What are requirements of precision injection molding for injection molding machines?

How is a precision injection molding machine defined?

Precision injection molding machines refer to molding machinery suitable for forming precision plastic products. How to measure or judge a precision injection molding machine?

Usually precision injection molding machines have two indicators: one is repeated size deviation of product; the other is repeated weight deviation of product. The former is difficult to compare due to size and thickness of product; the latter represents comprehensive level of injection molding machine.

Generally, weight error of ordinary injection molding machines is about 1%, and better machine can reach 0.8%. Below 0.5% is a precision machine, less than 0.3% is an ultra-precision machine. As mentioned above, precision injection molding machines require product dimensional accuracy to be generally within 0.01 to 0.001 mm.

Many precision injection moldings also require injection molding machines to have:

1. High injection pressure, high injection speed;

2. Clamping system has sufficient rigidity and clamping precision. So-called clamping precision refers to uniformity, adjustable, stable and repeatable of clamping force, position accuracy of opening and closing mold is high;

3. Pressure, flow, temperature, metering can be controlled to precise accuracy. Multi-stage injection can be used to ensure reproducibility of molding process and repeatability of product.

What are requirements of precision injection molding for injection molding machines?

1. Features of technical parameters:

1. Injection pressure:

Ordinary injection molding machine: 147 ~ 177Mpa; precision injection molding machine: 216 ~ 243 Mpa; ultra high pressure injection molding machine: 243MPa or more, usually between 243 ~ 392 MPa.

Benefits of precision injection molding machines for high pressure forming are:

A. Improve precision and quality of precision products.

Injection pressure has the most obvious effect on mold shrinkage of product. When injection pressure reaches 392 MPa, mold shrinkage rate of product is almost zero. At this time, accuracy of product is only affected by mold control or environment. Experiment proves that injection pressure is from 98 to 392 MPa, and mechanical strength of workpiece can be increased by 3 to 33%.

B. It can reduce wall thickness of precision product and increase molding length.

Taking PC as an example, ordinary machine has a injection pressure of 177 Mpa, which can form a product with a thickness of 0.8 mm, and a precision machine with a injection pressure of 392 MPa can form a product with a thickness of 0.45 mm or more. Ultra high pressure injection molding machines can obtain products with a larger flow length ratio.

C. Increasing injection pressure can fully affect injection rate.

In order to achieve rated injection rate, there are two ways: one is to increase maximum injection pressure of system; the other is to modify parameters of injection device or injection system, including screw parameters;

2. Injection speed:

Precision injection molding machines require high injection speeds. Injection speed of precision hydraulic injection molding machines is required to reach 200mm/s or more, and injection speed of all-electric injection molding machines can reach 300mm/above.

2. Characteristics of precision injection molding machine control:

1. Requirements for repeatability (reproducibility) of injection molding parameters are high, and multi-stage injection feedback control should be adopted.

1. Multi-level position control;

2. Multi-level speed control;

3. Multi-level pressure control;

4. Multi-stage back pressure control;

5, multi-stage screw speed control.

Accuracy of displacement sensor is required to be at least 0.1 mm, so that metering stroke, injection stroke and thickness of residual mat (injection monitoring point) can be strictly controlled to ensure accuracy of each injection and improve molding accuracy of product.

2. Barrel and nozzle temperature control should be accurate

When temperature control system is heated, overshoot amount is small, temperature fluctuation is small. Precision injection molding uses PID proportional, integral, and differential temperature control to achieve a temperature accuracy of ±1℃. Temperature control accuracy of ultra-precision injection molding machine is ±0.1℃, and temperature deviation can be stably controlled between ±0.2℃.

3. Injection molding machine hydraulic oil temperature needs to be controlled

Change of oil temperature of injection molding machine leads to fluctuation of injection pressure. It is necessary to use a cooling device for hydraulic oil to stabilize oil temperature at 50 to 55℃.

4. Control of mold temperature requirements

Mold temperature has a considerable influence on later heat shrinkage of part, affects apparent quality and crystallinity of part, and also affects mechanical properties of part. If cooling time is same, thickness of product having a low mold cavity temperature is greater than thickness of product having a higher temperature.

3. Hydraulic system of precision injection molding machine

1. Oil system requires use of proportional pressure valves, proportional flow valves or closed-loop variable pump systems.

