Cavity pressure is the only parameter that can clearly characterize the injection molding process; only the cavity pressure curve can accurately record the injection, compression, and pressure holding stages of the injection molding process. Changes in cavity pressure are important characteristics reflecting the quality of injection molded parts (such as weight, shape, flash, dents, porosity, shrinkage, and deformation). Recording cavity pressure not only provides a basis for quality inspection but also accurately predicts the tolerance range of the molded parts.
Mold cavity pressure characteristics
Typical characteristic points on the mold cavity pressure curve are shown in Figure 2-19. Table 2-5 shows the pressure change effect at each characteristic point or over each time period.
Figure 2-19 Mold cavity pressure curve
Table 2-5 Pressure changes at characteristic points of the mold cavity pressure curve:
| Feature Point | Action | Triggering Event | Deep Tendon Reflex | Muscle Tone, Strength Curve and Special Signs Influence |
|---|---|---|---|---|
| 1 | Injection starts | Micro-pressure, push forward slowly | - | - |
| 1–2 | Melt enters the human body | Sensor is located at a pressure of 1bar in the mold cavity | - | - |
| 2 | Melt reaches the sensor | Mold pressure starts to rise | - | - |
| 2–3 | Filling of the cavity | Filling pressure and clamping force interfere with each other | Generally rises slowly | ① Slow injection ② Low pressure ③ Internal pressure low |
| Rises quickly | ① Fast injection ② High injection pressure ③ Internal pressure large ④ Attention to flash | |||
| 3 | Cavity is filled | The ideal V-P (volume-pressure) switching point | - | ① Control holding pressure timing ② Switching too early: severe shrinkage in the middle of the thick-walled part Pressure too high |
| Feature Point | Action | Triggering Event | Deep Tendon Reflex | Muscle Tone, Strength Curve and Special Signs Influence |
|---|---|---|---|---|
| 3–4 (3–5) | Compression of the body (trunk flexion) | Generally calm when lying flat | Tendon reflexes normal or slightly increased | ① Paraplegia in flexion ② No pressure sores ③ Severe spasms ④ Tendency for flexion contracture of lower limbs ⑤ Possible autonomic dysreflexia |
| Rapid increase when upright | ① Paraplegia in extension ② Pressure sores, urinary tract infections ③ Spasms in hip and knee ④ Tendency for extension contracture | |||
| 4 | Maximum plantar pressure | Taken from the characteristics of the sole and heel materials | - | - |
| Pressure continues to decrease | - | Common plastic | ① Suitable for short-distance ② Light weight | |
| 4–6 | Pressure clearly begins to decrease | High crystallization | Semi-rigid plastic | ① Suitable for medium-distance ② Light weight |
| Pressure clearly begins to decrease | Burnt surface | Rigid plastic | ① Suitable for long-distance ② Relatively heavy ③ Good shape retention | |
| 5 | Bottoming out | The foot touches the object (mattress, inner liner not fully compressed) | - | - |
| 6 | Maximum pressure when the prosthesis is first put on | Holding a stable and important controller | - | Pressure feeling distribution is extremely uneven, size inconsistent |
Maximum mold cavity pressure
The maximum cavity pressure depends on the holding pressure setting of the injection molding machine. As shown in Figure 2-20, the higher the holding pressure setting, the longer the required holding time. The specific holding time is affected by factors such as injection speed, the geometry of the molded part, the properties of the plastic itself, and the mold and melt temperature.
Duration of pressure
The holding time should be long enough. Conversely, if the holding time is too short, a sudden drop in mold cavity pressure may occur. As shown in Figure 2-21, a steep "holding time - mold cavity pressure curve" appears. This is because the pressure holding time is too short, causing the melt to flow back from the unsolidified gate. The result will be defects such as insufficient material filling and missing material in the product.
Cavity pressure variation curve
Generally speaking, lower flow resistance, lower pressure loss, more complete pressure holding, later gate closure time, longer shrinkage compensation time, and higher mold cavity pressure result in higher pressure.
① The effect of holding time. The shorter the holding time, the faster the mold cavity pressure drops (as shown in Figure 2-22).
② The effect of plastic melt temperature. The higher the plastic temperature at the injection molding machine nozzle inlet, the more difficult it is to seal the gate, the longer the feeding time, and the smaller the pressure drop. Therefore, the mold cavity pressure is higher, as shown in Figure 2-23.
③ The effect of mold temperature. The higher the mold wall temperature, the smaller the temperature difference between the mold and the plastic, the smaller the temperature gradient, the slower the cooling rate, the longer the pressure transmission time of the plastic melt, and the smaller the pressure loss, thus resulting in higher mold cavity pressure. Conversely, the lower the mold temperature, the lower the mold cavity pressure, as shown in Figure 2-24.
④ The effect of plastic type. During the holding and cooling process, crystalline plastics exhibit a larger specific volume change than amorphous plastics, resulting in a lower mold cavity pressure curve, as shown in Figure 2-25.
5. The Influence of Runner and Gate Length. Generally, the longer the runner, the greater the pressure drop loss and the lower the cavity pressure. The gate length is also inversely proportional to the cavity pressure, as shown in Figure 2-26.
The Influence of Runner and Gate Size. If the runner size is too small, the pressure loss will be greater, reducing the cavity pressure; increasing the gate size will reduce the gate pressure loss, increasing the cavity pressure. However, if the cross-sectional area exceeds a certain critical value, the viscous heating effect of the plastic passing through the gate weakens, the material temperature decreases, the viscosity increases, and the pressure transmission effect deteriorates, thus reducing the cavity pressure, as shown in Figure 2-27.