Process Optimization for Medical Injection Molding

Processing continually proves to be my biggest passion since earning degrees in Plastics Engineering Technology and Manufacturing Management from Penn State University. I love successfully troubleshooting a pesky problem and the fulfilling thrill of qualifying a new mold and handing it over to the production team. We need to ensure a systematic approach to process development and troubleshooting methods to achieve our targeted OEE, and most of all, exceed our customers’ expectations.

Determining the Right Machine for the Mold

It is important to make sure the right-sized machine is selected for the mold, regardless of whether our customers permit press flexibility. Key considerations include:

Machine & Mold Spacing: will the mold physically fit between the tie bars and the daylight (the space between the mold halves when the mold is open)? Also, we need to ensure there is adequate space for the End-of-Arm Robot if we transfer parts from the machine and separate them by cavity.
Barrel Utilization: ideally, use 20-80% of the barrel to ensure a homogenous melt. If it’s below 20% of the barrel capacity, there could be degradation due to the long residence time, which is the amount of time the material is in the barrel. Using over 80% of the capacity does not thoroughly melt the plastic through shear heating and can lead to semi-molten pellets entering the mold, causing short shots. We can determine barrel utilization from the part model’s volume calculation before the mold is even completed.
Clamp tonnage requirements: using the part model, predict the tonnage requirements needed for the mold, while it’s still in developmental stages. Calculate the projected area of the part and runner at the parting line and multiply the product by the tonnage factor of the proposed material and be sure to consider the mold cavitation into this calculation. This will ensure the machine has adequate clamp tonnage for that specific part and mold cavitations. Ideally, the part or mold should not exceed 75% of the machine’s maximum clamp tonnage.
Special equipment needed: what sort of special equipment might be needed to run the mold? Does it need a high temperature control unit or a 48-zone hot runner controller? Will it run with the robot and end of arm tooling? These factors might determine the specific machine in which it runs.

Machine & Mold Spacing: will the mold physically fit between the tie bars and the daylight (the space between the mold halves when the mold is open)? Also, we need to ensure there is adequate space for the End-of-Arm Robot if we transfer parts from the machine and separate them by cavity.
Barrel Utilization: ideally, use 20-80% of the barrel to ensure a homogenous melt. If it’s below 20% of the barrel capacity, there could be degradation due to the long residence time, which is the amount of time the material is in the barrel. Using over 80% of the capacity does not thoroughly melt the plastic through shear heating and can lead to semi-molten pellets entering the mold, causing short shots. We can determine barrel utilization from the part model’s volume calculation before the mold is even completed.
Clamp tonnage requirements: using the part model, predict the tonnage requirements needed for the mold, while it’s still in developmental stages. Calculate the projected area of the part and runner at the parting line and multiply the product by the tonnage factor of the proposed material and be sure to consider the mold cavitation into this calculation. This will ensure the machine has adequate clamp tonnage for that specific part and mold cavitations. Ideally, the part or mold should not exceed 75% of the machine’s maximum clamp tonnage.
Special equipment needed: what sort of special equipment might be needed to run the mold? Does it need a high temperature control unit or a 48-zone hot runner controller? Will it run with the robot and end of arm tooling? These factors might determine the specific machine in which it runs.

Scientific Molding

Scientific Molding is defined as using a systematic approach to process development and troubleshooting. Our goal is to make quality parts, consistently, and at the quoted cycle time. When a new mold is delivered, these are the systematic steps we take to validate the mold:

Once the machine is cycling consistently and we produce about 90-98% full parts, we start an optimal fill study. During this study, we keep the fill-only parts visually the same while we change the injection velocity setting. We monitor any quality defects due to the faster or slower injection speeds. Our goal is to determine a good process window for injection speed while filling as fast and as consistently replicable as possible. The fill flow study replaced the velocity curve. It is not practical to reach a ten second fill time and the graph always shows the same thing; the material is non-Newtonian. The fill flow study observes the quality of the parts at the first step of process development.
Once the injection speed is set and we are back to 90-98% full parts, we determine our hold pressure window. We start by adding hold pressure to finish filling the parts and packing them all the way out. We do not want to have any flash which is excess material.
Next, we determine our hold time. Hold time is increased until the weight of the parts no longer increases and ensures our gate is frozen or sealed. When valve gates are utilized, the gate will never seal so we look for gate stabilization.
Last, we optimize our process by determining our cooling time needed for the parts to eject without deformation, optimizing our mold open and close speeds, and dialing in our robot program.

Several other steps are taken during the mold validation process include a melt temperature check to verify we are within the material’s recommended temperature range, checking and documenting our water flow rates, and typically running a Design of Experiments (DOE) to ensure we have a reliable process window for production. During the DOE, we usually test the interactions of the mold temperature, barrel temperatures, injection speed, and hold pressure. The DOE is subjective to customers’ requirements.

Training

At Plastikos, we have created a variety of internal training development programs, starting with our Career Opportunity and Readiness Education Program (CORE). The CORE program provides the highest level of how Plastikos operates and each department’s roles and responsibilities. This CORE program feeds our other development programs. The Processor Development Program has been designed to train employees with no prior experience to Process Technicians (PDP). We currently have 1 graduate and 3 going through this program. Systematic Molding is a key focus throughout the RJG series of classes that each PDP employee completes.

All the Process Engineers and Process Technicians use the same systematic approach to troubleshooting and receive a pairing of on-the-job and classroom training. The majority of our processing team is RJG’s Master Molder 1 certified. This structured training keeps everyone speaking the same language when it comes to troubleshooting and process development.

About the Author:

Danielle Bentley is the Medical Molding Manager and joined Plastikos in 2014. She holds a bachelor’s degree in Plastics Engineering Technology, a Medical Plastics certificate, and a Master of Manufacturing Management from Penn State University. Danielle is responsible for the manufacturing floor at Plastikos Medical, is an RJG Qualified Trainer, and loves working on continuous improvement projects. When Danielle is not at work she enjoys bowling, learning golf, yoga, and spending family time with her young daughters.