【Abstract】With interior of vehicle as a focal point, we have innovatively constructed a mold technical support specification for small batches of plastic parts, through "mold cavity core segmented processing", "simplification of lifter slider structure", "Cooling system construction", "simplification of surface treatment", "standardized borrowing" and "hot runner modular selection" have optimized mold structure of small batches of plastic parts. Practice has proved that use of mold technology for small batches of plastic parts effectively reduces mold development cycle and development cost, is an effective way to help value of interior parts break through.

With slowdown of global economic development, my country’s automobile production and sales have declined for two consecutive years since the first negative growth of my country’s automobile industry in 2018. Although automakers have adopted measures such as adjusting their own production capacity and de-stocking through terminal promotions, sales decline has tended to narrow, but overall sales are still in a sluggish state. At the same time, main body of car purchase has gradually shifted from post-70s and 80s to post-90s and even post-00s. Consumers have higher and higher demands for individualization and diversification. Faced with this trend, traditional high-volume strategy has been unable to meet individual needs of consumers, small-volume personalized customization of automotive products for consumers will bring new opportunities and competitive advantages for enterprises.
As matrix of plastic parts, molds are root of all innovations. Good molds are prerequisite for ensuring good quality. At the same time, molds are also bulk of investment and an important part of development cost of entire vehicle. Conventional mold strategy is generally based on premise of satisfying hundreds of thousands of dimensions (for example, PP injection mold requires 500,000 times of dimension), which requires a long mold life and good stability. However, mold investment is large, development cycle is long, project development schedule cannot be met in the face of small batch production. At the same time, it will also cause waste of costs and resources. How to control mold investment cost and mold development time in the process of small batch personalized customization while ensuring quality of plastic parts is an effective way to enhance competitiveness. This article takes automobile interior decoration as an example to discuss application of mold technology for small batch plastic parts in automobiles.

With adjustment of vehicle model strategy of auto companies, some high-end and limited-customized models are only in demand for a few thousand vehicles. If conventional molds are used for molding, it will be greatly over-designed and wasteful. Therefore, according to project planning, if it is a small-batch model, it is necessary to construct a mold technical specification for a small-batch plastic part. Combining specific life requirements of plastic parts, breaking conventional mold design methods, greatly simplifying structure of mold, avoiding concept of one-size-fits-all molds in the past, helping to greatly reduce development cost.

Generally, there are two types of cavity cores: integral type and combined type. Integral cavity and core are integrally processed by die steel. As shown in Figure 1, integral structure is simple, has no splicing positions, has high strength, good rigidity, and is not prone to deformation. It is suitable for small and medium-sized products with simple shapes. Combined cavity and core are usually combined by inlay, insert can be processed separately, which improves overall processing speed of mold. Combination of inserts can improve processing technology of mold and save high-quality steel. Inserts can use different steel materials from movable mold/fixed mold, which can solve problem that mold cannot be processed as a whole due to complexity of product, improve machinability of mold. For parts that are easy to wear during molding process, they can be made into inserts for combination, so that they can be replaced in time during later product molding process. In addition, use of combined molds can also achieve easier temperature control, reduce heat treatment deformation, and so on. Figure 2 shows core of combined mold. Combined insert is suitable for plastic parts with complex shapes.
In design of small batch molds for large and medium-sized plastic parts, cavity and core should be designed as a combined structure and processed in blocks in order to achieve purpose of shortening cycle. In addition, small batches of plastic parts are usually produced without soft molds for verification, design or engineering changes are prone to occur in actual production process. Use of insert-combined molds can promptly replace inserts in the case of engineering changes, avoiding overall scrap or replacement of mold cavities and cores, greatly saves cost of mold repairs caused by engineering changes in later period. However, if there are traces of inlaid lines on the surface of plastic part, it is necessary to communicate with plastic part designer during design of plastic part mold to confirm whether it will affect appearance quality of plastic part.

 

Figure 1 Integral type Figure 2 Combined type

In view of undercut position of plastic part, it is necessary to design a tilting mechanism that can be automatically ejected in mass production, as shown in Figure 3, to reduce molding cycle of plastic part. In order to simplify mold structure in small batch production, straight top (see Figure 4) can be used instead of lifter, which avoids complicated design of lifter structure. After product is formed, it is directly ejected by ejector rod, straight top of undercut position moves with plastic part, worker takes manual demoulding method to remove part from undercut position. By replacing lifter with a straight top, structure of mold is simplified and cost of mold development is reduced.

 

Figure 3 Hard mold lifter design Figure 4 Die lifter design of small batch plastic parts

In order to simplify mold structure and speed up mold production cycle for small-batch plastic parts, it is recommended that mold waterway for small-batch plastic parts is generally designed as a straight waterway + well, as shown in Figure 5. In mass production, in order to shorten production cycle of product, design of waterway arrangement is usually more complicated, and multiple waterway forms are used in combination, as shown in Figure 6, including use of expensive materials such as sheet copper to achieve rapid cooling purposes.

