1 Introduction

Microfluidic technology has developed rapidly in recent years, from the initial field of analytical chemistry to bioanalysis, diagnostics [1], drug discovery [2], food processing [3], and many other fields. However, high surface quality and form accuracy are needed for the application of microfluidic chips with high performance. The high requirement poses a great challenge to the manufacturing of microfluidic chips, especially considering the small structures. Wang et al. [4] fabricated microchannel structures with a width of 200 μm in quartz by picosecond laser. He et al. [5] introduced the micro- and nano-processing of glass 3D structures using femtosecond laser direct writing processing technology. The microchannel with a width of about 40 nm and a length of 40 μm was fabricated, which can be used for micro-/nanofluid biosensors, photo fluidic sensors, current element sensors, and surface-enhanced Raman scattering equipment with high added value. However, the high manufacturing cost of laser processing restricts its processed products from being widely used in microfluidic chips. The efficiency of the microfluidic manufacturing process based on the chemical etching process is limited by the reaction rate at a low level, which has been improved after several years of development. Peng et al. [6] reported a simple and low-cost method for fabricating microchannels on silicon substrates using direct ink writing as a stable mask and metal-assisted chemical etching. However, the chemical liquids used in the process, specially buffered hydrofluoric acid, were harmful to the environment and require further treatment before emission [7]. Ku et al. [8] proposed a processing method for glass microfluidic devices by immersing the glass substrate in cold water by micro-milling, realising the manufacturing of microchannels with a depth of 50 µm and a width of 300 µm. This method can effectively remove debris and extend the service life of milling tools, but the chip is still fabricated one by one which is difficult for mass production.

To improve the production efficiency, different methods have been proposed in recent decades. Polymers, such as polypropylene (PP), methyl methacrylate (PMMA), and polydimethylsiloxane (PDMS), are inexpensive and easily manufactured in volume. Hence, polymers have become the most commonly used materials for microfluidic chips [9]. The forming technology of polymer materials is an effective means of microfluidic product mass production. At present, the main forming technologies are 3D printing technology [10], hot pressing technology [11], and microinjection moulding technology [12]. Tiboni et al. [13] manufactured two kinds of microfluidic chips using polypropylene and applied them to the manufacturing of nanomedicine. 3D printing technology can greatly shorten the manufacturing process of microfluidic chips and meet the demand for small quantities of microfluidic chips with various complex structures. However, 3D printing technology is currently facing a success rate problem, resulting in high material waste and energy waste [14]. As a result, hot embossing and injection moulding are the most widely used methods in the industry [15]. Lin et al. [16] replicated microfluidic channels in PMMA by hot embossing and found that microchannels had good characteristic integrity and replication quality through a fluidity test. However, hot pressing requires a heating stage every time, and the forming cycle is long. The advantages of the microinjection moulding technology lie in the lower requirements for mould strength, longer mould life, higher production efficiency of microfluidic chips, and relatively low cost. Hence, microinjection moulding technology has a great potential to be applied in the mass production of microfluidic chips with low cost.

