Getting to know the details about injection molding pressure is key. Here’s a helpful guide.
Many forms of pressure are actively involved in creating a plastic injection molded component. Why is this so important, you ask? Balancing the injection, back, and holding pressure correctly is crucial as this plays a significant role in consistently making excellent plastic components.
Pressures primarily determine cycle times, finished part quality, and ultimately the cost per piece of the plastic component.
Understand Injection Molding Pressure
Plastic product developers are always eager to get or produce as many components per hour as possible. This is an efficient way of reducing the cost per piece of plastic parts as this is an essential consideration for large orders.
As a result, it is sometimes believed that increasing the injection molding pressure can considerably force the machine to produce more components faster.
Injection molding pressure is due to the force applied by a machined screw to push molten plastic material into a mold cavity. It is usually balanced against the machine’s clamping pressure.
Injection molding pressure refers to the melt pressure at the head of the screw during the injection process. It is employed to overcome the resistance of the molten material from the machine’s barrel to the cavity. This gives the molten material a particular filling rate and also compacts it.
Injection molding pressure is usually measured using a sensor positioned on the hydraulic pipeline or nozzle. It does not have a specific fixed value. The more challenging it is to fill the mold, the higher the injection molding pressure at that particular instance.
Injection molding is filling a mold cavity with the hot material flow or molten material. Theoretically, the faster the filling speed, the better, as long as you don’t consider the mold factors and product surface defects.
However, since the molten material generates resistance – i.e., the pressure within the mold cavity – during the flow process with the existence of the internal pressure within the mold cavity, the molding machine must render or supply a power equal to or greater than this resistance, i.e., hydraulic pressure. This should occur during injection in order to force the molten material to flow into the mold cavity.
Injection molding pressure is not constant from the beginning to the end of the entire process, even if the injection is uniform. As the molten material slowly fills the mold cavity, its contact surface or stress area expands—the injection molding pressure changes during this uniform injection.
What Happens During Initial Mold Filling Sequences?
Here’s what occurs during the initial mold filling sequence:
The mold is empty just before the initial injection of the molten material, meaning the resistance is negligible as the material is injected, and the cavity fills up quickly. However, three types of defects may occur if the cavity fills up too quickly.
1. The molten resin will shear under excess pressure, meaning the chemical bonds get destroyed, making part of the component highly defective.
2. The molten resin may spray right out of the gate in a jet instead of flowing smoothly. As a result, the resin will end up hitting the opposite tool wall, cool off, and solidify, turning into a defective component.
3. High injection molding pressure can quickly force the mold open just at the parting line, creating flashing on the component. But even more critical is the severe damage the mold tool will undergo.
This usually happens since the mold fills up very quickly at first – because it is empty – and there is very little internal resistance. But when the cavity is up to 95 percent full, the resistance, as well as the internal pressure, spikes. The pressure must then be scaled back in order to avoid totally destroying the mold tool.
When the injection molding equipment senses that it has reached the specified changeover point, it automatically minimizes the injection molding pressure and then switches over immediately to holding pressure, which performs a somewhat different function.
The Different Types of Pressure
As mentioned earlier, many forms of pressure are involved during injection molding processes. Here they are in no particular order:
- Clamping pressure
Clamping pressure – also known as a clamping force – keeps the mold closed tightly against injection molding pressure. The clamping pressure is usually much more than injection molding pressure and prevents the mold from opening during the injection process or stage.
- Back pressure
The return action of the screw after injecting the material creates back pressure. The latter usually varies according to the specific design of the screw, the type of plastic material, and the product quality requirements.
- Holding pressure
Holding pressure is used in finishing the filling of the injection mold. It is often required until the gates freeze. Holding pressure is equal to 50 percent of injection molding pressure.
- Cavity pressure
When molten material enters the mold cavity impression as forced by injection, the action generates a reverse force against injection molding pressure that tries to open up the mold. This pressure is referred to as ‘cavity pressure.’ The latter is usually maximum at the gate point.
- Line pressure
Line pressure, also referred to as gauge pressure or system pressure, is present within the pump’s main supply line.
- Nozzle contact pressure
Nozzle contact pressure is the pressure that acts between the sprue bush and the nozzle in the parallel direction of injection. This pressure is designed to counteract the injection molding pressure the machine can generate.
Higher injection molding pressure calls for higher nozzle contact forces for injection molding equipment of any size. Nozzle contact pressure generally requires at least 4 to 10 tons of force.
Conclusion
Injection molding pressure is one of the several types of pressure that come into play during injection molding processes. It is designed to balance the clamping force of the injection molding equipment during operation.
Injection molding pressure never remains constant from the start of the injection molding operation to the end, even if the injection is uniform. This is because the molten material’s contact surface expands as it fills up the mold cavity. This changes the injection molding pressure during the uniform injection of the material.