Hydraulic system of injection molding machine needs to use proportional pressure valve, proportional flow valve to achieve different working pressure and flow of system. Whether it is fixed pump hydraulic system or variable pump hydraulic system, there are proportional valves to control working pressure and working flow of oil pump output, achieve proportional linear adjustment.

2. In direct compression type clamping mechanism, mold section oil passage and injection section oil passage are separated.

This requirement is for a direct pressure injection molding machine because direct pressure injection molding machine starts to inject melt and fill mold cavity immediately after high pressure is applied. At the same time, oil must be filled with clamping cylinder to form a high clamping pressure. Otherwise, middle plate will retreat due to filling pressure and mold expansion force, which is not allowed. This requires that mold clamping oil line and injection oil circuit are separated. Otherwise, oil pump overload problem occurs due to oil filling clamping cylinder and injection cylinder at the same time, injection speed and system stability are affected.

3. Due to high speed of precision injection molding machines, response speed of hydraulic system must be emphasized.

Most of precision injection molding is done under high injection pressure and high rate of injection (connector or mobile phone button). Filling time is very short, generally only a few tenths of a second. This requires system to have high response performance, otherwise response will be delayed, eventually leading to a large deviation between output signal of system and input signal. Molding process is difficult to debug, which will directly affect size or weight stability of workpiece during production.

High responsiveness of injection molding machine depends on scan time of controller, response time of oil pump and oil valve. For hydraulic systems with servo valves, there are considerable advantages in terms of response and repeatability.

For variable pump injection molding machine, since pressure and flow output of oil pump are adjusted by swashplate angle of oil pump, it has high responsiveness and high responsiveness, repeatability of oil pump can reach 0.2% or less.

4. Hydraulic system of precision injection molding machine should fully embody machine-electric-liquid-integration project.

At present, injection molding machine generally adopts full computer control, and functions are gradually improved. It has intelligent functions such as “human-machine dialogue”, fault self-diagnosis, molding status monitoring, and SPC quality monitoring. Hydraulic system of injection molding machine is power component of each action of injection molding machine. Controller is hydraulic system control center, and all kinds of instruments are auxiliary monitoring part of running state of machine. Injection molding machine is actually an intelligent combination of machine, electricity, liquid and instrument.

4. Structural characteristics of precision injection molding machine

(1) Due to high injection pressure of precision injection molding machines. This requires a high stiffness of clamping system. Parallelism of moving and fixed templates is controlled in the range of 0.08 to 0.10 mm.

(2) It is required to protect low-pressure mold and control accuracy of clamping force. Because size of clamping force affects degree of mold deformation, it will ultimately affect dimensional tolerance of part.

(3) Speed of opening and closing mold should be fast, generally around 60mm/s.

(4) Plasticized parts: screw, screw head, check ring, barrel should be designed into a structural form with strong plasticizing ability, good degree of homogenization and high injection efficiency; screw drive torque is large and can be continuously variable.

Injection molding machine to achieve precision injection molding performance

As already mentioned, injection molded products have been applied to various fields and are widely used to replace high-precision metal parts, thereby placing strict requirements on dimensional accuracy, quality accuracy, apparent quality and mechanical properties of injection molded parts. At the same time, higher requirements have been placed on process factors affecting quality of injection molded products.

Ideal state of injection molding machine for product control is to directly control size, quality, apparent quality and mechanical properties of product to feedback control feedback signal. However, for the moment, direct measurement and conversion of these non-electrical quantities into electrical signals has not been solved, so it can only be solved by controlling controllable variables of injection molding machine that affect quality of above products, mainly including:

(1) Temperature of each section of screw melting cylinder, non-periodic signal, generally at least 4 sections of heating;

(2) Hydraulic oil temperature, non-periodic signal, detected by conventional thermocouple;

(3) Pre-plastic displacement of screw, and periodic signal is detected by displacement electronic ruler, thereby extracting speed signal;

(4) Injection system pressure, installed on injection cylinder with a strain sensor, detecting periodic signals, measuring injection pressure, holding pressure and back pressure signal;

(5) screw speed, use speed sensor to detect screw speed, keep back pressure constant;

(6) Screw retreat stop position is used for measuring injection amount and is detected by an electronic ruler;

(7) At least 4 sets of injection pressure and feedback signal detection for injection pressure, and feedback control;

(8) Injection speed is at least 4 levels of setting curve and feedback signal detection, displacement variable is converted to speed feedback, and feedback control is performed.

(By Plastic Mould - Ivy Cao)