Analysis of four reasons for insufficient filling of injection products

Time：2021-08-30 11:18:07 / Popularity： / Source：

1. Improper feed adjustment, lack of material or excessive material

Inaccurate feeding measurement or abnormal feeding control system operation, abnormal injection cycle caused by injection molding machine or mold or operating conditions, low pre-plastic back pressure, or low particle density in barrel may cause material shortage. For large particles, large voids, plastics with large changes in specific volume of crystallinity, such as polyethylene, polypropylene, nylon, etc., and high viscosity plastics such as ABS, amount of material should be adjusted higher, amount of material should be adjusted when material temperature is high.
When there is too much material at the end of barrel, screw will consume extra injection pressure to compress and push excess stock in barrel during injection, which greatly reduces effective injection pressure of plastic entering mold cavity and makes it difficult to fill product.

2. Injection pressure is too low, injection time is short, plunger or screw returns too early

Molten plastic has higher viscosity and poor fluidity at low working temperature, so it should be injected at higher pressure and speed. For example, when making ABS color parts, high temperature resistance of colorant limits heating temperature of barrel, which must be compensated by higher injection pressure and longer injection time than usual.

3. Slow injection speed

Injection speed is of great significance to some products with complex shapes, large thickness changes, and long processes, as well as plastics with high viscosity such as toughened ABS. When product cannot be filled with high pressure, high-speed injection should be considered to overcome problem of underfilling.

4. Material temperature is too low

Temperature of front end of barrel is low, and viscosity of molten material entering cavity rises too early to point where it is difficult to flow due to cooling effect of mold, which hinders filling of far end; Temperature in back section of barrel is low, and plastic with high viscosity is difficult to flow, which hinders forward movement of screw. As a result, it seems that pressure displayed by pressure gauge is sufficient, but melt actually enters cavity at low pressure and low speed;
If nozzle temperature is low, it may be that nozzle is in contact with cold mold for a long time and loses heat during fixed feeding, or nozzle heating ring is insufficiently heated or has poor contact, resulting in low material temperature, which may block mold’s feeding channel; if mold does not have cold slug wells, with self-locking nozzles and post-feeding procedures, nozzles can maintain necessary temperature; nozzle is too cold when it is just turned on, and sometimes a flame gun can be used for external heating to accelerate temperature of nozzle.
Entire injection process is divided into three stages, namely filling-feeding-holding pressure. Control of these three stages is not only closely related to product quality but also to whether process is stable, so thoroughly understanding know-how inside is a compulsory course for injection molding technicians. Let’s start with filling today!

1. Definition of filling:

Filling refers to process in which melt is injected to 90-95% of cavity from beginning, which is close to fullness, which is called filling stage. Also called the first stage.

2. Source of filling speed and pressure:

Many people don't think that filling speed is just a rough idea, you can set it as much as you want! But whether it is actually correct, as shown in effective viscosity curve in following table: When filling speed is gradually increased from 10% to 99%, it is found that “the higher injection speed, the higher shear rate generated, and the lower relative viscosity”, what problem does this indicate?
1. During high-speed filling, shear rate between two liquid levels is high, and viscosity of plastic decreases due to effect of shear thinning, which reduces overall flow resistance. Local viscous heating effect will also make thickness of solidified layer thinner. Therefore, in flow control stage, filling behavior often depends on the volume to be filled. That is, in flow control stage, due to high-speed filling, shear thinning effect of melt is often great, and cooling effect of thin wall is not obvious, so effect of speed prevails.
2. Low speed filling. When heat conduction controls low-speed filling, shear rate is lower, local viscosity is higher, and flow resistance is higher. Due to slower replenishment rate of hot plastic and slower flow, heat conduction effect is more obvious, heat is quickly taken away by cold mold wall. Coupled with a smaller amount of viscous heating, thickness of cured layer is thicker, and flow resistance at thinner wall is further increased.

3. Filling speed and pressure setting:

According to effective viscosity test result, in order to ensure that filling speed is less affected by viscosity, it should be selected in lower viscosity area, such as speed of 50%, corresponding pressure should not be less than 75Bar.

Gud Mould Industry Co., Ltd