Precautions for using injection molding machine


Matters needing attention


Five hazards of high temperature rise of injection molding machine: thermal deformation of machinery, reduction of oil viscosity, deformation of rubber seals, acceleration of oil oxidation and deterioration, but also reduce the component pressure of air.

One of the hazards: make the mechanical thermal deformation

The moving parts with different thermal expansion coefficient in the hydraulic components are stuck because of the smaller fit gap, which causes the action failure, affects the transmission accuracy of the hydraulic system, and leads to the deterioration of the working quality of the parts.

Hazard two: reduce the viscosity of the oil

Too high temperature rise of injection molding machine will lead to lower oil viscosity, increased leakage, pump volume efficiency and the efficiency of the whole system will be significantly reduced. As the viscosity of oil decreases, the oil film of moving parts such as slide valves becomes thinner and cut, and the friction resistance increases, leading to the aggravation of wear.

Hazard three: deformation of rubber seals

Excessive temperature rise of injection molding machine will deform rubber seals, accelerate aging failure, reduce sealing performance and service life, and cause leakage.

Hazard four: accelerate oil oxidation deterioration

Too high temperature rise of injection molding machine will accelerate the oxidation and deterioration of oil, precipitate asphalt material, and reduce the service life of hydraulic oil. The precipitate clogs the damping hole and gap valve port, resulting in the pressure valve stuck and unable to act, metal pipeline elongation and bending, or even rupture.

Hazard 5: Lead to poor working quality of components

The high temperature rise of injection molding machine will lead to the poor working quality of components, and the dissolved air in the oil will escape, resulting in air pockets, and the performance of the hydraulic system will be reduced. Hydraulic system of the ideal working temperature should be between 45 to 50 degrees, the reason is that the hydraulic system is designed based on a selected pressure oil viscosity, but viscosity changes with the discretion of the oil temperature, thus affecting work of the system components, such as oil cylinder, hydraulic valve, the control accuracy and response sensitivity is reduced, especially in the case of precision injection machine.

At the same time, the temperature is too high to accelerate the aging of seals, hardening, fragmentation; If the temperature is too low, the processing energy consumption is large, so that the running speed is reduced. Therefore, it is necessary to pay close attention to the working temperature of hydraulic oil. The causes of high oil temperature are various, but most of them are due to oil circuit fault or cooling system failure.

Processing method

(1) According to different load requirements, often check and adjust the pressure of the relief valve to make it just right.

(2) Reasonable selection of hydraulic oil, especially oil viscosity, in the case of conditions allow, try to use a lower viscosity to reduce viscosity friction loss.

(3) Improve the lubrication conditions of moving parts to reduce friction loss, which is conducive to reducing the workload and reducing heat.

(4) Improve the assembly quality and precision of hydraulic components and hydraulic system, strictly control the fit clearance of mating parts and improve lubrication conditions. The sealing material with small friction coefficient and the improved sealing structure are used to reduce the starting power of the hydraulic cylinder as much as possible, so as to reduce the heat generated by mechanical friction loss.

(5) If necessary, add cooling devices.

The selection

Generally speaking, customers who have been engaged in the injection molding industry for many years have the ability to judge and choose the appropriate injection molding machine to produce. But in some cases, the customer may need the manufacturer’s assistance to decide which size injection molding machine to use, or even the customer may only have a sample or idea of the product, and then ask whether the manufacturer’s machine can be produced, or which model is suitable.

In addition, some special products may require special devices such as accumulators, closed loops, injection compression, etc., to be produced more efficiently. Thus, how to decide the appropriate injection molding machine to produce, is a very important problem. The following information is provided for readers’ reference.

Usually, the important factors that affect the choice of injection machine include mold, product, plastic, molding requirements, etc. Therefore, the following information must be collected or available before making the choice:
Mold size (width, height, thickness), weight, special design, etc.;
The type and quantity of plastics used (single raw material or multiple plastics);
Appearance dimensions (length, width, height, thickness), weight, etc.; Molding requirements, such as quality conditions, production speed, etc.

After obtaining the above information, you can follow the following steps to select the appropriate injection machine:

1, choose the right type: by the product and plastic to determine the type and series.
Since there are many kinds of injection machines, it is necessary to correctly judge which kind of injection molding machine or which series this product should be produced by at the beginning, such as general thermoplastic or bakwood raw materials or PET raw materials, monochrome, two-color, multi-color, sandwich or mixed color, etc. In addition, some products need high stability (closed loop), high precision, ultra-high firing rate, high firing pressure or fast production (multi-loop), and other conditions, it is necessary to choose the right series to produce.

