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How to Choose an Injection Molding Machine

Dec 21.2021

How to choose an injection molding machine

The purchase of injection molding machines is not a small investment project; buying too large an injection molding machine is a waste, and buying too small is unsuitable; therefore, choosing an injection molding machine that meets the needs of your product becomes the most concerned content of every buyer .

There are too many types of injection molding machines on the market, how should we choose it? Today we specially wrote an article for you on how to choose an injection molding machine. If you plan to buy an injection molding machine, you must read this article!


Injection Rate

The injection rate refers to the amount (volume) of the molten material ejected from the nozzle per unit time. The calculated value is the product of the cross-sectional area of ​​the barrel and the speed.

Some injection molding machine manufacturers use injection speed instead of injection speed in the specification table. The injection rate is the maximum capacity of the screw per second during injection. It is calculated in cm3/sec or in G/S. Injection rate = injection speed x 3.1416 x (d/2) 2 x melt density of raw material x injection efficiency coefficient (d = screw diameter).

The injection rate is low, the melt filling time is long, the product is prone to cold joints, uneven density before and after, and high stress.

For injection products, while high-speed injection, low-temperature molds can be used to shorten the molding cycle. High-speed injection can also reduce the required clamping force without overfilling. However, if the injection speed is too high, when the molten material passes through the gate, large shear heat is generated, which is easy to form irregular flow, material burnt, and poor gas inhalation and exhaust. At the same time, under general open loop control , High-speed injection also cannot guarantee stable switching between injection and holding pressure, resulting in instability and causing product overflow or material shortage.

The minimum requirement for injection rate is: filling is completed before the plastic channel is solidified.

 

Screw Speed

The screw speed is an upper and lower limit expressed in revolutions per minute (rpm). The screw speed is less important than the screw surface speed. Both are related to the screw diameter. Screw surface speed (cm/min.)=3.1416×screw diameter (cm)×screw speed (rev/min).

Each plastic has its recommended maximum screw surface speed (linear speed), which should not be exceeded. For example, the surface speed of rigid polyvinyl chloride (PVC) should not exceed 12 m/min, and polypropylene PP can reach 60 m/min. , PC can allow 30 m/min.

For example: when using a 60mm screw to inject rigid polyvinyl chloride (UPVC), what is the maximum screw speed? The maximum screw speed = 1200/(3.1416*6) = 64 rpm.

Increasing the screw speed can greatly improve the production efficiency, but it will also increase the shear heat of the melt, which may cause the melt to overheat. At the same time, the increase of the screw rotation must be matched with a large displacement hydraulic pump and an enlarged motor, and the hydraulic system must also have a larger flow rate, which greatly increases the overall cost. (Individual low-viscosity plastics can reduce the displacement of the hydraulic motor to increase the speed)

 

Screw Drive Motor Torque

The hydraulic motor that drives the screw to rotate has a rated torque, which is sufficient for calculation in Newton-meters in the International System of Units (S1). It represents the maximum torque generated under the specified system pressure. Plastics with high viscosity require high torque and low speed, while plastics with low viscosity are the opposite. Large screws and smaller screws require more torque. Use a proportional pressure valve to adjust the motor torque during feeding to the required value.

On the same injection moulding machine, it is equipped with a fixed hydraulic motor. The general design practice is low viscosity materials, large and small screws have sufficient torque (PP, PS, PE and other low viscosity grades), higher viscosity materials, , B screw can be applied (ABS, TPR, etc., some high viscosity materials, only applicable to A screw (PA, PC, PET, PMMA, etc.).

In addition, the greater the aspect ratio, the greater the torque required.

 

Plasticizing Ability

Plasticizing capacity is the weight of general hard rubber (PS) that can be uniformly plasticized or raised to a uniform melting temperature per unit time under the highest screw speed and zero back pressure of an injection moulding machine. The unit is expressed in grams/second, or kg/hr. Plasticizing capacity (kg/hr.) = 1.29×D2×h1×density×rpm×60÷1000×efficiency D=screw diameter (cm), h1 = measuring section Depth (cm), the efficiency is generally 85%

Therefore, the plasticizing capacity of materials other than PS can be converted according to the difference in specific gravity. The size of the plasticizing capacity is related to the screw diameter, screw design, screw speed, etc. The plasticizing capacity marked on the machine is the maximum (PS), However, because the allowable linear speed (screw speed) of each material is different, the plasticizing ability of each material is also different.

In order to increase production and reduce cycle time, the plasticization time (storage) is best completed within the cooling time. In order to reduce energy consumption, the plasticization time should not be much shorter than the cooling time. The choice depends on the specific or most of the production It depends on the product.

 

Clamping Force

(Also called clamping force) An important parameter of injection molding machines on the market, that is, the clamping force applied by the injection molding machine to the mold.

The clamping force is the same as the injection volume, which reflects the ability of the machine to process the product to a certain extent; and is used as the main parameter to indicate the size of the machine; now most injection molding machine manufacturers use the clamping force (ton) as the Name the machine model.

When selecting the machine, try to use the value below the maximum clamping force. Sufficient clamping force is proportional to the projected area of the mold cavity, which is the area of the mold cavity projected on the mold separation surface. It should be noted that: insufficient clamping force will cause flashing (flopping) of the product during the production process or failure to form; and if the clamping force is too large, it will cause a waste of system resources. If the clamping force and template are much greater The mold needs will increase the template deformation, and reduce the life of the machine and the mold.

