Civilian propeller drones have taken the world by storm and have been widely used in geological prospecting, map surveying, aerial photography, disaster monitoring, patrolling of railways and high-voltage transmission lines. A large number of enterprises and capital have entered field of civilian drones. Due to rapid development and fierce competition in China, annual demand for civilian drones continues to grow at a high speed. It is expected that scale of civil drone market in China will exceed exceed 50 billion yuan in 2022. Blade is an important part of UAV, with strict requirements, huge demand, and promising market prospects. This article will explain design ideas and methods of blade precision mold, how to verify dynamic balance of blade.

Structure of plastic part shown in Figure 1 is a long sheet. Forward and reverse blades each have an internal thread of M6*1mm. Internal threads are right-handed internal thread (positive blade) and left-handed internal thread (reverse blade). Material is PA6+30%GF, specific requirements are as follows:

 

Figure 1 Structure of plastic parts of forward and reverse blades
a — —Positive blade: internal thread is right-hand thread
b — —Reverse blade: internal thread is left-handed thread
(1) Surface of plastic parts is smooth, there is no step difference, and dynamic balance deviation is less than 0.007g.
(2) Blades of plastic parts have no lack of glue, no filling dissatisfaction, no breaks, bubbles and peaks (burrs), no glue feeding points and ejector pins can be placed.
(3) Mold requires 2 cavities, namely 1+1.
(4) Injection molding cycle is controlled within 15s.
(5) Life of mold is ≥1 million times.

According to structure and process requirements of plastic parts, DCI-3545 fine nozzle mold base is selected, a single blade is designed with two small nozzles to large nozzles for injection. The most difficult points of this mold are as follows: ① Difficult to eject mold; ② How to pull right and left internal threads of plastic part synchronously; ③ Plastic part is easy to deform; ④ During mass production of mold, dynamic balance deviation control of blade is unstable. Mold structure is shown in Figure 2 and Figure 3.

 

Figure 2 Fixed mould diagram

1. Fixed mold base plate 2. Runner plate 3. Fixed template 4. Precision tie rod 5. Small tie rod 6. Fixed mold 7. Mold foot 8. Buckle machine 9. Precision straight lock 10. Squeeze block 11. Lock module

 

Figure 3 Mould drawing
1. Movable mold plate 2. Square iron 3. Support plate 4. Gear fixing plate 5. Movable mold base plate 6. Precision straight lock 7. Oil cylinder bracket 8. Oil cylinder 9. Movable mold 10. Precise guide post 11. Squeeze block 12. Mold foot 13. Balance block

Weight of positive and negative blades is equal, unit weight is 13g, surface of blade is required to be smooth and no glue point is allowed. According to mold flow analysis software MoldFlow to simulate injection molding process, combined with years of work experience and mold cost, 2-cavity mold uses a small nozzle to a large nozzle side nozzle, nozzle diameter of main runner is ϕ 2.5mm. One side has a 1° slope, bottom width of trapezoidal runner is 3.6mm, upper bottom width is 5mm, depth is 4mm. Single side design has a 10° slope, and bottom circumference is designed with R3mm rounded corners, which is good for processing and plastic filling. Diameter of nozzle of narrow nozzle is ϕ 1.5mm, and one-sided design has an inclination of 1°. Runner diameter of large nozzle is designed to be ϕ 4.5mm, glue inlet is a round gate with a slope, gate diameter is ϕ 0.6mm, and unilateral design slope is 30°. Specific details are shown in Figure 4 and Figure 5.

 

Figure 4 Gating system

1. Plastic parts 2. Main runner 3. Trapezoidal runner 4. Thin nozzle gate 5. Large nozzle runner 6. Glue port

 

Figure 5 Gate structure diagram
1. Plastic parts 2. Thin nozzle 3. Large nozzle runner 4. Round gate (single side slope 30°, glue port diameter ϕ 0.6mm)

Surface of plastic part is required to have no step, no peak (burr) and other injection defects, and dynamic balance of plastic part is very strict, so positioning system of mold is particularly important. Whether positioning system is reasonable will directly affect final assembly accuracy of mold and indirectly affect dynamic balance value of molded plastic part. In order to ensure effectiveness of positioning system, mold base, core and cavity adopt independent precise positioning mechanisms. Clamping guide of middle panel, runner plate and fixed mold plate of mold base adopts Misumi standard precision tie rod and tie rod guide bushing, fixed and movable mold clamping guide adopts Misumi standard precision guide pin guide bush and precision straight lock for combined positioning. Material of tie rod, tie rod guide sleeve, guide post and guide sleeve is SuJ2, with a hardness of 58~62HRC. Movable and fixed molds adopt concave and convex tiger mouth structures for precise positioning and coordination. Specific details are shown in Figure 6 and Figure 7.

