Injection molding process conditions

Injection molding is an engineering technology, it is related to the contents of the plastic into useful and can maintain the original performance of the products. The importance of injection molding processing conditions affect the plastics flow and cooling the temperature, pressure and the corresponding time for each role. 

 

 1, temperature control 

 

1, barrel temperature: injection molding process requires temperature control of barrel temperature, nozzle temperature and die temperature. The first two temperature mainly affected the plastics and plastic flow, the latter mainly affects the temperature of plastic flow and cooling. Each has a different plastic flow temperature, the same type of plastic, because different sources or brands, the flow temperature and decomposition temperature is a difference, this is due to different molecular weight and molecular weight distribution caused by different types of plastic injection machine plasticizing process is different, so select the barrel temperature is not the same. 

 

2, nozzle temperature: nozzle temperature is usually slightly lower than the highest temperature of the barrel, This is to prevent melt through nozzles may occur in the "drooling phenomenon." Nozzle temperature, not too low, otherwise it will result in the early melt coagulation and the nozzle block * or early condensate as feed into the mold cavity to affect the performance of products 

 

3, mold temperature: mold temperature on the product's inherent quality of performance and apparent great effect. Mold temperature is decided by the availability of the plastic crystal, the size and structure of products, performance requirements, and other processing conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.). 

 

Second, pressure control: the process of pressure injection pressure and injection pressure of two kinds of plastics and plastic directly affect the quality of plastics and products. 

 

1, plasticizing pressure: (back pressure) using screw-type injection machine, the screw top of the melt in the screw by turning the pressure back when called plasticization pressure, also known as back pressure. The size of this pressure is through the hydraulic system relief valve to be adjusted. The injection, the plasticization pressure, the size of screw speed with all the same, an increase of pressure that will increase the plasticizing temperature of the melt, but will reduce the rate of plasticizer. In addition, the increased pressure often make plastics melt temperature uniformity, pigment mixed evenly and melt in the gas discharge. Normal operation, the plastics pressure to decide in ensuring good quality products under the premise of the lower the better, its specific value is used in plastics with different varieties, but seldom more than 20 kg / square cm. 

 

2, injection pressure: in the current production, almost all of the injection machine injection pressure is to screw the top on the plastic plunger or by the pressure (the pressure from the oil line conversion to a) the yardstick. Injection pressure in injection molding is the role played by overcoming the flow of plastic from the barrel cavity flow resistance, given the rate of melt filling and compaction of the melt for. 

 

3, molding cycle 

 

Complete a course of time required for injection molding, said molding cycle, also known as the molding cycle. It actually includes the following components: forming cycle: cycle time directly affect labor productivity and equipment utilization. Therefore, in the production process should be the premise of guaranteeing quality, shorten the molding cycle, all the time. Throughout the molding cycle time for injection and cooling time is most important, the quality of their products have a decisive impact. Injection time of the filling time is directly inversely proportional to the filling rate, the production of filling normally about 3-5 seconds. 

 

Injection time of the holding pressure time is plastic on the cavity pressure time, the injection time in the proportion of large, generally about 20-120 seconds (thick workpieces up to 5 to 10 minutes). Frozen at the gate to melt before the holding time of how much impact on the accuracy of product size, if in the future, was not affected. Holding time is also the most favorable value, known it depends on the melt temperature, mold temperature and the main channel and the gate size. If the main channel and the gate size and process conditions are normal, and usually that is shrinkage ratio reached the minimum of pressure fluctuations in the value of subject areas. Cooling time depends mainly on the thickness of products, plastic thermal properties and crystal properties, and mold temperature, etc.. The end of the cooling time should be to ensure that when the product does not cause changes in ejection principle, the timing of the general cooling of about between 30 to 120 seconds, cooling time is too long not necessary, not only reduces the efficiency of production of complex workpieces will difficulties caused by stripping, forced ejection time and perhaps even stripping the stress. Other molding cycle time was whether the process and production automation, and continuous and continuous, and the degree of automation and so on. 

 

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

 

According to raw material suppliers of the information provided by the temperature range, the barrel temperature to the middle of the range, and adjust the mold temperature. The estimated amount of shooting rubber, injection molding machine will be transferred to the estimated two-thirds of the maximum amount of radio glue. Adjust the cable down (pumping glue) trip. Estimates and adjust the second injection time, the two injection pressure to zero. 

 

Initial injection pressure to adjust the level of half the limit of injection molding machines (50%) of; be raised to the maximum injection speed. Estimates and adjust the required cooling time. Will be transferred back pressure 3.5bar. Remove the barrel has been degraded resin. Using semi-automatic injection mode; start injection procedures, observe the action screw. 

 

Needed and appropriate injection speed and pressure regulator, to make filling time, increase the injection pressure. As mentioned earlier, because there will be a full process before filling, filling the final pressure can be adjusted to an injection pressure of 100%. Pressure will eventually be turned up high enough so that you can not reach the maximum speed limit set pressure. If flash can reduce the speed. 

