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29 March 2022

42 Pages

Computer Simulations of Injection Process of Elements Used in Electromechanical Devices

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1
Faculty of Mechanical Engineering and Computer Science, Czestochowa University of Technology, 42-201 Czestochowa, Poland
2
Faculty of Science and Technology, Jan Dlugosz University in Czestochowa, 42-200 Czestochowa, Poland
3
Faculty of Art, Technology and Communication, Vistula University, 02-787 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Materials2022, 15(7), 2511;https://doi.org/10.3390/ma15072511
This article belongs to the Topic Artificial Intelligence and Computational Methods: Modeling, Simulations and Optimization of Complex Systems
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Abstract

This paper presents the computer simulations of the injection process of elements used in electromechanical devices and an analysis of the impact of the injection molding process parameters on the quality of moldings. The study of the process was performed in Autodesk Simulation Moldflow Insight 2021. The setting of the injection process of the detail must be based on the material and process technological card data and knowledge of the injection molding machine work. The supervision of production quality in the case of injection moldings is based on the information and requirements received from the customer. The main goal of the analysis is to answer the question: how to properly set up the process of filling the mold cavities in order to meet the quality requirements of the presented molding. In this paper, the simulation was compared with the real process. It is extremely important to optimize the injection, including synchronizing all process parameters. Incorrectly selected values of the parameters may lead to product defects, leading to losses and destruction of raw materials, and unnecessary energy consumption connected with the process.

