How to set your injection molding machine parameters?

Time:2019-05-06 09:25:09 / Popularity: / Source:

Injection Molding Machine Introduction Technology: Polycarbonate injection molding is an engineering technology that involves transforming plastic into useful and original products. Important process conditions for polycarbonate injection molding are temperature, pressure and corresponding individual time of action that affect plasticizing flow and cooling.
  • Injection moulding machine setting - temperature control
1. Barrel temperature: Temperature to be controlled during moulding process is barrel temperature, nozzle temperature and mold temperature. First two temperatures primarily affect plasticization and flow of plastic, while 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. Plastics are injected in different types. Plasticizing process in machine is also different, so temperature of selected cylinder is also different.
2. Nozzle temperature: Nozzle temperature is usually slightly lower than maximum temperature of the 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 to block nozzle, or affect performance of product due to injection of early aggregate into cavity.
3. Mold temperature: 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 and structure of product, performance requirements, and other moulding process conditions (melt temperature, injection speed and injection pressure, molding cycle).
  • Injection moulding machine setting - pressure control
Pressure during moulding process includes plasticizing pressure and injection pressure, and directly affects plasticization of plastics and quality of products.
1. Plasticizing pressure: (back pressure) When using a screw type injection machine, pressure that top melt of tscrew 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. In injection, plasticizing pressure is constant 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 .
2 Injection pressure: 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 polycarbonate injection molding by overcoming flow resistance of plastic from barrel to cavity, giving melt filling rate and compacting melt.
  • Injection moulding machine setting - molding cycle
Time required to complete an moulding process is called molding cycle. It actually includes following parts:
Molding cycle: 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 utmost importance throughout molding cycle and they have a decisive influence on 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 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 melt at gate, amount of dwell time has an effect on dimensional accuracy of product. If after, 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 fluctuation range of product shrinkage is the smallest.
Cooling time is mainly determined by thickness of product, thermal properties and crystallization properties of plastic, and temperature of mold. End point of cooling time should be based on principle of ensuring product does not cause changes during demolding. Cooling time is generally between 30 and 120 seconds. It is not necessary to cool too long, which not only reduces production efficiency, but also causes difficulty in demolding complicated parts, and even mold release stress is generated when mold is released. Other times in molding cycle are related to continuity and 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 the middle of range and adjust mold temperature based on temperature range provided by material supplier's data.
Estimate amount of glue required to adjust injection molding machine to two-thirds of estimated maximum shot size. Adjust backstrip (pumping) stroke. Estimate and adjust secondary injection time and adjust secondary injection pressure to zero.
Initially adjust first-stage injection pressure to half (50%) of limit of injection molding 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 molding mode; start injection moulding process and observe action of screw.
Injection speed and pressure are appropriately adjusted as needed, and injection pressure can be increased if filling time is shortened. 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 that can be achieved and is not limited by set pressure. If there is a flash, you can reduce speed.
After each observation period, amount of glue and switching point are adjusted to set program so that filling of first stage can be achieved by 95-98% of weight of shot.
When injection volume, switching point, injection speed and pressure of first-stage injection molding 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 melt is completed just before cycle is completed and injection cycle is not limited.
Reduce cycle time to increase productivity
For most injection factory, 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:
Cycle begins - screw starts to advance, injection;
Article gate cooling
Screw starts to rotate - plasticizing stroke begins
Screw return is completed - screw rotation stops
Glue occurs if necessary
Mold open (may include pull out of core)
Part can be fully cooled to be ejected
Mold closure (may include return of core)
Mold closes - cycle restarts.
Automatic injection cycle is repeated in same order, same thing is repeated again and again. Cycle has three main parts:
Mold opening time; filling time; mold closing time; packing 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 molding machine that delays mold opening time must be repaired. In addition, if mold opening time is different for each injection, parts will be different.
Polycarbonate injection molding cavity(1-2)
For materials with better fluidity, this filling time should account for 1/10 to 1/8 of total 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, the 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, insufficient strength, or poor appearance due to uneven filling.
Holding time (2-3)
Holding time is the most important part of moulding 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.
Cooling time (4-7)
As melt enters mold cavity and hits metal surface, melt cooling step begins. Since solidification temperature of semi-crystalline material is high, time required for cooling is small. In case of a general module, module should have sufficient cooling time after melt is completed. If you find a problem when eject module, slowly extend cooling time until problem is solved.
Mold opening time (8-11)
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.
  • Factors affecting mold opening:
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 to allow part to break off mold before mold is closed again. Therefore, the shorter moving distance required to demold part, the less space it spends. When injection molding machine is in good condition, conversion from high speed opening to low speed ejection can be quite stable. 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, optimize high-speed section of stroke. 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:
Shorten mold opening stroke to minimum necessary for parts and runners to fall off
Exclude any factors that make it difficult to push out, like a flash around thimble
Shorten ejection stroke to minimum required
Use the fastest mold opening and closing speed, and at the same time slowly stop and close properly to prevent damage to mold
Look for all closed molds and create delays in clamping pressure, which indicate failure of mechanical or hydraulic valves
A large amount of insert activity in mold also increases mold opening time. A slight consideration of product design(reducing undercut) often makes ejection action automated or semi-automated
If this delay is caused by mold loss, it is reasonable to repair mold to reduce delays.
Develop a good operating habit of injection molding machine
Developing good injection molding machine operating habits is good for improving machine life and production safety.
  • Injection moulding machine setting - Before booting:
(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 polycarbonate 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.
  • Injection moulding machine setting - During operation:
(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.
3. At the end of work:
(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 such as molds to ensure production safety.
  • Injection moulding machine setting - some knowledge in the use of injection molding machines
First, function of back pressure
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; additive (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 (eg 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 molding machine and temperature of melt to rise. Magnitude of rise is related to set back pressure value. Large injection molding 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 temperature of melt in barrel to be too high, which is destructive for production of heat sensitive plastics.
Moreover, high back pressure also causes screw to be too large and irregular offside condition, so that amount of glue is extremely unstable. Offside is affected by viscoelastic properties of plastic. The more energy stored in melt, sudden backward jump of screw when it stops rotating, and jitter of some thermoplastics is more severe than other plastics, such as LDPE, HDPE, PP, EVA, PP/EPDM composites and PPVC. Compared with GPPS, HIPS, POM, PC, PPO-M and PMMA, it is more prone to beating.
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).
Second, opening and closing of mold
In general, mold opening and closing time used by most injection molding machines is slower than reference time (about 100-359%). This difference is related to weight, size and complexity of mold, safety protection of mold (prevent mold damage during operation).
Typical mold opening and closing times are as follows (tcm: time unit quoted by injection molding 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. Newer 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.
Operator of injection molding machine often does not pay attention to speed or time of board of a particular injection molding machine, sets 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. Learning techniques must be studied hard, and it takes time to improve their own technical level.

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