When choosing an injection molding machine, engineering materials and precision parts should choose an injection molding machine with high injection pressure as much as possible.
Injection rate and injection speed selection
The injection speed of the injection molding machine is the distance the screw moves forward per second, in cm/sec: S=Q/A
That is: injection speed = oil pump discharge volume ÷ injection cylinder effective area
S: injection speed cm/sec
A: injection cylinder effective area cm2
Q: oil pump discharge volume cc/rev (ml/rev)
We usually say that 300 injection speed, 500 injection speed, 1000 injection speed injection molding machine, generally refers to this speed. From the formula, we can see that when the cylinder diameter is small, the injection speed is greater, and when the oil pump displacement is large, the injection speed is large.
On what products do we have more urgent requirements for injection speed? When the product wall thickness is thinner, we require a higher injection speed to fill the mold as soon as possible. To increase the injection speed, on the one hand, the oil pump displacement is increased, and on the other hand, the cylinder diameter is reduced. When many customers choose injection molding machines, they ask injection molding machine manufacturers to increase the oil pump displacement by one or two levels, generally for this reason. Increasing the cylinder diameter has a very obvious side effect. The injection pressure will be reduced due to the increase in the cylinder diameter. When we produce thin-walled products, we need a higher injection pressure to overcome the material flow resistance. However, the method of reducing the injection pressure in order to increase the injection speed is mostly undesirable. This method is not recommended for non-general plastics.
Injection rate refers to the volume of plastic injected by the injection molding machine per second when injecting into the air. The unit is cm3/sec:
V=S*A
V: injection rate cm3/sec
S: injection speed cm/sec
A: cross-sectional area of the screw or barrel cm2
Injection rate is a unit volume expression, which means how much plastic can be injected per second. The larger the value, the more material can be injected into the mold. This parameter is very meaningful in some thick-walled and heavy products. For general products and thin-walled products, injection speed can better characterize the performance of the injection molding machine.
Injection molding machine machinery and lubrication system
The injection molding machine system is generally composed of a frame, three plates, a coring column, an injection system, an ejection system, a mold adjustment system, etc. The frame of a domestic injection molding machine is generally welded from steel, the three plates are cast iron parts, and the coring column, mold adjustment gear, and ejection mechanism are basically purchased and assembled from professional manufacturers. Many suppliers of injection molding machine manufacturers with guaranteed quality are supplied by the same manufacturers, and the quality difference is not large. The parts designed and manufactured by themselves or outsourced are mainly on several large parts such as the frame, three plates and oil circuit board. This is the part where the injection molding machine manufacturer directly controls its quality and cost.
By comparing several injection molding machines of similar models and different manufacturers, we can know which manufacturer's injection molding machine has a higher production cost. The clamping mechanism and the frame shooting platform of the injection molding machine account for 90% of the total weight of the machine. When the weight of a machine of one injection molding machine manufacturer is heavier than that of a similar model of another injection molding machine manufacturer, the money spent by this manufacturer is naturally higher than that of the other manufacturer. As an industrial product, heavier equipment means a more stable foundation and a more reliable working life, which is qualitatively different from handicrafts. Therefore, when we buy an injection molding machine, we must not ignore the weight of the injection molding machine itself.
In addition to referring to the stability of the injection molding machine itself, the durability of the injection molding machine also has an important consideration, that is, the quality of the injection molding machine lubrication system. Strictly speaking, the lubrication system of the injection molding machine needs to include manual lubrication and automatic lubrication. Strict injection molding machine manufacturers use brass tubes to connect the lubrication oil circuit, and some manufacturers use plastic thin tubes to connect to reduce costs. It is generally recommended to choose brass tube connection as much as possible, or choose brass tube connection in vulnerable or important parts, and use plastic tubes for parts that are not easy to damage or not so important. Lubricating oil should be selected as special lubricating oil. It is forbidden to use machine hydraulic oil instead. The recycled lubricating oil must be filtered before it can be used again. It cannot be put into use after simple machine-side filtering to prevent impurities from clogging the lubrication parts of the machine. When lubricating key parts such as the coring column, the four points of up, down, left and right should be lubricated in place. It is not possible to connect the active parts at will even if there is lubrication (some machines already have such a design, which is very wrong). Some injection molding machine manufacturers have cancelled the manual lubrication system, which is strictly undesirable. The traditional way of lubricating by hand butter is rarely used in injection molding machines now. The main reason is that it is difficult to avoid work pollution, workload and mistakes caused by improper operation.
Injection molding machine equipment control system
The injection molding machine control system is generally composed of computers, electrical components, hydraulic components, servo controllers, closed-loop control systems, etc. Due to cost reasons, domestic injection molding machines are not equipped with closed-loop control systems as standard. Unless there are special requirements, they are all open-loop control. The so-called servo motor closed-loop control is only the motor speed servo control, not the injection molding machine pressure and flow closed-loop control.
At present, domestic injection molding machine computers include imported and domestic products. If it is a general product, everyone's performance is similar, and the computer rarely breaks down. However, for products with high-speed thin-wall and high-precision requirements, the difference in computer response time comes out. The injection molding machine designed by the injection molding machine manufacturer according to the customer's product basically meets the performance requirements of its injection molding machine.
Compared with the injection molding machine computer, there are more problems with the servo controller, but there are not many domestic suppliers of servo control, so the performance difference of various brands in this regard is not large. Hydraulic control components are very mature abroad, and there are only a few manufacturers, and the failure rate is negligible. On the contrary, domestic manufacturers perform generally in this regard and have more problems. It's just that domestic hydraulic accessories are rarely used on injection molding machines, and most injection molding machines basically don't use them.
Injection molding machine stability verification method
How can technicians distinguish the quality of an injection molding machine? Here is a simple way to verify the repeatability accuracy of an injection molding machine.
When setting the injection molding parameters, set the injection holding pressure, speed, and time to 0. When injecting products, follow the normal parameter settings to first shoot the gate, then accelerate to the first section, and then fill to 90-95% of the product full of glue, to ensure that the product is in a state of missing a corner, and use the position injection method to inject glue. During the injection molding process, ensure that each injection time is completely consistent (inconsistency means that there is a problem with the adjusted parameters, and it cannot be guaranteed that the parameters are executed completely according to your own adjustments), ensure that the mold temperature does not change, repeat the manufacture of N products, weigh N trial pieces, and draw a product weight change chart, you can see the product weight change circuit diagram, when the average weight deviation of the product and N products exceeds 0.3%, the machine stability is poor, and if it exceeds 0.5%, it is recommended to overhaul the machine.