1. Introduction

The injection molding process is one of the most commonly used plastics processing technologies. It is carried out in injection molding machines and with such auxiliary devices as production thermostats, plastic dryers, grinders for grinding plastics, etc. Many papers studied primary molding conditions, from design creation to product manufacturing [1,2,3,4,5,6,7]. The injection process is influenced by various factors: properties of the material, wear of the mold, temperature fluctuations, parameters of the process [8,9,10,11,12]. Designers of processing machines introduce newer and newer solutions to increase the possibilities of regulating the injection process. In the injection molding process, plastics are processed in a plastic-fluid state. The most critical injection process parameters are temperature, pressure, and injection time. The selection of these parameters depends on the shape and size of the molded part, type and properties of the material used, the efficiency of the injection molding machine, and mold construction [13,14,15,16,17,18].
The material in the cylinder of the injection molding machine is heated thanks to heat supplied by the heating system, heat obtained as a result of frictional resistance of the material during rotation of the screw [9,19]. Mold temperature is the surface temperature of the mold cavities, and its range depends on the type of production and the type of material. In practice, it is assumed 60–120 °C in the production of crystalline materials, 10–20 °C in case of mass-products.
Mold temperature rise increases the crystallization of semicrystalline materials. It improves their usability, increases processing shrinkage and reduces uncontrolled secondary shrinkage, reduces the internal stress of the molded part, reduces collapses and cavities, increases the accuracy of the nest mapping, reduces flow resistance and pressure losses, extends the cooling time (approx. 2%/1 °C) [9,10].
Pressure changes in the injection mold and the accompanying pressure changes in the hydraulic system are shown in Figure 1. In the injection phase, the pressure in the hydraulic system (pHI) reaches its maximum value (approx. 10–14 MPa). There is, as a rule, pressure on the forehead of the snail ten times higher in this phase. This is called injection pressure [9,14,20,21].
Figure 1. The course of pressure changes in the mold during the injection cycle.
In the mold, during rapid flow, the pressure of the material changes depending on the flow resistance and is relatively low. However, immediately after filling the cavity, it directly increases. This growth compresses the material and is necessary for the complete filling of the cavity. On the other hand, too low injection pressure may cause shortcomings or temporary stoppages (supercooling) of the material in individual parts of the seats.
The surge in pressure increase can cause stresses of the molding, dimensional dispersion, reverse movement of the material to the cylinder, energy losses.
The only way to prevent an excessive pressure surge is to switch the pressure in the (pHI) hydraulic system to a lower pressure (pHII) at the time of compression. The switching point is the switching point (Ap).
In the injection molding cycle, there are several basic phases resulting from the specific features of the process (Figure 2): injection phase (5–6), pressure phase (6–7), plasticization phase (7–8).
Figure 2. Technological times diagram. Number 1–12 stagest of cycle.
The screw of the injection molding machine, when rotating, automatically moves backward, giving way to the plastic mass moving along its coils. In practice, its free sliding away is difficult because it is counteracted by damping the oil outflow from the working space of the cylinder. In this way, the so-called back pressure is called plasticizing or dispensing pressure. Its value sets the injection pressure at the level of approx. 0.1 [9,16,20].
When analyzing the economic aspects of the production of plastic moldings, it is stated that the essential component of the total production cycle time is the cooling time (Figure 3).
Figure 3. The structure of the most important technological times in the injection molding process.
Figure 4 presents a graph that determines the minimum time, the exceeding of which does not result in a noticeable increase in the mass of moldings. This time is taken as the optimal pressing time [9,21,22].
Figure 4. Weight method of determining the pressing time.
The p-v-T diagram illustrates the changes in the material’s properties during the injection process (Figure 5). The starting point is the variable state of the material in the mold, defined by changes in pressure p, temperature T, and flow velocity v [9,21,23].
Figure 5. p-v-T plot of polypropylene.
It can be concluded that the impact of pressure and the movement of the injection piston may increase the temperature of the material, there is a dependence of the internal pressure in the mold on the external pressure of injection for different temperatures of the material, as the injection temperature increases with constant external pressure, the internal pressure also increases, an increase in the external pressure causes an increase in the sealing pressure, an increase in mold temperature increases the time of the entire cycle.
The specific phenomena may occur in the sequence determined by the course of the cycle and the nature of the behavior of the material, both in the liquid state and during solidification in the mold space, as well as in the solid-state—as secondary shrinkage—for several hundred hours after removing the molded part from the mold [24].
Plastic injection molding is a cyclic process. The granular or powder material is plasticized in the cylinder of the injection molding machine by heat and then injected by a screw or piston into the molding cavities. There, the material solidifies at a reduced or increased temperature, maintaining the shape of the finished product—the molded part. After opening the mold, it is removed, and the process can be carried out again. This process is intended mainly for the processing of thermoplastics but is also used for the processing of thermo- and chemo-curable plastics. Injection molding is the basic process of manufacturing finished plastic products weighing from 0.01 g to 70 kg. Figure 6 shows a block diagram of the injection process parameters settings [9,16,20,21].
Figure 6. Block diagram of the injection process parameters setting.
A number of factors influence the quality of the molded parts: initial preparation of the raw material (drying), its quality (impurities), injection process parameters, the condition of the injection molding machine, level of its wear, arrangement of the hot runner system (cross-section of the channels, places where the material is deposited, etc.), arrangement of supply channels, the possibility of stopping the plugs from the nozzle, venting, etc., the share of pigments and dyes, the type of material being processed, preparation and knowledge of the personnel operating the injection molding machine and supervising the course of the technological process.
Table 1 presents the causes of mold defects and corrective actions aimed at correctly optimizing the injection process parameters [9,10,11,12,25,26,27].
Table 1. Reasons for defects in moldings and corrective actions to optimize the injection process.
The aim of the paper is to analyze the impact of the injection process parameters on the quality of the production carried out in Autodesk Simulation Moldflow Insight 2021, taking into account the technological data of the material and process as well as the operation of the injection molding machine. The main objective of the analysis is to determine the correct configuration of the process to meet the quality requirements of the presented molding, free from defects, presented in Table 1. The paper compares the simulation with the course of a real process and its technological conditions.