The performance of microfluidic chips obtained by microinjection moulding technology depends on the surface quality and form accuracy of the microstructure mould core. The micro-nano structure manufacturing in the microfluidic chip mould core mainly relies on laser beam processing [17], chemical etching [18], lithography technology [19], electrical discharge machining (EDM) [20, 21], and micro-milling [22]. Wang et al. [23] used picosecond laser processing technology to fabricate micro-convex block arrays with a diameter of 12.5 μm, height of about 21.9 μm, and spacing of 25 μm. Wang et al. [24] created a two-scale layered surface structure with a two-dimensional regular array of micro-bumps with nano-ripples on the nickel surface. However, as a guarantee of the machining accuracy of microstructure, ultra-fast laser power directly affects the machining efficiency, and there are still many problems such as high energy consumption and high technical difficulty. Gan et al. [25] proposed the use of femtosecond laser-induced chemical etching to produce micro-moulds based on triangular diffraction gratings. Guo et al. [26] proposed a method for manufacturing 100-nm concave two-dimensional silicon nano-moulds by side etching stripping, which can produce high-precision and large-size silicon nano-moulds. However, it is still very difficult to chemically etch microstructure with a high aspect ratio. Iwai et al. [27] fabricated microfluidic devices using multi-layer soft lithography and injection moulding processes and verified droplet formation performance. Ayoib et al. [28] used an inkjet transparent mask as a microfluidic photomask for the soft lithography of SU-8 negative photoresist cams, which improved the manufacturing economy of soft lithography of cams. Hung et al. [29] processed a microchannel with a depth of 600 μm, and a depth ratio of 1.2 on the surface of SUS316L stainless steel by using the die micro-EDM. The material removal rate was high, while the surface roughness was 0.715 μm, and the surface quality of the microchannel was poor. This is because the EDM depends on high-frequency pulse discharge, which can form the discharge pits on the machining surface, as well as the recasting layer, resulting in the deterioration of surface quality. Micro-milling can produce micro-parts with complex three-dimensional structures at relatively low cost, high efficiency, and good surface quality [30]. Behroodi et al. [31] combined the projection micro-stereolithography (PµSL) 3D printing with computer numerical control (CNC) micro-milling method to simplify the manufacture of microfluidic devices with high-resolution microstructures. Zhang et al. [32] used the manufactured polycrystalline diamond (PCD) micro-end mill to produce the microstructure on tungsten carbide through micro-milling. However, tool wear deteriorates the machining quality in the milling process. Ultrasonic vibration has a significant impact and plays an important role in assisting different manufacturing processes [33], which can effectively reduce the processing force and tool wear in the process of micro-milling, ensuring the processing quality [34,35,36]. Hence, ultrasonic vibration–assisted grinding (UVAG) can be conducted to improve the surface quality and form accuracy.

Another requirement is the post-processing of the structural surface in the mould core. A variety of polishing methods, such as copying tool polishing [37], magnetic field–assisted polishing [38], and fluid jet polishing [39, 40], were conducted in recent years. The key is to improve the surface roughness without reducing the precision of the characteristic structure shape. Chemical mechanical polishing is an effective polishing method to achieve sub-nanometre surface roughness [41,42,43]. Nevertheless, CMP only can be utilised to machine flat surfaces. Suzuki et al. [44] used the profile-shaped diamond wheel to grind the binderless tungsten carbide Fresnel die and obtained the form accuracy of 0.8 μm in peak-to-valley (PV) value and surface roughness of 14 nm. However, it is difficult to adapt this method to the polishing of complex structures. Guo et al. [45] used the magnetic field–assisted finishing (MFAF) method to finish the microfluidic die with a curved microstructure, reducing the surface roughness to about Ra 0.11 µm while ensuring the height of the microstructure. Kim et al. [46] polished the microstructural groove array on the silicon surface of electroplated copper by magnetorheological finishing. The surface roughness of the polished tetrahedral structure was reduced to one-tenth of the initial value, with the bottom roughness reaching 11.1 nm and the side roughness reaching 18.1 nm. Wang et al. [47] conducted a feasibility study on polishing V-groove structured surface with magnetic composite fluid slurry, and the surface roughness was reduced from 350 to 15–50 nm after 150 min of polishing. However, it is still difficult to maintain a high form accuracy with the magnetic field–assisted polishing method. Wang et al. [48] innovatively used the low-pressure fluid jet polishing method for the polishing of the sinusoidal structure surface and V-groove-structured surface without using the mask. The advantage of fluid jet polishing lies in its strong shape and material adaptability, which can be used for ultra-precision polishing of various materials [49], complex structures [50], and even inner cavities [51]. Secondly, it can ensure that the processed surface does not produce heat-affected zones [52]. Therefore, the fluid jet polishing technology is more suitable for polishing the microstructure in the microfluidic mould core surface. To enhance the polishing efficiency, multi-jet polishing technology was proposed by Wang et al. [53, 54], which can largely boost the polishing efficiency.

