Flash / excess glue:

Criteria for judging flash
1. Pressure in cavity is greater than 80MPa, and material with good fluidity is about 40Mpa
2. Filling volume shrinkage rate is negative (local pressure is too high)

 

Fender - Optimize product design to solve flash problem

 

Printer drawer handle

 

Comparative analysis of molding problems

 

 

Analysis of deformation causes
Main causes of deformation: uneven cooling and corner effect

 

Solution

 

 

Improved effect

 

Comparison of mold test results

 

Customer evaluation: Pass! Fusion lines, shrinkage, deformation, and flash issues have all been improved.
Void or/and Sink Mark Forming

 

Sink Mark or Void
Products
1. Rib is too thick
When rib is thick, area where rib and panel thickness is thicker. Plastic is concentrated here. When cooling, surrounding ribs and panels solidify first. Center of intersection of rib and panel remains liquid. Later solidified plastic shrinks on the earlier solidified plastic, which has a sucking-in effect on surrounding condensation layer. If condensation layer is weak at any point (usually on mold surface opposite to rib), it is possible that a sink mark will be formed there due to collapse. If condensation layer is strong enough, a shrinkage cavity will form in the center of intersection of rib and panel. Thickness of rib should be 50~75% of thickness of panel, or even thinner.
CAE simulation can predict sink index to understand impact of different rib thickness designs on sink marks or shrinkage cavities.
2. Stud (boss) hole is shallow or side wall is too thick
When stud hole is shallow or side wall is too thick, center of stud bottom or intersection of stud side wall and panel is too thick, and plastic is concentrated. When cooling, this place solidifies last, and has a sucking-in effect on plastic that has been pre-cured around it. If any condensation layer is relatively weak (usually on mold surface opposite to stud), it is possible that this place will collapse and form a dimple-shaped or ring-shaped sink mark. If condensation layer is strong enough, a shrinkage cavity will form in the center of above intersection.
CAE simulation can predict sink index to understand impact of different stud bottom and side wall thickness designs on sink marks or shrinkage cavities.
Car door lining (door panel), PP-TF15, panel 2.5t, boss side wall 1.3t, Ф2 bullhorn gate added to 2.5t frame edge, shrinkage mark.

 

3. Too much difference in wall thickness
When wall thickness difference is too large, shrinkage marks and shrinkage holes are easy to occur in thick wall.
Techniques such as coring-out should be used to make wall thickness as uniform as possible.

 

Mold
1. Temperature of mold surface opposite to rib is too high (easy to form sink marks)
If temperature of mold surface opposite to rib is higher than that near it (generally true, because melt is concentrated near it, heat load is large, and mold temperature remains high), condensation layer there is thin and rigidity is insufficient. When melt in the center solidifies, it is possible to pull thinner condensation layer inward to form a sink mark.
Mold surface opposite to rib must be cooled more and mold temperature at this location should be lowered so that condensation layer forms faster. When condensation layer is thicker, rigidity is greater and sink marks are not easy to form.
When setting mold temperature, you can start with recommended value of material manufacturer. Reduction (or increase) of each adjustment can be 6℃. After 10 injections and molding situation is stable, decide whether to make further adjustments based on the results.
CAE can predict sink index to understand effects of different cooling designs and mold temperatures on sink marks.
2. Sprue, runner or/and gate are too small
If sprue, runner or/and gate are too small, flow resistance will increase. If injection pressure is insufficient, cavity cannot be filled, melt density is low, and probability of shrinkage marks or shrinkage holes is high.
It is feasible to use CAE to simulate and analyze filling of different melt delivery systems (including sprues, runners and gates) on computer to find ideal sprue, runner and gate dimensions (including length and cross-sectional dimensions such as diameter, etc.).
Spoon, PS, sink marks

 

 

Scale parts, PA66

 

 

3. Improper gate location
Improper gate location makes flow length too long and flow resistance too large. If injection pressure is insufficient, cavity cannot be filled, melt density is low, and probability of shrinkage marks or shrinkage holes is high.
Gate should be placed at wall thickness as much as possible to ensure sufficient supply of compensation flow and reduce possibility of shrinkage marks or shrinkage holes.
It is a smart approach to use CAE to simulate and analyze different gate designs on computer to find the best number and location of gates.
Void

 

 

Injection molding machine
1. Injection time is too long
CAE simulation can find optimal injection time. With this injection time, molten plastic can be transferred to the end of cavity in the most labor-saving way, and more pressure can be transferred to plastic there, making it more tightly packed and less likely to produce shrinkage marks or shrinkage holes.
2. Insufficient holding pressure
Holding pressure or holding time is not enough. Plastic in the cavity is not filled solidly due to low pressure or insufficient supplementary material. Density is low and probability of sink marks or shrinkage holes is high.
CAE simulation can find ideal value of holding pressure and holding time. It is a smart approach to set it according to this value and then make fine adjustments.
Moldflow solves key indicators for shrinkage marks
1. Volume shrinkage value is 11%, difference in volume shrinkage between adjacent positions exceeds 5%, which is prone to shrinkage marks.
2. Amount of dents is > 0.03mm, and weld line is obvious under natural light conditions.
Judge whether product structure and wall thickness design will cause obvious dents on product surface.

