How to choose injection molding machine and general injection molding machine price cost



How to choose injection molding machine and general injection molding machine price cost
 


       The purchase of an injection molding machine is not a small investment project; it is wasteful to buy an injection molding machine that is too large, and it is too small and unsuitable; therefore, choosing an injection molding machine that meets the needs of its own products has become the most concerned content for every buyer Of course, the merchant recommends a suitable injection molding machine to the buyer, and introduces the buyer to how to choose the injection molding machine that meets the needs of his own product; this is the first lesson for the merchant to understand. In order to understand more systematically the various procedures for selecting an injection molding machine, I have written the following content and one of them.
       A good injection molding machine must make consistent plastic parts, and the same will happen the next time an order is redone. The criteria for selecting the weight by injection weight alone is oversimplification and the clamping force alone is not enough. This article explains the characteristics to consider when choosing an injection molding machine. The characteristics of injection molding machines are quantifiable and difficult to quantify. The former can be found on the injection molding machine specification sheet, and the latter can only be measured by yourself or learned from word of mouth. Only the influence of the quantifiable characteristics of the injection molding machine on the selection of the injection molding machine will now be focused on.

Quantifiable characteristics
       The quantifiable characteristics can be found on the specifications of various models of Haitian injection molding machines. In selecting these characteristics, they should be considered in their entirety and not individually. The meaning of each feature is explained in this chapter. In general, this chapter teaches you the specifications of injection molding machines. Most of the quantifiable characteristics of the specification table are the maximum permissible values ​​of the injection molding machine. In general, you can use this value or a smaller value.

1.1 Injection weight An injection weight is an important parameter of the injection device of the injection molding machine. It is the most commonly used parameter when selecting an injection molding machine, and it is also a key parameter. It is expressed in grams (g) or ounces (Note: 1 gram = 0.035 0.0 Division 1 蛊 Division = 28.35 g). Although this parameter is simple and easy to understand, it is also easy to be misunderstood. The reason is simple. When plastic is selected, the injection molder can determine the weight of the plastic part. Therefore, they can easily choose the machine based on the injection weight. In fact, the definition of injection weight refers to the maximum plastic injection weight that the injection device can reach when the injection screw makes a maximum injection stroke under the condition of air injection (without injection into the mold); this parameter reflects to a certain extent The processing capacity of the injection molding machine is measured; it can be measured or calculated from the theory (theoretical value and the measured value will have a certain degree of difference).

       The rubber used in the test is usually a general PS hard rubber with a specific gravity of 1.05. When the plastic of the plastic part is different from PS, the injection weight on the specification can be used after the following conversion. Injection weight of non-PS plastic = Wx × Vx / 1.05, (Vx = specific gravity of plastic Wx = injection weight in PS.)
For example: Saigang has a specific gravity of 1.42, which is on an injection machine with an injection weight (calculated as PS) of 227 grams
Injection. The injection weight of this machine should be 227g * 1.42 / 1.05 = 307g grams of steel.

       The specific gravity (Vx) of commonly used plastics is now listed in Table 2-3. Common plastic density (g / cm3)

Plastic name Density Plastic name Density
Unplasticized polyvinyl chloride (PVC)

1.35 ~ 1.45

High-density polyethylene (HDPE)

0.94 ~ 0.965

Modified plexiglass (372) (PMMA)

1.18

Polyethylene (PE)

0.92

Modified Polystyrene (204) (PS)

1.07

Polypropylene (PP)

0.9 ~ 0.92

Ultra high impact ABS

1.05

Polysulfone (PSF)

1.24

Low-temperature impact type ABS

1.02

Nylon 1010 (unreinforced) (PA)

1.04 ~ 1.06

High strength medium impact ABS

1.07

Nylon 1010 (glass fiber reinforced) (PA)

1.23

Heat-resistant ABS

1.06 ~ 1.08

Nylon 66 (PA)

1.14 ~ 1.15

Polyphenylene ether (PPO)

1.06 ~ 1.07

Polycarbonate (Unreinforced) (PC)

