Application of Moldflow in AUX--Process Introduction

 

Product Introduction
1. Product Overview: Product is a part of wall mounted air conditioner. Part is called air guide door. It is located on air outlet and plays a role of swinging air. When air conditioner is turned off, it is in a closed state and is a first-level appearance part;
Product size: 780x73x24;
Main wall thickness: 3.5 MM in the center, 2.5MM on the edge;

Product requirements:
First-level appearance parts, glossiness requirement is above 100;
Product cannot have shrinkage marks and S-shaped deformation;
Assembly clearance is required to be within 0.8MM;
Production cycle per mold is 55S;

 

1. Glossiness of product surface is less than 97 (required to be above 100);
2. Low production efficiency, cycle 60 seconds
3. After increasing mold temperature, glossiness is met, product is deformed and has large gaps, warping 8MM, gap 2MM (required deformation ±1.5, assembly gap ≤0.8 MM)

 

1. Current situation: front mold hot water 45℃, rear mold and slider connected to normal temperature water 25℃ production
2. Purpose: Reduce temperature difference between front and rear molds to prevent product deformation.
3. Defects: Product surface gloss is not enough; Low production efficiency;

 

1. Cause analysis: Front mold hot water temperature is 45℃, which is too low, resulting in insufficient gloss on product appearance
2. Improvement plan: Increase front mold water temperature
Concept: Water temperature of mold temperature controller is adjusted upward from 45℃, 5℃ each time;
On-site verification: After each adjustment, continuous production is carried out for 30 minutes (to allow mold to rise to a relatively stable temperature), and glossiness is measured until 60℃. It is found that glossiness of product reaches 103, which meets requirements (required to be above 100);
Further testing: Raise mold temperature to 65℃ again. After testing, glossiness has not increased significantly, but product has exceeded standard deformation after cooling.
3. Comparison between theoretical analysis and actual results:
Measured by infrared thermal imager, mold surface temperature is very close to theoretical analysis.

 

1. Factors affecting glossiness: Glossiness depends on mirror reflection ability of object surface to light.
2. Glossiness of product surface comes from replication ability of mold surface
1. The higher mold surface temperature, the higher glossiness (provided that product does not undergo ejection deformation);
2. Denser (fuller) product surface, the higher glossiness;
3. The better material fluidity and the more high-gloss material, the higher glossiness.
3. Experience Sharing: For mirror-polished molds, outer surface reaches 75℃, and glossiness of products produced by high-gloss ABS can reach 100.

When we raised front mold temperature from 45℃ to 60℃, we solved product gloss problem, but production efficiency was questioned! Production team leader proposed: In order to ensure deformation of product, cooling time was extended by 5 seconds after front mold temperature was increased.
At that time, everyone thought it made sense and it was safer, so we started production in this way. I was skeptical!

 

1. Theoretical analysis:
I immediately returned to office and ran Moldflow. Through comparison, I found that time to reach ejection temperature was not extended, but shortened by 9 seconds
2. On-site practice: Cooling time was directly reduced by 9 seconds, and production cycle was directly reduced from 60 seconds to 51 seconds, improving efficiency by 15%

 

Solve problem and summarize it
1. Find Moldflow at critical moment: Without in-depth analysis, it is really unreliable to make decisions based on your own ideas!
2. It is important to be rigorous in everything: In fact, for this product, cooling of rear mold and slider accounts for 70% of product surface area. Although front mold increases by 15℃, 70% area decreases by 10℃, and heat of product is taken away faster.

 

1. Kill two birds with one stone: When we raised temperature of front mold from 45℃ to 60℃, and lowered temperature of rear mold and slider from 25℃ to 15℃, we solved gloss problem of the and increased production efficiency by 15%
2. Lost one thing: Due to change in mold temperature, temperature difference between front and rear molds reached 45℃ (60-15), which is 2.25 times previous temperature difference of 20℃ (45-25), and product showed a large warping deformation.

 

 

Find cause: Run Moldflow: Input three main molding parameters on site into MF for analysis.
Analysis results show that:
Z-direction warping deformation is 8.7MM, and main causes of warping deformation are:
1. Uneven shrinkage accounts for 86%;
2. Uneven cooling accounts for 23%

 

Step 1: Determine solution
Uneven shrinkage (accounting for 86%)
Analysis of reasons: Main reason is large difference in product material thickness. Product is a two-layer structure, with upper layer material thickness of 1.9MM and lower layer material thickness of 3MM on average.
Difference in material thickness causes a 3% difference in volume shrinkage
Solution: Let upper layer shrink more.

 

Step 2: Solve main contradiction
Problem: Uneven shrinkage (accounting for 86%)
1. Improvement measures: Optimize holding time
Confirm that first holding time ends at 6.5 seconds
Confirm that second holding time ends at 14.5 seconds

 

2. Improvement measures: Optimize holding pressure
①. Determine first stage holding pressure to be 15MPa to prevent gate backflow;
②. Determine second stage holding pressure to be 80MPa;

 

Step 3: Conclusion of theoretical analysis
Effects of two pressure holding modes
1. High front and low back, product tends to shrink and deform toward side with higher temperature;
2. Low front and high back, product tends to shrink and deform toward side with lower temperature.

 

Step 4: Optimize parameters
Determine holding mode
1. From previous analysis results, front is low and back is high, and warpage deformation has reversed direction, proving that direction is correct!
2. Adjust second stage holding pressure to further improve deformation.

