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Introduction skills of injection molding machine adjustment: injection molding is an engineering skill, which involves transforming plastics into useful products that can adhere to their original functions. The important technological conditions of injection molding are the temperature, pressure and corresponding effect moments that affect the plasticizing activity and cooling.

I. temperature control

1. Barrel temperature:

The temperature to be controlled in injection molding process includes barrel temperature, nozzle temperature and mold temperature.

The first two temperatures mainly affect the plasticization and activity of plastics, and the second temperature mainly affects the activity and cooling of plastics.

Each kind of plastic has different activity temperature. For the same kind of plastic, its activity temperature and differentiation temperature are different because of different origins or trademarks. This is because the average molecular weight and molecular weight distribution are different. The plastic Plasticization Process in different types of injection machines is also different, so the temperature of the selected barrel is also different.

2. Nozzle temperature:

The nozzle temperature is generally slightly lower than the maximum temperature of the material barrel, which is to avoid the "salivation" of molten material in the straight through nozzle. The temperature of the nozzle shall not be too low, otherwise early solidification of the melt will be formed and the nozzle will be blocked *, perhaps because the early solidification will be injected into the mold cavity and affect the function of the product.

3. Mold temperature:

Mold temperature has a great influence on the intrinsic function and apparent quality of products. The mold temperature depends on the crystallinity of the plastic, the product size, structure, functional requirements, and other process conditions (melt temperature, injection speed, injection pressure, molding cycle, etc.).

II. Pressure control:

In the process of injection molding, the pressure includes plasticizing pressure and injection pressure, which directly affect the plasticizing of plastics and the quality of products.

1. Plasticizing pressure:

(back pressure) when the screw type injection machine is selected, the pressure encountered by the molten material at the top of the screw when the screw rolls back is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the overflow valve in the hydraulic system.

During injection, the size of the plasticizing pressure is constant with the rotation speed of the screw, so when the plasticizing pressure is added, the melt temperature will be increased, but the plasticizing speed will be reduced.

In addition, the addition of plasticizing pressure can often make the melt temperature uniform, the color mixture uniform and discharge the gas in the melt.

In general operation, the plasticizing pressure shall be determined as low as possible on the premise of ensuring excellent product quality. The detailed value varies with the type of plastic used, but it rarely exceeds 20 kg / cm2.

2. Injection pressure:

In the current production, the injection pressure of almost all injection machines is based on the pressure applied by the top of the plunger or screw to the plastic (converted from the oil circuit pressure). The effect of injection pressure in injection molding is to overcome the moving resistance of the plastic from the barrel to the cavity, give the melt filling rate and compact the melt.

III. forming cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually includes the following parts:

Forming cycle

Forming cycle直接影響勞動出產(chǎn)率和設(shè)備利用率。

Therefore, in the production process, we should try our best to shorten all relevant moments in the forming cycle on the premise of ensuring the quality. In the whole forming cycle, the injection time and cooling time are the most important, which have a decisive impact on the quality of products. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in the production is generally about 3-5 seconds.

The pressure maintaining time in the injection time is the pressure time on the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (for extra thick parts, it can be up to 5-10 minutes). Before the molten material at the gate is sealed and frozen, the pressure maintaining time has an impact on the accuracy of the product size. If it is later, it has no impact. The pressure maintaining time also has an optimal value, which is known to depend on the material temperature, mold temperature and the size of the sprue and gate. If the dimensions and process conditions of the sprue and gate are normal, the pressure value with the smallest fluctuation scale of the product shortening rate shall prevail. The cooling time mainly depends on the thickness of the product, the thermal function and crystallization function of the plastic, and the mold temperature. At the end of the cooling time, the principle should be to ensure that the product will not change during demoulding. The cooling time is generally about 30~120 seconds. It is unnecessary to have a long cooling time, which will not only reduce the production power, but also cause demoulding difficulties for complex parts, and even generate demoulding stress when forced demoulding. Other moments in the molding cycle are related to whether the production process is connected and automated, and the degree of connection and automation.

General injection molding machines can be adjusted according to the following procedures:

According to the temperature scale provided by the material supplier, adjust the barrel temperature to the middle of the scale and adjust the mold temperature.