 

After each observation cycle, he gave the shot glue and conversion point adjustment. Setup process, so that the injection in the first level has to be achieved by shooting rubber weight 95-98% of the filling. 

 

First-class injection molding, conversion point, the injection speed and pressure are adjusted properly, will provide a second-class packing pressure of adjustment procedures. 

 

Appropriate adjustments as necessary packing pressure, but not over-filled cavity. 

 

Adjust the screw speed to ensure the completion of the cycle just before the melt has been completed and the injection cycle has not been restricted. 

 

Shorten the cycle time to increase productivity 

 

 For most of the injection molding manufacturers, the injection cycle may directly affect the following two main purposes: 

 

1 day, get more from the mechanical workpiece; 2, workpiece meet the requirements of the guests. 

 

Injection molding cycle consists of the following components: 

 

Cycle starts - screw began to move forward, injection; 

 

Mould Gate cooling 

 

Screw began to turn - start plasticizing stroke 

 

Completed the screw back - screw rotation stop 

 

Such as glue, if necessary, pumping occurs 

 

Die Open (which may include mold core pull) 

 

Workpiece can be fully cooled ejection 

 

Ejection 

 

A closure (which may include punch-back position) 

 

Mold close - cycle restart. 

 

Automatic injection molding cycle is continued in the same order, the same thing over and over again. Cycle has three main parts: 

 

Mold time; filling time; mold closure time; 

 

Holding time 

 

The goal is to increase productivity in a very short time all the necessary actions to complete the top out, and to ensure that mold protection (including the pull out and return the slider and the lateral punch). Therefore, any delay time of mold or mold injection molding machine to repair the problem. In addition, if the time for each injection mold are not the same, would not the same workpiece. Injection mold cavity filling (from 1-2) 

 

To mobility in terms of better materials (such as Delrin? POM, Zytel? And Zytel? ST nylon, and Crastin?, Rynite? Polyester), which fill the total cycle time should be 1 / 10 to 1 / 8 time , filling the most critical is fast and stable screw forward time, and minimum and stable injection pressure. 

 

When the screw to move forward will melt by the nozzle from the feed tube, vertical flow, horizontal flow, gate, and then launched into the cavity, which will encounter resistance. This resistance is the nozzle diameter, flow size, gate size, thickness, and die impact of exhaust design. Flow resistance should be improved and reduce mold to achieve a balanced and stable fill. Otherwise, uneven as a result of filling the cavity by the injection of different products, different size, strength, or look bad. 

 

Holding time (2-3) 

 

When the plastic crystalline materials, packing time is the most important section of the course. This time by 99% melt filling the cavity between the start gate for the termination of solidification. Module strength and toughness are determined after the injection has to keep up the pressure to melt the components / gate solidification. Maintain the pressure has to advance to retain a short position of melting plastic in the screw before. This is to prevent the packing section of the hole after the melt solidification shrinkage, or gate location The weakness of the module as a result of insufficient strength of the key. 

 

Cooling time (4-7) 

 

When the melt into the mold cavity, hit the metal surface, the melt cooling has been started on the steps. As the POM, nylon, and polycarbonate and other semi-crystalline materials, a high solidification temperature, so it needs very little cooling time. If the general module, in melt is complete, module should have sufficient cooling time. If the ejection module and found problems, it can gradually extend the cooling time until the issue is resolved. 

 

Mold time (8-11) 

 

Mold open time is an important part of the cycle, especially for a piece of the mold assembly is especially true, even in relatively standard mold, the mold opening hours are often higher than 20% of the entire cycle. 

 

Factors affecting mold: 

 

The first consideration is the speed and movement distance of mold, the mold opens and the top of the workpiece in the process of moving the distance of movement should be reduced so as not to waste time, of course, die die mobile must be sufficient for the workpiece before the closure again smooth out mold, therefore, necessary to move the workpiece ejection shorter distance, then it's among the less spent, when the injection molding machine in good condition, from high to low to open the top of the converted can be very smooth. Equipment requires some maintenance to complete these changes in speed, but they can be molded to spend less time, save time and get a return many times. In order to achieve the minimum mold movement time, adjust the deceleration limit switch in order to advance the process of mold will not be too out of touch or damage parts, and optimize high-speed travel section. Furthermore, the appropriate periodic maintenance to ensure that the slowdown can be repeated each time. Produce the mold clamping pressure of time is another delay in opening hours, this time through mechanical wear and possible failure of hydraulic valves, so periodic maintenance to maintain good mechanical operating condition. 

