Plastic part structure is shown in Figure 1. It is a thick-walled bottle body made of PETG (polyethylene terephthalate glycol) material. It has comprehensive characteristics of high transparency, high strength and high toughness, with a shrinkage rate of 0.5%, a density of 1.2~1.3 g/cm3, general fluidity, easy to trap air, and is widely used in daily chemical plastic packaging, such as cream jars, extrusion blow bottles, injection blow bottles, etc.

 

Figure 1 Plastic part structure
The total height of plastic part is 31.9 mm, diameter of can body is φ41.7 mm, and wall thickness ratio is 1:2.5. The thinnest wall thickness is 2.42 mm, the thickest wall thickness is 6.1 mm, and maximum outer diameter is φ41.7 mm. It is qualitatively a plastic part with uneven wall thickness. According to structural characteristics of plastic part, mold is equipped with 5 main clamping positions: ① There is a convex arc ring with a diameter of 33 mm, a width of 0.8 mm, and a height of 0.3 mm on opening end face of plastic part. When cover thread is screwed, it is compressed with gasket inside cover to form a sealing function. It is a key part. It is required that this arc ring must be formed completely and without deformation. Mold is designed with a parting surface here. During injection, insert structure is used to assist exhaust to avoid insufficient filling defects caused by poor exhaust; ② Neck of plastic part is high and has a screw thread. Number of circles C=1.2. Mold is designed as a left-right symmetrical half core pulling structure; ③ Tank body is designed with an inward concave design, and φ37.1 mm is starting position of inward concave. Vertical direction of this place is the thickest position of plastic part wall. Therefore, mold is disassembled and parted here, which can improve flow marks caused by material problems during plastic part molding.

Parting line is also called parting line, commonly known as flash, burr, insert, etc. Gap formed at opening and closing of mold is specifically formed at parting surface of plastic part. Molten plastic will be squeezed into this gap during injection, and traces will be left after plastic part is taken out. Mold's own problems or adverse phenomena caused by influence of external factors are shown in Figure 2. Specifically, it is manifested as poor appearance of plastic part, such as misalignment, long edge material, unevenness, flake overflow, full circle flash, flake overflow and other appearance quality problems.

 

Figure 2 Formation of parting line
Classification problems: high material temperature, residual overflow, dirty material, insufficient clamping force, poor mold parting surface processing accuracy, insufficient mold assembly accuracy, insufficient mold part material hardness, mold molding deformation, molding injection (blow) material pressure is too high, molding time is too long, and mold parting surface is not clean, etc. are all factors that cause poor parting lines. Specific reasons are as follows.
(1) Material: dirty material, mixed material, high material temperature, overflow residue, etc. (masterbatch, additives in formula), which affect fluidity and smooth channel of injection process.
(2) Mold: insufficient processing accuracy of mold parting surface, polishing position loss angle, high main or side roughness, insufficient hardness of mold parts, deformation of molded parts, unclean mold parting surface, substandard mold assembly accuracy and other mold structure and mechanism technical problems.
(3) Equipment: injection molding machine mold clamping accuracy problem, that is, parallelism and verticality deviation of equipment and insufficient clamping force.
(4) Process: many process parameters are improperly set, such as: high mold temperature, too high injection pressure, too long injection time, too fast injection speed, low clamping pressure setting.

(1) Misalignment: Dimensional deviation of upper and lower molded parts at mold closing position, that is, misalignment, as shown in Figure 3 (a). Reason for its formation is a comprehensive problem of mold, mainly processing, precision, strength, hardness, compressive deformation, accessories, parts, assembly, etc. Comprehensive influence is an important factor causing misalignment.
(2) Long side material: No misalignment but long side material, as shown in Figure 3 (b). Main reasons for its formation are materials, processes, equipment, etc., and mold assembly accuracy is also a factor that cannot be ignored.

 

Figure 3 Classification of parting line
(3) Misalignment and long side material: Both of above reasons are present, and specific problems are analyzed specifically.

Variables: Distinguished by grade standards, 0.05 mm≤length≤0.38 mm, 0.05 mm≤thickness≤0.15 mm; Appearance: Not obvious visually; Function: Does not affect the most basic use function of plastic part, pay special attention to matching parts with assembly requirements; Advantages and disadvantages: Does not hurt hand when touched, especially scratched by blade. Due to different standards set by daily chemical industry and brands, acceptance standards are also different.

(1) Mold frame accuracy: Select reasonable materials, hardness, heat treatment, processing accuracy such as flatness, coaxiality, verticality and fit, etc. Mold frame accuracy must meet standards.
(2) Assembly accuracy: Main molding parts are shown in Figure 4, including fixed mold inserts 1, hot runner inserts 2, fixed mold cavity plates 3, positioning sleeves 4, dynamic mold cores 5, sliders 6, and slider bases 7. Combined with actual cases, conventional PP material guide mechanism and each movable part must ensure reasonable fit accuracy and dimensional stability.

