08/08/2022 - Injection molding under clean room conditions is playing an increasingly important role today.Different solutions are already available on the market to meet the increasing requirements of different industries and technical regulations.Recently, special attention has also been paid to the use of technical and high-temperature plastics.Especially with plastics with high mold temperatures, the thermal effects in the mold area can have a negative impact on the clean room flow.As a result, particles are distributed in an uncontrolled manner and a corresponding purity of the product cannot be guaranteed.Within the framework of comprehensive investigations, these effects were analyzed and solutions were developed.In VDI 2083, clean room technology is described as a "chain of all measures to reduce or prevent unwanted influences on the product or people" [1].In the injection molding process, we usually refer to the protection of the product.This can e.g.B. packaging (sterile or non-sterile) in the medical, pharmaceutical or food sector, components with high demands on the surface technology as is common in the automotive industry, optical components or technical medical products.In order to ensure different degrees of cleanliness depending on the requirement, the clean rooms are divided into different air cleanliness classes according to DIN EN ISO 14644-1.The purity of the classes is defined by the maximum number of particles of different particle sizes (see figure below).Air cleanliness classes according to DIN EN ISO 14644-1 [2] © TH RosenheimThe lowest class (ISO class 1) has the lowest particle load and therefore the highest purity.Just ten particles with a diameter of 0.1 µm are permitted per m³.In the least pure ISO class 9, particles smaller than 0.5 µm are not even considered.The only particle class in which particles of all relevant sizes are measured is ISO class 6. This class also reflects a technical transition area in clean room technology [3].In the less pure ISO classes 6 to 9, turbulent dilution flow (TVS) is mostly encountered.These clean rooms (with TVS) are characterized by individual twist outlets for the inflow of clean air.The clean air is mixed with the room air by the twist outlet, resulting in an inhomogeneous velocity profile with local flow reversal.The maximum flow rate is primarily limited by the thermal comfort of the personnel.From ISO class 1 to ISO class 6, clean rooms are designed in the form of a low-turbulence displacement flow (TAV).Clean rooms with TAV have a comprehensive inflow to enable a quasi-laminar flow.This requires a significantly higher effort, both financially for the clean room purchase and in operation, but leads to a significantly higher level of cleanliness.Mixed concepts are also used in the transition area between the clean room types and as a special form.In the injection molding area, clean rooms of ISO classes 7 or higher (less clean), i.e. with turbulent dilution flow, are mainly used [4].In addition to the specifications of DIN EN ISO 14644, the requirements of "Good Manufacturing Practice Medicinal Products for Human and Veterinary Use" (GMP for short) must also be met in the medical sector.In it, clean rooms are divided into four classes (A, B, C and D).Class A is for high-risk activities such as making aseptic connections.An air flow that is as laminar as possible is required here.Class A areas are often implemented as individual workstations for which Class B is defined as background environments.Classes C and D are defined as work areas for the less critical processes.The specified limit values ​​for airborne particles of the individual classes can be assigned to the air purity classes according to DIN EN ISO 14644-1 (see table).For clean rooms in the injection molding area, in addition to the normal requirements for a clean room, there is also the problem of integrating the injection molding machine (SGM).Various solutions are currently being implemented (see figure below).Machine installation variants for connecting the injection molding machine to a clean room with turbulent dilution flow.Pure areas are highlighted in green.The air flow is indicated with black arrows, thermal effects with red arrows.© TH RosenheimThe variant that is technically easiest to implement is the so-called "machine-in-room" variant.The injection molding machine is placed completely in the clean room.The advantage of this variant, in addition to the simple implementation, is the handling of the sprayed product.The product can be further processed directly in the clean room.However, the waste heat from the injection molding machine is expensive and energetically unfavorable.On the one hand, thermal effects can influence the clean room flow, on the other hand, the additional heat load must be dissipated by the air conditioning of the clean room.At the same time, the injection molding machine is an enormous source of particles.In order to reduce the entry of particles through the injection molding machine, these are increasingly connected to the clean room in such a way that only the closing area is in the clean room.With the "machine-in-room" variant, machine maintenance and tool changes usually have to be carried out in the clean room, which represents a further risk of contamination and is therefore significantly more complex