2. Organization of the Injection Process and Quality Control of Moldings

The quality assessment of the profile produced by the injection method is based on aesthetic control and visual and laboratory tests. The first visual assessment of the detail is made by the technologist when setting the production parameters of a given molded part. Most of the visible defects that may arise in setting up the process are quite easily noticeable, so any possible aesthetic defects that can be detected are corrected on an ongoing basis at the start of production. The injection molding machine operator can visually detect such defects as overspray, ejector marks, flash burns, material overheating, and the accompanying degradation of the material. These problems can usually be corrected by changing the injection parameters or the mold design.
Common problems that degrade the appearance and reduce the strength of moldings include color, craters, overexposure, traces of overheating, insufficient filling of the mold, joining lines in material [9,10,11,28].
The organization and management of injection molding processes are focused on the needs of a specific customer and their satisfaction with the ordered products. Their goal is to improve the effectiveness and efficiency of processes and the quality of moldings. The quality management system model is in the ISO 9001 standard [29].
The production of moldings can be consulted with the customer in accordance with the appropriate product production plan. When planning the implementation of the product, customer requirements and technical specifications are taken into account. Using the FMEA technique, hazard analysis is performed, trying to prevent errors in processes or defects in products. Acceptance criteria are defined for each phase of the process and can be approved by the customer. Based on information collected from the market, competition analysis, comparison with the best products available on the market, and contracts with suppliers of raw materials and with the customer, the quality requirements for products manufactured by the injection method are established. These requirements are based on technologies and applicable regulations. The requirements relate to the quality characteristics of products and the principles of product delivery and after-sales service. In the product/process development planning phase, the requirements not specified by the customer but necessary to ensure proper production or use of the product are defined. In a justified situation, compliance with the customer’s requirements for defining, documenting, and keeping “special characteristics” under control should be ensured. In the case of changes in the requirements/contract, the Quality Management System deals with the transfer of information to the departments affected by the changes.
Product realization phases, which include product and process design, are carried out through a multidisciplinary approach that, when necessary, ensures the involvement of different departments of the company. The interdisciplinary approach allows for the effective: definition, implementation, and tracking of unique characteristics, FMEA development, development, and review of control plans.
The input data for the design of the process are product design output data, productivity goals, process capability and costs, customer requirements, if any, and experiences from previous studies.
Production process design outputs are expressed in a way that can be compared with process design outputs and include process FMEA, drawings and specifications, process layout, control plan, work instructions, process validation criteria and objectives for quality, reliability, maintainability, analysis of measurement systems, error prevention and rapid measurement methods and response to non-compliance in the production process.
Information (drawings, specifications), appropriate production and measurement equipment, work instructions and controls ensure that the planning conditions for production activities are kept under control.
The devices that guarantee smooth production and the appropriate quality of the injected moldings include installation of the so-called chilled water (installation for cooling injection molds, injection molding machines and peripheral devices), mold thermo-regulating devices, plastic grinders, dye dosing devices, robots collecting profile from the injection mold. Figure 7 presents the Krauss-Maffei injection molding machine.
Figure 7. Krauss-Maffei injection molding machine.
The production process requires constant supervision and quality control of products in relation to dimensional tolerance, dimensional stability (SPC), shape (without deformations and sags), workpiece weight tolerance, deformation strength, specific properties in terms of the assumed features of the detailed structure [9,13,14,15,17].

3. Analysis of the Injection Process of Detail: A Flowmeter Using the Simulation Analysis of the Injection Process in Moldflow