2, put down: by the mold size to determine the machine “large column distance”, “die thickness”, “die minimum size” and “die disk size” is appropriate, to confirm whether the mold is put down.
The width and height of the mold should be less than or at least one side is less than the large column distance;
The width and height of the die should be within the size range of the die plate.
The thickness of the mold should be between the mold thickness of the injection molding machine;
The width and height of the mold should meet the recommended minimum mold size of the injection molding machine, too small is not acceptable;

3.Get: Determine whether the “die opening stroke” and “die supporting stroke” are sufficient for the finished product to be taken out by the mold and the finished product.
The die opening stroke shall be at least more than twice the height of the finished product in the direction of the switch die, and shall include the length of the vertical sprue;
The supporting die travel should be enough to eject the finished product;

4, lock: determined by the product and plastic “lock force” tons.
When the material is injected into the mold hole at high pressure, a clamping force will be generated, so the clamping unit of the injection molding machine must provide enough “clamping force” to prevent the mold from being stretched open. The calculation of the clamping force demand is as follows:
The projected area of the finished product in the direction of the switching die is obtained from the appearance size of the finished product.
The supporting force of the mold = the projected area of the finished product in the direction of the switching mold (cm2)× the number of mold holes × the pressure in the mold (kg/cm2);
The pressure in the mold varies with the raw material. The general raw material is 350 ~ 400kg/cm2.
The clamping force of the machine should be greater than the clamping force of the mould, and for the sake of safety, the clamping force of the machine should be more than 1.17 times the clamping force of the mould.
At this point, the specification of the clamping unit has been preliminarily decided, and the tonnage of the machine has been roughly determined. Then the following steps must be carried out to confirm which screw diameter of the injection unit is more in line with the needs.

5, shot full: by the weight of the finished product and the number of die holes to determine the required “injection volume” and choose the appropriate “screw diameter”.
The number of die holes (several holes per die) should be considered when calculating the weight of finished products.
For the sake of stability, the injection volume shall be more than 1.35 times of the finished product weight, that is, the finished product weight shall be less than 75% of the injection volume;

6, shoot well: judged by plastic “screw compression ratio” and “injection pressure” and other conditions. Some engineering plastics need higher injection pressure and appropriate screw compression ratio design, just have better forming effect, so in order to make the finished product shot better, in the choice of screw also need to consider the demand of injection pressure and compression ratio.
Generally, smaller diameter screws provide higher ejection pressure. As shown in Vertical Injection Molding Machine

7, shoot fast: and “injection speed” confirmation.

Some finished products need high ejection rate and rapid ejection to form stably, such as ultra-thin finished products. In this case, it may be necessary to confirm whether the ejection rate and firing rate of the machine are sufficient, and whether it needs to be equipped with accumulator, closed loop control and other devices. In general, under the same conditions, a screw that provides a higher pressure will usually have a lower rate of fire, and conversely, a screw that provides a lower pressure will usually have a higher rate of fire. Therefore, the choice of screw diameter, injection amount, injection pressure and injection rate (injection speed), need to cross consideration and selection.

In addition, a multi-loop design can also be used to shorten the molding time by synchronizing the composite action.
After the above steps, it is in principle possible to determine the injection molding machine that meets the requirements, but some special issues may also have to be considered, including:

The problem of size matching:

Under some special conditions, the customer’s moulds or products may be small but needed to shoot the quantity is big, or small amount of mould is big but needed to shoot and under this condition, manufacturers are preset standard may not be able to meet customer demand, and must carry out the so-called “size”, or “big wall small shot” or “small wall big shot”. The so-called “big wall and small injection” refers to the original standard clamping die unit with a smaller injection screw, on the contrary, “small wall and big injection” is the original standard clamping die unit with a larger injection screw. Of course, in the collocation may also be clamping and injection of several different levels.

The concept of a fast or high-speed machine:

In practice, more and more customers will ask to buy the so-called “high speed machine” or “fast machine”. Generally speaking, its purpose in addition to the demand of the product itself, most of the other is to shorten the molding cycle, improve the output per unit time, and then reduce production costs, improve competitiveness. In general, there are several ways to achieve this goal:
Injection speed: increase the motor and pump, or add accumulator (preferably with closed loop control);
Speed up the feeding speed: increase the motor and pump, or the feeding oil pressure motor to small, so that the screw speed up;
Multi-loop system: double circuit or three circuit design, in order to synchronize the composite action, shorten the forming time;
Increase the mold water, improve the cooling efficiency of the mold;

However, “there is no free lunch in the world”, the improvement and transformation of machine performance can increase the production efficiency, but often increase the investment cost and operation cost. Therefore, the benefit evaluation before investment needs to be carefully measured, so as to produce the highest benefit with the most appropriate model [1].
The point of choose and buy
Injection molding machine is a common plastic machinery, for the use and purchase of injection molding machine operators, first of all, should understand the performance of a good injection molding machine, need to evaluate from what aspects, in order to choose a satisfactory injection molding machine, let’s understand what aspects should be paid attention to to buy injection molding machine.

