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What is Injection Molding Back Pressure?

Discover the role of back pressure in injection molding, its effects on quality, and how to optimize it for better melt temperature control, uniformity, and production efficiency.

Injection molding manufactures high-quality plastic material using injections and molds. Plastic is heated at a very high temperature and injected into molds under high pressure. The plastic manufacturing process is complete when the final product is ejected from the molds. 

Now, let’s understand how back pressure is used in injection molding. Since pressure is important for injection molding, it is applied at different points and intensities inside an injection molding machine. 

Working of Back Pressure in Injection Molding

Back pressure or plasticizing pressure results from the melting and molding of plastics when injection molding. All the material gets pushed backward against the injection screw, where it gets mixed and pushed forward to the next section. This is where a device called metering in the mixing screw generates pressure. 

Injection molding machine in operation, illustrating the role of back pressure

The amount of pressure to be applied can also be controlled during molding. Pressure settings typically start at fifty PSI and can go up to ten PSI. The maximum hydraulic pressure limit required to manufacture equipment is 300 PSI.

So what do you think is the purpose of this? Let’s find out.

Effects of Back Pressure on the Injection Molding Process 

But what is the purpose of back pressure in injection molding? Basically, a high melt pressure leads to better melt temperature control during the process of molding. Additionally, great melt uniformity comes with higher back pressure. Due to the high plastic pressure, molten plastic can have a more uniform shape. 

With high pressure in plastic molding capacity, the additives and colors can be very nicely blended in the mold. You need to first craft an injection screw that can force out air with high pressure from the back. Due to the design and the pressure, you get the perfect gloss and shade of plastic.

Factors Affecting Back Pressure

The final quality of the plastic greatly depends on the proper application of back pressure during the molding process. Some important factors that are going to affect the quality of your product include:

  • Material Temperature 
  • Mold Design 
  • Injection Speed
  • Melt Density
  • Injection Pressure

How to Get the Right Amount of Back Pressure?

Injection Molding is a combination of different operations that take place one after the other. Resulting in high-quality and nicely crafted plastic products. All the processes have their own functions, and back pressure is important in filling up molds with ease. 

Let’s discuss in detail the factors responsible for ensuring the right amount of back pressure during injection molding: 

Compacting

Back pressure in injection molding helps compact the material effectively

Once the melted plastic is transferred to the mold, pressure is applied to pack all the material together. Here, all the material is squeezed to fill empty spaces. Back pressure again controls the resistance that is applied to the screw for pushing the material further. 

Applying higher resistance might lead to crafting denser materials. Leaving the material exposed to high temperature degrades the material and may lead to unwanted side effects like discoloration or loss of strength. 

Therefore, back pressure serves as a valuable tool by compacting the material. This helps in achieve a dense final product, but for further balance other factors also need to be optimized.

Mold filling

Throughout the process of molding, consistent pressure is required to ensure that the material evenly fills the cavities present inside it. Sometimes, the lack of adequate pressure leaves some space in the mold, which makes it difficult to cover the cavities. Initially, resistance is applied adequately to the screw during melt preparation.  

With back pressure, consistency is easy to achieve. Create force that keeps pushing back the melted plastic; pressure helps ensure the runner remains under pressure throughout the filling process.  

Shrinkage

Adding pressure also reduces the chances and impact of shrinkage on the material being injected into the mold. This is caused by the physical arrangement of the polymer molecules present in the melted matter. All these molecules are made to pack together closely during solidification. 

Back pressure has the ability to change the density of this final part. When back pressure compacts the melt during the screw preparation stage, it helps reduce the space between the polymer molecules. 

This denser packing due to back pressure translates to less shrinkage in the final molded part, resulting in the creation of parts that are closer to their intended dimensions.

Process timing

Every second is important during injection molding; missing even one second can lead to a change in the final outcome. The machine should have good control over the screw speed as the molten plastic moves forward. 

It encounters resistance due to back pressure, which makes it longer for the screw to return before the next injection cycle. While this time difference is generally very small, this is something only an experienced operator will take into account during fine-tuning. 

For instance, if a specific cycle takes longer for production, then a slight adjustment in back pressure might solve the problem.

Venting

Close-up of venting design in an injection molding component

It is not possible to stop the air particles or moisture from reaching the material or the mold; tiny particles or liquid from the environment somehow reach the mold. Back pressure helps reduce the amount of trapped air particles in the first place. 

A poorly mixed melt can also contain more air compared to a well-compacted melt achieved with proper back pressure. Additionally, back pressure helps maintain a higher pressure during the injection phase, which can sometimes help force residual air and gasses out of the mold, through the designated vents. 

This is very beneficial for molds having a complex geometry that might trap air more easily.