1.20

Polyoxymethylene (POM)

1.41

Polycarbonate (reinforced) (PC)

1.4 ~ 1.42

Polyethylene terephthalate (PET)

1.35



1.2 Determination of the injection volume-First calculate the weight of the product: W = product weight + gate system weight, where: (W = weight required for injection). All actual injection volume parameters are calculated using polystyrene as an example, so in the actual situation there are two cases to consider. When injection molded products are made of polystyrene (PS), the injection volume that the injection molding machine should have is Wps; Wps = (3 to 1.2) × (product weight + gate system total weight), of which, when the product quality requirements When the period is high or the period is short, the coefficient in the above formula should take a large value, otherwise it can take a small value. When the product is other plastic (named X plastic), the theoretical injection quantity of the plastic should be calculated according to the above method. Wx; Wx = (3 ~ 1.2) × (product weight + gate system total weight) ; And then convert the actual weight WPS of PS material according to the density of this plastic (X), the conversion formula is: Wps = Wx × 1.05 / Vx; according to the calculation result, compare Wps with the product sample, and select a suitable injection molding machine.
For example: suppose an injection molded product is made of polyethylene (PE). It has been calculated that the product itself weighs 185 grams. It is estimated that the runner system weighs 20 grams. Use the above formula to first calculate: Wx = 1.2 × (185 + 20) = 246 It is found from the manual or the above table that the specific gravity of PE material is Vx = 0.92, so Wps = 246 × 1.05 / 0.92 = 280.8 grams. For comparison parameter table, Haitian's HTF160W2-B or above injection molding machine is better.

1.3 The relationship between injection weight and injection volume-The injection weight is not the injection volume multiplied by the specific gravity of PS: the injection weight is measured and the injection volume is theoretical. The injection volume multiplied by the specific gravity of PS is larger than the injection weight, because plastic will flow into the gap between the barrel and the screw during injection. Also, the check valve needs to be moved back to reach the closed position. Therefore, manufacturers generally use the injection volume as the starting point for calculating the injection weight. The theoretical injection weight = injection volume × raw material melt density × injection efficiency coefficient (usually 0.86)

1.4 Choose an injection molding machine with sufficient injection weight. One should not choose an injection molding machine whose injection weight is just equal to the weight of the plastic part plus the weight of the runner plastic. In less demanding injection molding, such as toy portraits: the total weight should be 85% of the injected weight; in demanding injection molding; it is better to use less than 70%. (The plasticization is more uniform, there is less raw material)
1.5 Injection weight is too high or too small is not good: the total weight of plastic parts and runner plastic is generally between 30% and 85% of the injection weight. The lower limit is due to the following three considerations: the bending of the template; the residence time of the plastic in the barrel and the power consumption per kilogram of injection molded parts. The use of small molds for small injection molded parts can cause excessive bending of the template, causing him to flex (affect product quality), and even rupture the template. Using too large an injection molding machine to inject small plastic parts, melting too long will cause the plastic to decompose.
 1.6 The residence time in the barrel can be estimated by referring to the following formula. Dwell time in the barrel = (plastic weight in the barrel x injection cycle time / injection weight per mold. The plastic weight in the barrel is approximately 2 times the estimated injection capacity.
       For example: rigid polyvinyl chloride (UPVC) with a specific gravity of 1.38 is injected on an injection molding machine with a screw diameter of 55 mm, an injection stroke of 250 mm, and an injection weight (characterized by PS) of 570 grams. The injection cycle is 35 seconds, and the weight of each molding compound is 340 grams. How long is the dwell time? The melting capacity in the barrel is estimated by double the injection capacity = 2 * 3.1416 x 5.5 x 5.5 x 25/4 = 1188 cm3. Or directly take the injection capacity of the machine x 2 times (570 x 2 = 1140) and the residence time in the barrel is 1188 × 1.38 × 35/340 = 168 seconds. (Equivalent to the time of 5 molds) If replaced with a 50mm screw, the dwell time is 470 x 2 x 1.38 x 35/340 = 133 seconds. In addition, too short residence time will cause poor plasticization of the raw materials, which is also not good. In principle, it is ideal to use the same amount of material per minute as the injection volume. Using a multi-cavity mold or increasing the size of the mold can solve some problems of using large machines to inject small plastic parts. Lowering the barrel temperature can also ease the decomposition caused by long residence time.
       The above description mainly emphasizes the choice of the capacity of the injection molding machine. The factors to be considered are related to the material type, cycle, and quality requirements. When selecting, you need to consider the overall consideration.
1.7 Screw diameter: On the injection device of a machine, many manufacturers provide a variety of screw diameter options. The diameter of the screw directly affects the aspect ratio, injection pressure, and injection volume (and therefore also the injection weight).

