【Abstract】Process requirements of threaded plastic parts of medicine bottle cap were analyzed, design scheme of thread withdrawal structure of medicine bottle cap injection mold was determined. Mold opening sequence, thread withdrawal structure, forming thread, alignment, and brace gear rack thread withdrawal structure design are discussed, thread brace positioning method and precautions are explained.

Engineers often encounter some plastic parts with threaded features. For this type of mold design, some complicated mechanical principles need to be used. However, normally designed mechanism is basically linear motion, such as sliding block and lifter mechanism, which are all linear motions, and mold structure is relatively simple. However, some plastic parts with thread characteristics require rotation or rotation plus linear motion. Mold structure of these plastic parts is complicated, manufacturing and maintenance costs are high. After long-term practice, we has explored design method of internal thread mold of injection mold of medicine bottle cap, can complete design of mold quickly and at low cost, improve production accuracy and mold stability, reduce rejection rate of plastic parts, improve production efficiency, and save costs, which can significantly enhance competitiveness and economic benefits of company.

 

Figure 1 Picture of plastic parts

Existing plastic parts of a certain company: medicine bottle cap, material edible PP, shrinkage rate: 1.017. It requires one mold with two cavities, with high impact resistance and strong mechanical properties. Structure of plastic part is shown in Figure 1.
Analysis of plastic parts: plastic parts are common bottle caps, there are internal threads on side wall of inner hole, and internal threads can only adopt unthreaded structure. It can be seen from Figure 1 that outer shape of plastic part has an anti-rotation groove, top inner hole ϕ 10mm cannot be anti-rotation. According to normal classification analysis, movable mold has only a little R to prevent rotation, R is only 0.3mm, which cannot prevent rotation. Thread number of plastic part is double thread, and thread has only 1.4 teeth. Pitch is 2.5mm, lead is 5mm, threaded shaft stroke is 7mm.
Determining scheme: usually design of thread number is small, thread can be retreated with brace, rack stroke is not large, screw shaft rotates to exit plastic part under action of brace spiral, so that anti-rotation force does not need to be too large. Mould action: After moving and fixed molds are opened, cylinder drives rack to move linearly, linear movement of rack drives gear to rotate to drive threaded shaft to rotate. Threaded shaft rotates and moves linearly under action of threaded sleeve to exit plastic part. If you retract teeth before starting and fixing mold, plastic part may stick to mold.
For this plastic part, anti-rotation groove of movable mold is too shallow, only 0.3mm, which cannot guarantee that anti-rotation groove will not be damaged during rotation process. In addition, in order to reduce injection pressure of threaded sleeve, it is necessary to insert a ring of movable mold into a ring of bone, and at the same time prevent R0.3mm anti-rotation groove of movable mold from not reversing, you can add flat anti-rotation on inner ring of annular bone position, but this will cause new problems. After threaded shaft exits plastic part, plastic part will remain on movable mold insert, and an ejector pin needs to be added to eject plastic part.

According to design of mold opening sequence, mold opening sequence is shown in Figure 2.

 

Figure 2 Mold opening sequence

According to characteristics of plastic part and mold opening sequence, design braces withdrawal structure, which consists of thimble, inserts, braces, snap rings, gears, guide sleeves, threaded shafts and other parts. Specific design of braces withdrawal structure is shown in Figure 3.

 

Figure 3 Threaded back structure of braces

(1) Guide sleeve plays a role of guiding and positioning. Bearings cannot be used in this structure because shaft needs to rotate and move linearly.
(2) Bearings will hinder linear movement of shaft. Wall thickness of guide sleeve is 3~6mm. Guide sleeve is usually made of bronze or beryllium copper (also called bronze sleeve).
(3) Gears play role of precision transmission and force transmission. Gear modulus is determined according to tightening force. Gear on threaded shaft can be made as small as possible to reduce rank and thus reduce flow path.
(4) Snap ring is used for axial positioning of gear. Snap ring is a standard part, so when designing shaft, it should be designed according to standard size of snap ring. Gear is integrated with threaded shaft. There is no need to design snap ring. You can also drill screw holes on gear to fix gear with headless screws.

(1) Dimension A, distance between plastic part and bottom of cavity is 20~35mm, strength of cavity must be ensured.
(2) Dimension B, length of guide sleeve, should be equal to 1 times diameter of threaded shaft as much as possible. Too long will increase size of mold. At the same time, it is necessary to ensure that there is more than 10mm of pipe position after threaded shaft exits plastic part.
(3) Dimension C is determined according to size of plastic part, material and length of screw thread, etc. to determine tightening force on plastic part, generally 10~25mm. When structure is different, it will include stroke of threaded shaft, so that it will not be separated from matched gear after tooth is retracted.
(4) Dimension D, distance should be 1~3mm larger than threaded shaft stroke.
(5) Dimension E, this section cannot be too short. Injection pressure of threaded sleeve is borne by this thread. The larger projected area, the thicker thread and the longer length. Too long will increase mold thickness and increase size of molding machine. For plastic parts with a large area, try to consider middle insert to withstand injection pressure while reducing friction.
(6) Dimension G, positioning boss, function of positioning boss is to fix position of gear. It is mainly to fix position of threaded shaft in mold to prevent taper surface of threaded shaft from jamming with cavity, resulting in excessive torque of threaded shaft, generally 3~6mm.
(7) Dimension H, clearance between gear, mold plate and guide sleeve, at least 1mm.
(8) Dimension K, this section is local strength after snap ring groove is opened, at least 2mm, which is determined according to depth of snap ring groove, must be greater than depth of snap ring groove. Inserts can withstand injection pressure, which is convenient for water cooling.

