Today I will share an example of gas-assisted molding of a monitor shell. Through two different sets of solutions, we will explore optimal molding solution and analyze molding problems from data results. This will reduce trial mold cost and molding cycle before opening mold, thereby increasing profits. Mold flow analysis plays a vital role, and everyone can learn from it.

 

Product 3D model
Problem Description
This product is a gas-assisted molding case with dimensions of 450 x 340 x 60.
Analyze and verify gas-assisted molding for this product. Before filling, holding pressure and actually opening mold, you should first understand status of mold cavity.
Requirements: Product appearance must not have obvious welding lines or shrinkage marks.

 

Flow channel system and thickness distribution diagram
Picture shows that blue part represents the thinner area, and red part represents the thicker area. Except for thicker edge, thickness of most areas is 2.5mm, and product thickness is relatively uniform.

 

Air needle position diagram
Preliminary forming condition setting
Filling time: 2.5 sec. Melt temperature: 270 ℃. Mold temperature: 60 ℃
Filling/Gas pressure holding switching point: volume 100%; air intake time: 25 sec;
Mold opening time: 5 sec; Molding cycle time: 57 sec;
Material: Terluran GP-22

 

Flow wavefront short shot diagram
Wavefront flow diagrams are generally used to determine whether filling is balanced. Red represents the last filled location, blue represents the earliest filled area, and gray represents unfilled area.

 

Pressure curve distribution chart

 

Temperature Distribution Map
It can be seen from analysis results that when time is 2.7 seconds, bone position in the middle mesh area and the lower area of product has cooled to 130 ℃ (green area in picture), and average thickness temperature of product is maintained at about 265 ℃.

 

 

Plastic skin thickness ratio distribution chart
Red part represents 100% of thickness of plastic cortex, and thickness cross-section is in a solid state. Blue area represents the higher hollowing rate of plastic, showing a hollow phenomenon. Generally used to judge whether gas volume is appropriate for hollowing out.

 

There is no gas penetration in these areas (solid).

 

Gas Penetration Time Chart

 

Weld line location distribution diagram
This diagram shows position of welding line. You can compare colors to see angle at which wavefronts converge on both sides of welding line.
Through various data, suggestions are given. It is necessary to add gates and bring welding line closer to hole bone area to improve surface quality of product.
This is what we made later to improve gate situation

 

 

Gas nozzle position distribution and gate opening and closing conditions
Reforming condition setting
Filling time: 2.5 sec; Sol temperature: 270 ℃; Mold temperature: 60 ℃;
Filling/Gas pressure holding switching point: volume 100%; air intake time: 25 sec;
Mold opening time: 5 sec; molding cycle time: 57 sec;
Plastic: Terluran GP-22

 

Wavefront flow diagram

 

Pressure graph
Injection pressure is much lower than before.

 

Temperature distribution diagram
It can be seen from analysis results that when time is 2.7 seconds, bone position in the middle mesh area and the lower area of product has cooled to 130℃, and average thickness temperature of product is maintained at around 270℃. This one hasn't changed much.

 

Product volume shrinkage comparison

 

Welding line location diagram
As can be seen from picture above, welding lines on its surface are much lighter and need to be further optimized.
In conclusion:
Generally, closed airway loops should be avoided in gas-assisted designs, because closed loops will inevitably lead to incomplete gas penetration and form solid airway sections, which not only wastes plastic, takes long cooling times, and is prone to dents, but also increases possibility of warping due to uneven shrinkage.
It is recommended to add two latent gates to design, and use hot runner valve gate control to force joint line to the middle area of ribs in hole area.
From Moldflow gas-assisted analysis results, we can first understand whether arrangement of air channels, gas needles and gate positions still has room for optimization. If possible, we can use mold flow analysis software to conduct simulation analysis to understand product before opening mold. Possible problems and solving them based on experience will be able to effectively shorten development cycle and cost.