1.8 L / D Ratio: The ratio of the effective length of the screw to the diameter of the screw is the L / D ratio of the screw.
       If the injection molding machine has an optional screw, the screw aspect ratio is an important parameter in the selection process. An aspect ratio of 22: 1 or greater provides better mixing and more uniform heating in the compression zone of the screw. Highly required injection molded parts, such as injection engineering plastics, or high-precision injection within a tolerance of 0.01 mm, should choose a large aspect ratio. At a given screw length, a larger aspect ratio equals a smaller screw diameter. Therefore, the injection pressure increases, and the injection volume and injection weight decrease.
       The aspect ratio of 20: 1 is medium, which is suitable for general injection molding. The aspect ratio of 18: 1 or smaller is suitable for less demanding injection molding, and the larger injection weight is more important. The injection pressure is not high at this time.
       When the diameter of the screw is constant, a large aspect ratio can increase the length of each section of the screw and improve the plasticization quality. If the aspect ratio is too large and the screw length is long, the screw's own weight is increased, causing the front of the screw to sag and cause the screw and The gap of the barrel is uneven, and even when the screw rotates, the barrel is scraped, which reduces its service life. (Theoretically this is the case, but it can be overcome by materials and processes) The size of the aspect ratio will also be related to the raw materials used. Generally, high viscosity and heat sensitive materials are not recommended to use a large aspect ratio, generally 22 : 1 or less, such as PVC, PC, PMMA, and flame-retardant materials; for low-viscosity raw materials that require color mixing at the same time, it is recommended to adopt a large aspect ratio, and at the same time, the screw structure can be considered to increase the flow blocking device . Each minute requires a large amount of material, and when the injection molding machine has a small capacity, a large aspect ratio can also be used to improve the plasticizing ability.
       Some raw materials do not allow the compression ratio to be too large. (The larger the compression ratio, the greater the shear force). It also needs to be overcome with a large aspect ratio. For example PET, PBT and so on.

 1.9 Injection pressure: In order to overcome the flow resistance of the molten material flowing through the nozzle, runner and mold cavity, the screw applies pressure on the molten material; the injection pressure in the specification table of the injection molding machine is the highest pressure at the front of the barrel during injection. Not the highest pressure of oil pressure. The relationship between injection pressure and oil pressure is inversely proportional to the area of ​​the cross section of the screw and the area of ​​the injection cylinder.
       Usually, the injection pressure is about 10 times the highest oil pressure. If the injection device has a screw option, a smaller screw diameter produces a higher injection pressure.
The higher injection pressure facilitates the injection of engineering plastics. Because engineering plastics have higher viscosity, the higher the viscosity, the greater the flow resistance. For a certain type of screw, the maximum injection pressure is constant. If the injection pressure is set too high, the product may produce burrs, and it may be difficult to demould, affecting the smoothness of the product, and causing a large internal stress on the product.
       If the injection pressure is too low, it is easy to cause the material to fill the mold cavity, and even it cannot be formed. Generally low processing precision, good fluidity and low density plastics such as polyethylene and polyamide, the injection pressure can be less than or equal to 70-110Mpa: processing of medium viscosity plastics such as modified polystyrene, polycarbonate, etc. For products with a general shape but with certain accuracy requirements, the injection pressure should be 100-160Mpa; processing high-viscosity engineering plastics such as polysulfone and polyphenylene ether, etc., thin-walled long processes, uneven thickness and strict requirements for precision products The pressure is about 140-200Mpa; when processing high-quality precision micro products, the injection pressure can be more than 200-250Mpa.

