Preface

The main parameters of injection molding include drying temperature and time, injection volume, metering stroke (pre-plastic stroke), residual material, anti-delay, screw speed, back pressure, injection speed, injection speed and pressure, holding pressure and time, barrel temperature, mold temperature, cooling time, mold opening and closing speed and pressure, ejection speed, gas-assisted blowing pressure and time, etc.

This article will explain the injection molding process parameters in detail and propose adjustment methods for plastic injection molding process parameters.

What are the Injection Molding Parameters

Injection Molding Temperature

Temperature is a big deal when it comes to injection molding pressure. The injection molding machine’s barrel has 5 to 6 heating sections, and each material has its own processing temperature (for specific processing temperatures, check out the data from the material supplier).You gotta control the temperature when you’re injection molding.

If it’s too low, the melt doesn’t get plasticized well, which messes up the part and makes it harder to mold; if it’s too high, the raw material breaks down. In real life, the injection temperature is usually higher than the barrel temperature. The higher number depends on the injection speed and the material’s properties, and it can be as high as 30°C.

That’s because the molten material gets sheared when it goes through the injection gate, and that makes a ton of heat. There are two ways to deal with this difference when you’re doing mold flow analysis. One is to try to measure the temperature of the molten material when it’s in the air, and the other is to include the nozzle in the model.

Barrel Temperature

The temperature of the injection molding material, the melt temperature, plays a major role in the flow properties of the melt. Since plastic has no specific melting point, the so-called melting point is a temperature range in a molten state. The structure and composition of the plastic molecular chain are different, and thus the effect on its fluidity is also different.

Temperature has a more obvious effect on rigid molecular chains, such as PC, PPS, etc., while the fluidity of flexible molecular chains, such as PA, PP, PE, etc., does not change obviously with temperature. Therefore, the reasonable injection temperature should be adjusted according to different materials.

Baking Temperature and Time

Because most plastics are hygroscopic, they will absorb a small amount of moisture when exposed to the air. When the moisture content in the plastic is higher than a certain level, injection molding quality defects will occur, such as silver streaks, bubbles, brittle cracks, reduced mechanical properties and other defects. Therefore, the plastic needs to be dried before injection molding.

Most suppliers will provide recommended baking temperature and time values. However, the drying time should not be too long, otherwise the plasticity of the plastic will deteriorate, which will cause the material to become brittle.

For some materials with strong water absorption, such as PA, PBT, PET, PEI, and PSU, it is recommended to use a dehumidifying dryer for drying. For some materials with low hygroscopicity, such as PP, PE, PVC, POM, and other materials, if they are sealed in unopened bags or have been stored in a dry environment, they do not need to be dried.

Mold Temperature

Mold temperature. Some plastic materials need higher mold temperature because they have high crystallization temperature and slow crystallization speed. Some need higher or lower temperature because of size control and deformation or demolding needs.

For example, PC generally needs more than 60 degrees, while PPS sometimes needs more than 160 degrees mold temperature to get better appearance and improve fluidity. So, mold temperature has an inestimable effect on improving appearance, deformation, size, and plastic mold of the product.

Nozzle Temperature

The nozzle has the function of accelerating melt flow and maintaining melt temperature. During the injection molding process, the nozzle is in direct contact with the mold, which will cause the nozzle temperature to drop quickly, causing the molten material to condense at the nozzle and block the nozzle hole or the pouring system of the mold.

Moreover, the condensed material will affect the surface quality and performance of the product after being injected into the mold, so the nozzle temperature needs to be controlled.

Injection Pressure

The injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine.

Under the pressure, the plastic melt enters the vertical flow channel (also the main flow channel for some molds), the main flow channel, the branch flow channel of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate.

This process is the injection molding process, or the filling process.The purpose of pressure is to overcome the resistance in the flow of the melt, or conversely, the resistance in the flow must be overcome by the pressure of the injection molding machine to ensure the smooth filling process.