 

Figure 6 Mould base positioning diagram

1. Fixed mold base plate 2. Runner 3. Fixed mold plate 4. Movable mold plate 5. Square iron 6. Pallet 7. Gear fixing plate 8. Bottom plate 9. Precision body lock 10. Precision guide post 11. Ejector bottom plate 12. Ejector fixing plate 13. Precision body lock 14. Precision pull rod guide sleeve 15. Precision pull rod 16. Precision guide sleeve

 

Figure 7 Positioning diagram of moving and fixed molds
1. Fixed mold 2. Concave tiger mouth 3. Convex tiger mouth 4. Moving mold

Mold has 2 cavities, namely positive blade (internal thread is a right-handed internal thread) and reverse blade (internal thread is a left-handed internal thread), ejection direction of internal thread of positive and negative blades is opposite. Generally, internal thread has following commonly used structures: telescopic core structure, Huff structure, hydraulic motor chain structure and rack and pinion structure. Combining characteristics of this plastic part, a rack and pinion structure is used to draw internal thread, it is driven by a hydraulic cylinder (see Figure 8, Figure 9). By adding a transition gear between driving gear and reverse propeller gear to change direction of movement of reverse gear, internal threads of forward and reverse propellers can be drawn out at the same time (see Figure 10 for movement diagram). In order to ensure more stable rotation of forward and reverse propeller gears, braces are designed to ensure that movement is not eccentric, and auxiliary gears are added to balance centrifugal force.
Internal thread number of positive and negative blades of this plastic part is 6.5 turns (6.5 tooth profile). To fully extract internal thread, design rotation number of forward and reverse blade shafts should be 7.5 turns. Other relevant technical dimensions are as follows: ①Transmission ratio=3:1; ②Modulus of gear and rack: 2.5; ③Number of teeth of driving gear: 54 teeth; ④Number of teeth of driving pinion: 18 teeth; ⑤Number of teeth of blade gear: 18 teeth; ⑥ Number of teeth of auxiliary gear: 18 teeth; ⑦ Number of teeth of transition gear: 36 teeth; ⑧ Stroke of rack: =3.14*diameter of index circle of driving pinion*number of turns of thread / transmission ratio=3.14* 18*2.5*7/3=330mm; ⑨Stroke stroke of oil cylinder: 330+20 (buffer stroke of oil cylinder) = 350mm.

 

Figure 8 Schematic drawing of internal thread

1. Oil cylinder 2. Oil cylinder bracket 3. Gear rack structure

 

Figure 9 Structure diagram of rack and pinion

1. Positive propeller 2. Reverse propeller 3. Positive propeller barrel 4. Reverse propeller barrel 5. Positive propeller shaft 6. Auxiliary gear 7. Bushing 8. Anti-propeller shaft 9. Bushing 10. Rack 11. Auxiliary gear 12. Anti-propeller gear 13. Anti-propeller sleeve 14. Roller bearing 15. Ball bearing 16. Transition gear 17. Ball bearing 18. Drive pinion 19. Drive big gear 20. Roller bearing 21. Bush

 

Figure 10 Simplified diagram of rack and pinion motion
1. Rack 2. Drive pinion 3. Drive large gear 4. Transition gear 5. Reverse propeller gear 6. Spur gear 7. Auxiliary gear 8. Roller
Shafts of forward and reverse propellers are core components, their dimensions and tolerances are shown in Figure 11 and Figure 12.

 

Figure 11 Positive propeller shaft

 

Figure 12 Details of thread of propeller shaft

Surface of positive and negative blades of this plastic part does not allow any ejection mechanism to be placed. In order to ensure that plastic part is ejected smoothly and without deformation, forward and reverse blades are ejected by a barrel respectively. It also serves as a tube needle. After mold is opened, rack is driven by inputting high-pressure liquid (usually oil) into oil cylinder. Rack drives positive and negative propeller shafts to rotate through gear to draw out threads, then plastic part is ejected by cylinder.