 

 

Step 5: On-site verification (determine general direction)
Before improvement
Pressure holding curve: high in front and low in back
Deformation: warping about 8MM
After improvement
Pressure holding curve: low in front and high in back
Deformation: warping about 0.5MM

 

Step 5: On-site verification (determine the best solution)

 

 

 

Product Introduction
1. Product Overview: Product is front panel of a mobile air conditioner, which is a first-class appearance part;
Product size: 730x500x120;
Main wall thickness: 2.8MM;

2. Product requirements:
First-class appearance part, glossiness requirement is above 100;
Appearance cannot have shrinkage marks;
Product width deformation requirement is within 2 MM;
Assembly clearance requirement is within 0.7 MM;

 

Product has large arc deformation, difficult assembly, and large assembly gap. Outer extension is 8 MM, deformation is required to be 475±2, and assembly gap is ≤0.7 MM)

 

 

Promotion point:
Using different temperature differences between front and rear molds of product, freezing layer factors on the front and rear molds of product are different, front low and rear high holding pressure ideas are adopted to solve deformation problem

 

Analysis of long-term application benefits of Moldflow in enterprises
Previous similar products
Production efficiency: cycle 60 seconds (long)
Gloss: 96 (unqualified)
After using Moldflow
Production efficiency: cycle 51 seconds (shortened by 15%)
Gloss: 103 (qualified)
Calculated based on power of 800-ton injection molding machine 70 kWh, electricity cost savings = 10 million molds/60 molds per hour * 70 kWh * 1.2 yuan per kWh * 15% = 2.1 million RMB
Quality (gloss) improved, winning customer praise
Enterprise knowledge base construction: Construction of air guide door wall thickness standard

Share: In the early stage of new product mold opening, wall thickness is designed according to third wall thickness to ensure a wide range of deformation adjustment.

 

1. Background description
Uniform cooling is key factor to prevent product warping and deformation. In injection molding industry, temperature difference between front and rear molds is often controlled at 20~30℃. Usually as follows:
Due to appearance requirements of product, front mold core is usually connected to hot water at about 50℃;
Due to relatively complex structure of rear mold side, rear mold core is usually connected to warm water at about 20~30℃ (temperature is too high, which is not conducive to improving production efficiency);
2. Cause analysis:
Reducing temperature difference can effectively prevent insertion interference and burns;
Reducing temperature difference can reduce shrinkage difference of product, reduce warping and deformation.
3. Solutions
In design stage, calculate thermal expansion length of steel and avoid air at insertion position;
Based on production status, in mold matching stage: front mold is connected to hot water at 75~80℃ (considering that molten plastic transfers heat to mold), and rear mold core is connected to normal temperature water at 25~30℃
Protection measures: In relatively weak insertion position, it is designed in the form of an insert to prevent fatigue fracture in long-term production, and it is also convenient for subsequent production maintenance.
Practical results: Over past two years, more than 100 sets of molds have been counted, actual production is 60~65℃ for front mold and 15~18℃ for rear mold; mold temperature difference is 50℃, and there is no problem with mold bite.

 

Theoretical calculation method for thermal expansion of plastic molds
Take NAK80 steel as an example, thermal expansion coefficient is a=12.5*10-6
Front mold is 65℃, rear mold is 15℃, temperature difference △T =65-15=50
Take center of mold as 0 reference, distance in any direction =△L
Calculation formula is: thermal expansion length △d=△T *△L*a;
For example, thermal expansion distance of rubbing position 400 mm away from center position: △d=50*400*12.5 X10-6= 0.25 mm

 

Thermal expansion length △d=50*400*12.5X10-6=0.25 mm; that is, design size of mold at position A = 400-0.25=399.75mm; after mold is heated, expansion size just fits tightly with rear mold

 

Current mold is in a cooling state, and insertion position here is an interference fit, so copper or soft steel is used as an insert, screws are locked on the surface to facilitate disassembly and maintenance.

Experience:
In daily work, most of product warping and deformation we encounter are caused by uneven shrinkage.
1. Fully communicate wall thickness problem during new product development stage, and prevention is priority;
2. It is very necessary to restore site and use MF analysis. Only by thoroughly analyzing principle can we find correct way to solve problem.

 

1. Without deliberately adjusting holding pressure parameters, deformation trend of product is usually "temperature trend - the higher temperature is, the more likely product will shrink (opposite is true for glass fiber materials).
2. To reverse "temperature trend" deformation of product, holding pressure mode of low pressure at the front and high pressure at the back is often used. Specific time points and pressure sizes of segments need to be accurately analyzed by MF to reduce waste of debugging.
1. High pressure at the front and low pressure at the back, product is likely to shrink and deform to side with high temperature;
2. Low pressure at the front and high pressure at the back, product is likely to shrink and deform to side with low temperature.

Prediction of development of injection molding industry
With people's pursuit of quality of life and rapid development of Industry 4.0, more and more mold companies will develop in direction of high efficiency, high precision and high automation, which is inseparable from more and more realistic feasibility simulation analysis, especially strict requirements of warpage deformation and production cycle.
Extension of project topics
Mold design and Moldflow engineers need more interaction and pressure. Each cooling water channel requires precise analysis and rigorous consideration. Get rid of bad habit of "first ejector and then water channel" and elevate cooling layout to a strategic level!
Future direction of Moldflow application or practice in enterprises
I hope that Moldflow can automatically recommend pressure holding parameters; construct conformal water channels to liberate the brains and hands of MF people