Injection molding machine is also known as plastic injection molding machine or injection machine. It is the main molding equipment for making thermoplastic or thermosetting plastic into various shapes of plastic products by using injection molding molds. Accurate selection of injection molding machine equipment to match the scale of production or development of new products and other needs of the enterprise can maximize the economic benefits of limited funds. Several important elements of injection molding machine selection include: product weight, theoretical injection capacity, injection weight, product projection area, product wall thickness, cycle time, raw materials, clamping force, in-mold pressure, injection rate, injection power, etc. Today, we will teach you how to choose injection molding machine through five minutes.
1. Injection weight
On injection capacity (cm³) = screw cross-sectional area (cm²) × injection stroke (cm)
Injection weight (g) = theoretical injection capacity (cm³) × raw material density (g/cm³) × 0.86
Density of commonly used plastics
2. Screw diameter and speed
Usually there are 2 to 4 screws available for the same machine.
Smaller screws have larger L/D ratio, higher injection pressure, smaller injection volume, smaller injection rate, smaller melt rate, smaller torque required, and better mixability, while larger screws are the opposite.
When selecting a screw, the diameter of the screw should be determined according to the raw material, product weight, product wall thickness, flow length ratio, cycle time, and production requirements. Screw speed is the speed per minute (rpm) of the machine at no load.
Screw speed (rpm) = oil pump displacement (cc/min.) ÷ oil pressure motor displacement (cc.)
Under the allowed value, the faster the speed, the shorter the pre-molding time, but the worse the mixing, and the higher the energy consumption.
3. Injection pressure
The injection pressure is used to overcome the flow resistance. The injection pressure indicated on the machine brochure refers to the static pressure at the nozzle.
The injection pressure required for each product is difficult to predict accurately, and can only be simulated in advance using mold flow, or measured during molding.
Common plastic injection pressure range (mpa)
4. Injection speed and injection rate
Injection speed is the maximum forward speed of the screw during injection (mm/s);
Injection speed (cm/s) = oil pump displacement (cm3/s) ÷ effective cross-sectional area of injection cylinder (cm2)
Injection rate is the amount of melt injected from the nozzle per unit time (g / s)
Injection rate (g/s.) = weight of product (g) / filling time (sec.)
The injection speed of general injection molding machine is designed between 100 ~ 150(mm/s), and the speed of thin-walled high precision injection molding machine is designed between 200 ~ 500(mm/s).
Under the same injection pressure, the greater the injection speed, the greater the energy consumption, the injection speed more than 200mm/sec. It is better to add accumulator to save energy consumption; the greater the injection speed, the greater the pressure loss, so generally the greater the speed, the higher the injection pressure should also be.
5. Plasticizing capacity
Plasticizing capacity refers to the amount of molten plastic (g/s) that the machine can spit out per unit time under 0 back pressure.
Plasticizing capacity (kg/h) = 1.29 × screw diameter (cm) × metering section depth (cm) × density × screw speed (rpm) × 60 ÷ 1000 × 85%
Plasticizing capacity is related to screw diameter, screw design, and screw speed.
In addition to the plasticizing capacity, the plasticizing quality of precision high-grade products should be considered, including plasticizing degree, mixing uniformity, shear heat, etc.
General standard models of the total barrel material volume is about 2.4 times the maximum injection weight, packaging machines about 2.7 times.
For example, if the diameter of A-rod of 300h injection seat is 32mm and the injection weight is 106g, the total material volume in the screw barrel is about 106g x 2.4 = about 254g.
Material retention time (min.) = total barrel volume (g) / volume of material per minute (g)
The minimum retention time of most ingredients in the barrel is about 2 minutes (not less than 90 seconds) and the maximum is between 8 and 12 minutes. (The retention time is related to the temperature of the barrel)
For example: there is a product, 30 grams per mold, PS material, the choice of 300h B screw, injection weight 134 grams, the maximum production per minute: 134 grams × 2.4 / 30 grams / 1.5 minutes = 7.1 die times, equal to the fastest cycle about 8.5 seconds, the retention time of 2 minutes, the fastest cycle is about 11 seconds.
6. Clamping force
Clamping force refers to the maximum force that the injection molding machine can clamp the mold, also known as clamping force.
Most manufacturers use the clamping force as the machine type (several tons), and the clamping force is usually expressed in tons (1 ton = 1000 kg) or kilonewtons (KN)
Straight press.
Clamping force (kg/cm) = area of clamping cylinder (cm2) × hydraulic pressure (kg/cm2)
Machine hinge type.
Clamping force = clamping cylinder thrust × machine hinge magnification
Selection of clamping force
The simple way to determine the pressure in the mold.
General products: 300kg/ cm2.
High precision small products: 500kg/ cm2.
Thin-walled packaging products: 400kg/ cm2 - 600kg/ cm2.
Thin-walled IT product appearance parts: 600kg/ cm2 - 1000kg/ cm2
350 tons or more in-mold pressure simple way to determine: general appearance parts products clamping force of 250kg/ cm2 or more, other thin-walled parts refer to the above.
Clamping force (tons) ≧ product projection area (cm2) × in-mold pressure (kg/ cm2) ÷ 800
7. Holding space
The tolerance space should include the inner distance of the tie bar, the opening stroke, the maximum and minimum mold thickness, and the size of the template.
The inner distance of the tie bar is the inner size of the middle of the 4 tie bars, the mold will not fit beyond this size.
The required opening stroke is 2.5 times of the product height, and 4 to 5 times for in-mold labeling products.
8. Ejector
Ejecting force: the force (KN or tons) to eject the product, the general machine design ejecting force can be applied to most of the products, only a few special products need a larger ejecting force.
For example: PET bottle blanks need about 0.5 ~ 1 ton per point, and caps need about 400 kg or more per point for forced ejections.
Ejecting stroke: the maximum distance of the machine ejecting (mm)
Other ejecting requirements: synchronous ejecting, two-stage ejecting, ejecting holding, vibrating ejecting, delayed ejecting, rotating ejecting, etc.
9. Other factors
Nozzle stroke: the maximum distance of nozzle forward and backward.
Nozzle contact force (KN): the thrust of the nozzle against the mold. (Increase the nozzle diameter to pay attention to whether the thrust is sufficient)
Empty cycle time (dry cycle): measured by the European standard 6, the shortest cycle time in the state of no load. The length of this time can determine the configuration of the machine, hydraulic, electronic control system control capabilities.
Motor power: the power of driving oil pump or servo motor. (KW or HP) (1HP=746W)
Electric heat power: the total power of the barrel electric heaters. (KW)
Number of heating sections: The number of points that the barrel heating can be controlled separately.
System pressure: the maximum working pressure of hydraulic system (mpa, bar, kg/cm2), general grade: 140bar, 160bar, 210bar.(All-electric type has no system pressure, generally limited by the maximum value of individual design.)
Machine size: Mainly the external dimensions, as the basis of the factory arrangement.
Machine weight: the weight of the whole machine, as a reference for transportation, and plant load-bearing
Conclusion
Through the above calculation, you can choose the most suitable injection molding machine specifications, which can effectively help you choose the right injection molding machine.
Meanwhile, you can read the following articles to help you choose the right injection molding machine.
China Injection Molding Machine: Selection Guide
How to choose the right PET injection molding machine
How to choose high speed injection moulding machine in China
How to choose Bakelite injection molding machine