Based on the above introduction, most of the current studies focus on a single process in the microfluidic chip fabrication process, which makes it difficult to develop a mature fabrication process. Fewer studies focus on the large-scale fabrication process of polymeric microfluidic chips. Therefore, the process chain for low cost and mass fabrication of microfluidic chips is described in detail in this paper.

2 Methods

In this paper, the microstructure is first machined by milling and ultrasonic vibration–assisted grinding (UVAG) on the surface of the mould core using a milling cutter and ultra-fine grinding tool. The key parameters of micro-grinding are studied in detail to implement the machining of the microstructure with high surface quality and form accuracy. Then, the fluid jet polishing (FJP) technology is used to improve the surface quality of the mould core of the microfluidic chip while ensuring the shape of the microstructure. Finally, the polymeric microfluidic chip is efficiently fabricated by replicating the microstructure on the mould core surface to the polymer surface using microinjection moulding technology. Figure 1 shows the complete process chain for the mass production of polymeric microfluidic chips.

Fig. 1
figure 1

Process chain for the mass production of polymeric microfluidic chips

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3 Experiments

3.1 Mould core material

The machining performance of mould steel is better than materials with high hardness such as ceramics and hard alloys. Therefore, the mould core material is mould steel S136H. Its material properties are shown in Table 1, the mould steel has excellent corrosion resistance, wear resistance, and good machining and polishing performance.

Table 1 Material properties of S136H
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3.2 Experimental detail

3.2.1 Milling and grinding process of the microstructural mould core

To reduce the machining allowance of micro-milling and prolong the service life of micro-grinding rod tools, the machining process of the microstructural mould core can be divided into three steps. The first step is to use a carbide milling tool with a diameter of 1 mm to rough mill the mould to generate the micro-convex structure outline. In the second step, a carbide milling tool with a diameter of 0.5 mm was used to conduct semi-finish milling to improve the surface quality. In the third step, a cubic boron nitride (CBN) grinding tool with a grain size of #800 and a diameter of 0.3 mm (Resmo, Japan) was used to perform UVAG, and a microstructural mould core with high form accuracy and surface quality was produced, as shown in Fig. 2. The workpiece is not disassembled during the processing to avoid installation error, and only the length compensation of the grinding bar is needed. The process is carried out on the precision three-axis machining centre to complete the length compensation of the fine grinding bar to ensure the accuracy of the tool.

Fig. 2
figure 2

Schematic diagram of mould core machining process: a rough milling, b semi-finish milling, and c fine grinding

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The UVAG experiment was carried out on the three-axis ultrasonic machining centre (UHB-500), as shown in Fig. 3. The vibration frequency is 25–35 kHz with an amplitude of 4 µm. The fine-grained CBN electroplating grinding tool was used in the UVAG process experiment. To obtain the influence of different process parameters on grinding quality and realize the optimization of the UVAG process, the influences of grinding process parameters including spindle speed N, feed speed vf, grinding depth ap, and ultrasonic power Pa on the surface quality of microstructure machining were studied. Detailed processing parameters are shown in Table 2.

Fig. 3
figure 3

Experimental setup of ultrasonic vibration grinding machining centre: a machining picture and b machining diagram

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Table 2 Experimental parameters of ultrasonic vibration–assisted grinding of mould steel
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For the testing experiments in FJP, the design of the workpiece is simplified and scaled down. The surface roughness Ra of the top surface of the raised microstructure is between 0.182 and 0.199 µm, and the surface roughness of the bottom surface of the microstructure is between 0.481 and 0.503 µm.