 

Figure 5 Mold cooling water circuit for small batch of plastic parts Figure 6 Hard mold cooling water circuit
Design requirements for simplified waterway are as follows: ordinary ϕ 12mm quick-change water nozzles are used for water nozzles, which do not sink into mold plate; distance between waterway and product surface is 1.5D~2.0D (D is diameter of waterway, not less than 8mm); distance between waterway and waterway is 3~5D; it is recommended that all waterways pass through water directly to reduce internal serial connections. Small molds can be connected in series as one in and one out; for large molds, length of series connection is not more than 2m.

In order to avoid problems such as parting surface collapse and molding surface wear during mold life cycle, conventional injection molds usually heat mold steel to increase its surface hardness, reduce mold wear, and extend mold life. According to different heating and cooling methods, heat treatment of mold steel can be divided into two types: ordinary heat treatment and chemical heat treatment. Ordinary heat treatment is mainly annealing, tempering, quenching, annealing and other processes, which cause a certain transformation of structural composition of steel; chemical heat treatment is mainly surface treatment of steel, such as carburizing, nitriding, carbonitriding, etc., to change chemical composition of die steel surface to meet required performance requirements.
In small batch production, since mold is used within a few thousand times, surface of mold cavity does not need to be heat treated (quenched and tempered, quenched, carburized, blackened) like a large batch mold, to meet requirements of service life of mold. Conventional mold processing procedures usually include rough machining, semi-finishing (surface treatment), quenching heat treatment hardening, finishing, mold matching, polishing, etching, nitriding, post-treatment and other processes. In contrast, small batches of plastic molds can eliminate need for quenching, nitriding and other processes. At the same time, because mold hardness is not as high as that of conventional molds, mold polishing, etching and other processing become easier. By simplifying surface treatment process of mold, it can save 20% to 30% of mold processing cost and 10 to 15 days of mold processing cycle.

Standardization is development trend of industry, which can gradually improve mold standardization and digitization, and further enhance innovation capabilities. Main purpose of standardized borrowing is to use a common structure as much as possible to achieve it, avoid unnecessary processing and procurement costs, achieve rapid and efficient mold development, and achieve the best cost. However, due to ever-changing products, it is difficult to completely standardize mold plastic parts. Therefore, mold plastic parts that are not related to product are usually standardized. Mold standardization is mainly reflected in mold bases and common structural accessories. Standardization of mold base has more obvious advantages for mold manufacturing, especially for production of plastic parts with small production batches and more varieties. Use of standardization of molds has a positive effect on shortening design and manufacturing cycle, reducing mold costs. At the same time, use of standardized plastic parts such as general mold bases can greatly reduce labor intensity of workers.
In order to ensure quality of mold, standard parts of the mold have following requirements: all reused standard parts shall not be lower than requirements of batch product mold standard. In order to ensure safe production, lifting ring shall be implemented in accordance with mass product mold standard, shall not be simplified, and cylinder system components shall be shared as much as possible.

Core concept of hot runner modular development is to use same parts and combine different forms to achieve diversified hot runner designs. Hot runner modularization can increase sharing of parts, the versatility of plastic parts, increase standardization of plastic parts in production and assembly process, thereby accelerating speed of new products to market, improving operational efficiency and reducing costs. As shown in Figure 7, when traditional integral hot runner is used, opening frame of runner plate is larger, the overall pressure-bearing area of mold is less; manifold is a non-standard part, the overall structure needs to be adjusted according to position of hot nozzle; for development of new projects or change of glue feeding position of mold, runner plate cannot be reused and needs to be re-customized; processing time of runners with large diversions is longer, there may be certain errors and defects in smoothness of runner walls; all new project development needs to re-evaluate temperature balance, and there are certain errors. After adopting modular hot runner, the overall size of manifold is reduced, mold opening is relatively small, mold bearing area is larger. Modular manifold can adjust position of hot nozzle within a certain range, change position of mold glue, and develop new projects. This part can be reused in old mold. Standardized production, small-size parts processing, relatively low difficulty in runner processing, high controllability, can ensure smoothness of runner wall, and high recyclability. Modular development of hot runners can effectively ensure reuse of resources and reduce development cost of molds and plastic parts. Figure 8 shows two interchange states of modular hot runner.

 

Figure 7 Traditional hot runner-integral type

 

Figure 8 Modular hot runner
a — — Before modular hot runner interchange b — — After modular hot runner interchange

Based on above-mentioned mold development strategy for small-batch plastic parts, in response to demand for only a few thousand high-end and limited-edition models, Pan Asia applied it to development of a small-batch high-end models, optimized mold design in terms of mold design, processing, cooling system construction, surface treatment, standardization and hot runner selection, etc. By breaking “one size fits all’ approach of original plastic part mold, formulating specific mold development technology strategies and technical specifications according to needs of specific products, mold cost is reduced by 35% to 50%, mold development cycle is shortened by 1 to 2 months, enhance competitiveness of small batch plastic parts, and have very large economic benefits.

Through mold cavity core block processing, simplification of lifter slider structure, cooling system construction, surface treatment simplification, standardized borrowing, and hot runner modular selection, mold structure of small batches of plastic parts is further optimized. Through practice in the past two years, it has been proved that use of this small-batch mold strategy for plastic parts can greatly shorten product development cycle, reduce cost of plastic parts and mold development, achieve value breakthroughs by greatly assisting development and cost reduction, which has far-reaching significance for value of automotive interiors and even value of entire vehicle.