Injection moulding is an engineering technique that involves transforming plastic into a product that is useful and maintains its original properties. Important process conditions for injection moulding are temperature, pressure and corresponding individual time of action that affect plasticizing flow and cooling.

 

Temperature to be controlled during injection molding process includes barrel temperature, nozzle temperature and mold temperature. The first two temperatures primarily affect plasticization and flow of plastic, while the latter temperature primarily affects flow and cooling of plastic. Each plastic has a different flow temperature. Same plastic has different flow and decomposition temperatures due to different sources or grades. This is due to difference in average molecular weight and distribution. Plasticizing process of plastics in different types of injection machines is also different, so choice of barrel temperature is also different.

Nozzle temperature is usually slightly lower than maximum temperature of barrel. This is to prevent "flowing phenomenon" of melt in straight-through nozzle. Nozzle temperature should not be too low, otherwise it will cause early condensation of melt and block nozzle, or affect performance of product due to injection of early aggregate into cavity.

Mold temperature has a great influence on intrinsic properties and apparent quality of product. Temperature of mold depends on presence or absence of crystallinity of plastic, size, structure and performance requirements of product, other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).

Pressure in injection molding process includes plasticizing pressure and injection pressure, and directly affects plasticizing process of plastics and product quality.

(back pressure) When a screw type injection machine is used, pressure that top melt of screw receives when screw rotates backward is called plasticizing pressure, also known as back pressure. This pressure can be adjusted by a relief valve in hydraulic system. During injection, plasticizing pressure changes with rotation speed of screw, When plasticizing pressure is increased, temperature of melt is increased, but plasticizing speed is reduced. In addition, increasing plasticizing pressure often makes temperature of melt uniform, mixing of colorant uniform and discharge of gas in melt. In general operation, determination of plasticizing pressure should be as low as possible to ensure quality of product. Specific value varies with type of plastic used, but usually rarely exceeds 20 kg/cm 2 .

In current production, injection pressure of almost all injection machines is based on pressure exerted by plunger or top of screw on plastic (converted from oil pressure). Injection pressure plays a role in injection molding by overcoming flow resistance of plastic from barrel to cavity, giving melt filling rate and compacting melt.

Time required to complete an injection molding process is called molding cycle. It actually includes following parts:

Molding cycle directly affects labor productivity and equipment utilization. Therefore, in production process, relevant time in molding cycle should be shortened as much as possible under premise of ensuring quality. Injection time and cooling time are of the utmost importance throughout molding cycle and they have a decisive influence on the quality of product. Filling time in injection time is directly inversely proportional to filling rate, and filling time in production is generally about 3-5 seconds.
Holding time in injection time is pressure time of plastic in the cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (extra-thick parts can be as high as 5-10 minutes). Before melting of material at the gate, amount of dwell time has an effect on dimensional accuracy of product. If it is later, it has no effect. Holding time also has the best value, which is known to depend on material temperature, mold temperature and size of main runners and gates. If dimensions and process conditions of main runners and gates are normal, it is usually value of pressure at which range of fluctuation of product shrinkage is the smallest. Cooling time is mainly determined by thickness of product, thermal properties and crystallization properties of plastic, and mold temperature. End point of cooling time should be based on principle of ensuring that product does not cause changes during demolding. Cooling time is generally between 30 and 120 seconds. If it is too long, it is not necessary. It not only reduces production efficiency, but also causes difficulty in demoulding complex parts. Even mold demould stress is generated when demoulding. Other times in molding cycle are related to continuity, automation of production process, as well as degree of continuity and automation.
General injection molding machine can be adjusted according to following procedures:
Adjust barrel temperature to middle of range and adjust mold temperature based on temperature range provided by material supplier's data.
Estimate amount of material required to adjust injection moulding machine to two-thirds of estimated maximum shot size. Adjust backstrip (pumping) stroke. Estimate and adjust secondary injection time, adjust secondary injection pressure to zero.
Initially adjust the first-stage injection pressure to half (50%) of limit of injection moulding machine; adjust injection speed to the highest. Estimate and adjust cooling time required. Adjust back pressure to 3.5 bar. Remove degraded resin from barrel. Use semi-automatic injection moulding mode; start injection molding process and observe action of screw.
Injection speed and pressure can be appropriately adjusted as needed. To shorten filling time,  increase injection pressure. As mentioned earlier, final pressure of filling can be adjusted to 100% of primary injection pressure due to a process before full filling. Pressure must eventually be adjusted high enough so that maximum speed is not limited by set pressure. If there is a flash, you can reduce speed.
After each observation period, amount of injection material and switching point are adjusted. Setting procedure, so that a 95-98% filling of of shot can be obtained in the first stage of injection moulding.
When injection volume, switching point, injection speed and pressure of the first-stage injection moulding are properly adjusted, second-stage pressure-holding pressure adjustment procedure can be performed.
Adjust holding pressure as needed, but do not overfill cavity.
Screw speed is adjusted to ensure that melt is completed just before cycle is completed and injection cycle is not limited.
Reduce cycle time to increase productivity
For most injection molders, injection cycle can directly affect two main purposes:
1. Get more parts from machine every day;
2. Parts meet requirements of guests.

