The concept of backpressure is interesting. This in part because it is an elegant demonstration of thermodynamics. One of many such in the injection molding process. It is one that is often overlooked. You’ll often find descriptions of injection molding with no mention of backpressure. In the fundamental sense, it relates the pressure exerted on the system to the screw motion. But beyond this, it also affects several other aspects of the process. Injection molding is an interplay of different events culminating in a formed product. Several parameters need control and are monitored in the process. Many of these parameters relate to each other. One of such is backpressure. Backpressure is essential in injection molding. For an efficient injection molding run, back pressure needs controlling. Another name for backpressure is plasticizing pressure. When thought of as the latter it has a more positive ring to it. This article helps to understand the concept of backpressure. It discusses the role back pressure plays in injection molding and how it gets managed. We look at the components of the injection molding machine that handle backpressure. We also look at how backpressure affects other aspects of injection molding systems.
Back Pressure in a Nutshell
The pressure is an important physical concept in injection molding. Pressure control gets used at different points of the injection molding machine. There’s the sheer pressure on the melt as it forces its way between the screw and barrel. There’s the holding pressure that the molded part gets held at while it cools. There’s also the pressure applied on the screw which in turn exerts pressure on the melt. It is with this pressure the melt fills the runners and the mold. Somewhere around all this lies backpressure.
To understand back pressure one needs to have a good idea of how injection molding works. An injection molding machine mixes and melts plastics and molds them into shapes. The process begins when plastic pellets get fed into the hopper. The pellets get moved along a barrel which is in the typical case horizontal. As they pass through the heated barrel they get sheared by a plasticizing screw. The shearing occurs by the rotating action of the screw. As the screw rotates, it moves backward. This backward motion of the screw pushes the melted plastics to the front of the screw. There now exists this volume of fluid in front of the screw. This fluid exerts pressure on the screw causing its backward motion. The backward motion of the screw increases the volume in the melt chamber. This achieves the desired shot size. But this volume must get controlled to exert enough compression pressure on the melt. This is necessary for even well-compacted melt. So the pressure applied at the other end of the screw counters the pressure exerted by the melt in front of it. This pressure is backpressure. This pressure was generated by the plasticizing of the plastics. Hence its alternative name, plasticizing pressure.
How Back Pressure Affects Injection Moulding
Backpressure has a lot to do with how the injection process progresses. The screw does much of the work in the process. So the way it moves is crucial. The injection molding process centers around the back and forth motion of the screw. There are several parameters at play here. The rotation speed of the screw, measured in rotation per minute (rpm). The screw rotates anywhere between 20 and 60 rpm in most machines. The rotation speed for any given process depends also on the property of the plastics. Another parameter to consider is the size of the shot. The shot size gets determined by the size of the injection chamber. Since this is set by the limit switch which determines how far back the screw goes.
This pressure sends back the screw towards the hopper end of the machine. The speed at which the screw moves back depends on a balance of different factors. These are; the size of the screw, friction, and the net pressure. Once the screw hits the limit switch it gets forced forward by hydraulic pressure. This forward action injects melt into the mold. In injection molding timing is important. How does the back pressure affect timings? Here’s how. As the screw moves back the volume of the melt chamber increases. Meanwhile, the volume on the other end of the screw decreases. This results in increased pressure.
The rate at which the screw moves back gets controlled by the gradual increase in volume. So how much melt the screw pushes forward per unit time affects how fast it moves backward. The net pressure moving the screw back. It is a balance between the pressure exerted by the melt and the backpressure. The acceleration of the screw is proportional to the net pressure exerted. So to control how fast the screw moves back, control the backpressure. The illustration in the image provided below further explains this.
Diagram illustrating backpressure. P2 indicates the pressure exerted by the melt on the screw. P1 is the backpressure applied.
The type of mechanism used for controlling pressure varies. It depends on the design of the injection molding machine. In particular how it gets powered. This determines how the pressure gets applied. The injection molding machine can be hydraulic, mechanical, electrical, or a hybrid. In the hydraulic system, it occurs by controlling the flow of oil in and out of the cylinder. This gets done using valves controlling oil flow. In the electric type systems, this occurs through AC servo valves. Backpressure starts from around 50 psi. This then rises by 10 psi until a peak of around 300 psi. If the back pressure is too low, the screw’s backward motion is too easy. If the backpressure is too high the melt experiences too much shear.