Having already constructed a tool, an injection mold, and the task of launching serial production of molded parts—the main goal of the analysis is to answer the questions: how to properly set up the process of filling the mold cavities in order to meet the quality requirements of the presented molding (Figure 8). The analysis was performed in Autodesk Simulation Moldflow Insight 2021.
Figure 8. An example of the construction of a plastic feed system in an injection mold.
The assumption of the analysis is to obtain a balanced pattern of filling the cavities of the injection mold so as to select the appropriate parameters of the injection process and obtain a satisfactory quality of the molded part. The work includes analysis of the manufactured detail—housing of the flow meter of household appliances. Table 2 presents a summary of the detailed manufacturing parameters.
Table 2. A detailed description of the parameters.
The p-v-T diagram for the polymer material used is a condensed presentation of the interrelationships of three variables that affect the processing of the polymer: pressure, volume and temperature. The viscosity—shear rate relationship and p-v-T diagrams are illustrated in Figure 9, for amorphous and crystalline polymers.
Figure 9. Viscosity—shear rate relationship (a) and p-v-T diagrams (b).
In [30], the used material has the technological card. Image of the pressure distribution in the seat at the time of filling of 98% is presented in Figure 10.
Figure 10. Image of the pressure distribution in the seat at the time of filling of 98%.
Figure 11 shows the clamping force calculation. The clamping force is a function of the injection pressure and the protruding seat surface. A well-calculated clamping force should show that the maximum clamping force is lower than the maximum clamping force of the injection molding machine and should not exceed about 75% of the machine’s limit so that the remaining 25% is a safety factor. The requirements of the clamping force usually increase during the pressing of the seat, and therefore, when selecting the injection molding machine, this safety factor should always be taken into account.
Figure 11. Clamping force calculation.
Figure 12 shows visible flow fronts. This type of surface makes it easier to identify the swinging areas and highlights the areas where the streams meet. This analysis helps assess, for example, the correctness of the location of points initiating injection into the area of the mold cavity. The image helps to determine whether the distance of initiation points is not too far apart, which is certainly of great importance in setting the injection process and affects not only the aesthetics of the molded part but also results in the variability of the functional properties of the part and the quality of the manufactured product.
Figure 12. Plastic flow diagrams in the form—visible flow fronts.
Figure 13, Figure 14 and Figure 15 present the image of pressure distribution in the seat at the time of filling of 98%, the injection pressure course over time, and the clamping force calculation.
Figure 13. Image of pressure distribution in the seat at the time of filling of 98%.
Figure 14. Injection pressure course over time.
Figure 15. Clamping force calculation.
Figure 16 shows the temperature of the polymer when the flow front reaches a certain point (made at the center of the cross-section). The visible difference between the front temperature and the supply temperature by approx. 30 °C.
Figure 16. The temperature of the plastic flow front through the socket.
Another analysis performed using Moldflow shows the so-called weld lines—areas (lines) of joining the plastic stream that forms at the meeting angle (θ) from 135 degrees or less. Such a line is very disadvantageous for moldings, not only because of the visual characteristics of the molded part but also because of the weakening of the bonding joint. Figure 17 shows the areas where the streams meet. The color of the welding lines indicates the temperature at which they are formed. Colder temperatures may indicate areas of the difficult seam, which in turn will affect the appearance of the weld line. The strength of the colder seam lines may also be lower than at locations with higher seam line temperatures. The length of the entire weld connecting the lines should be kept to a minimum. Where possible, the injection path should be designed to prevent such areas from forming (Figure 17).
Figure 17. Temperature connection points of streams.
Marked areas indicate the possibility of creating the so-called siphon location (Figure 18). The air which is localized in this place should be eliminated.
Figure 18. The so-called siphons.
Shear rate measures how quickly layers of plastic slide past each other. If this happens too quickly, the polymer chains can break, and the material degrades. The higher shear rate should not exceed the maximum value recommended by the material supplier, and exceeding this value is likely to lead to polymer degradation. It is a good rule not to exceed 60% of the value recommended by the supplier for aesthetic applications.
In the analysis below (Figure 19), you can see a slight difference in the shear rate.
Figure 19. Shear rate.
The behavior of thermoplastics when applying or removing heat is completely different from the known and well-established behavior of other materials. The processing shrinkage of polymers is divided according to several criteria. Due to the time and place of its formation, processing contraction is divided into primary and secondary contraction. Primary shrinkage is understood as reducing the product’s dimensions during its cooling (for thermoplastics) or curing (for hardenable plastics) in the forming cavity of the processing tool and shortly after leaving it. The assumed time of ending the primary contraction is 16 h (Figure 20).
Figure 20. Volume shrinkage of tested moldings of the flow meter.
Shrinkage values should be uniform throughout the profile. For example, for a tool that has 0.8% shrinkage, the volumetric shrinkage is 0.8% × 3 = 2.4%. This is essential for good material packing, ensuring good structural and visual integrity of the parts. If the value is greater than three times, it can lead to undesirable deformation of the part and should be assessed in more detail in the warpage analysis.
The above analysis assumed the processing conditions (temperature, pressure with the switching point to the pressure). Considering only the data on the diameter of the plasticizing system nozzle, it is visible how the injection profile of the selected compact changes. The very distinct and noticeable change in flow rate affects the entire shape and illustrates the nest filling processes. It is also impossible not to notice that the very design of the mold, the construction raises minor reservations—the location of the nozzle opening points in the injection mold seat provides many difficulties in setting the correct parameters for the injection process of such a complex molding.
The maximum relative errors between simulation and experimental data are as follows: clamping force 5%, plastic flow in the mold 3%, pressure distribution in the cavity during filling, injection pressure 3%, Pressure drop in the mold 3%, shrinkage 1.5%.