When you’re doing injection molding, the pressure at the nozzle of the injection molding machine is the highest because you have to overcome the flow resistance of the melt throughout the whole process. Then, the pressure gradually decreases along the flow length to the front end of the melt wave front. If the exhaust inside the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.

There are many factors that affect the melt filling pressure, which can be summarized into three categories: Material factors, such as the type and viscosity of plastic; Structural factors, such as the type, number and position of the gating system, the cavity shape of the mold and the thickness of the product; Process elements of molding.

Holding Pressure

Press holding is when you squish and make the melt smaller in the mold after you fill the mold. The pressure you use to do this is called holding pressure.

In actual production, the holding pressure can be set to be equal to the injection pressure, and is generally slightly lower than the injection pressure. When the holding pressure is high, the shrinkage rate of the product decreases, the surface finish and density increase, the weld mark strength increases, and the product size is stable.

The disadvantage is that the residual stress in the product is large during demolding and it is easy to produce overflow.

Press Holding Time

The holding time is the time for compacting and shrinkage compensation of the plastic in the mold cavity, which accounts for a large proportion of the entire injection time. For products with simple shapes, the holding time can also be very short.

The length of time you hold the melt at the gate before it freezes has a big effect on how good your part is. If you hold it for a short time, the part will be low density, small, and have sink marks. If you hold it for a long time, the part will have a lot of internal stress, be weak, and be hard to get out of the mold.

Also, the holding time is related to the temperature of the material, the temperature of the mold, the size of the main flow channel and the size of the gate. If the process parameters are normal and the gating system is reasonably designed, the best holding time is usually the time when the shrinkage fluctuation range of the product is the smallest.

When you’re figuring out how long to hold the plastic in the mold, you need to think about a few things. First, you need to think about the kind of plastic you’re using and how well it works.

Second, you need to think about the conditions, like what you’re making and what the mold is like. Third, you need to think about other stuff that’s going on in the injection process, like how hot it is, how much pressure there is, how fast the plastic is going in, how fast the screw is turning, and other things like that.

Back Pressure

Back pressure is the pressure that the screw has to overcome when it reverses and retreats to store material. High back pressure is good for color dispersion and plastic melting, but it also makes the screw take longer to retract, makes the plastic fibers shorter, and increases the pressure of the injection molding machine.

So, the back pressure should be lower, generally not exceeding 20% of the injection pressure. When you’re injecting foam plastics, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will be pushed out of the barrel.

Some injection molding machines can program the back pressure to compensate for the reduction in screw length during melting, which will reduce the input heat and reduce the temperature. However, since the result of this change is difficult to estimate, it is not easy to make corresponding adjustments to the machine.

Metering Stroke (Pre-Plastic Stroke)

After each injection instruction is terminated, the screw is at the front end of the barrel. When the pre-plastic instruction is issued, the screw starts to rotate, and the material is transported to the screw head. The screw retreats under the back pressure of the rubber material until it hits the limit switch.

This is called the metering process or pre-plastic process, and the distance the screw goes back is called the metering stroke or pre-plastic stroke. So, the volume of rubber material at the screw head is the metering volume made by the screw going back, and its metering stroke is the injection stroke. How well the metering stroke repeats will affect how much the injection volume fluctuates.

Residual Material

After the screw injection is done, the molten material at the screw head cannot be completely injected, and some needs to be retained to form a residual material.

This way, on one hand, it can prevent the screw head and the nozzle from touching and causing a mechanical collision accident; on the other hand, this residual material pad can be used to control the repeatability of the injection volume to achieve the purpose of stabilizing the quality of the injection molded product . Generally, the residual material is set to 1.5~2.5mm alarm.

Anti-Delay (Loose Retreat)

Anti-delay refers to the process in which the screw metering (pre-plasticization) is in place and then retreats a certain distance in a straight line, so that the internal pressure of the melt in the metering chamber decreases and the melt is prevented from flowing out of the metering chamber (through the nozzle or gap).