than outside the clean room.In some clean rooms, however, the machines can be completely removed from the clean room for maintenance using elaborately installed rail systems.With the so-called "outside drop" variant, this disadvantage does not apply.The machine is completely outside the clean room.In order to still guarantee cleanroom-compatible production, the mold area is equipped with special filter fan units (FFU), also known as laminar flow modules (LFM).This means that the mold area is flown through with hall air that has been filtered to clean room level.Special connections to a clean room are required for further handling of the molded part in order to rule out particle contamination during part transport.If higher degrees of cleanliness are necessary or if there are special requirements for a component, a combination of the variants shown is also used.With the so-called "room-in-room" machine installation, the machine is located in the clean room - either completely or only the tool area - and is also equipped with an FFU.Here, too, machine maintenance and tool changes are complex, but higher levels of cleanliness can be achieved in the tool area than in the rest of the clean room.Tab. summarizes the advantages and disadvantages of the individual variants.With all installation variants, the waste heat from the injection molding tool causes additional difficulties.Since the size of this is not known and is difficult to determine, ventilation and air conditioning systems must be dimensioned with a safety factor.This leads to an oversizing of the plants with an associated plant operation outside the optimal range.With increasing processing and mold temperature of the plastic, the influence of the waste heat on the clean room flow also increases.The air in the immediate vicinity of the tool is heated and forced upwards by thermal buoyancy.The thermal buoyancy directly counteracts the clean room flow defined downwards.If the thermal buoyancy exceeds the flow of the clean room or the FFU, a chimney effect occurs and the particle-laden air from the tool area flows upwards and is distributed uncontrolled in the clean room.Both Khalid according to Oberauer [6] and Schöngruber [7] defined a mold temperature of 40 °C as the limit.From this temperature, according to her statement, the clean room flow in the “machine-in-room” variant is no longer sufficient to ensure that the mold area has clean air flowing through it.The aforementioned filter fan units can help here.Schöngruber [7] thus established a uniform flow through the mold area at mold temperatures of up to 90 °C.The following two hypotheses can be derived from this:These were checked as part of a research project at the Technical University of Rosenheim using detailed flow analyses.The results are presented belowClean room at the Technical University of RosenheimFor a better understanding of the tests carried out, the test environment, the clean room of the Technical University of Rosenheim, is first described.This was built in 2010 and has been implemented as a "machine-in-room" clean room.It is designed as an ISO class 7 clean room and accordingly equipped with a turbulent dilution flow.The ventilation system is designed for an air exchange rate of 36 h-1 at a clean room overpressure of 22 Pa.The Engel VC200/80 injection molding machine was also equipped with a filter fan unit.This allows different installation variants to be examined: the "machine-in-room" variant with the FFU open, the "room-in-room" variant with the FFU closed and the outside-drop variant by deactivating the clean room and opening the doors .The clean room also has an extensive monitoring system for monitoring the operating data.In addition to the relevant climate data of the clean room (air temperature and humidity) and the overpressure in the clean room, it is possible to continuously monitor the particle concentration.The ventilation concept is designed as a recirculation concept with a total volume flow of supply air of 6,200 m³/h and a proportion of fresh air of 1,000 m³/h.The supply air is introduced horizontally into the clean room via eight ceiling twist outlets.The personnel and material locks are each supplied with clean air via overflow openings.The air is extracted at seven exhaust air openings, five of which are in the clean room and one in each of the two locks.In clean rooms with low-turbulence displacement flow (TAV), the air flows evenly over the entire ceiling surface into the clean room.This results in a quasi-laminar flow pattern in the entire room.It is often assumed that the clean rooms with turbulent dilution flow (TVS) relevant to injection molding also have a uniform flow.However, since the air does not flow over the entire ceiling surface, but only through individual twist outlets, the result is a more complex flow pattern that is difficult to visualize.Due to the turbulent flow, fog evaporates too quickly in the visualization to be able to make statements about the overall flow in the room.On the other hand, selective visualizations for individual areas are very possible.In order to still get a visualization of the room flow, a flow simulation (CFD) was carried out.This could be validated using selective visualizations and speed measurements in critical areas.Since initially only the flow in the clean room without thermal