According to shape and characteristics of plastic part, size of molded thread is designed. Specific shape is shown in Figure 4.

 

Figure 4 Formed thread
(1) Formed thread and power thread must have same pitch and different tooth profile, which is mainly to facilitate processing power thread to reduce cost and ensure accuracy. Formed thread has same size and shape as plastic part.
(2) When forming thread is a double-thread or multi-thread thread, power thread is only designed as a single thread, pitch is same as lead of forming thread, to avoid lead difference, reduce and optimize design mold, reduce cost.
(3) Major diameter of power thread should be spaced from top diameter of braces, single side should be at least 0.1mm. Small diameter transition fits, spiral surface also transitions fit, avoidance position is shown in Figure 5.

 

Figure 5 Avoidance position

(3) Leave 30mm pipe positions at both ends of threaded shaft and insert, avoid gap in the middle, 0.2mm on one side, reduce contact area (avoid gap on threaded shaft), and reduce length of finishing. Avoidance diagram is shown in Figure 6, inserts will not affect movement and appearance at the same time. If structure allows, you can also add bronze sleeves at both ends, effect is better.

 

Figure 6 Schematic diagram of avoiding air

Distance between plastic part and plastic part cannot be too small or too large, otherwise runner will be wasted and mold cost will increase. Technical design considers following 3 points:
(1) To facilitate arrangement of runner sprue sleeves.
(2) Rotation between two gears must not be affected, there is a thimble in the middle, gears must not interfere with thimble, and there must be a clearance of at least 1mm.
(3) If plastic parts are very small, it is necessary to consider that there should be at least 3mm of safety steel between bearing and bearing.
Based on above technical points, design ranking is shown in Figure 7.

 

Figure 7 Layout design drawing

Number of teeth in the gear of this plastic part are all 40, gear matching is shown in Figure 8. Design of tooth withdrawal structure of braces and racks is shown in Figure 9. Driving wheel rotates once, gear on threaded shaft also rotates once, transmission ratio is 1:1. However, there are more calculations for structure of braces and racks. Compared with other structures, following 4 points need to be considered for structure of braces and racks:
(1) Measure and calculate thread pitch. After thread is cooled, thread pitch will change due to shrinkage, and there will be several decimal points.
(2) Count number of thread turns, measure thread length, determine number of thread withdrawal turns and thread axis stroke.
(3) According to number of thread turns and size of plastic part, determine gear module, determine number of gear teeth matched with rack, calculate gear indexing circumference.

 

Figure 8 Gear fit

 

Figure 9 Structure of braces and rack withdrawing teeth
1. Insert 2. Guide sleeve 3. Bearing 4. Driving gear 5. Shaft 6. Rack and pinion 7. Formed thread 8. Driven gear 9. Block 10. Screw
(4) According to number of thread turns and indexing circumference, select gear transmission ratio. It is also necessary to take into account size of mold base to facilitate arrangement of parts such as rack and pinion cylinder seat, which requires constant adjustment.

Prerequisite for cooperation of rack and gear refers to that gear index circle is tangent to rack index line. Rack and pinion design is shown in Figure 10.

 

Figure 10 Gear rack coordination
Rack parameters: addendum height A is equal to 1 modulus, tooth root height B is equal to 1.25 modulus, tooth pitch is equal to circumferential pitch, circumferential pitch is equal to indexing circumferential length/number of teeth, C=D=1/2 circumferential pitch, C+D=week section, weekly section=modulus×3.1415, C=D=1/2M×3.1415.

Braces need to be debugged and assembled. Main reason is that spiral line cannot be as accurate as design on drawing when thread is processed. It is difficult for lathe to align profile reference when rotating thread, gear positioning groove can be used for positioning, as shown in Figure 11. Gear is a standard part, positioning method is reliable and easy to adjust. The more number of teeth, the smaller angle difference. When braces are worn, angle can be adjusted to continue using. This is the biggest advantage of using gear positioning.

 

Figure 11 Gear positioning

Based on author's many years of practical experience and proof of mold trial, design of threaded mold for this plastic part has a scientific and efficient tooth withdrawal structure, which simplifies mold structure, stabilizes production process, and reduces production costs. Moreover, operation is simple, rejection rate is reduced, production efficiency is greatly improved, and it has promotion value.