Another purpose of anti-backflow is to reduce the pressure of the nozzle flow channel system and reduce internal stress; and to facilitate the withdrawal of the material rod when opening the mold. The setting of the anti-backflow depends on the viscosity of the plastic and the condition of the product.

Excessive anti-backflow will cause bubbles to be mixed in the melt in the metering chamber, seriously affecting the quality of the product. For materials with high viscosity, no anti-backflow is required. The anti-backflow is generally set to 1~2% of the screw retreat stroke.

Injection Time

The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding auxiliary time such as mold opening and closing.

Even though the injection time is short and doesn’t affect the molding cycle much, adjusting the injection time is important for controlling the pressure of the gate, runner, and cavity. A reasonable injection time helps the melt fill well, which is important for improving the surface quality of the product and reducing dimensional tolerances.

The injection time is much shorter than the cooling time, which is about 1/10 to 1/15 of the cooling time. This rule can be used as a basis for predicting the total molding time of plastic parts.

When performing mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely pushed by the screw to fill the cavity.If the screw pressure holding switch occurs before the cavity is filled, the analysis result will be greater than the setting of the process conditions.

Injection Speed

Injection speed refers to the speed at which the melt in the barrel (also known as the screw propulsion speed) (mm/s). The injection speed determines the appearance, size, shrinkage, flow distribution, etc. of the product.

It is generally slow first-fast-then slow, that is, first use a faster speed to make the melt pass through the main channel, the branch channel, and the gate to achieve the purpose of balanced injection, and then quickly fill the entire mold cavity, and then use a slower speed to supplement the insufficient glue caused by shrinkage and backflow until the gate freezes, which can overcome the poor quality such as burning, gas marks, and shrinkage.

Screw Speed

The screw speed affects the thermal history and shear effect of the injection molding material during transportation and plasticization in the screw, and is an important parameter that affects factors such as plasticization capacity, plasticization quality and molding cycle. With the increase of screw speed, the plasticization capacity, melt temperature and melt temperature uniformity are improved.

The screw speed setting is determined by the screw diameter. Each plastic material has a maximum O.D. (outer diameter) linear velocity value, usually expressed in m/s. When converted to screw speed, the screw speed range is generally 30~120 RPM.

The specific maximum linear velocity is shown in Figure 31 below. For different plastic materials, the material supplier will recommend the screw speed setting value in the specification number.

For small screws, the screw groove depth is relatively shallow, so the rubber absorbs heat quickly, which is enough to soften the rubber in the compression section. In addition, the friction heat between the screw and the barrel is small, so a higher speed can be used. For large screws, on the contrary, it is not easy to use a high speed to avoid uneven plasticization and excessive friction heat.

For heat-sensitive plastics (like PVC, POM, etc.), use a low screw speed to avoid material decomposition; for plastics with high melt viscosity (like PC, PSF, PPO, etc.), use a low screw speed too.

Opening and Closing Speed and Pressure

The clamping speed usually needs to be set with two speeds for opening and closing. First, the mold is closed quickly, and then it is closed slowly before the front and rear molds touch to avoid damaging the mold.

Locking force setting point: low pressure brings the front and rear molds together to protect the mold from low pressure; then use high pressure to lock the mold.

Ejection Force and Speed

When the product is taken out of the mold, you need to use some external force to overcome the adhesion between the product and the mold. This external force is called the ejection force. If the ejection force is too small, the product cannot be taken out of the mold; if the ejection force is too large, the product will be deformed, or even damaged.

Also, the speed and distance of the ejection also affect the ejection. If the ejection speed is fast, the product is prone to warping and damage; if the ejection distance is short, the product is prone to warping and damage. The product is not easy to take off.

Injection Molding Processes Parameter Adjustment Method

Temperature Control

Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, you set the temperature you want, and the sensor display will be compared with the temperature generated at the set point.