Blade surface is not allowed to have weld marks, dynamic balance deviation is less than 0.007g. Uneven size and arrangement of cooling water channels are extremely easy to cause deformation of plastic parts, make dynamic balance deviation too large and difficult to correct. After cooling deformation simulation of mold flow software and combined with previous design experience, 6 independent sets of cooling water are designed for fixed and moving model cores, cavities of each blade. Cooling water diameter is designed to be ϕ 8.5mm, distance between cooling water channels in length direction is 40mm, evenly arranged. Distance of cooling water path from surface of plastic part in height direction is controlled at 10~12mm. Specific details are shown in Figure 14.

 

Figure 13 ejection system

1. Positive propeller 2. Positive propeller tube 3. Positive propeller shaft 4. Reverse blade 5. Reverse blade barrel 6. Reverse blade shaft

 

Figure 14 Cooling layout diagram
1. Plastic parts 2. Fixed mold cooling water circuit 3. Moving mold cooling water circuit
By adjusting temperature and flow rate of each group of cooling water channels, mold temperature is well controlled, cooling is uniform and sufficient, appearance and dimensional tolerances of plastic parts are controlled within requirements, dynamic balance deviation is relatively easy to maintain stability.

Venting is a problem that cannot be ignored in design of injection molds. Insufficient venting will cause many molding defects, such as short shots, scorching, pores, voids, bubbles, surface silver lines, weld lines and other obvious appearance problems. In view of structural characteristics of plastic part, arrangement of runners and technical requirements, exhaust groove on core is made a full circle of exhaust grooves along shape of blade surface, as shown in Figure 15. Width of primary exhaust slot in core is designed to be 5mm, depth is 0.015mm, length of primary exhaust slot is 3mm, and the distance between the two exhaust slots is 20mm, evenly distributed. Depth of secondary exhaust groove on the periphery of core is 0.4mm, and width is designed to be 4mm. In addition, unilateral gap between shaft in movable model core and cylinder ejector rod is designed to be 0.005~0.01mm, which can also play an auxiliary exhaust effect.

 

Figure 15 Core exhaust system
1. Plastic parts 2. Primary exhaust slot 0.015mm 3. Secondary exhaust slot 0.4mm

Since material of blade is PA6+30%GF (GF: glass fiber), surface of blade is required to be smooth and free of burrs, mold life is ≥1 million times, which requires core to have good polishing performance, excellent wear resistance and certain toughness. According to above requirements, material of core and cavity inserts adopts imported steel material 1.2344 of German Gritz, which is hardened to 48~50HRC. Shaft material is SKD61, hardness is 48~50HRC, cylinder material is SKH51, hardness is 56~60HRC, material of gear and rack is domestic 718, hardness is 29~33HRC, material of shaft sleeve and braces is tin copper, hardness is 38~40HRC, mold base material is S50C, and hardness is 18~22HRC. Although steel selected for above main parts can meet demand for mass production, to maintain stability of long-term production, mold must be regularly and comprehensively maintained.

This plastic part is front and back blades of drone. Blade is an important part of drone. If dynamic balance deviation of blade is too large, it will cause drone to fly unstable or control failure and cause a crash. Dynamic balance deviation of plastic parts must be controlled below 0.007g. Due to processing accuracy and assembly tolerances, dynamic balance deviation of plastic parts cannot be achieved through pre-design, and can only be corrected later through mold trial. There are many methods for correcting dynamic balance deviation. This article will describe a simple and practical method for correcting dynamic balance deviation, and cost is relatively low. Method steps are as follows:
Step 1: Choose a qualified blade model.
Step 2: Place blade on balancer to test deviation value, find position of deviation and mark blade.
Step 3: Take off blades and stick a special tape on mark.
Step 4: Repeat steps 2 and 3 until deviation of dynamic balance of blade is <0.007g.
Step 5: According to sample of paddle attached to ape, area corresponding to cavity is re-grinded and polished.
Step 6: After cavity is less polished, blade prototype is produced by injection.
Step 7: Place re-qualified blade sample board on balancer to test deviation value. If deviation value does not meet requirements, repeat steps 2 to 6 until dynamic balance deviation value is <0.007g.
According to past experience, dynamic balance of blade can be corrected according to above method, dynamic balance deviation of blade can be corrected to <0.007g by a maximum of 3 cycles of operation.

In actual production process, dimensional tolerances, appearance performance and dynamic balance values of blades completely meet customer requirements. Moreover, correction method of dynamic balance deviation values is simple, fast and accurate, which solves problem of mold design and correction of dynamic balance of such plastic parts. Its technical and economic indicators have a strong competitive advantage in mold industry, have a promotion and demonstration role in same industry.