Key points for setting operating conditions at each stage in injection molding

Time:2022-12-31 08:45:44 / Popularity: / Source:

Plasticizing stage of solid plastics

Tank temperature and nozzle temperature

(1) Gradually heat up from the first stage (feeding place) to third stage (before nozzle), temperature setting value of each stage differs by about 5~10 ℃, and gradually heat plastic to appropriate processing temperature. Various plastics have its different processing temperatures.
(2) For low-viscosity plastics, in order to avoid material leakage when machine base retreats, nozzle temperature can usually be set to third stage temperature or slightly lower (about 5℃), but too low will cause cold material, prone to flow marks during injection. For high-viscosity plastics, setting value can be about 5~10℃ higher than temperature of third stage, which can be used to heat runner system during injection.
(3) Melt is heated by friction due to rotation of screw, which often causes temperature of melt in third stage to be higher than set value of heater, generally even 15~30℃ higher, so it should be avoided that temperature setting is too high to cause thermal degradation.

Screw back pressure

· Back pressure of screw is generally about 3~10kg/cm2. Main purpose of back pressure setting is to resist screw. When material is fed, plastic is transported to front end of screw, and melt pressure is generated to make screw slowly retreat.
· If back pressure of screw is large, plastic will be transported from feeding port to front end of screw, and it will take a long time, so degree of plasticization is more complete, and gas is not easy to enter, but melt pressure generated at the front end of screw is also large, which is easy to cause material leakage when machine seat retreats. On the contrary, if back pressure of screw is small, plastic is transported from feeding port to the front end of screw, and time elapsed is shorter, so degree of plasticization is less complete, and gas is easier to enter.
· In plasticizing stage of solid plastics, because screw retreats due to pressurization of melt chamber during feeding process, length of screw experienced by plastic entering later will be shortened, and melting capacity of plastic will be insufficient. Therefore, if incompletely melted rubber particles enter melting chamber and become suspended, it will seriously affect product quality. Therefore, with progress of feeding stroke, backward speed of screw should gradually slow down, and backward rubber must have enough time to melt in screw groove. For this purpose, screw back pressure should be gradually increased as feed stroke progresses, and screw speed should be gradually decreased.
setting operating conditions at each stage in injection molding 

Screw speed

· Faster screw speed rpm means more feed and higher shearing heat, but if it is too fast, it may cause shearing of plastic molecules
· For plastics containing glass fiber, if screw speed is too fast, glass fiber will be cut, resulting in a decrease in physical properties
· With progress of feeding stroke, screw speed should be gradually reduced in order to allow backward rubber to melt in screw groove for enough time.
setting operating conditions at each stage in injection molding 

Cavity filling stage

Feed stroke

When feeding, screw rotates and retreats at the same time. When screw retreats to set point of feeding stroke, feeding is stopped.
Feeding position should include residual amount and advance amount of screw during filling and pressure holding stages.
If setting of feeding stroke is about 60~70% of maximum injection volume (catalog value), it is the most appropriate choice of injection machine.
If feeding stroke is set too much, amount of melt at high temperature of third stage will be very large, and it will be shot after many molding cycles, which may cause discoloration or thermal degradation.
If feeding stroke is set too small, there will not be enough residual amount after each injection, so pressure transmission will be unstable in later stage of pressure holding stage.

Filling speed

(1) During filling process, if speed is fast, injection pressure required for filling cavity will be high, and if speed is slow, injection pressure required for filling cavity will also be high due to formation of solidified layer. Therefore, under a certain filling speed, injection pressure required for filling cavity will be the smallest, and filling speed at this time is the most appropriate. Relative filling time is called "optimal filling time".
(2) During filling process, melted glue forms a solidified layer on the surface of mold, which determines surface properties of finished product. If wavefront speed is too fast, molecules will be stretched seriously and solidified layer will be too thin, even resulting in slippage and bright marks on the surface of finished product.
If wave front speed is too slow, solidified layer will thicken and increase flow resistance of plastic entering later, so it is easy to form wave marks on the surface. Therefore, it is the key to ensure surface quality of finished product to properly control advancing speed of screw in multiple stages, so that melt wave front can maintain a stable and consistent speed in the process of filling mold cavity. For example, in the product shown in figure, since wavefront area (A1) is constantly changing with filling process, in order to have a stable and consistent speed (V1), speed of screw (Vs) must also change relatively, and relationship is as follows:
setting operating conditions at each stage in injection molding 
As (screw cross-sectional area) is a fixed value, and V1 is also expected to remain fixed in design, so wavefront area (A1) is proportional to speed of screw (Vs).
setting operating conditions at each stage in injection molding 
Filling stage is speed control, screw moves forward according to set speed, as long as pressure does not rise to pressure set value (generally called first stage pressure), screw will inject sol into mold cavity at set speed, but if pressure rises to set value of injection pressure during filling process, pressure will no longer rise, and screw will continue to be pushed at set pressure, so speed of screw will decrease.
setting operating conditions at each stage in injection molding 

Filling time

1. Actual filling time during injection process (time of screw in speed control mode) is not necessarily filling time (t1) set on machine, but actual filling time (t2) is
setting operating conditions at each stage in injection molding 
"Screw advancing distance" referred to here refers to distance from initial position of screw to filling/holding transition point.
2. When t1>t2, it means that screw has reached filling/pressure-holding transition point at t2, so it is switched to pressure control at pressure-holding stage.
However, if screw has not reached filling/holding transition point at time t1 (that is, t1<t2), set filling time will also be exhausted, that is, pressure control will be switched to pressure holding stage at this position, which is so-called early switching. When t1=t2, it means that screw reaches filling/holding pressure transition point at time t1, and it is also switched to pressure control in holding pressure stage at this position.

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