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How To Choose and Buy Injection Molding Machines Is Very Important!

Jan. 06, 2021


Generally speaking, most customers who have been engaged in injection molding industry for many years have the ability to judge and choose the appropriate injection molding machine for production. However, in some cases, the customer may need the assistance of the manufacturer to decide which injection molding machine to use, and even the customer may only have samples or ideas of the products, and then ask whether the manufacturer's machine can produce or which model is more suitable.


In addition, some special products may need to be matched with special devices, such as pressure accumulator, closed loop, injection compression, etc., in order to produce more efficiently.


Thus, it is an extremely important issue how to choose the appropriate injection molding machine for production. The following information is provided for readers' reference.


Usually, the important factors affecting the selection of injection molding machine include mold, product, plastic, molding requirements and so on.


Therefore, the following information must be collected or available before making the selection:


① Die size (width, height, thickness), weight, special design, etc

② Types and quantities of plastics used (single raw material or multiple plastics)


③ Appearance dimensions (length, width, height and thickness) and weight of injection molded products


④ molding requirements, such as quality conditions and production speed.


After obtaining the above information, you can choose the appropriate injection molding machine according to the following steps:


1.Selecting the right model: the model and series are determined by the products and plastics.


Since there are many kinds of injection molding machines, it is necessary to correctly judge which injection molding machine or series should be used to produce this product, such as general thermoplastic plastics or bakelite raw materials or PET raw materials, which are monochrome, bicolor, multicolor, sandwich or mixed colors.


In addition, some products need high stability (closed loop), high precision, ultra-high firing rate, high firing pressure or rapid production (multi-loop), etc., and suitable series must be selected for production.

Ii. fit: judge whether the "big column inner distance", "die thickness", "die minimum size" and "die plate size" of the machine are appropriate according to the die size to confirm whether the die can fit.


(1) the width and height of the mold should be less than or at least one side should be less than the distance between large columns


② The width and height of the die should be within the size range of the die plate


③ The thickness of the mold should be between that of the injection molding machine


④ The width and height of the mold should conform to the minimum mold size recommended by the injection molding machine, even if it is too small.


3. Obtaining: Determine whether the "mold opening stroke" and "mold supporting stroke" are enough for the finished product to be taken out by the mold and the finished product.


(1) the mold opening stroke should be at least more than twice the height of the finished product in the direction of mold opening and closing, and the length of sprue should be included


(2) The stroke of die support should be enough to eject the finished product


IV. Locking: The tonnage of "clamping force" is determined by products and plastics.


When the raw materials are injected into the mold cavity at high pressure, there will be a force to support the mold, so the mold locking unit of the injection molding machine must provide enough "clamping force" to prevent the mold from being opened.


The clamping force requirement is calculated as follows:


(1) calculating the projection area of the finished product in the direction of the switch die from the appearance size of the finished product


(2) die supporting force = projected area of finished product in the direction of opening and closing die (cm2)× die cavity number × die pressure (kg/cm2)


③ The in-mold pressure varies with the raw materials, and the general raw materials are 350 ~ 400 kg/cm2


④ The clamping force of the machine needs to be greater than the supporting force, and for the sake of safety, the clamping force of the machine usually needs to be more than 1.17 times of the supporting force.


So far, the specifications of the mold clamping unit have been preliminarily determined, and the tonnage of the model has been roughly determined. Then, the following steps must be carried out to confirm which injection unit has a screw diameter that meets the requirements.


5. Shot saturation: Determine the required "shot quantity" and select the appropriate "screw diameter" according to the finished product weight and the number of die holes.


(1) to calculate the weight of finished products, the number of die holes should be considered


② For the sake of stability, the injection quantity should be more than 1.35 times of the finished product weight, that is, the finished product weight should be within 75% of the injection quantity.


6. Good shooting: judging the conditions of "screw compression ratio" and "shooting pressure" by plastic.

Some engineering plastics need higher injection pressure and suitable screw compression ratio design to have better molding effect. Therefore, in order to make the finished products shoot better, the injection pressure and compression ratio should also be considered when selecting screws.


Generally speaking, a screw with a smaller diameter can provide a higher ejection pressure.


VII. Fast shooting: confirmation of "shooting speed".


Some finished products need to be shot at high shooting rate and speed in order to form stably, such as ultra-thin finished products. In this case, it may be necessary to confirm whether the shooting rate and shooting speed of the machine are sufficient, and whether it is necessary to match the pressure accumulator, closed loop control and other devices. Generally speaking, under the same conditions, the screw that can provide higher firing pressure usually has lower firing rate, on the contrary, the screw that can provide lower firing pressure usually has higher firing rate. Therefore, when selecting the screw diameter, the injection quantity, injection pressure and injection rate (injection speed) need to be considered and chosen.


In addition, a multi-loop design can be adopted to shorten the molding time by synchronous compound action.


After the above steps, the injection molding machine that meets the requirements can be determined in principle, but some special problems may have to be considered again, including:


1, the size of the problem:


Under some special conditions, such as the customer's mold or product may have a small mold size but a large amount of shot, or the mold may have a large size but a small amount of shot. In this case, the standard specifications preset by the manufacturer may not meet the customer's needs, but the so-called "size matching" must be carried out, that is, "big wall small shot" or "small wall big shot".


The so-called "big wall shooting" refers to matching the original standard die clamping unit with a smaller injection screw, whereas the "small wall shooting" refers to matching the original standard die clamping unit with a larger injection screw. Of course, it is also possible that there is a difference of several levels between clamping and injection.


2, the concept of fast machine or high speed machine:


In practice, more and more customers will ask to buy so-called "high-speed machines" or "fast machines". Generally speaking, the purpose is to shorten the molding cycle and increase the output per unit time, so as to reduce the production cost and improve the competitiveness.


Generally, there are several ways to achieve the above objectives:


Speeding up the injection speed: increase the motor and pump, or add pressure accumulator (preferably add closed loop control)


Accelerating the feeding speed: increase the motor and pump, or reduce the feeding hydraulic motor to speed up the screw speed


Multi-loop system: Double-loop or three-loop design is adopted to synchronously perform compound actions and shorten molding time


Increase the waterway of the mold to improve the cooling efficiency of the mold.


Although the improvement and transformation of machine performance can increase production efficiency, it often increases investment cost and operation cost. Therefore, the benefit evaluation before investment needs to be carefully measured in order to produce the highest benefit with the most suitable machine type.


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