effects is to be examined, the machine is at a standstill.The TVS is clearly visible in the sectional view of the flow vectors.There are both upward and downward currents.In the tool area there is a turbulent flow without a uniform direction.Various methods are used to analyze the flow conditions in the area of ​​the injection molding tool.The absolute flow velocity is determined with flow sensors (thermoanemometer).With so-called bi-directional flow sensors it is also possible to determine the flow direction.These sensors are used to monitor whether there is an upward or downward flow.A fog generator is usually used to visualize the air flow.Most fog generators are operated with a special fog fluid.However, since these fog fluids mean an additional particle entry in the clean room, fog generators based on liquid nitrogen or water (distilled water) are recommended for use in the clean room.All fog generators have in common that it is not possible to examine the current without influencing it.The fog is conveyed out of the generator by a fan and, depending on the orientation of the fog opening, already has its own direction of flow.In addition, the fog has a different density than the surrounding air.Especially in the area of ​​thermal lift, where small differences in density in the air are to be examined, misinterpretations can arise due to the influence of the fog.In order to visualize the air flow without the disturbing influence of the fog, another measuring method, the so-called Schlieren photography, is used.With Schlieren photography, density gradients in the air are visualized.Since this is an optical process, it enables visualization without influencing the flow.The basic structure consists of a point light source that is positioned slightly offset in the focal point of a parabolic mirror.This creates a focused point of light next to the point light source.A refractive edge serves as a "filter" and is placed in such a way that part of the point of light hits the "filter".The remaining light behind the refraction edge is captured by a camera.If there is a change in the refractive index of the air (e.g. density differences due to thermal effects), light rays that would get past the "filter" are deflected into the "filter" and create dark streaks in the image.With those rays of light that get through the refraction through the "filter", bright spots appear in the image.This enables the visual representation of density differences and air flows in real time without influencing the flow.The larger the mirror used, the larger the area that can be examined.A parabolic mirror with a diameter of 400 mm is used for the measurement setup.This size enables the flow in the area of ​​one half of the mold to be visualized and can be flexibly integrated into the mold area.If the mirror is placed in the tool area, there is not enough space in the clean room to implement the structure as described.To save space, the light has to be deflected on its way to the parabolic mirror and back.For this purpose, a plane mirror is placed next to the machine at an angle of 45° to the parabolic mirror.The figure on the right shows a sketch of the structure of the streaks in the clean room of the Technical University of Rosenheim.Left: Flow visualization using Schlieren photography using a tea light as an example.Right: Top view of the Schlieren system in the tool area of ​​an injection molding machine: (1) Point light source (2) Tool area (3) Concave mirror (4) Refractive edge (5) Camera (6) Plane mirror for deflection © TH RosenheimFirst, the flow in the tool area is considered analogous to the room flow without thermal effects.In order to be able to assess the other types of machine installation in addition to the "machine-in-room" variant, the measurements are carried out both with and without a filter fan unit (FFU).The mist is introduced horizontally over the tool from the nozzle side.Thermoanemometers are installed centrally between the mold halves to determine the flow rate.With FFU, the speed is 0.4 m/s.The influence of the fog is taken into account.The figure below clearly shows how the mist is discharged downwards between the opened mold halves.If the FFU is omitted, the flow is turbulent.The mist spreads throughout the tool area.The measured flow velocities vary in the range from 0 m/s to 0.4 m/s.During the measurement period (120 s), a flow reversal was recorded several times.Such a turbulent flow without heat input from the tool suggests that the flow through the tool area is not uniform regardless of the operating state.The hypothesis formulated in the problem statement "at low mold temperatures, the clean room flow is sufficient to flush the mold area with clean air in the "machine-in-room" variant" can be refuted in this case.Nebula visualizations in the tool area with filter fan unit (left) and without (right).The visualizations were carried out without temperature control and with the machine at a standstill.