Solution to poor exhaust in injection molding of thick-walled plastic parts

Time：2024-07-30 08:26:25 / Popularity： / Source：

1 Plastic part structure and mold design

Structure of control handle is shown in Figure 1. Material is PA66, which is a typical thick-walled part with a mutation structure. The thickest wall thickness is 8mm. Control position at the right side of handle with a width of 20mm has a fine linen surface with a matte effect to ensure touch feel. At the same time, this part also belongs to appearance surface. There must be no defects such as whitening, burns, yin and yang patterns. The overall parting surface of plastic part is not allowed to have flash, and step difference is required to be less than 0.1mm.

injection molding of thick-walled plastic parts

Since most areas of plastic part are use surface and appearance surface, there are restrictions on gate position. Gate position can select area and initial design of parting surface as shown in Figure 2. Mold adopts a 1-mold 4-cavity layout, and side gate is fed. Size and shape of side gate can be adjusted within specified area according to trial mold situation, as shown in Figure 3.

2 Analysis of causes and solutions for poor exhaust

2.1 Numerical simulation of poor exhaust area

Through mold flow simulation analysis of MoldFlow software, it was found that although gate size and shape were adjusted many times in specified area, filling state did not change much. Final cavity layout and analysis of causes of poor exhaust are shown in Figure 4, that is, there is always poor exhaust at the right side of handle.

2.2 Vent opening

When using PA66 material for injection molding, melt temperature is 290℃; because appearance of plastic part has a fine hemp surface, good replication accuracy is required during molding, so a higher mold temperature range (70~90℃) is selected for testing. In addition, due to good fluidity of material and small overflow value, mold exhaust groove depth will produce flash when it exceeds 0.015mm, which is set to 0.01mm. Exhaust groove position is shown in Figure 5. After mold test verification, when exhaust groove depth is within allowable range of material overflow value, poor exhaust phenomenon is still serious, and surface of plastic part will be scorched. After slightly deepening exhaust groove, sample is obtained as shown in Figure 6.
As can be seen from Figure 6, even when deepening of exhaust groove leads to appearance of flash, appearance of operating position (i.e., hemp pattern area) at the right side with a width of 20mm is still poor. Affected by high-temperature gas in mold cavity, surface is shiny and hemp pattern is not visible. A careful observation of mold cavity surface shows that hemp pattern surface of cavity surface is attached with a lot of products of material decomposition caused by retained high-temperature gas, as shown in Figure 7.

2.3 Parting surface adjustment

Test results show that only opening exhaust groove cannot solve problem of poor exhaust in molding process of plastic part, because in the area where problem occurs, dynamic and fixed mold cavities occupy different volumes on both sides of original parting surface, and shape trend is also different. Exhaust process is inconsistent, and resistance to gas escape on fixed mold side is large. If parting surface is moved to fixed mold side, it is possible to improve poor exhaust situation. Therefore, position of parting surface is changed, and parting surface is partially moved up, as shown in Figure 8. When position of parting surface is modified, additional insert also increases exhaust channel.

Sample after mold test is shown in Figure 9. There is no burning phenomenon in hemp area, and no flash is generated on parting surface. However, hemp surface area is bright and does not achieve required matte effect. This indicates that there is still a small amount of trapped gas during injection molding, that is, there is still a small amount of high-temperature gas in hemp area that hinders adhesion of polymer melt to mold cavity surface, resulting in failure to completely replicate micro-nano-level hemp characteristics.

It can be seen from above tests that passive exhaust alone cannot completely eliminate various gases in cavity and cannot obtain an ideal matte hemp surface. In the case of passive exhaust, it is necessary to reduce injection speed, injection pressure and holding pressure, shorten holding time to reduce risk of burns caused by failure to quickly and quickly eliminate high-temperature gas due to excessive injection speed and excessive injection pressure, but this also increases risk of material shortage and shrinkage marks in molded plastic parts.

2.4 Vacuum injection technology plan and test verification

Based on above analysis, it is decided to use local vacuum in mold to improve poor exhaust according to actual situation. For comparative analysis, two diagonal cavities were selected for vacuum injection test, as shown in Figure 10. A sealing groove was added to parting surface around cavity for installing sealing strip, and two exhaust holes were added. Edge of parting surface in hemp-grained area was set with an exhaust groove connected to exhaust hole.

In vacuum technology, sealing of vacuum channel is the key to forming a vacuum in mold cavity. Generally, a sealing ring is used for sealing. However, push rod in mold is a moving part, and position of push rod hole is not suitable for sealing with a sealing ring. Scheme adopts sliding sleeve method, which can achieve purpose of sealing without affecting sliding of push rod.
Because sealing ring is set on the outside of four cavities, in fact, all four cavities will be affected by vacuum. Exhaust groove added at corresponding position of test cavity makes vacuuming smoother. Difference in final test results is shown in Figure 11. Plastic part of test cavity has uniform hemp grain and a matte surface, while control cavity still has a shiny area.

Gud Mould Industry Co., Ltd