 

Note: 

 

Reduce the mold opening trip to the minimum necessary to workpiece and the flow off 

 

Difficult to exclude any factors that make the top out, like thimble fly around the edges (Phi front) 

 

Shorten the trip to the top of the minimum necessary 

 

With the fastest speed of mold, and closed modules, taking the appropriate suspension and closed slowly to prevent damage to die 

 

Find all the closed mold and produce clamping pressure of the delay, they said the failure of mechanical or hydraulic valve 

 

In the mold assembly of a large number of pieces of activities open mold growth. Little consideration of product design (less Kau) on top of the movement often makes automatic or semi-automation 

 

If this delay is caused by the die wear and tear, repairs should die, to reduce delays. 

 

Injection molding machine operator to develop good habits 

 

To develop good habits to improve the operation of injection molding machine and production machine life are good for security. 

 

Before a start: 

 

(1) Check the electrical control box if there is water and oil into, if electrical damp, do not boot. Maintenance personnel should be dry before turning on power to electrical components. 

 

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

 

(3) Check the emergency stop switches, safety door switch is normal before and after. Verify motor and pump rotation directions are the same. 

 

(4) Check the cooling pipe is smooth, and oil cooler and the barrel end of the cooling water jacket cooling water access. 

 

(5) Check whether the active portion of lubricating oil (grease), and add a little oil. 

 

(6) Open the electric, the various sections of the heating barrel. When the temperature of each meet the requirements, the longer holding period to keep the machine temperature to stabilize. Holding time depending on the requirements of equipment and plastic raw materials vary. 

 

(7) in the hopper to fill it enough plastic. According to various plastic injection molding requirements, some of the best first, dried materials. 

 

(8) to cover barrel on the shrouds, it saves energy, but also to extend the heating cycle and current contactor life. 

 

 2 during operation: 

 

(1) Do not chosen the easy, free to cancel the role of the security door. 

 

(2) to observe the temperature of the oil pressure, oil temperature should not exceed the prescribed range. The ideal operating temperature of hydraulic oil should be maintained at 45 ~ 50 ℃, and the average in the range of 35 ~ 60 ℃ more appropriate. 

 

(3) Note adjust the travel limit switches, avoid the impact generated when the machine in action.

 

3 end of the work: 

 

(1) stop before the barrel of the plastic machine should be clean, to prevent oxidation of surplus material or long-term thermal decomposition. 

 

(2) should open the mold, so long at the closed toggle state agencies. 

 

(3) The workshop must have a lifting device. Assembly and Disassembly Mold and other bulky parts should be very careful to ensure production safety. 

 

Injection molding machine to use some of the knowledge 

 

First, the function of back pressure 

 

The application can ensure that the screw back pressure in the rotation reduction, can generate enough mechanical energy, the plastic melted and mixed. Back pressure has the following purposes: 

 

The volatile gases, including air emissions outside shot cylinder material; the additives (such as toner, color species, anti-static agent, talc, etc.) and melt evenly mixed together; to melt flows through the screw length of uniform technology; provide uniform and stable plastic materials, finished products to obtain accurate weight control. 

 

 The back pressure value chosen should be as low as possible (for example, 4-15bar, or 58-217.5psi), as long as the proper melt density and uniformity, and no air bubbles inside the melt, volatile gases and is not fully plastic of plastic can. The use of back pressure to the pressure of injection molding machine temperature and melt temperature rise. Have increased and the value set by the back pressure. Larger injection molding machine (screw diameter of more than 70mm/2.75in) of the oil line back pressure can be up to 25-40bar (362.5-580psi). But pay attention, too high back pressure caused by the fire barrel melt temperature inside is too high, this situation is very sensitive to heat plastic production is destructive effect. 

 

Too high back pressure and also cause excessive and irregular screw offside situation, so shooting glue unstable. Offside how much plastic is suffering from the influence of viscoelastic properties. Melt the stored energy the more the screw in the stop rotation, resulting in a sudden backward beating, the beating of a number of thermoplastic plastic compared with other severe, such as LDPE, HDPE, PP, EVA, PP / EPDM compounds and PPVC, Compared to the GPPS, HIPS, POM, PC, PPO-M and PMMA are prone to flicker. To get the best production conditions, the correct back pressure setting is important, so that melt can be properly mixed, and screw the offside area will not exceed 0.4mm (0.016in). 

 

2, mold opening and closing 

 

In general, most of the mold injection machine used in opening and closing times slower than the reference time (about 100-359%), the difference with the mold of weight, size and complexity, but also on the security protection and die (in the open co-operation to prevent mold damage) the. 

 

A typical mold opening and closing hours are as follows (tcm: injection molding machine reference unit of time): 

 

The traditional two-plate mold :1-2tcm 

 

Composite mold (including side-mode core and rotating the device to use) and the multi-plate mold :2-3 .5 tcm 

 

If mold opening and closing time than the actual operation of more than 15% of the time, then need to modify the mold or use of another injection molding machine to shorten the time. Newer injection function provides faster opening and closing speed, low mold opening and closing (mold sensor) the pressure to start the joint tight mold clamping force.