Melt temperature

Close-up of mold components highlighting melt temperature in injection molding

Pressure and temperature are directly related; both need to rise simultaneously. Back pressure generates heat within the melted plastic due to the frictional forces created by the mixing action of the screw. While this temperature rise may be very minor, it is still a factor to consider, especially with very heat-sensitive materials. 

Excessive heat can degrade the plastic, leading to problems like discoloration or loss of mechanical strength. Therefore, it’s important to find a balance where the back pressure is high enough to achieve good mixing. 

It can still not be so high that it causes the melt temperature to exceed the optimal level for every specific type of plastic being molded. In some cases, if the material is particularly heat-sensitive, a slightly lower back pressure setting might be used to avoid overheating the melt.

What is the right back pressure for Injection Molding?

There is no specific number that suggests what the right amount of back pressure should be for the best results. A balanced amount of pressure at an optimal level will give you the most positive results. 

This parameter needs to be carefully considered and adjusted based on several factors specific to each molding process. Here’s a breakdown of what influences the ideal back pressure:

Type of Material

Close-up of an injection mold, showcasing the type of material used

Thickness: Some materials are of higher thickness than others and usually require higher back pressure to ensure proper mixing, overcome resistance to flow, and achieve a uniform melt. This is like stirring a thick soup compared to water – you need more force for the thick soup.

Ingredients of Melt: Sometimes, the material used in the melted plastic includes additives like colorants, flame residues, or fillers (like glass fibers) that can affect the melt density and influence the level of back pressure being applied. 

These additions could also increase the resistance within the screw channel. It is very important to adjust the back pressure to ensure adequate mixing of the material and prevent any sort of degradation.

Sensitivity to Heat: Some delicate plastics are more resistant to heat and could degrade quickly if exposed to excess temperatures for longer periods. While using such heat-sensitive materials, it is beneficial to use a slightly lower back pressure setting. This will help in minimizing overheating and prevent issues like discoloration or loss of mechanical strength.

Goal of end-product

Structured Applications: For many applications intended for strength, where dimensional accuracy is paramount, a higher back pressure might be necessary. This helps minimize internal stresses, reduces shrinkage, and ensures parts are crafted with superior mechanical properties. 

Look of the product: Certain products demand good appearance, for such applications, precise control over the amount of back pressure is often necessary. Sufficient back pressure can prevent marks or reduce the visibility of flow lines on the product. 

However, excessively high back pressure can sometimes contribute to swirling or color inconsistencies due to excessive shear in the melt. Finding a balance is very crucial.  

Production rate: If you need a faster production rate, the amount of control over back pressure becomes more critical. While the effect on overall cycle time is often minor, well-adjusted back pressure can contribute to efficient screw recovery. This can translate to faster production rates and higher production throughout. 

Quality required

Dimensional requirements: Strict attention to dimensional accuracy might require using higher back pressure to ensure consistency in the amount of material injected and uniform packing of the plastic into the mold cavity. Minimizing variation in these factors translates to greater control over the dimensions of the final product.

Managing Defects: The right amount of back pressure reduces the chances of aesthetic defects, including flash excess plastic leaking, voids (air bubbles), sink marks (depressions), and short shots (incomplete filling). A well-optimized back pressure setting helps maintain the overall quality of the molded parts.

Long-lasting Products: Several products are designed for a longer life, and by putting the proper back pressure on them, you can help minimize internal stresses within the part. By reducing these stresses, you can improve resistance to warping, cracking, and premature failure, particularly in demanding environments.

Finding the Optimal Pressure Settings

Injection molding machine and products demonstrating optimal pressure settings

Consider these additional tips to ensure you are using the right amount of pressure in your applications. 

  • Try the recommended instructions from the manual. 
  • Observe the final results and make changes accordingly
  • Keep making tweaks to your process. 

Positive vs Negative Aspects of Back Pressure

Back pressure is significant in the process of injection molding, from the design to the final product. Just like other processes, back pressure has its pros and cons, too. 

There are several positive and negative aspects of back pressure in injection molding, so you will be well-equipped to find the optimal setting for specific molding processes.

Positive EffectsNegative Effects
Size of each molded part needs to be similar Overheating may damage delicate plastics
Melted plastic gets evenly mixedThis may create some damage to the machine 
Prevents minor errors like holes, dentsThe size of each molded part needs to be similar 
Speeds up the molding processRequires other settings to be adjusted 

Conclusion 

It is back pressure that makes it possible for the manufacturers to deliver good quality plastic products. Back pressure influences everything during the process, starting from the design of the plastic model to ensuring consistency. 

Where too little can be dangerous, exerting extra pressure can damage the appearance completely. 

To address all these problems, it is very important to follow only the recommended settings for the machine. With experienced manufacturers, you get to know the right amount of pressure required by different materials.

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