Basic knowledge of the most popular injection mold

Basic knowledge of injection molding process conditions

injection molding is an engineering technology, which involves transforming plastics into useful products that can maintain their original properties. The important process conditions of injection molding are the temperature, pressure and corresponding action time that affect the plasticizing flow and cooling

I. temperature control

1. Barrel temperature: the temperature that needs to be controlled in the injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of plastic, and the second temperature mainly affects the flow and cooling of plastic. Each kind of plastic has different flow temperature. For the same kind of plastic, due to different sources or brands, its flow temperature and decomposition temperature are different, which is due to different average molecular weight and molecular weight distribution. The plasticization process of plastic in different types of injection machines is also different, so the temperature of material barrel is also different

2. Nozzle temperature: the nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the possible "salivation" of molten material in the straight through nozzle. The nozzle temperature should not be too low, otherwise it will cause early solidification of molten material and block the nozzle *, or affect the performance of the product due to the injection of early solidification material into the mold cavity

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

II. Pressure control: the pressure in the injection molding process includes plasticizing pressure and injection pressure, which directly affects the plasticization of plastic and the quality of products

1. Plasticizing pressure: (back pressure) when the screw injection machine is used, the pressure on the molten material at the top of the screw when the screw rotates and retreats is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by the overflow valve in the hydraulic system. In injection, the plasticizing pressure does not change with the rotation speed of the screw, so increasing the plasticizing pressure will increase the temperature of the melt, but will reduce the plasticizing speed. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the pigment mixed evenly and the gas in the melt discharged. In general operation, the plasticizing pressure should be determined as low as possible on the premise of ensuring good product quality. The specific value varies with the type of plastic used, but usually it rarely exceeds 20 kg/cm2

2. Injection pressure: in 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 role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the melt filling rate and compact the melt

III. molding cycle

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

molding cycle: the molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the forming cycle should be shortened as far as possible on the premise of ensuring the quality. In the whole molding cycle, injection time and cooling time are the most important, which have a decisive impact on the quality of products. The mold filling time in the injection time is directly inversely proportional to the mold filling rate, and the mold filling time in production is generally about seconds

the pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about seconds (for extra thick parts, it can be as high as 5~10 minutes). Before the molten material at the gate is sealed and frozen, the holding time has an impact on the dimensional accuracy of the product, but it has no impact in the future. Pressure holding time also has an optimal value, which is known to depend on material temperature, mold temperature, and the size of sprue and gate. If the size and process conditions of the sprue and gate are normal, the pressure value with the smallest fluctuation range of the product shrinkage is usually taken as the standard. The cooling time mainly depends on the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end point of the cooling time should be based on the principle of ensuring that there is no change when the product is demoulded. 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 efficiency, but also cause difficulty in demoulding for complex parts, and even produce demoulding stress when forced demoulding. The other time in the molding cycle is related to whether the production process is continuous and automated, and the degree of continuity and automation

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

according to the temperature range provided by the raw material supplier, adjust the barrel temperature to the middle of the range, and adjust the mold temperature

estimate the required glue injection volume, and adjust the injection molding machine to two-thirds of the estimated maximum glue injection volume. Adjust the reverse cable (glue extraction) stroke. Estimate and adjust the secondary injection molding time, and adjust the secondary injection molding 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 maximum. Estimate and adjust the cooling time required. Adjust the back pressure to 3.5bar. Remove the degraded resin in the barrel. Adopt semi-automatic injection molding mode; Start the injection procedure and observe the action of the screw

it is necessary to properly adjust the injection speed and pressure. If you want to shorten the filling time, you can increase the injection pressure. As mentioned above, there will be a process before full mold filling, and the final mold filling pressure can be adjusted to 100% of the primary injection pressure. Finally, the pressure should be adjusted high enough so that the maximum speed that can be achieved is not limited by the set pressure. If there is overflow, the speed can be reduced

after each observation cycle, adjust the glue injection amount and the conversion point. Set the program so that% mold filling can be obtained 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 stage injection molding are adjusted properly, the adjustment procedure of the pressure holding pressure of the second stage can be carried out

