form, and Figure 3 shows gate residue after plastic part is ejected.
Figure 2 Change form of forward glue Figure 3 Residual gate material

After understanding problems of plastic parts and molds, not affecting functions of all aspects of plastic parts, after communicating and confirming with customer, following improvement plans are proposed.
(1) Plastic part is changed from 4 undercuts to 3 undercuts. Undercuts on square holes on short side are cancelled, two undercuts on long side are changed to middle of plastic part. At the same time, length of undercut of plastic part are increased. Size is shown in Figure 4 and Figure 5, and improved plastic part structure is shown in Figure 6.
Improved plastic part not only enhances strength of buckle position, makes two buckles tighter after assembly of plastic part. At the same time, plastic part also changes from original 4 buckles to 3 buckles, which simplifies mold structure. In terms of mold cost, it saves about 8,000 yuan in mold processing costs.

 

Figure 4 Improved front undercut position Figure 5 Improved front two side undercut position

 

Figure 6 Improved undercut position

(2). Glue feeding method has been changed from original submersible ejector stick glue feeding method to horn glue feeding method (see Figure 7). Thereby reducing manual trimming of gate, reducing labor and costs, improving production efficiency, and improving aesthetics of plastic parts.

 

Figure 7 Feeding method of horns
Of course, changing to horn feeding method will inevitably increase difficulty and cost of mold processing, but increase in mold cost is negligible compared to labor and injection molding.
Following is a comparison of mold cost and labor cost: ①Increased mold cost for horn injection: a plastic part needs to dismantle two horn injection inserts, a total of 8 cavities plastic parts, a total of 16 horn injection inserts are added. Calculated at cost of about 70 yuan for an insert, cost of 16 inserts has increased by about 1,120 yuan; ②Labor cost saved by horn feeding: cost of a worker is about 200 yuan a day, including wages, five insurances and one housing fund.
From above cost comparison, it can be seen that as long as cost of trimming gate for a worker for 6 days is reduced, it has already exceeded cost of additional horn gate inserts. For a plastic part to be produced for a long time, cost saving is very considerable.

(3). Square hole of plastic part is ejected by a fixed mold slider (see Figure 8), which improves aesthetics of plastic part. Because, mold is ejected in the form of a slider, parting surface of slider and inner mold is in the form of collision (two-phase assembly surface is vertically bonded). Compared with inclined surface insertion form of lifter (two assembly surfaces are inclined to fit inclined surface), parting surface of mold will touch more tightly, and plastic parts are not prone to drape, burrs, etc. At the same time, with slider structure, parting surface of mold is on the inner side of plastic part. Even if there is a little drape (fuzzy), it is difficult for user of plastic part to get it. If mold is ejected with an lifter, drape is on the appearance surface of plastic part, which is easy to be noticed by user, which may cause discomfort. From perspective of mold strength, after adopting slider structure, because it is not limited by size of inner frame of plastic part, strength and smoothness of mold movement are greatly enhanced, mass production and production efficiency of mold are improved.

 

Figure 8 Fixed mold slider

(4). Three undercuts inside plastic part are in the form of core-pulling by movable mold and sliding block out of mold. After adopting form of inner slide block, the overall structure of mold is more compact, because inner slide block does not need to be extended to ejector plate like an lifter, so that slide block can be made very short and relatively large. Figure 9 and Figure 10 show size comparison diagrams of slider and lifter. During working process of inner sliding block, sliding block does not push up plastic part, which also reduces its force. At the same time, after changing to slider structure, linear sliding of slider will be much smoother than oblique sliding, coefficient of bending deformation will be greatly reduced. From perspective of plastic part demolding, use of inner slider mold structure avoids situation that plastic part sticks to lifter, plastic part can be taken out with a robot to realize automation of production. This also greatly improves mass productivity and production efficiency of mold. Structure is shown in Figure 11.

