General flow pattern-fountain-like laminar flow
Features of fountain flow:
1. The fastest flowing area on the section is located at the center of section.
2. Rubber entering from gate first forms a solidified layer on the surface.
3. Later entering material flows in the center of section.
4. Flow mode directly affects orientation of glass fiber and molecular chain.

 

Schematic diagram of fountain flow
Flow pattern & molecular orientation on cross section
1. Molecular orientation is mainly affected by shear flow and surface tension
2. Area with the largest shear appears near solidified layer, resulting in high orientation

 

Influence of molecular or glass fiber orientation on bending strength
1. Orientation of molecules or glass fibers will affect bending strength of product
2. Orientation of molecules or glass fibers will increase strength of material in orientation direction, but load-bearing capacity in vertical orientation direction will decrease.
3. Choice of filling style must consider load loading method under working conditions.

 

Molecular chain or fiber orientation and bending strength

During entire injection process, melt reaches every corner of cavity at a fixed wavefront velocity (MFV) at the same time. However, in reality, it is often difficult to fill with a uniform wavefront velocity.

 

Reasons for change of melt wavefront velocity
1. Difference in wavefront velocity of melt causes plastic molecules (indicated by dots) to stretch in different ways, leading to difference in orientation of molecules and the fibers, which results in the difference in shrinkage or warpage.
2. At the same time, difference of melt wavefront velocity will lead to difference of shear rate, which will lead to temperature difference and shear stress difference of shear heat.
3. Excessive shearing heat will cause material to heat up too fast and cause degradation or brittleness; insufficient shearing heat will cause material to cool too fast, causing cold material or short shots.

 

Changes in wavefront velocity caused by changes in wavefront area
Volumetric flow rate = Melt wavefront velocity (MFV) * Melt wavefront area (MFA); (Melt flow rate is constant)

Filling balance is basic principle of filling. Balanced filling can obtain uniform pressure distribution, shrinkage distribution, temperature change and density distribution. Relatively small warpage (out-of-plane deformation).

 

Typical unbalanced filling
Basic specifications for filling and balancing mold design:
1. One-way stable filling style
2. Balanced filling style
3. Constant pressure gradient
4. Minimum shear stress
5. Avoid the hysteresis effect
6. Avoid undercurrent
7. Stable shear heat

Multi-cavity, multi-gate mold:
Filling balance under multiple cavities or multiple gates must be considered. Balanced filling can obtain:
1. Uniform shrinkage and uniform or uniform deformation;
2. The smallest injection pressure and clamping force;
3. The smallest residual stress of product;

 

Gates are not the best choice for melt flow controllers:
1. Hysteresis is prone to occur at gate;
2. Pressure drop of melt flow at gate is greatly affected by shear heat, and balance is relatively unstable;
3. Processing error and friction and wear of gate have a great influence on pressure drop of melt flowing through gate;
4. Molding window is very small, small fluctuations in process parameters will have a significant impact on balance between mold cavities;

 

Unbalanced gate

Flow balance
Original plan
Product description
Material PA66+30GF;
Mold is 1*2, three-plate mold;
Requirements: Plane runout of bottom surface should be controlled within 0.2mm.
Original design: Based on original design of unbalanced filling, in field test, size of two gates on the left was manually changed from 1.0mm to 1.6mm, and size of three gates on the right was changed from 1.6mm to 2.0mm.

 

Filling analysis
From results, filling imbalance of product has been greatly improved, due to difference in gate size, subsequent imbalance of holding pressure and uneven shrinkage of product will be caused. Generally speaking, filling quality of product is acceptable, but there will be certain problems in follow-up.

