In injection molding production, product quality assurance is a systematic project, which requires not only guarantee of equipment accuracy and operation stability, but also support of injection molding process. At the same time, condition of mold and rationality of structure also play a vital role. Now, with gradual improvement of quality requirements for injection molding products, in addition to ensuring injection molding process, we also turn our attention to previous process of injection molding, that is, design and manufacture of mold.
Due to factors such as unreasonable mold structure and poor use, it is difficult to ensure product quality, defects such as weld marks (water marks) and shrinkage are prominent, and production efficiency is also greatly affected.

Among many defects of injection molded shells, defects that have a greater impact on quality of spraying processing and are more numerous are mainly poor weld marks (water marks).
Severe water marks will cause spraying to be unable to cover, and surface of shell will give people a cracked feeling. After repeated repairs and spraying, problems such as rough, astringent, and dirty surfaces will occur, which greatly increases costs and labor intensity, causing great waste.

When molten plastic meets inserts, holes (including grids), areas with discontinuous flow velocity, and areas where filling material flow is interrupted in cavity, it cannot be completely fused, resulting in linear or hemp-shaped weld marks. In addition, due to differences in structure and material flow balance, poor exhaust will also produce more serious water inclusions.

Water marks on the surface of shell are caused by different structural differences in different parts of mold, so causes and shapes of marks are also different.

For example: 25A11 shell, 2165 (I) shell

 

Cause:
Due to obstruction of mesh pins on the front, flow rate of molten material is lower than that on the side; and gaps between mesh pins form several flow channels. (As shown in figure)
In addition, due to influence of ribs and flow channels (air channels) at the corner frame on material flow, part of gas in mold cavity will be wrapped at the end of mesh, eventually forming a hemp-shaped water pattern.

 

Improvement measures
Weld existing airway (flow channel) and block it. Reopen airway (flow channel) from four edges/corners, thereby completely changing flow direction of molten material and increasing flow speed of molten material at edges.

 

For example: 25189/29189/25289 shells and other 189/289 series

 

Cause:
Shell of this series of products has a gate near middle of button hole. When molten material flows through button hole molding column, it is blocked by each button hole molding column, forming two streams of molten material joining in material flow direction. In addition, due to small wall thickness (1.2m) at arc surface of button hole, molten material flows slowly due to factors such as temperature, thus forming a deeper curved water pattern.
Improvement measures:
Increase wall thickness of button hole forming part of movable mold by 50um. This will increase flow rate of molten material and slow down cooling rate of front ends of two molten materials, thereby reducing depth and width of water lines.

 

For example, 128/228 series with two process grooves on the front of 29128 shell and 21228 shell

 

Causes:
There are two process grooves (groove depth 1.2mm-1.3mm) on the front of product, and two forming edges on mold (fixed mold). When molten material flows, forming edges become two dams that block flow, slowing down material flow rate;
In addition, wall thickness at process groove is reduced (about 1.5m), and flow rate of molten material at this location is further reduced;
Low edge has a larger wall thickness (corner thickness is up to 5.6mm), which plays almost same role as flow channel, and material flow rate is relatively fast. Three flow paths on both sides of front and bottom edges do not disappear at the edge of mold cavity parting surface, but meet below process groove. Gas is wrapped below process groove, and finally forms a hemp-shaped water pattern. If a large amount of gas is trapped, weld mark will be thicker and deeper.
Improvement measures:
1. Increase wall thickness between two lower gates (generally about 30~50pm), and add a clearance groove at process groove on mold core to speed up flow rate of front molten material.
2. Weld bottom edge to reduce glue and reduce wall thickness to slow down flow rate of molten material in this area.

 

Such as 29211, 25211, 25289 and most other shell products

 

Causes:
1. Poor gas discharge.
2. Mold temperature is relatively low, front material temperature of two melts is reduced, and fusion effect is poor.
Improvement measures: Add exhaust grooves on the surface of bladder, and open exhaust grooves at corresponding positions of water lines.

 

Such as: 29211, 2165, 21228, 25228, 25C41, 29C41 shells and other products

 

Cause:
Molten material on the front side flows slower than side due to relatively thin wall thickness or obstruction, part of gas in mold cavity is trapped and formed.
Improvement measures:
1. Thicken wall thickness of speaker mesh on both sides (generally about 30~50 μm);
2. Grind and thicken parts corresponding to water pattern on the side of movable mold and surrounding area;
3. Grind off raised edges that hinder material flow on the front side to speed up flow rate of molten material on the front side.
Side water marks 2
Such as: 25289 shell, 2165 shell and most other shell products

 

Cause: It should be a normal weld mark between two gates, but due to poor venting, it causes thicker and deeper water marks, and even hemp-shaped water marks as shown in picture.
Improvement measures:
Open and deepen exhaust groove at corresponding position of ejector plate.

 

Such as: 25A11 shell, 21A11 shell and other A11 series products

 

Cause:
When molten material flows through power button hole molding column, it is blocked and molten material is divided into two streams. After flowing to lower edge, it first fuses and wraps air. Molten material flows further and melts. Air is compressed, affecting fusion of molten material, thus forming a deeper water mark (even a hemp ball-shaped water mark).

 

Improvement measures:
Grind and thicken area around moving mold button forming hole (about 1mm). This will increase flow rate of molten material in this area and ensure that there is no air wrapped in this area, thereby improving fusion effect.

 

Improved mold and product quality
According to incomplete statistics, 17 sets of molds have been rectified so far. Serious condition on each shell reflects that prominent water marks have been basically eliminated, achieving goal of no or less sandpaper polishing of injection molded semi-finished products. Quality of injection molded shells has been greatly improved. According to feedback from injection molding, proportion of water marks has dropped from 3.1% in May 2005 to 0.9% in October 2005. It provides high-quality incoming materials for processing workshop and lays a solid foundation for "improving first-pass rate of injection molding."

It can be seen that rationality of mold design and precision of manufacturing play a vital role in ensuring quality of injection molded products. A good mold should have a large adjustment range for injection molding process during production, so that production can be carried out smoothly and continuously.
When designing molds now, software simulation is required to determine structures such as number and position of gates, runners, and airways. However, in actual production, due to influence of many factors such as temperature, process, and equipment, simulation cannot be completely consistent with actual situation. Therefore, there are still some problems in design and manufacture of molds, which still need to be further adjusted and modified in subsequent injection molding production.