Generally, the indication of clamping force is expressed in a metric ton (equal to 1000 kg) or kilonewton force, and 1 metric ton is approximately equal to 10 kilonewtons.

 

Mold Opening Stroke

The mold opening stroke is the displacement distance of the moving template from mold closing to mold opening. The mold opening stroke determines the maximum height H of the injection molded part. The relationship is: mold opening stroke ≥ 2H + nozzle length L. If using hot runner system, L=0. The above inequality provides space for gravity, manipulators or human hands to take away the injection molded parts.

 

Mold Height (thickness)

The mold height is leftover from the era of vertical clamping devices. In the horizontal mold clamping device, the more appropriate name is mold thickness. In the specifications of a hinged injection molding machine, the modulus is expressed in a range, which represents the minimum to maximum mold thickness that the injection molding machine can accommodate. The difference between them is the modulus adjustment of the injection molding machine.

In the specifications of an injection molding machine with direct hydraulic clamping, the modulus is represented by a number, which represents the minimum mold thickness that the injection molding machine can accommodate. The available mold thickness should be greater than the minimum modulus, so that the injection molding machine can close and lock the mold. Otherwise, use a smaller injection molding machine (in fact, it should be a smaller clamping device). The available mold thickness should be smaller than the maximum modulus, so that the mold can be put down. Otherwise, a larger injection molding machine should be used. This parameter determines the size of mold opening space and product depth. The greater the modulus, the deeper the product depth, on the contrary, the shallower the product depth.

 

The Maximum Opening Distance of Template

The maximum opening distance of the template is the maximum distance between the fixed template and the movable template when the clamping device is retracted. Its relationship with mold opening stroke and maximum/minimum modulus is as follows

On the hinged injection molding machine, the maximum opening distance of the template = opening stroke + maximum modulus.

On a direct pressure hydraulic mold clamping injection molding machine, the maximum template opening distance = mold opening stroke + minimum modulus tolerance.

 

Tie Rod Spacing

The distance between the guide posts determines the overall size of the mold, and the mold can be used only when it can be accommodated in the tie rod space. The drawbar space is expressed in horizontal and vertical dimensions, as shown in the following icons. If the mold is hoisted from above, its width should be smaller than the horizontal tie rod space. If the mold is pushed in from the side, its length should be less than the vertical tie rod space.

It is recommended to leave more than 25 mm of space on both sides of the mold, and 50 mm of space on both sides of the large mold. It is also necessary to prevent the heavy mold from colliding with the tie rod during installation, causing it to dent and affecting the smooth movement of the subsequent mobile template.

 

Template Size

The template is a thick plate supported behind the mold. It is recommended that the mold should not exceed the template size to prevent the mold from bending during injection. It is generally recommended that the shape of the template is not greater than the center distance of the tie rods.

The bumper of an injection molded car is an example of the use of a very wide template. Some manufacturers can provide enlarged special templates.

If the mold is too small, it will produce excessive bending stress on the template, and even break the template. The smallest mold should occupy two-thirds of the inner distance of the tie rod.

 

Diameter of Tie Rod

Except for small machines (with a clamping force of 30 tons or less) that have two tie rods, most injection molding machines with tie rods use four tie rods. Their common pulling force locks the mold to cope with the reverse thrust formed by the cavity pressure during injection.

If the tensile force is evenly distributed on the four tie rods, the stress of each tie rod is: Stress = clamping force × 1000/(3.1416 × (d2/4) * 4) = clamping force × 1000/(3.1416 ×d2). (The stress is calculated in kg/mm2, the clamping force is calculated in tons, and the diameter d is calculated in mm.)

The fracture stress of high tensile steel exceeds 90 kg/mm2. Low carbon steel is only 20 kg/mm2. However, considering a safety factor, the general safety factor is set at more than 10 times. That is to say, in the case of using high tensile steel, the general design stress is below 9 kg/mm2. When the breaking stress is exceeded, the tie rod will break.

Reasons for tie rod breakage include:

a. Uneven mold thickness;

b. The cavity is not symmetrical;

c. The mold adjustment device is out of adjustment.

They all cause uneven pull of the tie rod.

Another reason is that the mold temperature rises to cause the mold to expand, and the tension of the tie rod is therefore higher than when the mold is adjusted. The tie rod fractures are usually at the bottom of the screw thread because of stress concentration there. In particular, the occurrence rate of the bottom of the screw in the first three teeth of the fixed template is as high as 90%. For this reason, Haitian has a special elastic extension structure for the nut at the fixed template to improve it.

 

Conclusion

The above is the detailed explanation about choosing the injection molding machine. If you want to buy the injection molding machine with reasonable price and good quality, please visit our official website: www.sunbun-machine.com

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Zhejiang Sunbun Intelligent Equipment Co., Ltd (Sunbun Machine) is the branch of Sunbun Group. Which Founded in 1993. Through nearly 30 years development Sunbun Group now have Wenzhou, Hangzhou, Shanghai , Xuzhou , Quzhou 7 Production Base.
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