 

Injection cycle consists of following item:
Cycle begins - screw starts to advance, injection;
Article gate cooling
Screw starts to rotate - plasticizing stroke begins
Screw return is completed - screw rotation stops
Drawing injection material occurs if necessary
Mold open (may include pull out of core)
Part can be fully cooled to be ejected
Eject
Mold closure (may include return of the core)
Mold closes - cycle restarts.
Automatic injection cycle is repeated in same order, and same thing is repeated again and again. Cycle has three main parts:
Mold opening time, filling time, mold closing time, holding time
Goal of increasing productivity is to complete all necessary actions in a very short time, complete ejection, and ensure that mold is protected (including pulling out, retracting slider and side core). Therefore, any problem with mold or injection moulding machine that delays mold opening time must be repaired. In addition, if mold opening time is different for each injection, parts will be different.

For materials with better fluidity, filling time should account for 1/10 to 1/8 of entire cycle. The most critical for filling is fast and stable screw advance time, the lowest and stable injection pressure.
When screw advances, solution is subjected to resistance from material pipe through nozzle, vertical flow path, horizontal flow, gate, and then into cavity. This resistance is affected by nozzle diameter, runner size, gate size, product thickness, and mold venting design.
Flow resistance should be improved and reduced in mold to achieve filling balance and stability. Otherwise, products molded by different cavities are different in size due to uneven filling, insufficient strength, or poor appearance.

Holding time is the most important part of process when injecting a crystalline material. This period of time starts from 99% of melt-filled cavity until gate solidifies to a stop. Strength and toughness of module are determined by whether or not pressure is maintained on melt after injection molding until part/gate is solidified. Keep a small amount of melt position in front of screw before holding pressure. This part of pressure holding is the key to prevent strength of module from being insufficient due to void after solidification and shrinkage of melt, or weak position of gate.

As melt enters mold cavity and hits metal surface, melt cooling step begins. Due to high solidification temperature of semi-crystalline material is high, time required for cooling is small. In the case of a general molded products, molded products should have sufficient cooling time after melt is completed. If you find a problem when you eject molded products, you can slowly extend cooling time until problem is solved.

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

 

The first item to consider is speed and distance of mold. Distance that mold moves during opening and ejection of part should be reduced to avoid wasting movement time. Of course, mold movement must be sufficient for part to be smoothly separated from mold before mold is closed again. Therefore, the shorter moving distance required to demold part, the less space it spends. When injection moulding machine is in good condition, transition from high speed opening to low speed ejection can be quite smooth. Equipment requires some maintenance to accomplish these speed changes, but these costs can be reduced from molding time, saving time and getting multiple returns. In order to achieve minimum mold movement time, adjust deceleration limit switch so that mold does not over-contact or damage part during pre-out process, and optimize high-speed section of stroke. Again, proper periodic maintenance to ensure that this deceleration can be repeated each time. Generation of clamping pressure time is another retardation throughout mold opening time, which may be affected by mechanical wear and hydraulic valve failure, so that periodic mechanical maintenance can maintain good operating conditions.
Note:
1 Shorten mold opening stroke to minimum necessary for parts and runners to fall off.
2 Exclude any factors that make it difficult to eject, like flash around thimble.
3 Shorten ejection stroke to minimum required
4 Use the fastest mold opening and closing speed, and at the same time slowly stop and close properly to prevent damage to mold 
5 Find delays in clamping pressure and all closed molds, they represent failure of mechanical or hydraulic valves.
6 A large number of insert activities in mold also increase mold opening time. A slight consideration of product design (reducing undercut) often makes ejection action automated or semi-automated.
7 If this delay is caused by loss of mold, it is reasonable to repair mold to reduce delay.
8 Develop good injection moulding machine operating habits
9 Developing good injection moulding machine operating habits is good for improving machine life and production safety.