Thus far, we have based the discussion on backpressure on thermoplastics. But bear in mind that the same principle applies to thermosets. The difference is that the plastics get fed in as liquid resins. This is unlike thermoplastics which get fed in as solid pellets or granules.
6 Things to Know About Back Pressure
Knowing how backpressure occurs in the injection molding process is important. It gives a better understanding of how the process gets controlled. The injection molding process is a combination of well-orchestrated events. Every event has an impact on the quality of the final process. Here are 6 roles of back pressure in injection molding.
Compacting
To get uniform smooth materials pressure must get exerted on the melt. A higher pressure brings the molecules closer together. This improves density and makes for a better product. If the melt is not compact enough it could result in insufficient flow down the line. So the back pressure does more than control the screw motion. It also maintains the compacting pressure needed. The image below is an illustration of compaction. As the pressure gets increased the molecules move closer together.
Image illustrating compaction under increased pressure (say for example three times the pressure)
Venting
Even where the product gets dried it is difficult to avoid air getting into the melt. This can be from the condensation of volatiles in the plastic. It could also be from condensed air or liquid from the environment. When the pressure gets exerted, trapped air escapes. This gets driven by a pressure gradient which moves the trapped air from the melt chamber. This trapped air gets moved to the lower pressure region and out of the system. Trapped air can result in loss of design precision or surface imperfection. Even when not visible on the product it can result in loss of functionality. Often the injection molding machine gets fitted with a venting and decompression zone. The back pressure aids the effectiveness of such features.
Process timing
Timing gets managed to even fractions of a second in injection molding. A one-second difference in one cycle can have an impact on profits. Accurate control of backpressure reflects the inaccuracy of screw speed. While the screw moves back, another process is taking place. The melt in the mold from the previous cycle is cooling. The backward motion of the screw thus needs to be in sync with the ejection of the product. Such that by the time the screw stops moving back that product has left and mold gets closed. This going wrong by even a second could halt the whole process.
Shrinkage
Exerting pressure on the melt helps address shrinkage. It encourages isotropic shrinkage. It also reduces the impact of shrinkage. Where the melt is under pressure a reduction in total volume will result in fewer gaps. Compared to where there is less pressure exerted. So back pressure allows for better dimensional stability. This is not to say that shrinkage will not occur, it will. But its effect will be less pronounced.
Mold filling
The pressure at which the melt gets held while filling the mold is important. This is more so in multi-cavity systems. The pressure must remain as the melt travels through the runners. The backpressure contributes to the speed and pressure of mold filling.
Melt temperature
The pressure relates to temperature. As the pressure of a system increases so does temperature. As melt gets pushed forward into the melt chamber, the volume of melt there increases. But the volume of the space available to the melt gets reduced by the backpressure. By physical laws, the system responds by increasing temperature. The chaos generated by fitting more melt into a smaller space gets converted to heat. This is good for managing the cost of energy. The increased temperature gets achieved in this zone by applying backpressure. For many plastics, this increase in temperature decreases viscosity. This is good for more effective mold filling and product formation. If backpressure is too high the temperature rise, as a result, can lead to degradation of the plastic. Bear in mind that this is only for plastics that are temperature sensitive. Or when working with quite a high backpressure. Temperature increase from back pressure is not as high as that from screw rotation.
Filled Plastics/ Composites
In materials filled with fibers, care is necessary not to have a detrimental effect. High backpressure can reduce fiber length. Long fibers in particular are liable to damage. The compaction and increased shearing from back pressure can result in breakage. Plastics get filled with long fibers to improve their properties. The length and arrangement of the fibers within the plastic affect these properties. Many plastic products for injection molding have recommendations from the manufacturer. For such plastics, take note of the pressure the material can withstand.
Conclusion
Backpressure, amongst other things it does, controls the screw backward motion. Besides this, it contributes to other important factors crucial to product quality. Good control over backpressure has a significant impact on product formation. Properties like melt uniformity and dimensional stability get improved through backpressure.
