Basic knowledge of injection molding process conditions

Plastic molding is an engineering technology that involves transforming plastic into products that are useful and maintain their original properties. The important process conditions for injection molding are the temperature, pressure and corresponding action time that affect plasticization flow and cooling.

1. Temperature control

1. Barrel temperature: The temperatures that need to be controlled during the injection molding process include barrel temperature, nozzle temperature and mold temperature, etc. The first two temperatures mainly affect the plasticization and flow of plastics, while the latter temperature mainly affects the flow and cooling of plastics. Each plastic has a different flow temperature. The same plastic has different flow temperatures and decomposition temperatures due to different sources or grades. This is due to different average molecular weights and molecular weight distributions. Plastics in different types of injection molding have different flow temperatures and decomposition temperatures. The plasticizing process in the machine is also different, so the temperature of the barrel is also different.

2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum temperature of the barrel. This is to prevent the "salivation phenomenon" that may occur in the straight-through nozzle. The nozzle temperature cannot be too low, otherwise it will cause premature solidification of the molten material and block the nozzle, or the premature solidification material will be injected into the mold cavity and affect the performance of the product.

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

2. Pressure control: The pressure during the injection molding process includes plasticizing pressure and injection pressure, and directly affects the plasticization of plastic and product quality.

1. Plasticizing pressure: (back pressure) When using a screw injection machine, the pressure on the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted through the relief valve in the hydraulic system. During injection, the plasticizing pressure remains constant with the screw speed. Increasing the plasticizing pressure will increase the temperature of the container, but will reduce the plasticizing speed. In addition, increasing the plasticizing pressure can often make the temperature of the solution uniform and the mixing of color materials

Combine evenly and discharge the gas in the container. In general operations, the plasticizing pressure should be determined as low as possible while ensuring excellent product quality. The specific value varies with the type of plastic used, but it usually rarely exceeds 20 kilograms square centimeters.

2. Injection pressure: In current production, the injection pressure of almost all injection machines is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the molten material a filling rate and compact the material.

3. Molding 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:

Molding cycle: Molding cycle directly affects labor productivity and equipment utilization. Therefore, during the production process, the relevant times in the molding cycle should be shortened as much as possible while ensuring quality. In the entire molding cycle, injection time and cooling time are the most important, and they have a decisive impact on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate. The filling time in production is generally about 3-5 seconds. The holding time in the injection time is the proportion of the pressure time of the plastic in the cavity to the entire injection time, which is generally about 20-120 seconds (for extra thick parts, it can be as long as 5-10 minutes. At the gate Before the molten material is frozen, the holding time has an impact on the dimensional accuracy of the product. If it is measured later, it has no impact. The holding time also has an optimal value. It is known that it depends on the material temperature, mold temperature, and the main channel and gate. Size. If the size of the main channel and gate and the process conditions are normal, it is usually based on the pressure value with the smallest fluctuation range of the shrinkage rate of the product. The cooling time is mainly determined by the thickness of the product, the thermal properties and crystallization properties of the plastic, and Mold temperature, etc. The end point of the cooling time should be based on ensuring that no changes are caused when the product is demoulded. The cooling time is generally about 30 to 120 seconds. Too long cooling time is not necessary. It not only reduces production efficiency, but also affects complex parts. It will also cause difficulty in demoulding, and even demoulding stress will occur when forced demoulding. Other times in the molding cycle are related to whether the production process is continuous and automated, and the degree of continuity and automation.

A general injection molding machine can be adjusted according to the following procedures:

According to the temperature range provided by the raw material supplier's information, adjust the material temperature to the middle of the range and adjust the mold temperature.

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

Preliminarily adjust the primary injection pressure to half (50%) of the limit of the injection molding machine; adjust the injection speed to the highest. Estimate and adjust the required cooling time. Adjust the back pressure to 3.5b. Clear the degraded material in the barrel Resin. Use semi-automatic injection molding mode; start the injection molding process and observe the movement of the screw.

If necessary, adjust the injection speed and pressure appropriately. If you want to shorten the mold filling time, you can increase the injection pressure. As mentioned before, since there is a process before full mold filling, the final filling pressure can be adjusted to 100% of the first-level injection pressure. The pressure must eventually be adjusted high enough so that the maximum speed that can be achieved is not limited by the set pressure. If there is any posthumous material, the speed can be reduced. After each observation cycle, adjust the injection amount and conversion point. Set the program so that 95-98% mold filling based on the injection weight can be achieved in 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 second-level holding pressure adjustment process can be carried out. Adjust the holding pressure appropriately as needed, but do not overfill the mold cavity.