 

Handle of rice cooker lid---Optimize product design to solve shrinkage problem

 

Light guide column---optimize gate size to solve shrinkage problem

 

Threaded products

 

 

Dent Moldflow judgment criteria

 

Dent Moldflow judgment criteria:
1. Difference in adjacent volume shrinkage exceeds 5%
2. Dent amount is > 0.03mm, which is easy to be obvious
Weld Line

 

 

 

Simple product, one gate

 

 

Joining line / weld line:

 

Linear marks formed on the surface of product (mostly at 2 or more gates or places with holes)

Weld Line
Product
1. Front edge meeting angle is too small
When front edge meeting angle is small, molecules on both sides diffuse less and quality is poor. When meeting angle increases, surface traces of weld mark gradually disappear.
Increase in meeting angle can be achieved by adjusting product thickness, improving gate position and number, changing runner position and size. This can be verified with help of CAE.
Meeting Angle between Two Melt Fronts Forming Weld Line

 

Weld line depth measurement and relationship between depth and butt angle

 

Experimental determination of relationship between butt angle and visible weld line

 

As can be seen from above figure, when butt angle is 75 degrees, depth of weld line is exactly 2 microns. 2 microns is boundary between visible and invisible weld lines, so 75 degrees is boundary between visible and invisible weld lines.
Moldflow's weld line results describe butt angle of weld line
This butt angle is used to evaluate whether weld line is visible.
Moldflow analysis of seam line judgment indicators
I. Weld line butt angle is >75 degrees
2. Wavefront temperature is 20 degrees lower than melting temperature
3. There is obvious air entrapment in weld line area

 

Seam line
Seam line - mark produced when two streams of material meet and merge

 

Optimization ideas:

 

It can be seen that flow mode of RR thickened by 0.6mm is the best
Optimize product design to solve weld line problem

 

Refrigerator and fruit cabinet - optimize product design to improve exhaust and seam lines
Original solution
Material: Polystyrol 165H, BASF (PS)

 

Optimization plan

 

Optimize gate position to solve weld line problem
Product: Transparent juice cup body
Material: PC
Problem: Weld line problem
Result: Solve weld line problem

 

Optimize product design to solve weld line problems

 

Actual Results

 

In-depth study
>Resin temperature is too low
When two resin flows meet, fusion occurs. At this time, the lower temperature of the two, the more obvious fusion. Since two resin flows at fusion point do not mix with each other (because they are semi-solidified while moving forward in jet), if temperature is too low, surface layer will become thicker, texture will be obvious, and strength will be reduced. This is because adhesion between the two is weakened. On the contrary, if temperature of two resin flows is high, adhesion will be strengthened and appearance will become less obvious.
>Low pressure
At fusion point, two molten resins are squeezed, and adhesion here depends on pressure applied there. The lower holding pressure, the more obvious fusion and the lower strength. As solidification proceeds, pressure transmission becomes more difficult. In addition, if gate runner size becomes smaller and gate position becomes worse, appearance and strength of fusion will deteriorate.

 

(2-3) Weak exhaust at exhaust port
Fusion is confluence point of resin and may also be end of flow. At this time, if a vent is not set well at this position to exhaust gas, appearance and strength of fusion will deteriorate.

 

Figure 5. Exhaust of fusion part should be sufficient
Note: (1) Quality of fusion line of plastics without reinforcement is significantly higher than that of plastics with reinforcement.
(2) Quality of fusion line area is closely related to type and content of fillers and reinforcements. Additives such as processing aids and flame retardants have an adverse effect on quality of fusion line.
(3) Containing fiber reinforcement, arrangement direction of fibers in fusion line area is perpendicular to flow direction, which will significantly reduce mechanical properties of part at this point.
Eliminate weld lines
Methods to improve weld lines
Offset weld lines by diverting or blocking flow.
Guide weld lines by adding cold wells.
Improve appearance by increasing melt temperature and mold temperature
Improve convergence angle of melt lines by adjusting wall thickness