(Updated June 2019)
In a manufacturing environment filled with predictive and data-based methods, why should your molding process stray away from what seems to be the normal buzz words over the past decade? Or why should OEM’s allow injection molders in their supply chain that don’t use more advanced molding techniques then the remaining 70% of the suppliers out there?
While there are groups of people in plastics that don’t see the benefit behind always using scientific molding principles to the fullest extent of decoupled III, there’s no denying that implementing scientific molding has its benefits.
What Is Scientific Injection Molding
Scientific injection molding is a disciplined approach to using data backed decisions to setup consistent and repeatable molding processes. By creating these extremely robust molding processes, predictive quality and consistent repeat ability is the outcome.
This is accomplished through 4 phases:
- Material Selection
- Part Design
- Tooling Design
- Process Development
While each of these phases is important, most people typically think solely of the 4th phase (process development) when they think of scientific injection molding. To fully implement scientific molding, it has to begin with the material selection and through part design to give the tool and production side the highest likelihood of success.
Scientific Molding vs Traditional Molding
Traditional molding principles use a single constant pressure across all of the fill/pack/hold stages, which in turn can lead to inconsistency from “shot to shot” having a process engineer chasing their tail based on the variations. With all of the variables in the injection molding process; melt flow, barrel temperature, hydraulic pressure, mold temperature, etc. utilizing a scientific molding approach curbs the fluxuations in your finished molded product.
In scientific injection molding, the fill and pack/hold stages are separated and is the basis for the terminology “decoupled”.
By separating out the first two stages of the process, this limits the shot-to-shot variation and improves consistency by using the non-Newton fluid behavior of the thermoplastic. Decoupled Molding is further broken down into various forms (Decoupled I, Decoupled II, and Decoupled III) and the usage of these levels is based on the injection molders equipment (eDART®) or their training level.
The injection molding process is mainly broken down into 3 separate phases:
- Fill : The cavity of the molding tool is filled between 95-99%. This partial filling of the tool establishes the foundation for consistency between shots.
- Pack/Hold : During the Pack/Hold phase the remaining volume is filled and the plastic is compressed to fully “pack” the cavity.
- Cooling/Recovery : During the final phase, Cooling/Recovery, the part is allowed to cool and become dimensionally stable.
The traditional molding process fills the plastic into the mold and pack it during the first stage, and then switches to a smaller pump during the hold phase, primarily to conserve energy. The holding pressure was left either at the same pressure as the first stage or, in some cases, slightly lower to minimize over packing at the gate.
The benefits behind scientific over traditional molding at the highest level are significant for shot-to-shot consistency, predictable quality, lower scrap rates and a more robust part.
Scientific Molding Levels & Certifications
With the different molding levels within scientific injection molding comes different levels on monitoring, control and investment (financial and time) from the injection molder.
Per RJG, the different classification definitions are below:
- Decoupled I : An improved technique of molding, which can be achieved on certain types of parts, is Decoupled I. This technique was used in the 1970's when cavity pressure control was initiated. With this technique, the mold is filled at a controlled velocity until the mold is volumetrically full. At this point, the machine is transferred to a set holding pressure and melt inertia (kinetic energy and the decompression of the melt) is used to pack the mold. Filling is disconnected from packing, but the inertia of the first-stage fill is the major component of the packing process. This is a process that requires a high degree of machine repeatability and is not for the faint of heart. It is generally only used in a very limited set of specific applications.
- Decoupled II : If we are to achieve faster fill rates to take advantage of rheology, we must be able to fill quickly and consistently. The only way to do this is to fully separate the filling phase from the packing phase. If we do not separate the fast fill from the sudden stop at the end (when the cavity is volumetrically full), the melt inertia will cause a rapid buildup of pressure when the plastic hits the end of the cavity, producing flash. This is analogous to driving your car into the back wall of the garage to stop it.
A better approach is to slow down before hitting the end of the cavity, thus a decoupling the fast fill stage from the packing stage. Using Decoupled II, this is accomplished by transferring from fill into second-stage pressure when the mold is 95% to 98% full. This is analogous to driving fast on the way home from work and slowing down before parking in the garage.
- Decoupled III : The latest evolution of the Decoupled Molding technique has been to separate the process into three distinct stages: fill, pack, and hold. The first stage, fill, is achieved at one or more velocities (multiple speeds may be necessary depending on part geometry).
The packing phase is decoupled from the filling phase; however, instead of simply squeezing the plastic in under second-stage pressure, packing is done using a low-speed, controlled velocity stage until a pressure set point inside the mold cavity is reached. This low packing rate absorbs most of the melt inertia and allows precise levels of packing to be achieved. This is similar to driving slowly into the garage and stopping exactly when your windshield touches the tennis ball hanging from the ceiling. Hold is then used to prevent back flow of plastic out of the mold until the gate is sealed (putting the car in park and setting the brake, to extend the analogy).
Certifications and Training
One of the most well-known training companies for scientific injection molding is RJG out of Michigan, and their robust training schedule and sequences are top notch. For those looking to get a taste of the fundamentals they have specific classes in systematic molding and specific works shops for various operations, purchasing, engineering, processing and production floor personnel.
Their Master Molder® I and Master Molder® II courses are designed to put your employees to the test with 2 solid weeks of training and testing to receive the certification. For an injection molding company to consistently practice scientific molding, staying up to date with key employees’ certification is essential.
At Crescent Industries, we have 3 employees at Master Molder® I certification and 5 employees with Master Molder® II certifications. Crescent is committed to investing in our employee resources to continue training with the latest technology behind scientific injection molding, to ensure that our customers are getting the most value and highest quality.
Check out Part II where we discuss the equipment and detailed processing to bring the theories of scientific molding into reality.
For additional information, please click below to read our white paper "Tips to Assist You in Selecting an Injection Molding Partner".