 

Figure 4 Molding parts combination
1. Fixed mold insert 2. Hot runner insert 3. Fixed mold cavity plate 4. Positioning sleeve 5. Moving mold core 6. Slider 7. Slider base
(3) Self-contained precision positioning: Coaxiality of main molding part, positioning mechanism between mold frame and its various plates and frames must ensure that their positioning accuracy and dimensional accuracy are reasonable, mold parts move smoothly and durable without burning, pulling, or straining.

 

Figure 5 Precision positioning combination

Further improve mold parting surface structure, add reasonable and effective precision positioning parts (mechanisms) to main molding part of mold, so as to achieve good mold closing accuracy, reduce generation of mold closing lines, improve appearance quality and accuracy of plastic parts. Specific implementation method is shown in Figure 6.

 

Figure 6 Precision positioning of main molding parts
Multiple precision positioning structure: Design secondary and multiple mold closing multiple positioning to achieve precision positioning, as shown in Figure 6. Structural design forms: round, square, cross, elliptical, special-shaped, etc.; durable technical forms: open grooves, open vents, pre-surface hardening treatment, durable coating surface treatment, etc.; main design positions: main parting surface, side parting surface, inlaid parting surface, etc.

Precision positioning is a structural design used in injection molds to improve closing accuracy of mold parting surfaces. Its main principle is to set additional positioning devices on parting surface of mold to ensure that upper and lower molds or movable and fixed molds can be accurately aligned when mold is closed, reducing appearance of parting line or reducing obvious degree of parting line.
Common precision positioning structures include tiger's mouth precision positioning, taper precision positioning, cylindrical precision positioning, etc. Taking tiger's mouth precision positioning as an example, it is usually to set mutually matching pits and protrusions at four corners or key positions of mold. When mold is closed, protrusions are accurately inserted into pits to achieve precise fitting positioning.

(1) Improving molding quality of plastic parts: Reasonable precision positioning design can effectively reduce gap at mold line, prevent plastic melt from squeezing into gap to form flash during mold closing, improve appearance quality and dimensional accuracy of plastic parts. For some daily chemical packaging plastic parts with high appearance requirements, such as cosmetic bottles and skin care product containers, precision positioning can meet consumers' demand for high quality.
(2) Enhancing mold stability: Precision positioning structure can make mold more stable during mold closing process and reduce risk of damage caused by mold misalignment. Especially in high-speed injection or large molds, precision positioning can withstand greater mold closing force and ensure long-term stable operation of mold.
(3) Adapting to molding of complex-shaped plastic parts: For daily chemical packaging plastic parts with complex shapes, traditional positioning methods may not meet high-precision requirements. Precision positioning design can be customized according to shape and structural characteristics of plastic part to ensure that mold can be accurately closed in all directions.

(1) Advantages: ① Reduce influence of parting line. Precision positioning can control parting line within a smaller range, making surface of plastic part smoother and flatter, and improving appearance quality of plastic part. For transparent or translucent daily chemical packaging plastic parts, precision positioning can reduce influence of parting line on light refraction, making plastic part more beautiful. ② Extend service life of mold. Since precision positioning can reduce misalignment and wear of mold, it can extend service life of mold and reduce production costs. Especially in high-speed injection and mass production, role of precision positioning is more obvious. ③ Enhance versatility of mold. Some precision positioning structures can be designed to be adjustable to adapt to plastic parts of different sizes and shapes, improve versatility of mold, and reduce development cost of mold.
(2) Disadvantages: ① Increase mold manufacturing cost. Design and manufacture of precision positioning structures require higher precision and technical requirements, which will increase manufacturing cost of mold, especially for some complex precision positioning structures, such as taper precision positioning and cylindrical precision positioning, which are difficult to process and have relatively high costs; ② Difficult to maintain. Precision positioning structures are usually delicate and easy to wear and damage. Therefore, regular maintenance and inspection are required during use of mold to ensure accuracy and performance of precision positioning, which also increases maintenance cost and workload of mold; ③ High requirements for mold design and manufacturing. Design of precision positioning needs to consider multiple factors such as structure of mold, shape of plastic part, and injection process. Therefore, technical level and experience of mold designer are required to be high. Manufacture of precision positioning also requires high-precision processing equipment and technology to ensure accuracy and quality of positioning structure.