     Table 2-5 lists the commonly used plastic injection pressure ranges.


Plastic
Free-flowing thick-walled products

Medium flow, general products

Difficult to flow, thin-walled narrow gate products

ABS

80-110

100-150

120-200

Polyoxymethylene (POM)

85-100

100-130

120-200

Polyethylene (PP)

70-100

100-120

120-200

Polyamide (PA)

90-110

110-140

> 140

Polycarbonate (PC)

100-120

120-150

> 160

Plexiglass (PMMA, 372)

100-120

120-150

> 160

Polystyrene (PS)

80-100

100-120

120-200

Unplasticized polyvinyl chloride (PVC, UPVC)

100-120

120-150

> 160

Thermosetting plastics (MF, PF, BMC)

100-140

140-175

175-230

Elastomers (TPR, EVA)

80-100

100-120

120-160

       Plastic suppliers publish the maximum and minimum injection pressures available for plastic in the plastic specification sheet.

2.0 Injection Stroke: For a given screw diameter, increasing the injection stroke can increase the injection volume. However, increasing the injection stroke will increase the injection time and therefore the injection cycle. It also reduces the effective screw length and therefore the aspect ratio. The advantage of high aspect ratio is lost. From the specifications of the injection machine with an aspect ratio of 18: 1, statistics show that the injection stroke is about 4 times the screw diameter. When the aspect ratio is increased, the injection stroke can be appropriately lengthened.
It is recommended that the injection stroke of an injection molding machine be no greater than 5.5D. When choosing an injection molding machine, you must pay attention to the fact that the large injection stroke and the high injection volume and weight are actually sacrificing the injection time and the length-to-diameter ratio. ).

2.1 Injection Volume-The injection volume is theoretical and it is equal to the cross-sectional area of ​​the screw multiplied by the injection stroke.
Injection volume (cm3) = (d2 / 4) × i; (d = screw diameter is equal to the diameter of the inner hole of the barrel, i = injection stroke is calculated in cm). Due to the melt backflow and backflow of the check valve, the actual injection volume is approximately 97% of theory. To calculate the injection weight from the actual injection volume, the change in the specific volume of the plastic in the solid and dissolved states should be considered. Under normal circumstances, the actual weight is 85% of the theoretical volume X specific gravity (solid weight)
2.2 Injection speed: In the specification table of the injection molding machine, the injection speed is the fastest speed of the screw during injection, and it is calculated in cm / s. The injection speed affects the injection time. High injection speeds are required when injecting thin-walled parts to prevent the melt from cooling and solidifying when the cavity is not full. By controlling the flow of pressure oil, the injection molding machine can have multiple injection speeds during injection. The constant-speed leading-edge theory states that the best injection molding is when the molten front is advancing at a constant speed in the mold cavity.
       Because the cross-sectional area of ​​the mold cavity is not consistent, multiple injection speeds are needed to achieve a constant leading edge flow during injection. Some injection molding machines have a rate of fire of up to 10 segments. Some injection molding machines can add accumulators to speed up injection. The accumulator stores high-pressure pressurized oil during the low oil consumption phase of the injection cycle for use in the high oil injection phase. It equalizes the load of the electric motor and reduces its overload. Although increasing the motor and oil pump (alternative devices provided by some injection molding machine manufacturers) can increase the injection speed by about 25%, the accumulator can generally increase the injection speed by about 3 times.
       The higher the injection speed, the higher the control capacity of the hydraulic system is required, otherwise it will cause runaway or instability. Generally, the rate of fire is 150mm / S. The following can be controlled by general open loop. The rate of fire is 150mm / S to 200mm / S. In the following, directional proportional closed-loop control is used, 200mm / S. The above is better controlled with a servo valve.
       Some customers will require a high injection speed when selecting a machine, but in essence, what is really needed is the injection rate, because from the injection filling, how long does the product need to complete the filling. At the same injection speed, the smaller the screw, the smaller the injection rate, and vice versa.