Adjust the screw speed to ensure that the glue volume is completed just before the cycle is completed, and the injection cycle is not limited.

Shorten cycle times and increase productivity

For most injection molding manufacturers, the injection molding cycle can directly affect the following two main purposes: 1. Get more parts from the machine every day; 2. The parts meet customer requirements.

The injection molding cycle consists of:

At the beginning of the cycle, the screw starts to advance and injection occurs;

Part gate cooling

The screw starts to rotate and the plasticizing stroke begins.

The screw return completes and the screw rotation stops.

If necessary, glue extraction occurs

Mold opening (may include pulling out of the mold core)

The product can be ejected after it is fully cooled.

Push out

Mold closing (may include return of mold core)

The mold closes - the cycle begins again.

An automatic injection molding cycle is one where the same things repeat over and over again in the same sequence. The cycle has three main parts:

Mold opening time; filling time; mold closing time; holding time

The goal to increase productivity is to complete all necessary actions to complete ejection in a very short time and ensure that the mold is protected (including pulling out and retracting the slider and side core). Therefore, any problem with the mold or injection molding machine that delays the mold opening time

The problem must be repaired. In addition, if the mold opening time for each injection is different, the parts will also be different.

Injection molding fills the cavity (from 1-2)

For materials with good fluidity (such as Delrin polyacetal, nylon, and polyester), this filling time should account for 1/10 to 1/8 of the entire cycle. The most critical thing during filling is fast and stable screw filling.

Rod advancement time, and minimum and stable injection pressure.

When the screw advances forward and the melt flows from the material tube through the nozzle, vertical flow channel, cross flow, gate, and then into the mold cavity, it will encounter resistance. This resistance is affected by the nozzle diameter, runner size, gate size, product thickness, and mold exhaust design.

Flow resistance should be improved and reduced within the mold to achieve filling balance and stability. Otherwise, due to uneven filling, the injection molded products in different mold cavities may have different sizes, insufficient strength, or poor appearance.

Holding time (2-3)

When injection molding crystalline materials, holding time is the most important part of the process. This period of time starts when the volume fills 9% of the mold cavity and ends when the gate solidifies. The strength and toughness of the module are determined by whether the pressure is maintained on the container after injection molding until the part/gate solidifies. When maintaining pressure, a small section of glue-holding position should be reserved in front of the screw. This pressure-holding period is the key to preventing cavities after the melt solidifies and shrinks, or weaknesses in the gate position, which lead to insufficient module strength.

Cooling time (4-7)

When the melt enters the mold cavity and hits the metal surface, the melt cooling step has begun. Since the solidification temperature of semi-crystalline materials such as polyoxymethylene, nylon, and polycarbonate is very high, very little cooling time is required. For general modules, after the glue melting is completed, the module should have sufficient cooling time. If a problem is found when ejecting the module, the cooling time can be slowly extended until the problem is solved.

Mold opening time (8-11)

The mold opening time is an important part of the entire cycle, especially for molds with embedded parts. Even in comparative molds, the mold opening time is often higher than 20% of the entire cycle.

Factors affecting mold opening:

The first thing to consider is the speed and movement distance of the mold. The distance the mold moves during the process of opening and ejecting the part should be reduced to avoid wasting movement time. Of course, the mold movement must be enough to allow the part to smoothly escape from the mold before the mold is closed again. , Therefore, the moving distance required to release the part from the mold is short and the measurement time is small. When the injection molding machine is in good condition, the transition from high-speed opening to low-speed ejection can be quite smooth. The equipment requires some maintenance to accomplish these changes in speed, but the cost is repaid many times over in reduced molding time and time savings. In order to achieve the minimum mold movement time, adjust the deceleration limit switch so that the mold does not contact or damage the part too much during the pre-ejection process, and optimize the high-speed section of the stroke. Furthermore, proper periodic maintenance is required to ensure that this deceleration can be repeated every time. The time to generate clamping pressure is another delay in the entire mold opening time. This time may be caused by mechanical wear and hydraulic valve

The effects of failure, therefore periodic mechanical maintenance can maintain good operating condition.