3.2.2 Fluid jet polishing (FJP) of the microstructural mould core

The ZEEKO IRP200 polishing machine was selected for the fluid jet polishing experiment, which has a 7-axis polishing system equipped with a sapphire nozzle, as shown in Fig. 4a. In FJP, the polishing slurry containing water and abrasive is pressurised and pumped out of the nozzle to generate the fluid jet, and the fluid jet impinges the target surface to generate the micro-/nanometre scale material removal induced by the erosion of the high-speed abrasive in the slurry, as shown in Fig. 4b. Because it adopts the flexible jet as the carrier, FJP has a high adaptability to the surface with complex and irregular structures. The polishing slurry used in the experiment was silicon carbide abrasive (GC #1000, Fujimi) with a concentration of 6 wt.% and an average abrasive radius of 11.9 µm. The effect of some critical parameters in FJP is investigated, including the feed speed, fluid pressure, impinging angle, and stand-off distance. Table 3 shows the detailed experimental conditions of the polishing process.

Fig. 4
figure 4

Experimental setup and the principle of fluid jet polishing. a Photograph of the experimental setup. (b) Schematic diagram

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Table 3 Experimental parameters of fluid jet polishing process for mould core
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3.2.3 Microinjection moulding of polymeric microfluidic chip

BABYPLAST 6/10P injection moulding machine was selected for the microinjection moulding process of the microfluidic chip, as shown in Fig. 5a. The principle of injection moulding is shown in Fig. 5b. Polymer particles flow into the plasticizing chamber through the hopper, melt at high temperature, and then inject into the mould cavity with micro-raised array structure mould core through the gate. After the pressure holding and cooling stage, the pattern on the microchannel mould core was transferred to the polymer and solidified. Finally, the polymer sample was pushed out by the thimble after the front and back mould separation, to achieve the purpose of polymer microfluidic chip scale replication forming. The selected material was polypropylene (PP, LG B310), which possesses excellent heat resistance, bending fatigue resistance, and corrosion resistance. It is widely used in microinjection moulding of polymer microfluidic chips. In this experiment, the influence of key injection process parameters such as melt temperature, injection speed, holding pressure, and holding time on the surface quality and form accuracy of polymeric microchannel was studied. Table 4 shows the experimental parameters of the microinjection moulding process.

Fig. 5
figure 5

Experimental setup and the principle of the microinjection process. a Photograph of the experimental setup. b Schematic diagram of microinjection moulding process

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Table 4 Experimental parameters of the microinjection process
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3.3 Measurement and characterisation methods

3D surface profiler (Keyence VK-X260) was used to measure the processed surface of mould steel by UVAG. The surface roughness of mould steel was obtained according to the measured 3D topography, and the average value of 10 measured data was selected as the roughness Ra. The surface roughness (Sa) and morphologies of the bottom surface of the microstructural mould core were measured by using a white light interferometer (ZYGO Nexview) and scanning electron microscope (SEM). The mean values of two different areas on the underside of the microstructure for each polishing parameter during measurement were selected.

The roughness at the bottom of the polymer microchannel structure was measured by a stylus profiler (KLA-Tencor D-300). Five sample lengths were selected from the bottom profile of the polymer microfluidic chip to calculate the roughness at the bottom of the flow channel, and their average value was taken as the roughness value Ra of the bottom surface of the polymeric microfluidic chip. The form of the convex microstructure was measured by a non-contact 3D laser confocal microscope (Keyence VK-X260). The micro-convex structure was designed with a 6° structural slope to facilitate the demolding of the injection moulding workpiece.