2.3 Injection rate: The injection rate refers to the amount of dissolved material (volume) ejected from the nozzle in a unit time. The calculated value is the product of the barrel cross-sectional area and speed.
       Some injection molding machine manufacturers use the injection rate instead of the injection rate in the specification table. The injection rate is the maximum capacity of the screw per second during injection. It is calculated in cm3 / second or G / S.
       Injection rate = injection speed × 3.1416 × (d / 2) 2 × melt density of the raw material × injection efficiency coefficient (d = screw diameter)

       The injection rate is low, the dissolving material is filled for a long time, and the product is prone to remaining cold joints, uneven front and rear density, and large stress. When injecting products, at the same time 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 forming overfill conditions. However, the injection speed is too high. When the molten material passes through the gate, it generates a large amount of shear heat, which is easy to form irregular flow, scorching of the material, and poor intake of gas and exhaust. At the same time, under the general open-loop control , High-speed injection can not guarantee the stable switching between injection and holding pressure, which results in instability and causes the product to appear overflow or lack of material.

       The minimum injection rate requirement is that the filling is complete before the plastic channel has solidified.

2.4 Screw speed: The screw speed is an upper and lower limit expressed in revolutions per minute (rpm). Screw speed is less important than screw surface speed. Both are related to screw diameter. Surface speed of the screw (cm / min.) = 3.1416 x screw diameter (cm) x screw speed (rev / min). Each plastic has its recommended maximum screw surface speed (line speed) and should not be exceeded. Such as rigid polyvinyl chloride is called PVC) The surface speed should not exceed 12 m / min, polypropylene PP can reach 60 m / min. , PC can allow 30 m / min.
       For example: What is the maximum screw speed when using a 60mm screw to inject rigid polyvinyl chloride (UPVC)? The maximum screw speed = 1200 / (3.11416 * 6) = 64 rpm.
       Increasing the screw speed can greatly improve 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 improvement of screw rotation must be matched with a large-displacement hydraulic pump and an enlarged motor. The hydraulic system must also have a larger flow rate, which greatly increases the overall cost. (Some low-viscosity plastics can reduce the displacement of the hydraulic motor to increase the speed)

2.5 Screw drive motor torque: The hydraulic motor that drives the screw has a rated torque, which is enough to calculate in Newton meters in the International Unit System (S1). It represents the maximum torque produced at a specified system pressure. High-viscosity plastics require high torque and low speed, while low-viscosity plastics do the opposite. Larger screws require more torque than smaller screws. Use proportional pressure valve to adjust the motor torque during feeding to the required value.
     On the same injection molding machine, it is equipped with a fixed hydraulic motor. The general design practice is low viscosity materials, the large and small screws have sufficient torque (PP, PS, PE and other low viscosity grades), higher viscosity materials, A B screw can be used (ABS, TPR, etc., some high viscosity materials, only A screw (PA, PC, PET, PMMA, etc.). In addition, the larger the aspect ratio, the greater the required torque.



2.6 Plasticizing capacity: Plasticizing capacity is the weight of general hard rubber (PS) that an injection molding machine can uniformly plasticize or raise to a uniform melting temperature per unit time at the maximum screw speed and zero back pressure. 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), efficiency is generally 85%, so the plasticizing ability of materials other than PS can be converted according to the difference in specific gravity. The size of the plasticizing ability is related to the screw diameter, screw design, screw speed, etc. The plasticizing capacity is the maximum value (PS), but because the allowable linear speed (screw speed) of each material is different, the plasticizing capacity of each material is also different. In order to increase the production volume, reduce the cycle, and the plasticizing time (Storage) is best completed within the cooling time. In order to reduce energy consumption, the plasticizing time does not need to be much shorter than the cooling time. How to choose depends on the specific or most of the products produced.

