Scientific Molding Thinking: Molding is a Science not an Art

Introduction: Why do We Introduce Scientific Molding?

Injection molding is the process of converting plastic materials into flowable molten plastic materials by an injection molding machine, and injecting them into a mold cavity where they are cooled and solidified to form the final product. Plastic materials are less expensive to obtain compared with other materials such as metal, wood, and ceramics. In addition, it is easy to produce complex geometric products in large quantities with plastic materials, so people constantly extend the application of plastic products to various types of products. However, the complex flow behavior of plastics due to their viscoelastic property has led to many specious and erroneous experiences in injection molding, which may easily mislead the direction of problem solving.

Injection molding is a science, not an art. Molding conditions cannot be set based on feelings or experiences. There should be a systematized set of knowledge, so that each implementation step is completed with an objective basis, not based on subjective determinations. This systematized knowledge is scientific molding. Scientific molding system is centered on learning key molding principles and theories, which covers raw materials, part structure, molds, equipment and molding conditions needed for stable molding operation mechanism. The strategic application and management of this system is systematic molding which can shorten the time needed to build a stable and repeatable molding process, resulting in increased machine operating time, product production time, scrap rate and so on. A scientific molding system can simplify the complex things and repeat the simple things (standardized).

When molders are able to master the scientific molding system, they can use this system to establish the process of molding qualification. Molding qualification is a process that includes "establish a plan, obtain information, record results, interpret data". This process is divided into three stages: IQ, OQ and PQ (known as 3Q*). According to the scientific molding system and molding qualification process, a data collection system can be used to establish and implement efficient and stable molding operations data management and confirmation, which is the foundation for AI molding data application.

*Note: IQ, OQ and PQ (3Q) are the English abbreviations of Installation Qualification, Operation Qualification, and Performance Qualification respectively.

In the mass production process of injection molding, many controllable and uncontrollable factors may change the melt viscosity, resulting in the variation in molding quality. For example, when the same manufacturer produces plastics of the same type but in different batches, the viscosity variation may reach ±10~20% change. Therefore, melt viscosity stability control is particularly critical. This paper will focus on "segmented molding condition setting technique and melt viscosity stability control", which are the core foundation of the scientific molding system. Segmented molding condition setting technique utilizes the characteristics of "plastic rheological shear thinning" and special separation/disconnection techniques to plan the molding conditions for the machine to effectively perform operations in each stage (including filling, pressure-holding, feeding/cooling and other molding processes). This helps to reduce the influence of the results of the condition settings and the variations in melt viscosity fluctuations. Figure 1 illustrates the relationship between the key stages of scientific molding and the injection molding conditions and setting logic.

Figure 1. Setting Conditions for Key Stages of Injection Molding

As people say, quality is "designed and built", it does not come from inspection! Does the quality generated by our injection molding condition settings come from design or from inspection?

Influence of Melt Temperature on Viscosity Variation

Each plastic has a temperature range recommended by the material supplier for the melting process. In order to reduce the influence of melt temperature on viscosity fluctuation, a reasonable value to be set in this range can be selected to complete filling, resist disturbances and achieve stable molding at the same time. As shown in Figure 2, in the molding process, the higher the melt temperature, the higher the MI value and the better the flow characteristics. When the melt temperature is set at a higher level, the slightest temperature variation during mass production will bring about drastic viscosity changes, which is not conducive to stable molding in the long term. Therefore, if the melt temperature is set in the middle of the range recommended by the material manufacturer, stable viscosity variations can be achieved, but the disadvantage is that the melt flow deteriorates.

Figure 2. The Relationship Between Melt Temperature and Flow Characteristics

Increased Filling Speed is Conducive to Stable Mass Production

For stable and long-term production, while lowering the melt temperature may result in deteriorated plastic flow, it can be improved by adjusting the "filling rate".

In Figure 3, for example, the viscosity change (η1) obtained for variation of this plastic in the recommended processing temperature range of 200~230°C is much lower than the viscosity change (η2) due to shear rate variation. In other words, it is more effective to vary the shear rate (i.e., filling rate) than to vary the temperature in order to obtain high flow characteristics of the melt.

Figure 3. Relationship Between Shear Rate, Temperature and Viscosity

As show in Figure 4, during mass production, increasing the melt filling speed not only achieves better flow characteristics, but also allows the melt flow wavefront to maintain a stable viscosity when the speed changes(*). Therefore, as long as the injection pressure is sufficient, when you increase the filling speed to be set, the viscosity will not be drastically varied, and it also solves the problem of high viscosity due to lower melt temperature setting that is unfavorable for filling.

*Note: It should be noted that a high resistance in flow wavefront may disrupt this phenomenon.

Figure 4. Relationship Between Melt Filling Velocity and Flow Characteristics

Summary: Filling speed has much greater influence on viscosity than melt temperature. Therefore, increasing the filling speed can make up for the poor flow of plastic caused by low melt temperature, which is favorable to the long-term stable production.

Reduced Loss of Transfer Pressure May Reduce Defective Products

At the end of the plastic filling stage, the low viscosity of the melt also reduces the pressure transfer drop in the pressure-holding stage to make the internal pressure of the melt uniform; on the contrary, if the viscosity of the melt is high, the plastic does not flow well and the high pressure transfer drop causes uneven shrinkage, which is prone to cause problems such as warping and deformation of the product.

Summary: Under scientific molding conditions, low melt temperature + high filling speed + sufficient filling pressure can reduce the loss of pressure transfer, so that the product can obtain better shrinkage uniformity during the pressure-holding stage.

Increasing Mold Temperature Improves Dimensional Stability of the Product

In general, a low mold temperature helps to shorten the molding cycle length, but the mechanical properties and dimensional stability of the product after ejection are not ideal, thus the quality tested before shipment may be a temporary illusion. On the contrary, if the mold temperature is set at a high level, recrystallization and residual internal stresses can be avoided, and the mechanical properties and dimensional stability of the product can be greatly improved after ejection.

Influence of Mold and Melt Temperature Setting and Cycle Length

In the relationship between mold temperature and melt temperature, traditional molders would usually set lower mold temperature and higher melt temperature to control the molding cycle length of the product. However, according to the logic of scientific molding, high mold temperature combined with low melt temperature can achieve correct mechanical properties and dimensional stability. Although it seems that the cooling time will be increased, from the time consumed by the heat removal, under the same cooling efficiency, it takes less time to remove heat for the scientific molding temperature setting than the traditional temperature setting. Therefore, the overall molding cycle may not increase, and even all the conditions are set in line with the characteristics of plastics, and in the long run, the stability of melt viscosity variation can be truly grasped.

Importance of Scientific Molding Thinking

Injection molding is part of the traditional manufacturing process. With the development of Industry 4.0, the injection molding field is now equipped with many networking applications, moving towards smart manufacturing.

For example, the information of injection machine in production can be captured in real time. Stable molding conditions are set up by the molding program. With the technique of setting up segmented molding conditions, the molding process data will be highly linked with the product quality, and the molding program will be able to identify the quality acceptance window with OQ, which can be used to monitor the molding quality of the mass production to avoid continued production of defective products. Or, we can apply real-time monitoring and automatic correction of molding conditions to minimize the cost of downtime for troubleshooting. These are the automated production management derived from the application of big data in the injection molding stage.

The basic foundation for these AI-intelligent molding data applications is to establish and implement efficient and stable production processes based on scientific molding system.

Figure 5. Scientific Molding Thinking

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