The secret of plastic injection molding process adjustment

Five techniques of injection molding machine adjustment:

• 1. Temperature: barrel temperature, material temperature, mold temperature, drying temperature, oil temperature, ambient temperature, etc.
• 2. Pressure: injection pressure, holding pressure, back pressure, demolding pressure, mold opening pressure, mold clamping pressure, etc.
• 3. Time: injection time, holding time, cooling time, drying time, measurement delay time, etc.
• 4. Speed: injection speed, carriage return speed, mold opening and closing speed, demolding speed, etc.
• 5. Location: measurement position, ejection position, mold opening position, etc.

Proportional control of injection speed has been widely adopted by injection molding machine manufacturers.

Although computer-controlled injection speed segmented control systems have long existed, the advantages of this machine setup are rarely brought into play due to limited information. This article will systematically explain the advantages of applying multi-speed injection, and briefly introduce its use in eliminating shortcomings, trapped air, shrinkage and other product defects.

The close relationship between injection speed and product quality makes it a key parameter for injection molding.

By determining the beginning, middle, and end of the filling velocity segment, and achieving a smooth transition from one setpoint to another, a stable melt surface velocity can be ensured to produce the desired molecular interrogation and minimal internal stress.

We recommend the following speed segmentation principle for injection moulding:

• 1) The velocity of the fluid surface should be constant.
• 2) Quick glue should be used to prevent the melt from freezing during the glue injection process.
• 3) The setting of injection speed should take into account that the speed is slowed down at the water inlet while filling quickly in critical areas (such as flow channels).
• 4) The injection speed should be guaranteed to stop immediately after the cavity is filled to prevent overfilling, flashing and residual stress.

Considering the shape of the products in injection moulding

The basis for setting the speed segment must take into account the mold geometry, other flow restrictions, and instability. The speed setting must have a clearer understanding of the injection molding process and material knowledge, otherwise the product quality will be difficult to control. Because the melt flow rate is difficult to measure directly, it can be indirectly calculated by measuring the screw advance speed or the cavity pressure (make sure that the check valve is not leaking).

Considering the mateiral properties in injection moulding

Material properties are very important, because polymers may degrade due to different stresses. Increasing the molding temperature may lead to severe oxidation and degradation of the chemical structure, but at the same time the degradation caused by shear becomes smaller, because high temperature reduces the viscosity of the material. Reduced shear stress. Undoubtedly, the multi-stage injection speed is very helpful for forming heat-sensitive materials such as PC, POM, UPVC and their blending ingredients.

The maximum injection speed is required at the thin wall in injection moulding process

The geometry of the mold is also the determining factor: the maximum injection speed is required at the thin wall; the slow-fast-slow speed curve is needed for thick-walled parts to avoid defects; the injection speed setting should ensure the melt forward velocity to ensure that the part quality meets the standard constant.

The melt flow speed is very important for injection moulding

because it will affect the molecular arrangement direction and surface state in the part; when the melt reaches the cross-section structure in front, it should slow down; for complex molds with radial diffusion, the melt throughput should be guaranteed Increase evenly; long runners must be filled quickly to reduce the cooling of the melt front, but injection of high viscosity materials, such as PC, is the exception, because too fast speeds can bring cold material through the water inlet into the cavity.

Adjusting the injection speed can help eliminate defects when moulding.

1. When the melt reaches the water inlet through the nozzle and the flow channel, the surface of the melt front may have cooled and solidified, or the melt will stagnate due to the sudden narrowing of the flow channel until sufficient pressure is established to push the melt through the inlet Water inlet, which will cause a peak shape of the pressure through the water inlet.
2. High pressure will damage the material and cause surface defects such as flow marks and scorch at the water inlet. This situation can be overcome by decelerating just before the water inlet. This deceleration can prevent excessive shear at the water inlet, and then increase the rate of fire to the original value. Because it is very difficult to precisely control the rate of fire to slow down at the inlet, deceleration at the end of the flow path is a better solution.
3. We can avoid or reduce defects such as flash, scorch, trapped air, etc. by controlling the speed of the last shot. Deceleration at the end of filling can prevent over-filling of the cavity, avoid flashing and reduce residual stress. The trapped air due to poor exhaust or filling problems at the end of the mold flow path can also be solved by reducing the exhaust speed, especially the exhaust speed at the end of the shot.
4. Short shots are caused by slow speeds at the water inlet or local flow obstruction caused by melt solidification. Accelerating the injection speed just after passing through the water inlet or local flow obstruction can solve this problem.
5. Defects on heat-sensitive materials such as flow marks, scorching at the water inlet, molecular cracking, delamination, and peeling are caused by excessive shear when passing through the water inlet.
6. Smooth parts depend on the injection speed, and glass-fiber fillers are particularly sensitive, especially nylon. Dark spots (wavy lines) are caused by unstable flow due to viscosity changes. Twisted flow can lead to wavy or uneven haze. What kind of defects are produced depends on the degree of flow instability.
7. When the melt passes through the water inlet, high-speed injection will cause high shear, and the heat-sensitive plastic will scorch. This scorched material will pass through the cavity, reach the flow front, and appear on the surface of the part.
8. In order to prevent jetting, the setting speed of the glue must ensure that the runner area is filled quickly and then passed through the water inlet slowly. Finding this speed transition point is the essence of the problem. If it is too early, the filling time will be excessively increased, and if it is too late, too much flow inertia will cause the appearance of ejection. The lower the melt viscosity and the higher the barrel temperature, the more pronounced the tendency for such ejection to occur. Because small inlets require high-speed and high-pressure injection, they are also an important factor that causes flow defects.
9. Shrinkage can be improved through more efficient pressure transmission and smaller pressure drops. The low mold temperature and the slow screw advance speed greatly shorten the flow length and must be compensated by the high rate of fire. High-speed flow will reduce heat loss, and frictional heat due to high shear heat will cause the melt temperature to rise, slowing down the thickness of the outer layer of the part. The cavity crossing must be thick enough to avoid too much pressure drop, otherwise shrinkage will occur.

In short, most injection defects can be solved by adjusting the injection moulding speed, so the technique for adjusting the injection process is to reasonably set the injection speed and its segmentation.