4 Results and discussions

4.1 Precision ground surface of the microstructural mould core

Figure 6 shows the surface roughness Ra of the mould steel under different ultrasonic vibration–assisted grinding process parameters. It can be seen that the surface roughness Ra gradually decreases with the increase of spindle speed as shown in Fig. 6a. This is induced by that the effective abrasive grains involved in grinding per unit of time gradually increase, resulting in an effective improvement of the machined surface quality. There is no increase in surface roughness due to the excessive wear of abrasive grains at higher rotational speeds. It indicates that even when using smaller abrasive grains for machining mould steel, the ultrasonic vibration–assisted machining technology is invoked to effectively reduce the abrasive grain wear and maintain the sharpness of abrasive grains at higher rotational speeds. As can be seen from Fig. 6b, with the increase in the feed rate, the roughness of the machined surface of mould steel presents a rising trend. When the feed rate is 5 mm/min, the surface roughness Ra reaches the lowest value of 0.195 µm. Since the experiments were conducted with grinding rods having a finer grit, the lower feed rates contributed to effective material removal and improved surface quality. As shown in Fig. 6c, the machined surface roughness decreases with the increase of the cutting depth when the cutting depth is less than 2 µm and increases when the cutting depth ap exceeds 2 µm. The main reason for this phenomenon is the ‘ploughing phenomenon’ when the cutting depth ap is lower [55]. The material flow sideways when the abrasive particles squeeze the workpiece surface, reducing the grinding surface quality. Figure 6d shows the relationship between the ultrasonic power and the processed surface quality of mould steel. With the increase of ultrasonic power, the surface roughness first decreases and then increases. When the ultrasonic power is 70%, the machined surface roughness of mould steel reaches a small value of 0.303 µm. In conclusion, the machined surface roughness Ra of mould steel varies from 0.195 to 0.458 μm under different grinding processes. When the spindle speed N, feed rate vf, cutting depth ap, and ultrasonic power Pa are 30,000 r/min, 5 mm/min, 2 μm, and 90%, respectively, the machining quality is better.

Fig. 6
figure 6

Influences of ultrasonic vibration–assisted grinding process parameters on surface quality: a spindle speed N, b feed rate vf, c grinding depth ap, and d ultrasonic power Pa

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Figure 7 shows the macroscopical picture and overall 3D morphology of the micromachined mould core with a micro-convex array structure. It can be seen from the figure that the microstructure processed by UVAG has a clear overall outline and smooth surface, which achieves higher sidewall verticality and higher processing quality. According to the inspection data, the bottom roughness Ra of the micro-convex structure is about 0.346 μm, the top roughness Ra of the micro-convex structure is about 0.136 μm, the average width of the microstructure is 237.352 μm, and the average height is 168.407 μm.

Fig. 7
figure 7

Photo and 3D morphology of micro-convex array structured mould core: a macroscopical picture and b 3D morphology

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4.2 Precision polished surface of the microstructural mould core

Figure 8 shows the SEM photographs of the microstructure. Figure 9 shows the surface morphologies and surface roughness of the mould after polishing under different conditions. The colour represents the surface height at different positions. The results of the 3D surface roughness results have been summarised in Fig. 10. As shown in Fig. 10a, the influence of the feed rate on the polished surface quality is demonstrated, with the range of surface roughness variation in surface mean height (Sa) from 0.063 to 0.049 µm. Figure 8a shows the surface morphology of the mould core under different feed rates. Surface roughness Sa tends to decrease and then increase with increasing feed rate. At a lower feed rate, more material is removed from the surface of the microstructure even after the grinding marks are removed. Larger polishing grains produce more processing marks on the surface, resulting in poor polishing quality and uniformity. In addition, excessive material removal can destroy the integrity of the microstructure boundary and even change the microstructure contour. As shown in Fig. 8a, when the feed rate is lower than 50 mm/min, the shape of the microstructure contour changes greatly. Especially when the feed rate is at 10 mm/min, the width of the microstructure is greatly reduced due to the high material removal per unit time. When the feed rate is increased to 50 mm/min, microstructure surface quality is better, indicating that the surface machining marks can be removed well at this time. Under this condition, the edges of the microstructure are clear, and the machining marks are not obvious. Meanwhile, there is no obvious form change. Therefore, the form accuracy of the microstructure can be guaranteed, and the polishing quality is the best, as shown in Fig. 8a. The polished surface quality decreases again after 50 mm/min. In Fig. 8a, the feed rate F larger than 50 mm/min shows no significant change in the microstructure either, indicating that the material removal per unit time of the fluid jet decreases with the continuous increase of feed rate. This results in the inability to completely remove the marks produced by milling and grinding.

