The experimental conditions were chosen in order to test a variety of effects, such as the refill level, the refill portion and the powder’s properties on the overall performance of the feeding process (with a focus on the mass flow rate deviations and the feed factor behavior) in both the original and redesigned hoppers in the KT20 LIW twin-screw feeder. The configuration of the feeder was kept the same for all trials presented in this Section, which allowed a one-to-one comparison of the two hopper designs.

First, to exclude the effect of material properties, feeder configuration and process parameters on the LIW feeder’s performance, both hoppers were tested using the free-flowing, poorly compressible M200 powder (see Table 3). No feeding issues were expected due to the excellent flowability. The mass flow rate was set to 15 kg/h. 80% of the maximum hopper capacity were filled (corresponding to 12 kg, the maximum fill level in the M200 trials in Table 2). When the pre-defined minimum refill level (10 kg in the M200 trials, see Table 2) was reached, 2 kg refill portions were added manually to study the refill effect in the hoppers. The refill was repeated at frequent intervals to test the reproducibility.

The mass flow rate based on both the feeder and the catch-scale, the feeder’s net weight and the feed factor are shown in Fig. 3 for the original (a) and redesigned (b) hoppers. The feed factor was used in this study to compare changes in the powder’s bulk density in the hopper-screw interface zone of the original and redesigned hoppers (for the same feeder configurations and material). For instance, a feed factor curve with a significant gradient indicates greater density changes (powder densification in the screws) and a higher feed factor, higher stress (i.e., higher bulk density) on the particles in the hopper. The main goal of redesigning the hopper was to provide a constant bulk density in the feeder screws that would maintain the mass flow rate setpoint constant during the refills. The results showed a generally higher feed factor for M200 in the original hopper. Moreover, this hopper resulted in slightly higher mass flow rate deviations (see Table S1 and Table S2 in the supporting material) and a more pronounced start-up effect compared to the redesigned one, as can be clearly seen from the comparison of the feed factor curves.

Process data for M200 feeding: a) original hopper, b) redesigned hopper

A lower feed factor was observed for the redesigned hopper (Fig. 3-b) in comparison to the original hopper (Fig. 3-a) with low feed factor deviations from the start-up phase to the “steady-state” phase of feeding. In fact, the feed factor remains more or less constant throughout the feeding tests. Less feed factor fluctuations indicate less changes in powder bulk density in the screws. This is supported by a lower mass flow rate deviation during the start-up phase for the redesigned hopper.

A density change (densification) in the screws arises from the increased inter-particle stresses (since incoming material compresses the powder at the bottom) in the original hopper [13]. Therefore, the results show that the redesigned hopper was successful in homogenizing and reducing stresses on the particles in the hopper-screw interface zone.

The 2 kg refills were repeated at frequent intervals in both hoppers. The quick manual refills (6—7 s) increase the compressive forces on the particles in the hopper-screw interface zone since the powder height (and therefore weight) increases above the screws. Depending on the flow pattern in the hopper, increased material load leads to powder density changes (densification) in the screw. However, in the redesigned hopper, due to its unique design for homogenization of the stress on powder particles, this effect was mitigated, i.e., a lower feed factor deviation was observed. Based on the free-flowing nature of the used powder, the feed factor deviation can be considered representative for powder density changes in the feeder screws.

For better visibility, in Fig. 4, the average feed factor and average mass flow rate (calculated from the catch-scale signal) are compared for all feeding cycles of M200 in both original and redesigned hoppers. The results confirm that the mass flow rate and feed factor deviations (standard deviation shown as error bars in Fig. 4) were reduced in the redesigned hopper. The average feed factor is lower for the redesigned hopper in every feeding cycle. Again, this can be attributed to the reduced and homogenized stress applied on the hopper-screw interface, i.e., less powder is forced into the screw flights compared to the original hopper. Consequently, less mass flow rate and feed factor deviations arise in the case of the redesigned hopper.

Comparison of feeding performance of original and redesigned hoppers for all feeding cycles of M200: (top) average feed factor with standard deviation (error bars), (bottom) mass flow rate calculated from the catch-scale signal with standard deviation (error bars); The red dotted line is the mass flow rate setpoint (15 kg/h)

All in all, the average mass flow rate and standard deviation (SD) for M200 in both hoppers are in the acceptable range, but an improvement for the redesigned hopper especially during the start-up phase is apparent.