Difficulties in setting and stabilizing the mold injection process, where two moldings have a different shape, are added because the mold is 3-part, and the design of the flow system through the mold can cause considerable difficulties for people who are not experienced in such designs. Therefore, when analyzing the collected materials, it is necessary to thoroughly review the collected data, which will significantly facilitate the assessment of the situation before starting to set the parameters of the injection process of the above-mentioned part.
When setting up the detailed injection process, you also need to consider the order of the material feed channels opening. The practice also shows that not everything can be predicted in setting the parameters on the machine, but very often, they differ from the adopted data for analysis.
Setting the process of injection of the detail must be based on the data provided in the form of material, technological card, knowledge of the injection molding machine, the so-called setting parameters of devices associated with the process (thermostats, heated channel regulators, etc.). Setting up the process also requires familiarization with the construction of the mold, the construction of the cooling system, and hot channels. When initiating a trial batch, the parameters are saved in the so-called memory injection parameter cards (Figure 21).
Figure 21. Injection parameters card—moldings of the body and cover of the flowmeter.
Supervision of production quality, in this case, injection moldings, is based on the information and requirements received from the customer. Information (drawings, specifications), requirements for instrumentation, production and measurement conditions, work instructions and controls, ensuring the correct course of all production activities play an essential role. For each product a production control plan is developed. The control plan includes output from the product and process FMEA, control of unique characteristics, and response plans when the process becomes unstable. The injection molding product control plan is a living document, i.e., it is updated in the event of changes in the process, product, and in the event of major changes affecting the process capability. Table 3 shows the activities and scope compliant with the control plan.
Table 3. Image illustrating the flowmeter product inspection plan.
An important task of the control plan is to fully and clearly express all activities aimed at appropriate verification and control of profile to obtain and maintain customer satisfaction in the end. A product that is new, for example, must have full dimensional documentation of the detail-molded part so that it is possible to determine and make a decision on whether to accept it for serial production. Meanwhile, at each subsequent launch of the production series (according to the order), the dimensional control plan covers only important and essential dimensions, the non-compliance of which may indicate errors in the injection process or, e.g., damage to the injection mold tool, which results in the cessation of production. The restart may take place after eliminating the cause of the incompatibility.
Table 4 and Table 5 and Figure 22, Figure 23, Figure 24, Figure 25, Figure 26, Figure 27, Figure 28 and Figure 29 show the self-check instruction and process.
Table 4. GAA analysis—stocks in closed status.
Table 5. GAA analysis—actions in open status—no actions were taken.
Figure 22. XR card—illustrating the production process of the flow meter detail.
Figure 23. Defect analysis image.
Figure 24. The image showing the parameters adjustment—the effect of reducing the weight of the part.
Figure 25. Image showing the onset of injection instability.
Figure 26. Histogram showing an anomaly in the process.
Figure 27. Histogram showing the consistent process.
Figure 28. Histogram showing an anomaly in the process.
Figure 29. Distribution histogram for single values.
Another important document for the correct assessment of the injection-molded parts is the card called the “Self-Test Manual”. It is a working instruction intended for operators responsible for producing a given detail. These instructions are closely related to the control plans for a specifically defined molding with other documents of the production process used to assess the manufactured production correctly. Instructions are available at the workstation.
The “Self-inspection” manual contains guidelines for the injection molding machine operator, whose duty is to inspect the workpiece visually. The essence of the instructions mentioned above is:
  • determination of important areas of visual inspection (i.e., marking areas where the non-compliance has occurred or could occur in the future, e.g., the aesthetically unacceptable line connecting the streams of material, tears caused by improper setting of the process of releasing the detail from the mold cavity—traces of ejectors in difficult points, location of the area formulated by hydraulic cores, location of gas traps, etc.),
  • determination of the amount of inspection frequency, which is important, for example, when injection molding parts with low grammage and usually multiple cavities, and a fairly fast cycle. Then the detail does not require 100% inspection. Of course, when inspecting a detail during the process, we analyze a set of working sockets
  • determination of the method and means of control (e.g., only visual inspection or the necessity to use some kind of test).
The above graphs show the beginnings of the process instability. You can also see fluctuations until the parameters and conditions of processing are stabilized. From the moment of changing the parameters, the process has been stabilized, which is shown in the image of the above graph and the histogram of individual values in Figure 24, Figure 25, Figure 26 and Figure 27.

4. Conclusions

The molding production process requires the knowledge, precision, and strong commitment of a team of people who, using appropriate devices, can ensure the quality of the required product. It is extremely important to optimize the injection, including synchronizing all process parameters. Incorrectly selecting their values may lead to product defects, leading to losses and destruction of raw materials. Behind the production, success requires good company organization, employee competencies and control, and product verification. The study assessed the injection process on the example of producing a specific detail. The research part presents, on a specific example, the injection process, its course, and, consequently, its impact on the quality of the presented production. A study based on simulations of the injection process was shown, and the simulation was compared with the real process.

Author Contributions

Conceptualization, A.G., A.K.-K. and P.L.; methodology, A.G., A.K.-K. and P.L.; formal analysis, A.G., A.K.-K. and J.K.; investigation, A.G., A.K.-K., E.K. and P.L.; resources, A.G. and P.L.; writing—original draft preparation, A.G., A.K.-K. and P.L.; visualization, A.G., A.K.-K., E.K., P.L. and J.K.; supervision, A.G., A.K.-K., E.K., P.L. and J.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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