Fig. 8
figure 8

SEM images of microstructure under different polishing process parameters: a feed rate F, b fluid pressure P, c impinging angle A, and d stand-off distance T

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Fig. 9
figure 9

3D topographies at the bottom of the microstructure under different polishing process parameters: a feed rate F, b fluid pressure P, c impinging angle A, and d stand-off distance T

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Fig. 10
figure 10

Effects of polishing process parameters on surface quality: a feed rate F, b fluid pressure P, c impinging angle A, and d stand-off distance T

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Figure 10b shows the influence of the fluid pressures on polished surface quality. With the increase in fluid pressure, the average surface roughness Sa at the bottom decreases first and then increases, with a range of 0.077 ~ 0.061 µm. Figure 8b shows the surface topography of the mould core under different fluid pressures. Fluid pressure mainly affects the impact velocity of the abrasive fluid in the process of FJP, which can have a significant impact on polishing performance. According to Fig. 10b, the surface roughness is 0.061 µm when the fluid pressure is 8 bar. When the fluid pressure is less than 8 bar, the impact velocity decreases, and the number of abrasive particles per unit of time also decreases. This limits the material removal rate per unit of time, resulting in inadequate polishing and poor surface quality. In this situation, no significant deformation is found in the morphology of the microstructure mould core when the fluid pressure is 6 bar or 8 bar as shown in Fig. 10b. When the fluid pressure is greater than 8 bar, more abrasive particles per unit of time participate in the polishing process together with higher energy per particle, resulting in excessive material removal of abrasive particles. Higher impact velocity leads to deeper indentation of abrasive particles on the polished surface, resulting in higher surface roughness, especially when the fluid pressure reaches 12 bar. According to Fig. 8b, it is noted that the microstructure also has large shape changes, indicating that excessive polishing occurs.

Impinging angle reflects the angles between the fluid jet and the target surface in the process of machining. Different impinging angles will affect the direction of abrasive particles impacting the target surface material, which is an important factor in the FJP process. Figure 10c shows the influence of different impinging angles on the polished surface quality. While Fig. 8c shows the changes in the surface morphology of the mould core at different impinging angles. With the increasing impinging angle, the average surface roughness Sa at the bottom rises first, then decreases, and tends to be stable, with a range of 0.063 ~ 0.050 µm. As can be seen from Fig. 8c, when the impinging angle is 45°, the surface quality is best, and the average surface roughness Sa is 0.050 µm. However, according to the surface topography, the inconsistency of polishing quality appeared on both sides of the microstructure, which affects the polishing quality. When the impinging angle is 90°, over-polishing occurs due to excessive material removal, which not only seriously affects the surface quality, but also seriously damages the form accuracy. Therefore, a larger impinging angle is easy to adversely affect the surface roughness and form accuracy. Hence, a smaller impinging angle should be selected in the process of FJP to obtain better surface quality.

Stand-off distance reflects the distance between the nozzle and the target surface, which is also an important parameter in FJP. Figure 10d shows the influence of the stand-off distance on polished surface quality, and Fig. 8d shows the surface topography of the mould core under different stand-off distances. No obvious relationship between the surface roughness and the stand-off distance is found in this study, which means that the effect of the stand-off distance on the surface roughness is not obvious. This phenomenon is consistent with the reference [56].