Technical parameters of injection molding machine for purchase. Users can choose injection molding machine by longitudinal or transverse comparison to distinguish. The so-called “longitudinal” is the injection molding machine main technical parameters should meet industry standards, according to industry standards. The so-called “transverse” is based on the domestic and foreign similar injection molding machine technical parameters are compared. So it’s a very careful scientific work. According to the material, shape, structure and even the application field and specific occasion of injection molding products; According to the structure, quality, precision, cavity number, flow channel type and its structure, shape, size; According to the product day, month, year production capacity automation degree to choose injection molding machine parameters at all levels.

Select suppliers of injection molding machine of choose and buy, had better be to choose a nearby service manufacturers purchase, because the injection molding machine structure complex, hard to avoid is not a problem, quick after-sales service is the key, don’t seek low prices when purchasing, injection molding machine accessories, home-made and imported price difference is very big, so you need to try to find out the manufacturer use what brand of accessories. Of course, some accessories you can’t see, such as oil seals, bearings, etc., so at this time the manufacturer’s reputation and reputation are very key.

Choose the type of injection molding machine. The two main parameters of injection molding machine are injection volume and clamping force. In order to ensure the quality of the processed products, the actual weight of a mold product should be within 70% of the theoretical injection volume of the machine. The required clamping force is calculated according to the projected area of the product. Special attention should be paid to the distance between the columns of the machine, the die stroke and the minimum die thickness is appropriate with the die.

The man-machine dialogue ability and operability of the optional injection molding machine. Injection molding machine should make its operation and management more humanized, operation should be convenient, display should be clear, easy to understand, at a glance. With the function of parameter selection, product quality control, mold data storage and automatic reproduction of each parameter, it can quickly diagnose and eliminate the fault, reduce the material waste in the shortest time, and restore the normal operation of the system. The key points of the purchase of injection molding machine machine work stability, reliability, safety and service life. The stability of the machine is mainly reflected in whether the clamping mechanism and injection molding system run smoothly in each cycle, and work in the quiet without impact, and the noise shall not exceed the industry standard. These situations are related to the manufacturing and assembly accuracy of the machinery, as well as the reasonable design and structure of the elbow connecting rod.

The reliability and service life are mainly reflected in the system rigidity of the clamping mechanism, and related to the structure, size, material and heat treatment process of the template, tie rod, connecting rod and its pin shaft; Injection molding machine through the system such as cylinder, screw, screw head, reverse ring and screw driving spindle structure and manufacturing technology, these parts are the main force parts and wear parts of injection molding machine, directly affecting the service life. The safety of injection molding machine is very important. The reliability of low voltage protection system is related to man-machine safety when the die closing mechanism is opening and closing. Modern injection molding machine requires the implementation of electrical, mechanical, hydraulic combined safety protection system, this protection mainly reflects two aspects: one is the safety protection of the mold, the other is the safety protection of the human body.

Before closing the mold, when the safety door is not closed, the closed mold should not be able to move; In the process of die closing, if the safety door is not closed to the correct position, the template action will stop or automatically open the die. If foreign bodies, such as embedded parts, fall into the mold cavity by mistake due to vibration or other reasons, or other foreign bodies, such as starting, are wrongly placed into the mold, the mold should stop the closing action, or automatically open the mold and give an alarm.

Another safety link of the injection molding machine is to prevent the cold start of the pre-molded screw. When the material of the screw does not reach the specified temperature and holding time, it is forbidden to start, otherwise an alarm will occur. Spatter protection of injection molding machine nozzle is required to prevent scalding.

The key points of injection molding machine to buy a high degree of automation, complete functions to improve, high production efficiency. Modern injection molding machine can realize manual, semi-automatic, automatic operation, as long as the mold design is reasonable, can realize the automatic operation from injection mold filling to product ejection fall, can also be equipped with manipulator to realize unmanned operation. The functions of core pumping, hydraulic nozzle control and fast heating channel probe can be programmed into the automatic program of injection molding cycle for automatic circulation. High production efficiency is mainly reflected in that each mold product in the normal injection process conditions, the molding cycle should be short. This is a reflection of the mechanical, electrical, hydraulic system comprehensive performance indicators of an injection molding machine, therefore, must use high quality hydraulic, electronic components, the use of precision manufacturing technology to improve the sensitivity of injection molding machine system and the repeated accuracy, this is the injection molding system efficient work is reliable guarantee.

Energy-saving and environmental protection of the key points of the purchase of injection molding machine are the indicators that can not be ignored in judging modern injection molding machine. Energy SAVING mainly reflects the WATER-saving AND POWER-saving ability of injection molding machine in heating system (electromagnetic heating), power drive system (inverter, servo, etc.) and heat dissipation system under normal process conditions compared with similar machines. The design of modern injection molding machine should fully consider the theme of “environmental protection”. Injection molding machine should prevent hydraulic oil leakage or excessive noise, to work quietly. In addition, the injection molding machine itself should also be more environmentally friendly materials.

The key points of the purchase of injection molding machine maintenance, maintenance to facilitate. The installation position of various machines such as mechanical, hydraulic, electronic and electrical components should be conducive to the maintenance and overhaul of the machine. At the same time, the universality and standardization of injection molding machines are also very important to users.