In this Thermocouples are also widely used as sensors in temperature control systems. On the control instrument, the required temperature is set, and the sensor display will be compared with the temperature generated at the set point. simplest system, when the temperature reaches the set point, it will turn off, and the power will turn on again after the temperature drops. This system is called on-off control because it is either on or off.

Temperature

Temperature measurement and control are super important in injection molding. It’s pretty easy to measure temperature, but most injection molding machines don’t have enough temperature sampling points or lines.

Most injection molding machines use thermocouples to sense temperature. A thermocouple is two different wires connected at one end. If one end is hotter than the other, a small electrical signal is generated. The hotter it gets, the stronger the signal.

Melt Temperature

The melt temperature is very important, and the temperature of the injection cylinder used is only a guide. The melt temperature can be measured at the nozzle or using the air injection method. The temperature setting of the shot cylinder depends on the melt temperature, screw speed, back pressure, shot size and injection cycle.  

If you don’t know what temperature to use for a particular plastic, start with the lowest setting. The shot cylinder is divided into zones, but they’re not all set to the same temperature.

If the operation time is long or at high temperatures, set the temperature of the first zone to a lower value. This will prevent the plastic from melting and diverting prematurely. Before injection begins, make sure the hydraulic oil, hopper closer, mold and shot cylinder are at the correct temperature.

Injection Pressure

This is the pressure that causes the plastic to flow and can be measured by a sensor on the nozzle or hydraulic line. It has no fixed value, but the more difficult the mold is to fill, the higher the injection pressure. There is a direct relationship between injection line pressure and injection pressure.

When you’re filling the mold, you might need to use high pressure to keep the injection speed where you want it. You don’t need high pressure after the mold is full. But sometimes, when you’re injecting certain semi-crystalline thermoplastics (like PA and POM), changing the pressure will mess up the structure, so you don’t need to use packing pressure.

Clamping Pressure

To counteract the injection pressure, you need to use clamping pressure. Don’t just automatically select the maximum value available, but consider the projected area and calculate a suitable value. The projected area of the injection molded part is the largest area seen from the direction of the clamping force.

For most injection molding situations, it’s about 2 tons per square inch, or 31 meganewtons per square meter.But this is only a rough rule of thumb and should be used as a very rough rule of thumb, because once the injection molded parts has any depth, the side walls must be considered.

Back Pressure

This is the pressure that needs to be generated and exceeded before the screw retreats. Although high back pressure is beneficial to uniform color distribution and plastic melting, it also prolongs the return time of the middle screw, reduces the length of the fiber contained in the filled plastic, and increases the stress of the injection molding machine.

Therefore, the lower the back pressure, the better. In any case, it cannot exceed 20% of the injection pressure (maximum rating) of the injection molding machine.

Injection Speed

This is about how fast the mold fills up when the screw is used like a punch. When you shoot thin-walled products, you have to shoot fast so the mold fills up before the plastic gets hard and makes a smoother surface.

We use different shooting speeds when we fill the mold to avoid problems like splashing or air getting trapped. We can inject the plastic into the mold using either an open-loop or closed-loop control system.

Nozzle Pressure

Nozzle pressure is the pressure inside the nozzle. It is the pressure that makes the molten plastic. It does not have a fixed value, but increases as the difficulty of mold filling increases. There is a direct relationship between nozzle pressure, line pressure and injection pressure.

On a screw injection molding machine, the nozzle pressure is about 10% less than the injection pressure. On a piston injection molding machine, the pressure loss can reach about 10%. On a piston injection molding machine, the pressure loss can reach 50%.

Conclusion

Injection molding is a very common process method, and the setting of process parameters is very important to the performance and quality of the product.

In the injection molding process, the reasonable setting of parameters such as temperature, pressure, and speed can effectively improve the physical properties, size, appearance, and surface quality of the product.So, in real life, you gotta make some changes to the product to make it better.