In summary, the optimised polishing process parameters are feed rate F = 50 mm/min, fluid pressure P = 8 bar, impinging angle A = 45°, and stand-off distance T = 4 mm. The results show that the FJP technology can be used to polish the microstructure of complex microchannel mould cores. The surface quality of the microchannel mould core was improved greatly after FJP based on the optimised polishing process parameters. Figure 11a, c demonstrates the 3D topography of the microstructure before and after polishing, respectively. And Fig. 11b, d presents the SEM photographs of the microfluidic mould core before and after polishing. It can be seen from Fig. 11b that there are burrs on the edge of the side wall of the microstructure before polishing, as well as obvious milling and grinding marks. After FJP, the surface has been improved by removing the burrs and milling/grinding marks on the side wall. The average surface roughness of the microstructure is Sa 0.051 µm at the top and 0.230 µm at the bottom. The results show that FJP can significantly improve the surface quality of the microstructure.

Fig. 11
figure 11

3D morphology and SEM images of mould core before and after polishing: a 3D morphology before polishing, b SEM images before polishing, c 3D morphology after polishing, and d SEM images after polishing

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As shown in Fig. 12a, the height deviation e of the two curves can be obtained by comparing the section profile of the mould core micro-convex structure with that of the theoretical machined section. The PV value of the deviation curve can be used to calculate the form accuracy of microstructural mould core The form deviation PV value of the machined mould core with a micro-convex structure can be calculated as 15.681 µm by the PV value of the height deviation curve, as shown in Fig. 12b.

Fig. 12
figure 12

a Section profile curves of theoretical and machined microstructural mould core. b Form accuracy of microstructural mould core

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4.3 Microinjection moulding quality of polymeric microchannel

Figure 13 shows the influence of different injection process parameters on the roughness Ra at the bottom of the polymeric microchannel. Overall, melt temperature T and holding time t have more significant effects on the roughness Ra at the bottom of the polymeric microchannel. Figure 13a reflects that the roughness at the bottom of the microchannel decreases with the increase in melt temperature. When the melt temperature increases to 245 °C, the roughness Ra at the bottom of the microchannel can reach a minimum value of 0.029 µm. Figure 13b shows that with the increase of injection velocity V, the roughness of polymer microinjection presents a slight downward trend. The main reason is that the structural changes caused by the microstructure’s protrusion side walls and rounded corners at the bottom make the microinjection moulding filling process more difficult. The faster injection speed allows the molten polymer to fill the cavity more quickly, reducing the effect of too fast temperature drop. It can be seen from Fig. 13c that the roughness Ra at the bottom of the micro-flow channel changes not significantly with the increase of the holding pressure P, ranging from 0.033 to 0.030 µm. When the holding pressure exceeds 5 MPa, the surface roughness of the workpiece increases. As can be seen from Fig. 13d, the roughness Ra at the bottom of the micro-flow channel gradually decreases with the increase of the holding time T. When the holding time exceeds 5 s, the roughness at the bottom of the polymeric microchannel increases slightly and then reaches 0.027 µm when the holding time reaches 9 s.

Fig. 13
figure 13

Influences of microinjection process parameters on the roughness of microchannel bottom: a melt temperature T, b injection speed V, c holding pressure P, and d holding pressure t

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Figure 14 reflects the different microinjection moulding process parameters on the form accuracy PV of the polymeric microchannel. The polymeric microchannel still maintains a high form accuracy when the mould core microstructure has a higher perpendicularity and the fillet radius is smaller. With the change of moulding process parameters, the form accuracy of the polymeric microchannel ranged from 8.594 to 16.180 µm. Melting temperature and holding time have a great influence on form accuracy. As shown in Fig. 14a, the form accuracy PV of polymer injection parts decreases sharply at first and then tends to be stable with the increase of melt temperature T. This is because the low melt temperature makes the polymer melt flow insufficient, resulting in insufficient filling of injection parts and affecting the form accuracy of the polymeric chip. As can be seen from Fig. 14b, the form accuracy PV of polymeric microchannel rises slowly first and then decreases slightly with the increase of injection velocity V. As shown in Fig. 14c, the form accuracy PV of microchannel maintains at a stable level with the increase of holding pressure P. As shown in Fig. 14d, the form accuracy PV of the microchannel decreases sharply first and then slowly with the increase of the holding time T. When the holding time is 9 s, melt temperature is 235 °C, injection speed is 80 mm/s, and holding pressure is 5 MPa, the form accuracy PV of the microchannel reaches a minimum value of 8.594 µm, and the roughness Ra of the microchannel at the bottom is 0.027 µm. The microstructure characteristics of polymer chips processed by UVAG have higher surface quality and form accuracy.