 

Injection pressure analysis

 

Injection pressure distribution map
It can be seen from results that pressure loss on gating system is relatively large, close to 43Mpa, which is similar to filling pressure of product. At the same time, due to imbalance of flow, pressure distribution on product is not uniform, indicating that distribution of gate position and gate size is not very reasonable, which will cause uneven distribution of product material density.
Volume shrinkage analysis

 

Product volume shrinkage distribution chart
Volume shrinkage reflects shrinkage of product after holding pressure and cooling. Inconsistent shrinkage will lead to product warpage, size and difference of shrinkage will determine appearance of product, such as shrinkage marks, surface depressions or bumps.
It can be clearly seen from shrinkage cross-section that there is a large difference in shrinkage in wall thickness direction of product, which is also reason for large shrinkage mark in corresponding position of product; at the same time, due to inconsistency of gate size, difference in shrinkage of each area of product, such as difference in shrinkage of two gate areas as shown in small picture. In addition, there is an extremely uneven wall thickness near hole in product, which leads to large shrinkage in this area and possible shrinkage.
Overall deformation trend

 

Deformation trend graph
Warpage analysis allows engineers to understand deformation trend and approximate deformation range of product before mold is manufactured, find main cause of deformation, and take effective measures to improve it.
Result of small image shows deformation trend of product after deformation effect is magnified by 10 times.
As can be seen from small picture, original plan. Center hole has detailed eccentricity, which will affect flatness of bottom surface and dynamic balance of product, and eccentricity after optimization of flow balance has been greatly improved.

 

Changes in flatness
1. Moldflow can directly obtain deformation fluctuation of product surface or edge, which is convenient for us to investigate flatness or straightness of product.
2. It can be seen from analysis result of above figure that flatness of top surface is about 0.27mm.
3. Due to difference of material parameters, value has a certain deviation.

 

Optimization and improvement
Improvement plan
According to simulation results of original scheme, dimensions of sub-runners, runners, and gates were designed to be balanced, as shown in following figure:
Filling analysis

 

A good design scheme can ensure that product obtains a uniform filling speed, entire product is filled smoothly and balanced.
From results, filling of product is relatively balanced, which will facilitate subsequent pressure retention and reduce unevenness of product shrinkage.
Injection pressure analysis

 

After optimization: injection pressure distribution map
It can be seen from results that pressure loss on gating system is large, nearly 70Mpa, and pressure in product cavity is close to original, but pressure distribution on product of new solution is even, which will improve uneven distribution of product material density .
Size of flow channel can be further adjusted to reduce injection pressure.
Volume shrinkage analysis

 

It can be clearly seen from contracted cross-sectional view that contraction is more uniform in the area except for position of mesopore.
Overall deformation trend

 

Deformation trend graph
Warpage analysis allows engineers to understand deformation trend and approximate deformation range of product before mold is manufactured, find main cause of deformation, and take effective measures to improve it.
Result on the left shows deformation trend of product after deformation effect is magnified by 5 times.
Note that it can be seen from small picture that center hole is eccentric, which will affect flatness of bottom surface and dynamic balance of product.

 

Flatness changes
It can be seen from analysis results after flow optimization that flatness of top surface is about 0.12mm

Difference in flow resistance caused by wall thickness or structure in flow direction will affect flow pattern.
1. Features such as horn mesh, screw post, etc.;
2. Ribs in the vertical flow direction;
3. Local mutation of wall thickness;
4. Opening and closing of the needle valve gate;
5. Screw column, etc.;
When structure cannot be adjusted, try to keep gate away from area with larger flow resistance, so that it can be filled at last!
Due to problem of cold material, hysteresis area will be brittle after molding, so we should try to avoid welding lines

 

Case Studies
Filling stagnant flow
product description
Product name: Terminal cover for vehicle
Material used: Super impact-resistant PA66
Melting material temperature: 290℃
Mold temperature: 70℃
Problem: No matter how process is adjusted, product is prone to breakage when twisting (red circle area in figure below)

 

Original plan analysis result

 

Occurrence of hysteresis can be clearly seen from filling mode. And weld line appears in the position with the most severe hysteresis, and strength is weak!

 

Peak temperature of material at hinged part drops below material solidification temperature (240℃), and melt stops flowing!!
Optimization

 

Instructions for adjusting gate position:
1. One side or a single flow direction
2. Adjust flow balance so that twisted part is filled only from one side
Comparison of filling modes after improvement:

 

Improve front filling animation

 

Filling animation after improvement
Problem of lag in twist is avoided!!
Improve position comparison of front and rear welding lines

 

There is no weld line at twist!!
Material peak temperature comparison

 

Avoid brittleness of material caused by cold material!!