Notice:

Shorten the mold opening stroke to the minimum necessary to allow the part and runner to fall off, except for any factors that make ejection difficult, such as flash around the ejector pin) Shorten the ejection stroke to the minimum necessary

Use the fastest mold opening and closing speeds while stopping and closing appropriately and slowly to prevent damage to the mold. Look for delays in all mold closing and clamping pressures. They indicate mechanical or hydraulic valve failure in the mold. The insert assembly activity also increases the mold opening time. With a little more consideration of product design (reducing undercuts), the ejection action can often be automated or semi-automated.

If the delay is caused by mold wear, the mold should be repaired to reduce the delay.

Develop good injection molding machine operating habits

Developing good injection molding machine operating habits is of great benefit to improving machine life and production safety.

1 Before starting up:

(1) Check whether there is water or oil entering the electrical control box. If the electrical appliance is damp, do not turn it on. Maintenance personnel should blow dry the electrical parts before turning on the machine.

(2) Check whether the power supply voltage is consistent, which should generally not exceed ±15%.

(3) Check whether the emergency stop switch and front and rear safety door switches are normal. Verify that the rotation directions of the motor and oil pump are consistent.

(4) Check whether each cooling pipe is smooth, and pass cooling water into the oil cooler and the cooling water jacket at the end of the barrel.

(5) Check whether there is lubricating oil (grease) in each moving part, and add enough lubricating oil.

(6) Turn on the electric heat to heat each section of the barrel. When the temperature of each section reaches the requirement, keep it warm for a period of time to stabilize the machine temperature. The holding time varies according to the requirements of different equipment and plastic raw materials.

(7) Add enough plastic to the hopper. According to the requirements for injection molding of different plastics, some raw materials are best dried first.

(8) Cover the heat shield on the barrel, which can save electric energy and extend the life of the electric heating coil and current contactor.

2During operation:

(1) Do not cancel the function of the safety door at will just for the sake of convenience.

(2) Pay attention to the temperature of the pressure oil and do not exceed the specified range. The ideal working temperature of hydraulic oil should be maintained between 45~50℃, and generally a range of 35~60℃ is more suitable.

(3) Pay attention to adjusting each stroke limit switch to avoid collision when the machine moves.

3At the end of the job:

(1) Before stopping the machine, the plastic in the barrel should be cleaned to prevent the remaining materials from oxidizing or decomposing by heat for a long time.

(2) The mold should be opened to keep the toggle mechanism in a locked state for a long time.

(3) The workshop must be equipped with lifting equipment. Great care should be taken when assembling and disassembling bulky parts such as molds to ensure production safety.

Some knowledge in using injection molding machine

1. The function of back pressure

The application of back pressure can ensure that the screw can generate enough mechanical energy to melt and mix the plastic when it rotates and resets. Back pressure also has the following uses: to discharge volatile gases, including air, out of the injection cylinder; to uniformly mix additives (such as toners, colors, antistatic agents, talcum powder, etc.) and dissolved materials; to make the flow through The material content of the screw length is evened out; providing uniform and stable plasticized material

materials to obtain precise finished product weight control. The selected back pressure value should be as low as possible (for example, 4-15bar, or 58-217.5ps), as long as the material has appropriate density and uniformity, and there are no bubbles, volatile gases and incomplete plasticization in the dissolved material. Chemical plastic will do. The use of back pressure increases the pressure temperature and material temperature of the injection molding machine. The amplitude of the rise is related to the set back pressure value. The oil circuit back pressure of larger injection molding machines (screw diameter exceeds 70mm/2.75im) can be as high as 25-406ar (362.5-580s). However, it should be noted that too high backpressure will cause the material temperature in the injection molding station to be too high. High, this situation is damaging to the production of heat-sensitive plastics.

Moreover, too high back pressure also causes excessive screw size and irregular offside conditions, making the injection amount extremely unstable. The amount of offside is affected by the viscoelastic properties of the plastic. The energy stored in the material is too much. When the screw stops rotating, it will suddenly jump backwards. Some thermoplastic plastics jump more severely than others, such as LDPE, HDPE, PP, EVA, PP/EPDM composites and PPVC. , compared with GPPS, HPS, POM, PC, PPO-M and PMMA, they are all relatively

Jumping phenomenon is prone to occur. In order to obtain the best production conditions, it is important to set the correct back pressure so that the material can be properly mixed and the screw's off-set range will not exceed 0.4mm (0.016i).

2. Opening and closing of the mold

Generally speaking, the mold opening and closing time used by most injection molding machines is slower than the quoted time (about 100-359%). This difference is related to the weight, size and complexity of the mold, and also to the safety protection of the mold (during opening). Preventing damage to the mold during closing operations).