Fig. 14
figure 14

Influences of microinjection process parameters on form accuracy PV of polymeric microchannel: a melt temperature T, b injection speed V, c holding pressure P, and d holding pressure t

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Figure 15 shows the photograph and morphology of the polymeric microchannel when the melt temperature is 235 ℃, the injection speed is 80 mm/s, the pressure holding pressure is 5 MPa, and the pressure holding time is 8 s. Figure 15a shows the macroscopic photograph of the polymeric microfluidic chip. Figure 15b, c shows the 3D morphologies of the polymeric microfluidic chip and microchannel, respectively. The side wall of the microchannel has high verticality and clear and complete boundary due to high injection moulding accuracy. In Fig. 15d, e, the SEM images of the polymeric microfluidic chip and microchannel can be seen, respectively. The surface at the top and bottom of the microchannel is very smooth. The bottom surface roughness Ra is 0.027 µm. The structure and the edge of the microchannel have no obvious shrinkage and flow marks or other defects. The microinjection moulding quality is considerably high.

Fig. 15
figure 15

a Photograph of the polymeric microfluidic chip; b 3D topography of polymeric microfluidic chip; c 3D topography of polymeric microchannel; d SEM image of polymeric microfluidic chip; e SEM image of polymeric microchannel

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The section profile of the micro-convex structure of the mould core was compared with that of the polymeric microchannel, and the former was obtained along the direction h of the mould core micro-convex height, as shown in Fig. 16a. The height deviation e is obtained by the difference between the two curves, and the PV value of the deviation curve can be used to calculate the form accuracy of the polymeric microchannel structure, as shown in Fig. 16b.

Fig. 16
figure 16

a Section profiles of microstructural mould core and polymeric microchannel. b Form accuracy of polymeric microchannel by injection moulding

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5 Conclusions

In this paper, a process chain of microfluidic chips including milling, ultrasonic vibration–assisted grinding (UVAG), fluid jet polishing (FJP), and injection moulding is introduced in detail, which can not only realize low cost and mass production of the polymeric microfluidic chips but also reduce energy consumption. The processing parameters of UVAG, FJP, and injection moulding were optimised and systematically analysed based on the form accuracy and surface quality. The main research results and conclusions are as follows:

  1. (1)

    UVAG and FJP can be combined to realize the precision machining of the mould core for the microfluidic chips with controllable form accuracy and surface quality.

  2. (2)

    In UVAG of the mould steel, the surface roughness decreases with the increase of spindle speed and increases with the increase of feed rate. In addition, the roughness decreases first and then increases with the increase of cutting depth and ultrasonic power.

  3. (3)

    In the FJP of the mould core, the surface roughness Sa firstly decreases and then increases with the increase in feed rate and fluid pressure. The roughness increases first, then decreases, and tends to be stable with the increase in injection angle. There is no obvious relationship between the stand-off distance and the surface roughness. It was found that the FJP is effective to remove the debris and grinding marks left on the channel surface, improving the surface quality.

  4. (4)

    In the microinjection process of the polymer chips, the surface roughness and form accuracy of the microchannel decrease with the increase of melt temperature and holding time, while the effect of injection speed and holding pressure is gentle. The form accuracy of the microchannel reaches 8.594 μm, and the roughness of the microchannel is 0.027 μm.