© TH RosenheimTo assess the thermal influences, the tool with active temperature control is considered.In order to visualize the air flow without the disturbing influence of the fog, Schlieren photography is used here.Figure below shows the results at a mold temperature of 90°C.In the "machine-in-room" variant (without FFU), the air rises straight up, driven by convection on the tool surface, and spreads uncontrolled in the clean room.If an FFU with a speed of 0.45 m/s is used, a mixed flow pattern appears.The air rises, but is pushed aside by the FFU flow.This results in a highly turbulent mixed flow.If the flow rate of the FFU is increased to 0.8 m/s, the air is directed downwards.Here the flow of the FFU is the dominant flow.The limit temperature for the FFU of 90 °C defined in the hypothesis can be exceeded at increased flow rates.Schlieren photograph of flow at the top edge of the nozzle side of the tool.The mold temperature is 90 °C.The red arrow shows the direction of flow © TH RosenheimThe tool movement is considered as a further factor influencing the air flow.For this purpose, the machine is running dry, the plasticizing unit is not active and unheated.Due to the vibrations caused by the machine movement, a view using Schlieren photography is unfortunately not possible here.Mist is introduced horizontally over the nozzle side.Bi-directional thermal anemometers are mounted on the tool to monitor the pressure and suction effects occurring.These effects only occur in the immediate vicinity of the tool and cannot be seen with fog visualizations.Their influence on the overall flow was therefore considered to be negligible.The visualizations with fog show an image analogous to the results of Schlieren photography.Without FFU, there is a continuous upward flow.Due to the larger heat transfer surface, this is particularly pronounced when the mold is open.When the FFU is active, the air is directed downwards independently of the tool movement.According to the results of the investigations with Schlieren photography, the FFU is operated with a flow rate of 0.8 m/s.During the tool movement, no separate influence can be determined for either of the two variants.Mist visualization when the nozzle side is running dry, viewing direction from the extraction side.The mold temperature is 100 °C.Effects on energy consumption © TH RosenheimEnergy consumption measurements in the clean room at the TH Rosenheim have shown that energy savings can be achieved by using an FFU compared to the machine-in-room variant.Since the critical mold area is supplied with clean air by the FFU, the number of air changes in the clean room can be reduced.In the clean room of the Technical University of Rosenheim, the possible energy savings by reducing the fan output were examined as an example.The control of the clean room only allows a minimum air exchange rate of 34 h-1, the standard recommends a minimum air exchange rate of 30 h-1 for ISO class 7 clean rooms.The results of the measurements can be seen in Fig. 9.If you reduce the number of air exchanges from 36 to 32 h-1, the power consumption is reduced from 2,460 W to 1,580 W. If you also take into account the power consumption of the FFU of 140 W, this results in an energy saving of 30%.Power consumption of the fan depending on the number of air changes © TH RosenheimConclusion It can be deduced from the measurements that a filter fan unit (FFU) above the mold area is always recommended, regardless of the machine installation variant.The flow velocity should be at least 0.8 m/s and the outflow opening of the FFU should span the entire tool area in order to achieve a flow that is as uniform as possible.In existing systems, the FFU can also contribute to a reduction in energy consumption.Since the critical tool area is supplied with clean air by the FFU, the number of air changes in the clean room can be reduced.In the clean room at Rosenheim University of Applied Sciences, the ventilation system required 30% less power.The investigations have also shown that there is no "best" machine installation variant.It should always be checked on a case-by-case basis which variant can be implemented most effectively and cost-effectively.The hypothesis "at low mold temperatures, the clean room flow is sufficient to flush the mold area with clean air in the "machine-in-room" variant" could be refuted.The limit temperature specified in the second hypothesis "Filter fan units ensure an even flow through the mold area at mold temperatures of up to 90 °C" could also be exceeded with an increased flow rate.With a flow rate of 0.8 m/s, a complete flow took place.However, since the investigations were only carried out on one tool, no new limit temperature for the FFU could be specified.Authors Stephan Puntigam and Prof. Dipl.-Ing.Peter Karlinger, Technical University of Rosenheim, Germany⇒ Acknowledgments The results were obtained as part of the HTempRe research project (funding reference ZF4383603WO7) at the TH Rosenheim.We would like to thank you for the support from the AIF and the Central Innovation Program for SMEs - ZIM.We would also like to thank our industrial cooperation partner, Petek Reinraumtechnik GmbH, for the intensive and successful cooperation, and Engel Austria GmbH for support with the system technology.Current topics from the process and process industryOpinion barometer for the chemical industry> CHEMonitor - All outputsAdvertising opportunities Print, digital & content solutionsCurrent topics from the process and process industryOpinion barometer for the chemical industry> CHEMonitor - All outputsAdvertising opportunities Print, digital & content solutionsWell informed every week - subscribe to the CHEManager newsletter here!