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

adjust the screw speed to ensure that the glue melt has been completed just before the cycle is completed, and the injection cycle is not limited

shorten cycle time to improve productivity

for most injection molding manufacturers, injection molding cycle can directly affect the following two main purposes: 1. Get more parts from machinery every day; 2. The products meet the requirements of the guests

injection cycle consists of the following:

cycle start -- screw starts to move forward, injection

part gate cooling

screw starts to rotate -- plasticizing stroke starts

screw return is completed -- screw rotation stops

if necessary, glue extraction occurs

mold opening (which may include mold core pulling)

the part can be ejected when it is fully cooled

ejection

mold closing (which may include mold core return)

mold closing - cycle starts again

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

mold opening time; Filling time; Mold closing time; The goal of improving productivity is to complete all necessary actions in a very short time, complete ejection, and ensure that the mold is protected (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 parts will also be different

injection molding filling the mold cavity (by)

for materials with good fluidity (such as Delrin? POM, Zytel? And Zytel? St nylon, as well as crastin? And Rynite? Polyester), the filling time should account for 1/10 to 1/8 of the whole cycle. The key during filling is the fast and stable screw advance time and the lowest and stable injection pressure

when the screw is pushed forward, the solution will be injected into the mold cavity from the feed pipe through the nozzle, the vertical flow channel, the horizontal 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 flow resistance should be improved and reduced in the mold to achieve filling balance and stability. Otherwise, due to uneven filling, the products injected into different mold cavities have different sizes, insufficient strength, or poor appearance

holding time ()

when injecting crystalline materials, the holding time is the most important process. This period starts from the 99% filling of the mold cavity by the melt to the solidification of the gate. The strength and toughness of the module are determined by whether the pressure is maintained on the melt after injection molding until the part/gate solidifies. When maintaining the pressure, it is also necessary to reserve a small section of molten glue in front of the screw in advance. This pressure maintaining stage is the key to prevent the cavity after the melt solidification shrinkage or the weak position of the gate from causing the insufficient strength of the module

cooling time ()

when the melt enters the mold cavity and touches the metal surface, the melt cooling step has begun. Due to the high solidification temperature of semi crystalline materials such as polyoxymethylene, nylon, and polycarbonate, there is little cooling time. For general modules, the module should have enough cooling time after the completion of glue melting. If a problem is found when ejecting the module, the cooling time can be extended slowly until the problem is solved

mold opening time ()

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

factors affecting mold opening:

the first thing to consider is the speed and moving distance of the mold. The moving distance of the mold in the process of opening and ejecting the workpiece should be reduced to avoid wasting the moving time. Of course, the mold movement must be enough to make the workpiece leave the mold smoothly before the mold is closed again. Therefore, the shorter the moving distance required to demould the workpiece, the less time it takes. 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 needs some maintenance to complete these speed changes, but these costs can be reduced from molding time, saving time and getting multiple returns. In order to achieve the minimum mold movement time, adjust the deceleration limit switch so that the mold will not contact or damage the workpiece too much during the pre ejection process, and optimize the high-speed section of the stroke. Moreover, proper periodic maintenance should be carried out to ensure that the deceleration can be repeated every time. The time to generate mold locking pressure is another delay in the whole mold opening time. This time may be affected by mechanical wear and hydraulic valve failure, so periodic mechanical maintenance can maintain a good operating state

note:

shorten the mold opening stroke to the minimum necessary to prevent the parts and flow channels from falling off

eliminate any factors that make ejection difficult, such as the flash (CAPE) around the thimble

shorten the ejection stroke to the minimum necessary

use the fastest mold opening and closing speed, and at the same time, stop and close appropriately and slowly to prevent damage to the mold

look for all mold closures and delays in mold locking pressure, They indicate a mechanical or hydraulic valve failure