(Updated June 2019)
Furthering our discussion around scientific injection molding, we will take a deep dive into the equipment and how the processing works. If you missed part I, you can read it here.
Not like buying a new iPhone and skimming through the new features in the latest IOS, the equipment used in scientific injection molding requires previous training to fully utilize it. While there are various levels of equipment and different approaches of implementing the decoupled processing, we will review how Crescent Industries has been using it.
The main two areas of equipment used in scientific injection molding by RJF is the eDART® System and the in-mold sensors. These two pieces are used to communicate together along with the machine to give the injection molder a view into what's happening in the process that they aren't aware of.
In a traditional molding environment, processes are based on machine input. The process engineer sets up the machine based on the inputs they enter; where it's position, time, temperature, pressure, etc. The injection molding machine is supposed to execute on those inputs, however without secondary equipment you don't really know whether or not those inputs were actually achieved.
The scientific molding approach instead focuses on outputs. Regardless of what your input is, by focusing on the output you can setup a more robust process based on actual data from the cycle. To see what the output is, you need the eDART® System and the in-mold sensors to communicate with the machine and tell you what's happening in the mold during the cycle.
The brain power behind decoupled II processing equipment is the eDART®. This essentially acts as the data collection hub and then translating that data into readable graphs, charts, outputs, etc. The equipment is connected with the machine and the sensors (if applicable). All training on the eDART® system and how to use it is covered in the Master Molder® II course at RJG. Per RJG, this course covers the below areas:
Participants will learn to apply cavity pressure control strategies to accomplish Decoupled III techniques using instrumentation and data acquisition and gain a deeper understanding of the improvements Decoupled III provides for process repeatability and robustness. This course also covers the proper use of instrumentation and how to achieve process control solutions utilizing Decoupled III processing techniques. You will learn how monitoring techniques are used to detect and contain suspect parts and how to do accurate machine and mold qualifications.
Participants will get extensive hands-on training on the RJG eDART® System. This includes system set-up, troubleshooting and data analyzer.
· Cavity pressure control using Decoupled III molding techniques
· Reading and interpreting cavity pressure curves
· Matching processes on different presses using cavity pressure data
· Proper sensor location and placement
· Evaluation of molding machine performance
· Calculating process improvement
By gaining the knowledge using the eDART® System, your process engineers can now significantly enhance their implementation of scientific injection molding into the “normal” processing standards.
The cost of entry isn’t cheap to purchase, and since you need an eDART® System to run Decoupled III processes, it is a requirement to at least have one.
At Crescent Industries, we have created a cart system that has our eDART® mounted on it, along with any other equipment needed for process validation or process troubleshooting. Currently Crescent Industries has 3 eDART® Systems on 3 separate carts. This allows us maximum mobility since the equipment does not have to be directly mounted to a machine, we can move the system from each machine as it’s needed during setup and production.
To gain insights into what’s happening in the mold during processing, you need to install in-mold sensors to take measurements and translate that information into the eDART®. The in-mold sensors are broken into two categories, pressure sensors and temperature sensors. As outlined by RJG, there are benefits to each type of sensor:
Cavity Pressure Sensors
Pressure data can help identify:
· Dimensional Variation
· Chemical Resistance
· Warp or In-Molded Stress
· Cooling circuit variation
· Imbalance or blockage—warp due to semi-crystalline shrinkage behavior
· Improper melt temperatures
Scientific Injection Molding
As with any new project, you have to first validate the tool and establish your baselines/setup to make a “good part” that is dimensionally stable as well as meeting the standards of the customer. By using a scientific approach and focusing on the outputs of the machine instead of the inputs, there is a heavier emphasis on data collection to establish baselines and a robust process of establishing the most repeatable part quality.
At Crescent Industries, we have taken our training and history in the scientific injection molding principles to apply a specific process that works best with the customers that we serve.
Our internal 6 step mold qualification worksheet walks us through all of the steps required to establish a robust process. This starts with the initial viscosity curve and balancing the cavities, then onto pressure drop, process window and gate seal study. Once common argument within the scientific injection molding community is whether or not you need/can always achieve a gate seal. While it is a goal to achieve one, sometimes the part design/functionality doesn’t require one. The last step of the mold qualification is the cooling study.
While we are going through our internal mold qualification process, we use specific documents to track our process and retain the values. Once these documents are saved and stored on our internal servers, we have the ability to reference back to initial trials years down the road. By working with FimmTech we have leveraged their knowledge in scientific injection molding to assist with establishing robust processes as outlined by them below:
Viscosity Curve - Output
Looking at a typical curve, one can notice that the viscosity stays fairly constant after about 60% of the injection speed. Therefore, setting the injection speed to 70% would ensure that the filling stage of the process will stay consistent. Any small natural variations will not cause large changes in viscosities resulting in shot to shot variations.
Shot to shot variations should be reduced in order to achieve repeatable quality of parts. This is especially important in case of tight tolerance parts and multi cavity molds. Optimizing the injection speed through In-mold rheology is only the first step to achieving a robust process. Later, the holding phase and the cooling phase must also be optimized.
Cavity Balance - Output
Check the %variation between the maximum and the minimum fill for the cavities at each of the fill percentages. Record this info and perform the window study as described in the later section.If the parts can be packed out and the process window is large, check to see if the parts are in spec. if they are the imbalance if any is acceptable.
If the process window is very small and you get flash on the cavity that fills first while the other is short or has sink, investigate the reason for the imbalance.
Pressure Drop - Output
The maximum pressure used in the process should never be equal to the maximum available pressure at the machine. For example, if the maximum available hydraulic pressure is 2200 psi, then the end of fill pressure should not be equal to 2200. If this is the case, it means that the screw needs more pressure to move at the set injection speed and cannot do so because of the limited pressure. Such a condition is called ‘Pressure Limited’. Typically, you should a maximum of about 90% of the maximum available pressure. Therefore, in this case, where the maximum is 2200, the end of fill pressure should not be more than 1980 psi.
In the generated graph, if you are pressure limited or more than 90% of the maximum, look for steep increases in the pressure and try to reduce these. For example, if the secondary runner section shows a steep increase, then this means it takes a lot of force to move the plastic through this section. Increasing the diameter of the runner will help in reducing this pressure.
Process Window - Output
The process window is an indicator of how much can you vary your process and still make an acceptable part. An ideal situation is you have a wide process window. If the process window is very narrow, then there is always a danger of molding parts with defects. For example, if the process window is very small, then one could get occasional short shots or occasional flash due to the natural variation in the process. A robust process is one that has a large process window and accommodates the natural variation.
While our team is documenting the process, we are recording all of the molding calculations in a spreadsheet along with the data from the scientific injection molding equipment. This is the best way to establish a repeatable and robust process.
Scientific injection molding is a methodology that has been changing the molding industry for the past 20 years. As the technology continues to advance, Crescent Industries is committed to investing, ensuring that our customers are getting the most value and highest quality.
For additional information, please click below to read our white paper "Tips to Assist You in Selecting an Injection Molding Partner".