The mass flow sensitivity to powder refills was evaluated by feeding the blend (a compressible formulation, see Table 3). The effect of refill level and reproducibility were studied by executing multiple refills (manually) at various refill levels. The mass flow rate was set to 20 kg/h and refill levels were chosen to be 6, 8, and 10 kg with refill portions of 2 kg (see Table 2). For the lowest refill level of 6 kg, refill portions of 4 kg were added to the experimental plan in order to study an extremen case with regard to the effect of the refill portions on feeding performance. Feeding performance results are shown in Fig. 5 for the original (a) and redesigned (b) hoppers.

Process data of the blend feeding (compressible formulation): a) original hopper, b) redesigned hopper

In general, the highest mass flow rate and feed factor deviations occur when the hopper is refilled at the lowest refill level of 6 kg with the higher refill portions of 4 kg. Thus, it is recommended to refill the hopper more frequently at a higher refill level to reduce the deviations, as is also reported in the literature [18], since refill with higher refill portion increases the compressive forces on the powder at the bottom of the hopper in the screws. This leads to densification (increase in powder bulk density) in the screws and overfeeding since the feeder is operating in volumetric mode. After the refill ends, the gravimetric controller of the feeder takes action and adjusts the screw speeds to obtain the mass flow rate setpoint.

From Fig. 5 is can be clearly seen that the effect of refill level and refill portions on the feed factor variability is lower in the redesigned hopper than the original hopper. The feed factor is lower with smaller deviations for the redesigned hopper at all refill levels and refill portions comparing Fig. 5-a and Fig. 5-b. The most significant difference in feed factor deviations is at the lowest refill level of 6 kg (with both refill portions of 2 and 4 kg). In general, the feed factor shows start-up effects in both hoppers, but the start-up effect is less pronounced for the redesigned hopper.

Mass flow rate and feed factor deviations for both hoppers at each refill level are summarized in Table S1 and Table S2 in the supporting material for the original and redesigned hoppers, respectively. The results for each refill level are highly reproducible (RSD is low – 0.7–0.8% in mass flow rate on the catch-scale for the redesigned hopper). The bucket on the catch-scale must be changed during long feeding processes. The bucket changing introduces disturbances to the catch-scale signal. The results for feeding cycles where bucket changing happened are excluded from evaluation and shown as NA (not available) in the tables. The results show an overall good feeding performance for both hoppers. The actual average mass flow rate is close to the setpoint mass flow rate of 20 kg/h, and the deviation (RSD%) is below 2% for both the original and redesigned hoppers at each refill level. However, an improvement in feeding performance (both mass flow rate and feed factor deviations) for the trials with the redesigned hopper can be seen at each refill level.

Based on the measured data, the total dosed mass during refill (5 s before refill starts until 295 s after, since the refill effects persist for an extended amount of time) was calculated from the catch-scale net weight. During refill, the feeder operates in volumetric mode, where the screw rotational speed is kept constant. Hence, the feeding relies on volumetric displacement and not the powder weight loss in the feeder. Therefore, the bulk density of the powder is highly relevant during refill, and it should remain as constant as possible. The catch-scale results can be used for analyzing the feeding performance during refills and to determine the extent of deviations during the refill phase.

The results of total dosed mass during refill for the compressible formulation and the poor compressible material are summarized in Table 4. The dosed mass is compared to the theoretical or expected dose at the same time window for the respective MF setpoint and expressed in percentage deviation.

Data in Table 4 shows a slight overdosing for the compressible formulation with both hoppers. However, the deviation of dosed mass for feeding with the redesigned hopper is clearly smaller than the original hopper at all refill levels, except for 6–8 kg refill level (slightly higher). For the poorly compressible material M200, there is a slight underdosing (comparing the dosed mass to the expected dosed mass) for feeding with the original hopper. However, in this case, the deviations are minor. In general, the SD and deviation are more consistent for the redesigned hopper than the original one. The results confirm that the redesigned hopper minimizes the material density changes which may occur during refill events (at different refill levels and for different refill portions).