Measure the required injection volume, and adjust the injection molding machine to two-thirds of the estimated maximum injection volume. Adjust the reverse cable (glue extraction) stroke. Measure and adjust the secondary injection time, and adjust the secondary injection pressure to zero.

Start to adjust the primary injection pressure to half (50%) of the limit of the injection molding machine; Adjust the injection speed to the maximum. Measure and adjust the required cooling time. Adjust the back pressure to 3.5bar. Remove the degraded resin in the barrel. Select semi-automatic injection molding form; Start the injection procedure and investigate the screw action.Adjust the injection speed and pressure properly. If you want to shorten the filling time, you can add the injection pressure. As mentioned above, because there will be a process before full mold filling, the mold filling pressure can be adjusted to 100% of the primary injection pressure. After all, the pressure must be adjusted high enough so that the maximum speed that can be reached is not limited by the set pressure. If there is overflow, the speed can be reduced.

After each investigation cycle, adjust the glue injection amount and conversion point. Set the procedure so that 95-98% mold filling can be achieved according to the injection weight at the first stage of injection molding.

When the injection volume, conversion point, injection speed and pressure of the first level injection molding are all adjusted properly, the adjustment procedure of the second level pressure maintaining can be carried out.

Adjust the holding pressure properly as required, but do not overfill the mold cavity.

Adjust the screw speed to ensure that the melt is finished just before the end of the cycle, and the injection cycle is not limited.

Shorten cycle time to improve productivity

Injection cycle

For most injection molding manufacturers, the injection cycle can directly affect the following two primary purposes:

1. Get more parts from machinery every day;

2. The products meet the requirements of the guests.

Injection cycle由以下組成

Cycle start - the screw starts to move and inject;

Part gate cooling

Screw start rolling -- plasticizing stroke start

Screw return completion - screw rolling abort

If necessary, glue pumping will occur

Mold opening (perhaps including core pulling out)

The workpiece can be ejected after sufficient cooling

Ejection

Mold closure (possibly including core return)

Mold closed - cycle restarted.

The automatic injection cycle is repeated again and again in the same sequence. The cycle has three primary parts:

Mold opening time; Filling time; Mold closing time; Pressure maintaining time

The goal of productivity improvement is to complete all necessary actions in a very short time, complete ejection, and ensure that the mold is maintained (including pulling out and returning the slider and side mold core).

Therefore, any mold or injection molding machine problems that delay the opening time must be repaired. In addition, if the opening time of each injection is different, the workpiece will also be different.

Injection filling cavity (from 1-2)

For materials with good mobility, the filling time should account for 1/10 to 1/8 of the whole cycle. The key to filling is the fast and stable screw driving time and the lowest and stable injection pressure.

When the screw is pushed forward, the solution is injected into the mold cavity from the material pipe through the nozzle, the vertical channel, the cross flow, the gate, and then the resistance will be encountered. This resistance is affected by nozzle diameter, runner size, gate size, product thickness, and mold exhaust design.

The movement resistance should be improved and reduced in the mold to achieve filling balance and stability. Otherwise, because of uneven filling, the products injected into different mold cavities have different dimensions, insufficient strength, or poor appearance.

Pressure maintaining time (2-3)

When injecting crystalline materials, the pressure holding time is the most important process. This time is from the beginning of 99% melt filling in the mold cavity to the stop of gate condensation. The strength and toughness of the module are determined by whether the pressure is applied to the melt after injection molding until the component / gate solidifies. Keep a small section of the melt orientation in front of the screw when pressing. This pressure maintaining section is the key to avoid the cavity after the melt condensation is shortened, or the weak position of the gate orientation, which leads to the insufficient strength of the module.

Cooling time (4-7)

When the melt enters the mold cavity and touches the metal surface, the step of melt cooling has begun. Because the condensation temperature of semi crystalline materials is very high, the cooling time is very few. For general modules, the module should have enough cooling time after the glue is melted. If a problem is found when the module is ejected, the cooling time can be extended gradually until the problem is solved.

Mold opening time (8-11)

The mold opening time is an important part of the whole cycle, especially for the molds with inserts. Even in the more standard molds, the mold opening time is often higher than 20% of the whole cycle.

Source: Jiangmen engineering plastics http://xrtsm.com.cn/