2.7 Clamping force: (also called clamping force) An important parameter of injection molding machines on the market, that is, the clamping force exerted by the injection molding machine on the mold. The clamping force is the same as the injection volume, which reflects the capacity of the machined product to a certain extent; and is used as the main parameter to indicate the size of the machine specifications; most injection molding machine manufacturers now use clamping force (tons) as the Machine model name. When selecting the machine, try to use the value below the maximum clamping force. Sufficient clamping force is proportional to the projection 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 the product to produce flashing (flipping) or cannot be formed; if the clamping force is too large, system resources will be wasted, and if the clamping force and template are far greater than Needed by the mold, will increase the amount of template deformation, and reduce the life of the machine and mold. The general clamping force is expressed in metric tons (equal to 1000 kg) or kilonewtons. One metric ton is approximately equal to 10 kilonewtons. There are several methods to estimate the clamping force.
1. Calculate the clamping force F based on the vertical projection area of ​​the injection molded product on the template (head plate or two plates):
Clamping force (ton) = In-mold pressure (kg / cm2) X Projected area of ​​product (cm2) ÷ 1000
That is, F = P XS, where F = clamping force (t), S—the vertical projected area of ​​the product on the template (cm2), P—in-mold pressure (kg / cm2), and the P value is listed in the table below (in-mold pressure) (Unit: kg / cm2)


Plastic name
General products

Thin-walled, precision, high quality

Ultra-thin (below 0.6) high flow length ratio

PS, PE, PP, ABS
PET.

150

200 ~ 300

500 ~ 1000

PA, POM, PBT

200

200 ~ 300


PC, PMMA,

300

300 or more

600 or more

for example:
       Suppose that the projection area of ​​a common product in the vertical direction of the head plate or the second plate is 410 cm2, and the product material is PE, and the required clamping force is calculated.

       Calculate from the above formula as follows: F = P.S = 150X410 ÷ 1000 = 61.5 (tons). According to Haitian's model table, our company's 60-ton injection molding machine, HTF60W, should be used. (Lower configuration)
       Reasonable clamping force selection, after the calculation method listed above is calculated, it is ideal to divide it by a safety factor of 0.8. For example: a general hard plastic cup with a diameter of 79 mm is to be injection molded, and the wall thickness of the cup is 0.6 mm. Find sufficient clamping force. The projection area of ​​the cup (and runner) is 3.1416 × 7.9 × 7.9 / 4 = 49 cm 2. This cup belongs to the category of thin wall. The conservative clamping force is 49 X 500 ÷ 1000 ÷ 0.8 = 30.6 tons.
3. It is a more accurate method to consider the process and wall thickness in the estimation.

  The 3.1 flow is the longest distance o from the gate to the mold cavity. If the wall thickness of the injection-molded parts is different, take the minimum wall thickness.
       For example: the flow of the same hard plastic cup is 104 mm. Find a more accurate clamping force. Flow wall thickness ratio = 104 / 0.6 = 173. From Figure 2, the cavity pressure at a wall thickness of 0.6 mm is 550 bar.
       Clamping force = 550 × 1.02 × 49 ÷ 1000 ÷ 0.8 = 34.5 tons

       The above estimates do not take viscosity into account. But the estimation is still correct, because the viscosity factor of general hard rubber is 1.0.
       For example: the same plastic cup is now injection molded with super non-breakable plastic (ABS) to find the required clamping force. With a viscosity factor of 1.5, the required clamping force = 1.5 x 34.5 tons = 52 tons.
4. A more accurate estimation of the clamping force is the clamping force calculated by computer simulation when the mold is designed.

3.2 Mold opening stroke-The mold opening stroke is the displacement distance of the moving template from the mold clamping to the mold opening. The 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 vacancy to the center of gravity, and the robot or human hand can remove the injection molded part.

     Based on the above, you can basically choose the right injection molding machine. At present, the injection molding machine with a clamping force of 90 tons in China is usually around 100,000 to 120,000. Based on this price depreciation, plus electricity consumption, labor, plant, and management costs, the hourly cost of the injection molding machine can be calculated. Currently, 90T The cost in the industry is 40-45 yuan / hour.