 

Figure 9 Inner slider

 

Figure 10 Inclined roof

 

Figure 11 Slider structure
1. Plastic parts 2. Slide block Ⅰ 3. Slide block Ⅱ 4. Slide block Ⅲ 5. Core-pulling block
Mold opening method is to move core-pulling block down and then pull slider to translate into plastic part to realize plastic part's reverse release. Of course, use of slider structures (including rubber mold sliders and movable mold sliders) is not without disadvantages.
Disadvantage 1: After adopting fixed mold slider structure, when opening mold, you must ensure that upper panel of mold is opened first. If upper panel is not opened first, fixed mold slider is not separated from buckle of plastic part, slider will cause damage to plastic part, and even cause serious consequences of mold slider.
Disadvantage 2: After adopting movable mold sliding structure, when opening mold, it is necessary to ensure that mold opening force of molding machine is used to pull out core block of movable mold slider before ejector plate is ejected, so that inner slider buckle is separated from plastic part. Otherwise, when plastic part is ejected, plastic part will be damaged, or even inner sliding block will be damaged, ejector rod will be stuck in the top bend.
In order to avoid above problems, design sequence of mold opening is as follows:
a. In the first step, upper panel is pulled apart, fixed mold slider is separated from plastic part.
b. In the second step, fixed/moving mold plate is separated, and plastic part is separated from fixed mold.
c. In the third step, continue to open mold and support plate is pulled apart, movable mold plate is separated from support plate, and inner sliding block is separated from internal buckle of plastic part.
d. In the fourth step, ejector pad is ejected, plastic part and gate are ejected from movable mold.
e. In the fifth step, manipulator grips plastic part and runner to leave mold.
f. In the sixth step, ejector backing plate is returned, mold is closed again, and next cycle is entered.
In order to meet above mold opening sequence, as shown in Figure 12: ①Add 4 springs and 4 fixed distance screws under fixed mold base plate and fixed mold plate, use spring force to push fixed mold plate; ②Fixed mold plate and movable mold plate are connected with a pull plate; ③Movable mold plate and supporting plate are connected with a fixed distance screw so that fixed mold plate and movable mold can be opened.
(5) In order to reduce gate diameter and increase utilization rate of raw materials, mold adopts mold structure of hot filling nozzle to cold runner, so that at the same time, shrinking molding cycle increases production efficiency. Although this increases cost of mold: cost of the entire hot runner is about 6,000 yuan, including hot nozzle and temperature control box. However, compared with saved gate material and improved efficiency, this cost is acceptable at a cost of several thousand yuan.
Basic structure of improved mold is shown in Figure 12.

 

Figure 12 The overall structure of mold
1. Hot nozzle 2. Spring 3. Oblique wedge 4. Square hole slider 5. Inner slider 6. Core pulling block 7. Fixed distance screw 8. Movable mold seat plate 9. Ejector backing plate 10. Ejector fixing plate 11. Spacer plate 12. Support plate 13. Fixed distance pull plate 14. Movable mold plate 15. Fixed mold plate 16. Fixed distance screw 17. Fixed mold base plate
(6) In order to prevent mold from being burnt, core and cavity materials are hardened. At the same time, slider and mold are made of different materials: core and cavity materials are S136 materials, hardness after hardening is 50±2HRC. Block is made of SKD51 material, hardness after hardening is about 60HRC. Reason for this is to prevent slider from being burned, because two parts of same hardness are more likely to cause burns and jams when they rub against each other or collide.

In improved production of mold, production cycle is reduced from original production cycle of 45s to 29s, gate is automatically broken, and two labors are reduced. Mold realizes fully automatic production, which greatly reduces production cost. In the production of molds for more than half a year, there has not been a case of mold repair due to poor mold operation, no delay in the delivery of customer orders due to mold problems. At the same time, quality of plastic parts has been greatly improved. In the early experiments and later use of plastic parts, there was no single case of plastic parts falling apart due to poor inter-matching of plastic parts, which greatly improved safety of plastic parts and customer experience.

No matter what kind of plastic parts and molds, as long as you fully understand and analyze plastic parts in a scientific and rigorous manner, conduct strict evaluation and certification of mold problems, I believe that design and improvement of plastic parts and molds will achieve satisfactory results. .