In order to intuitively understand impact of precision positioning on parting line, after actual application, data comparison in production is selected for analysis. Following are some data comparisons that can be used as reference indicators.
(1) Parting line width. Before applying precision positioning, average width of mold line of plastic part was measured to be 0.35 mm; after applying precision positioning, average width of mold line of plastic part was measured again to be 0.15 mm.
(2) Appearance quality score of plastic parts. Invite professional quality inspectors to score appearance quality of plastic parts. Scoring criteria may include degree of obviousness of mold line, surface flatness, glossiness and other aspects. Before applying precision positioning, average appearance quality score of plastic part was recorded as S2 second-class product; after applying precision positioning, average appearance quality score of plastic part was recorded as S1 first-class product.
(3) Mold service life. Number of production times or service life of mold before applying precision positioning was recorded as 750,000 molds; after applying precision positioning, number of production times or service life of mold was continued to be recorded as 1 million molds.
(4) Production efficiency. Average production cycle of mold before applying precision positioning was measured to be 57~60 s; after applying precision positioning, average production cycle of mold was measured to be 53~56 s.
If precision positioning structure is reasonably designed, it will improve mold closing speed and stability, shorten production cycle, and improve production efficiency. It should be noted that above data comparison is for reference only. Effect in actual application may vary due to factors such as mold structure, plastic material, injection process, etc. When comparing data, other variables should be controlled as much as possible to ensure accuracy and reliability of data.

Table 1 shows accuracy reference for preventing flash in existing conventional daily chemical packaging injection molds. For information on accuracy of some common injection molds, please refer to accuracy range in Table 1.

Table 1 Precision reference for existing conventional daily chemical packaging injection molds
(1) Clearance of mold parts is an important factor affecting generation of flash. The smaller clearance, the lower possibility of plastic melt overflowing to form flash. Due to different properties of different plastics such as fluidity and viscosity, clearance of mold parts varies. For example, PP has better fluidity, so clearance can be appropriately larger, but it cannot exceed 0.05 mm; while for transparent materials such as PCTG and PCTA that have high requirements for appearance, clearance needs to be controlled to be smaller.
(2) Dimensional accuracy grade (IT) reflects precision of mold manufacturing. The lower grade, the higher precision. For daily chemical packaging, appearance and dimensional accuracy requirements are high. Generally, a higher accuracy grade is selected. For example, ABS and PS are often used for cosmetic bottles and caps. Their dimensional accuracy must reach IT7-IT9 to meet requirements.
(3) Flatness accuracy. Flatness accuracy of mold parting surface has a great influence on molding quality of plastic parts. If flatness is poor, gaps are likely to appear during mold closing, resulting in flash. For example, acrylic materials are often used in high-end cosmetic bottle caps and bottle packaging. Their flatness accuracy requirements are high and need to be controlled within (±0.015~±0.05) mm.
(4) Shrinkage range. Molten plastic will shrink to a certain extent after injection molding, and shrinkage rate will affect dimensional accuracy of plastic parts. Different plastics have different shrinkage rates. Impact of shrinkage rate must be considered during mold design and manufacturing, and size of mold parts must be adjusted accordingly. For example, shrinkage rate of PE is large, so a large shrinkage margin should be reserved during design.
(5) Surface roughness of mold parts affects appearance quality and demolding performance of plastic parts. The smaller surface roughness, the smoother surface of plastic part and the easier it is to demold. For daily chemical packaging, in order to ensure appearance quality of plastic parts, surface roughness of mold parts is required to be relatively high. For example, surface roughness of molds made of transparent materials such as PCTG and PCTA needs to be controlled within a relatively low range to ensure transparency and roughness of plastic parts.

Mold structure is shown in Figure 7. The overall dimensions of mold are 650 mm×500 mm×520 mm. 2×4 arrangement has a total of 8 cavities. Because bottle mouth has its own screw thread, mold is designed as a 2-row side-by-side half-way split core-pulling structure. The overall structure adopts a 2-time parting design. Bottle body is made of high-transparent PETG material for injection molding. Robot suction cup is used to automatically pick up parts and intelligently align them for loading into cartons to prevent appearance problems such as bruises and scratches on 8-cavity plastic parts in each mold.

 

Figure 7 Mold structure
1. Runner plate 2. Runner plate 3. Cavity pressure plate 4. Cavity plate 5. Push plate 6. Support plate 7. Support plate 8. Moving mold seat plate 9. Hot runner plate 10. Hot runner insert 11. Fixed mold insert 12. Fixed mold cavity plate 13. Columnar precision positioning part 14. Slider 15. Inclined guide column 16. Inclined wedge 17. Moving mold core 18. Limiting pull rod 19. Push rod 20. Guide column 21. Columnar precision positioning part 22. Reset spring
Under opening force of T160 injection molding machine, moving and fixed mold parting surfaces I of mold are separated, and bottle body plastic part is separated from fixed mold cavity plate 12. When injection molding machine ejector indirectly pushes push rod 19 to push out, parting surface II is separated. When push plate 5 is pushed out, slider on support plate 6 slides horizontally on both sides under reverse thrust of inclined guide column 15, and threaded part of plastic part is completely separated, realizing Haff slider thread core pulling action. After push plate 5 and support plate 6 continue to push out 35 mm, they stop under action of limit pull rod 18. After action is completed, push plate 5 and support plate 6 are reset to initial state under action of reset spring 22. At this time, plastic part still stays on moving mold core 17, in a loose demoulding and semi-suspended state. At this time, robot enters mold to absorb plastic part, take it out and place it in specified carton, and mold parts are reset to complete an injection molding cycle.