(1) Check if there is water or oil in electrical control box. If appliance is damp, do not turn it on. Electrical parts should be blown dry by maintenance personnel before turning them on.
(2) Check if power supply voltage is consistent, generally should not exceed ±15%.
(3) Check emergency stop switch, whether front and rear safety door switches are normal. Verify that motor and oil pump are rotating in same direction.
(4) Check whether cooling pipes are unblocked, and pass cooling water to cooling water jacket at oil cooler and end of barrel.
(5) Check if there is any lubricating oil (fat) in each active part and add enough lubricant.
(6) Turn on electric heating and warm various sections of barrel. When temperature of each section meets requirements, it is kept for a further period of time to stabilize machine temperature. Holding time varies according to requirements of different equipment and plastic materials.
(7) Add enough plastic to hopper. According to requirements of different plastic injection molding, some raw materials are preferably dried first.
(8) Cover heat shield on barrel to save energy, extend life of electric heating coil and current contactor.

(1) Don't arbitrarily cancel role of security door for convenience of coveting.
(2) Pay attention to temperature of pressure oil. Oil temperature should not exceed specified range. Ideal working temperature of hydraulic oil should be maintained between 45~50℃, generally in the range of 35~60℃.
(3) Pay attention to adjusting each travel limit switch to avoid impact of machine during action.

(1) Before stopping machine, plastic in barrel should be cleaned to prevent oxidation of residual material or long-term thermal decomposition.
(2) Mold should be opened to keep toggle mechanism locked for a long time.
(3) Workshop must be equipped with lifting equipment. Care should be taken when installing and removing bulky parts to ensure production safety.

 

Application of back pressure ensures that screw produces sufficient mechanical energy to melt and mix plastic during rotary reset. Back pressure also has following uses:
Volatile gases, including air, are discharged from shooting cylinder; additives (such as toner, color, antistatic agent, talc, etc.) and melt are uniformly mixed; melt flowing through length of screw is homogenized; provides a uniform and stable plasticized material for precise finished weight control.
Selected back pressure value should be as low as possible (4-15 bar, or 58-217.5 psi). As long as melt has proper density and uniformity, there are no bubbles, volatile gases and plastics that are not fully plasticized in melt.
Use of back pressure causes pressure temperature of injection moulding machine and melt temperature to rise. Magnitude of rise is related to set back pressure value. Large injection moulding machine (screw diameter over 70mm / 2.75in) can have a back pressure of up to 25-40bar (362.5-580psi). However, it should be noted that too high back pressure causes melt temperature in barrel to be too high, which is destructive for production of heat sensitive plastics. Moreover, too high back pressure also causes large and irregularly off-position of screw, so that  amount of material is extremely unstable. Offside is affected by viscoelastic properties of plastic. The more energy stored in melt, the sudden backward jump of screw when it stops rotating, and the jitter of some thermoplastics is more severe than other plastics, such as LDPE, HDPE, PP, EVA, PP/EPDM composites and PPVC are more prone to beating than GPPS, HIPS, POM, PC, PPO-M and PMMA.
In order to obtain optimum production conditions, it is important that correct back pressure setting is made so that melt can be properly mixed and offside range of screw does not exceed 0.4 mm (0.016 in).

In general, mold opening and closing time used by most injection moulding machines is slower than reference time (about 100-359%). This difference is related to weight, size and complexity of mold, as well as safety protection of mold(prevent mold damage during operation).
Typical mold opening and closing time are as follows (tcm: time unit quoted by injection moulding machine):
Traditional double-plate mold: 1-2tcm
Composite molds (including use of side cores and screw-out devices) and multi-plate molds: 2-3.5tcm
If mold opening and closing time is 15% longer than actual operation time, then you need to modify mold or use another injection molding machine to shorten time. New injection molding machine provides faster opening and closing speeds, using low mold opening and closing (mold sensing) pressure to initiate clamping force to tighten mold.
Injection moulding machine operators often do not pay attention to speed or time of board of a particular injection moulding machine, set mold opening and closing time with personal experience, which often makes operation time long. A one-second reduction in a ten-second operation immediately yields a 10% improvement. This improvement is often difference between profit and loss.