Plastic pressure is one of the four basic plastic variables. Its accurate control is vital…
By Donald C. Paulson
(As published in Plastics Business Magazine)
More and more we hear the term Scientific Molding used to describe the ideal injection molding process. But what does it mean exactly? Opinions vary widely. Allow me, as a long-time researcher and developer of the four key plastic processing variables to add my thoughts to the discussion.
What is Scientific Injection Molding?
Scientific injection molding (SIM) is the practical application of the laws of physics as they apply to molded parts properties. To understand molding as a science you must first discard the idea that the machine control settings determine plastic part properties. Part properties, whether good parts or rejects, are determined by just four conditions, the melt temperature, the plastic flow rate in the mold, the pressure of the plastic in the mold and the cooling rate in the mold.
The key to understanding plastic processing is to understand how each of these four basic variables affect the plastic and how changes in plastic behavior affect molded parts. Listen to the molecules. When you have part problems, are you putting them at the wrong temperature, the wrong pressure, the wrong flow rate or the wrong cooling rate?
How do we get to understand molding as a science?
First, the initial requirement is that our molding machines, molds and any auxiliary equipment be capable and consistent in their operation. SIM specifies several machine/mold tests to accurately gauge the capability of the equipment. Examples include: viscosity curves, cavity balance studies, pressure drop studies, and gate seal studies. Procedures for these tests are widely available. The term “Scientific Injection Molding” is often used for these tests. But these tests are just one component and the beginning of a truly scientific process. If injection molding is to get the benefit of actual science there must be more to it than just machine/mold tests.
Second, a scientific injection molder must understand “molding from the plastics point of view”. Each of the 30+ machine controls has an effect on one or more the four plastic variables. Today’s machines have the capability to control all of the plastic variables, but not directly. For example, there is no single control adjustment for melt temperature. Actual melt temperature (not barrel temperatures) is the result of many factors, including screw rpm, back pressure and heater settings. The value of understanding molding from the plastics point of view is that the knowledge tells you what set of machine controls to adjust and which ones to ignore.
The proof that plastic flow rate in the mold affects directional shrinkage, warp and part cracking was one of the reasons why machine builders added fill rate control. The development of velocity to pressure transfer setpoint (VPT) was also a direct result of research. Cavity pressure is much more consistent if a VPT setting near 95% of fill is set. Then pack/hold pressure is used to adjust part dimensions.
If we step back and ask, what is the goal of a scientific understanding in the injection molding process? It is the same as any innovation, to increase productivity and reduce problems.
The term I coined decades ago is “molding from the plastics point of view.” What does it take to become a scientific molder? An understanding of plastic behavior; that it is compressible with pressure, molecules orient when they flow, temperature affects how easily they flow and their distance apart, cooling a plastic reduces pressure in the mold because the molecules moved closer together. If it’s crystalline plastic the crystal sizes are affected by how fast the plastic is cooled.
The Four Plastic Processing Variables
These four plastic variables determine final part properties, good or bad. There is no single machine control that affects each of these variables independently of the other.
One, the plastic melt temperature affects the distance between the molecules. When melt temperature changes, the molecules distance from each other changes. This affects the pressure loss they flow. Typically flow viscosity decreases as melt temperature increases. For some materials, this effect is dramatic.
Two, the plastic pressure in the mold determines part dimensions, shrinkage and stress.
Three, the plastic flow rate. Flow speed differences across the flow path forces the plastic molecules to line up (orient) in the flow direction. Oriented molecules have different properties in the flow direction versus the non-flow direction. This affects strength properties and differences in shrinkage and pressure loss during flow.
Four, the plastic cooling rate. Faster cooling will freeze in molecular orientation, slower cooling allows molecules to “relax” reducing stress in the part. With crystalline materials, cooling rate directly affects crystal size and formation.
In summary, all of the properties of molded plastic parts are determined by one or more of these four processing variables. To the uninitiated, this sounds heretical. After all, there are at least 30 Machine Controls that seem to have the ability to affect part properties. And it’s true, they do. But from the plastics point of view each machine control adjustment affects one or more of the four basic variables. Our goal then is to develop a multi-tiered production team, all who have a scientific understanding of the molding process applicable to their job.