How to Set Back Pressure in Injection Molding

Geospace Technologies Contract & Manufacturing Division

December 4, 2019

An Overview of the Injection Molding Process 

Injection molding is the process of transforming plastics into a variety of different products that we use every day. Resin pellets and powders are mixed and fed into a hopper. This material is fed into a screw that pushes the material forward while the heaters melt the plastic. Heaters along the line raise the melt temperature while the pressure is built up. A valve at the front of the screw prevents melted resin from moving forward until the mold is ready. The injection cylinder pushes the material into the mold at a high pressure. The valves close off the mold and the material is formed into the desired shape. The mold temperature controller runs cooling fluid around the mold’s circuits to balance out the heat flux experienced by the mold during each cycle. There are often two different mold temperature control units for each half of the mold. This provides greater process control. 

The mold will eventually be opened and the newly formed part removed. Then the injection molding process repeats, creating another “shot” of material to be pushed into the mold. The cycle time is the time it takes for the process to repeat itself. Depending on the parts, it could be anywhere from seconds to minutes. 

Occasionally, a purging compound is used to clean the extruder and molding machine. Proper back pressure can help keep the screw on the bottom while the purging occurs. And back pressure will help remove the old colorants during a color change. 

 

What is Back Pressure? 

Back pressure in injection molding, also called plasticizing pressure, is a normal result of melting and molding plastics via injection molding. The material is pushing back against the injection screw as the material is mixed and pushed forward. The back pressure is generated by the metering section of the mixing screw. 

It is possible to control the hydraulic back pressure during the injection molding process. The back pressure settings typically start at fifty PSI and increase in ten PSI increments. The maximum hydraulic pressure for the equipment is normally 300 PSI. You rarely want pressures higher than this, since it can shear the plastic and cause thermally degraded plastic. This will alter the mechanical properties of the molded part. The parts may be brittle, and they’re more prone to fail. 

Back pressure is different from the injection pressure, the pressure at which the mold fills. That is sometimes called the first stage of pressure. The holding pressure is maintained after the molten plastic fills the mold and is maintained until the gate freezes or a timer releases it. Clamp pressure is the force applied to the mold itself to hold it closed. That force has to be strong enough to oppose the separating force which is caused by the injection of plastic into the mold. The clamp pressure force will depend on the material type and depth of the part. 

complete turnkey solutions banner

 

The Purpose of Back Pressure in Injection Molding 

What is the purpose of back pressure in injection molding? High melt pressure yields better melt temperature control, and you aren’t increasing the melt temperature to get more uniform temperature through the material. There is greater melt uniformity with higher back pressure. Higher plastic pressure gives the molten plastic a more uniform consistency. 

You’ll get a better blending of additives and colors at a higher plastic pressure. This requires having a properly designed screw. With higher back pressure, you’re extruding the air inside the melt material. This improves the plastic’s gloss and eliminates internal bubbles.

You’re less likely to get black specks and material degradation where the flights meet the root of the screw. Screw augering caused by pellets wedged between the flight land and feed throat is dramatically reduced. This is a more common issue with small screws. Regardless of the injection unit, you’ll have improved shot size position control. With the ideal back pressure, you’ll also reduce issues with shrinkage on the surface of the product. 

There are times you’ll want to lower the back pressure. Too much pressure can break fibers in reinforced resins. Lower pressure will prevent vent flooding in a vented barrel. These vents are often used to remove moisture and trapped air and other volatiles. 

 

How Does Back Pressure Affect Shot Size? 

High back pressure increases shot size. However, this can cause problems during the injection molding process. Higher pressure can lead to excessive pressure which contributes to accelerated wear and tear in injection molding machines. This can eventually result in imperfections along the parting line of the mold or places where the mold created a boundary for the part. 

 

How Does Back Pressure Affect Melt Temperature? 

Back pressure does not raise the melt temperature. However, melt temperature will rise as the screw speed is increased. This is because seventy to ninety percent of the energy needed to melt the plastic for the shot comes from the screw rotation. 

Too many injection molding machines fail to take screw rotation speed into account when they’re designing a new manufacturing process. Temperature settings and back pressure are considered more important. Unfortunately, they often set the screw speed higher than necessary. This can create process variability since you want the screw speed and recovery time to be the same across your operation. Yet back pressure will impact screw rotation time, too. This is why it is essential to take all process variables into account when designing an injection molding process. 

 

How to Set Back Pressure for Injection Molding

How do you set back pressure in injection molding? It will primarily be controlled by adjusting the return throttle valve on the injection cylinder. These valves are installed on the back of the injection cylinder.

 

If you would like to learn more about back pressure or the injection molding process, don’t hesitate to reach out to the professionals at Geospace Technologies. Since 1980, we have stayed at the forefront of engineering and manufacturing innovation. Contact us or submit a form about your inquiry.

complete turnkey solutions banner

713-986-4444