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How About The Effects Of Pressure On Plastic Injection Molded Parts

Date: Mar 31 2020 | By:LC Rapid
Tag:Plastic Injection Molding,

When manufacturing precision plastic injection molding parts, a variety of pressure types are involved. We have explained the significance of clamping pressure, at present let's take a look at injection, hold, and backpressure. It is significant for product developers to appreciate how the forces cooperate together and ultimately balance how they affect part quality, product costs, and number of parts per hour.


What is the injection pressure?

The injection pressure is a force applied to the reciprocating screw, pushing the molten plastic resin into the mold cavity, with a capacity of about 95%. It balances the clamping pressure of the machine and is calculated according to the structure and dimension of the parts.

Are quick injections always better?

Often, product developers need to take as many parts as possible because it reduces labor costs, which is a significant consideration for big orders. But there’re some unavoidable limitations.

When the mold cavity is empty, the flow resistance is small, so the cavity fills fast and simply at the starting of the cycle. Although, when approximately 95% is full, the pressure suddenly rises, causing flickering or forcing the mold to open prematurely. It may break your parts as same as the machine. To escape from these situations, the injection pressure must be reduced in a controlled manner at the so-called conversion point. At this stage, the cycle switches to hold pressure.


What is keeping pressure?

The hold or packing pressure is significant to fill the last 5% or so of the mold tool cavity. This pressure is reduced from the first injection pressure and remains for longer due to several significant reasons.

First, the gas trapped in the mold must be given enough time in purpose to escape through the vents in the mold. In addition, the packaging promotes the molecules in the resin to align and reach their maximum density. Third, thin-walled parts need some time to fully fill. At last, maintaining pressure and great tool design are essential to reduce part pressure.

However, maintaining pressure should be balanced with another limit. in the gate is "frozen" and blocked by the solidified resin, the cavity needs to be filled fast. Process engineers need to carefully adjust these forces over some time determined by the chemical factors of the plastic and temperature.

What is back pressure?

The back pressure is the resistance in the injection moulding barrel, preventing the screw from returning to its starting position. This resistance is intentional and occurs when the operator controls the valve on the machine to limit the speed at which hydraulic oil can return to the reservoir.

Why do I need back pressure?

Back pressure is significant to get extra time to make the heavy groove on the reciprocating screw mix the next lens of the resin. The groove's cutting action heats the resin and maintains a How Pressure Affects Your Plastic Injection Molded Parts

There are many types of pressure involved when making a plastic injection molded part. We’ve explained the importance of clamping pressure, so now let’s look at injection, holding and back pressure. It’s important for a product developer to understand how these forces work together and ultimately how balancing these forces affects part quality, pieces per hour and your product cost.


What is Injection Pressure?

Injection pressure is the force applied by the reciprocating screw to push molten plastic resin into a mold cavity, up to about 95% of capacity. It’s balanced against the clamping pressure of the machine, and is calculated based on the size and shape of the part, as well as the size of the gate opening.

Is Faster Injection Always Better?

Normally product developers want to get as many parts per as they can because this will reduce labor costs, an important consideration for larger orders. But there are some limits that can’t be avoided.

First, when the mold cavity is empty, there is little resistance to flow so the cavity fills quickly and easily at the beginning of the cycle. However, when about 95% full, there will be a sudden spike in pressure that can cause flashing, or forcing the mold open prematurely. This can damage your parts as well as the machine. To avoid this, the injection pressure must be reduced in a controlled fashion, at the so-called changeover point. At this stage, the cycle switches over to holding pressure.

What is Holding Pressure?


Holding, or packing, pressure is necessary for filling the leftover 5% or so of a mold tool cavity. This pressure is decreased from the initial injection pressure, and held for a longer time for a few important reasons.

For one, trapped gases in the mold must be given time to escape through vents in the mold. Also, packing helps the molecules in the resin to align and achieve their highest density. Three, thin-walled sections in the part also need time to fill completely. And finally, holding pressure is essential, along with good tool design, for relieving stress in the part.

However, holding pressure needs to be balanced against another limitation. The cavity needs to be filled quickly before the gate “freezes off” and becomes blocked by solidified resin.So the process engineer needs to calibrate these forces carefully within a period of time determined by the temperature and chemistry of the plastic.

What is Back Pressure?

Back pressure is resistance in the barrel of the injection molding machine that impedes the return of the screw to its start position. This resistance is deliberate, and is created when the operator controls a valve on the machine to restrict the rate at which hydraulic fluid is returned to the reservoir.

Why Is Back Pressure Necessary?

Back pressure is necessary to allow extra time for the heavy flutes on the reciprocating screw to mix the next shot of resin. The cutting action of the flutes heats the resin and maintains accordant density from one shot to the next. They also help to thoroughly mix any masterbatch coloring pigments that are in the resin.

As with the other parameters, this pressure must be controlled by the process technician so that the resin isn’t overmixed, which can degrade it due to excessive sheering.


How Do These Pressures Affect Your Parts?

You want to maximize the number of parts per hour in order to reduce production costs and of course to get your parts to market more quickly. But responsible process control technicians need to carefully balance multiple parameters within the machine to control quality and provide consistency from one cycle to the next.

Understanding a little more about how this process works can help you communicate more effectively with your supplier, and also reinforces the importance of good mold tool design. Learn more when you upload your CAD designs for a free quotation.

density from one pin to the next one. It helps to exhaustively mix any masterbatches in the resin.

As with other parameters, this pressure must be controlled by a process technician so that the resin is not over-mixed, thereby reducing the degradation of the resin due to excessive purity.

How do the pressures affect your parts?

You should increase the appropriate number of parts per hour to reduce production costs of product, and also, take your products and parts to market faster. Although, responsible process control technicians required to carefully balance multiple parameters in the machine to control quality and provide consistency from one cycle to the next.

A better understanding of how the process works can increase your communication more effectively with suppliers and strengthen the significance of great mold tool design. Upload your CAD designs to get free quotes.

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