Short shot, Air entrapment, Brittleness, Burning, Flash, Delamination, Flow marks, Silver streaks, Dent marks, Weld marks, Molding cycle, Warping

Short shot refers to a phenomenon in which mold cavity cannot be completely filled. Causes of short shot:
1. Mold temperature, material temperature or injection pressure and speed are too low
2. Uneven plasticization of raw materials
3. Poor exhaust
4. Insufficient fluidity of raw materials
5. Part is too thin or gate size is too small
6. Polymer melt solidifies prematurely due to unreasonable structural design

 

Short shot-solution

 

Fill time result can accurately predict filling pattern of plastic melt and area of short shot.

Air entrapment refers to air being trapped in cavity, causing bubbles in part.
It is caused by inability of gas to be discharged from parting surface, ejector pin or exhaust hole when two melt fronts meet. Gas trapped in cavity cannot be discharged in time, which can easily lead to surface blistering, air inclusion inside part, and unsatisfactory injection.
Air entrapment-solution

 

Moldflow can accurately analyze distribution of trapped air. We can discharge gas in cavity in time by optimizing wall thickness of product or setting ejector pins or exhaust ports on mold.

Brittleness of parts means that parts are prone to cracking or breaking in certain parts. Analysis of brittleness causes:
1. Drying conditions are not suitable; too much recycled material is used
2. Incorrect injection temperature setting
3. Improper gate and runner system settings
4. Low strength of melt mark

 

Brittleness-Solution

 

Through Moldflow analysis, cause of front end cracking of product can be accurately determined: obvious undercurrent appears in weld line area, and residual stress is large, which makes it easy to crack.
Based on simulation analysis results, customer reduces stress value by guiding R angle inside product and appropriately increases wall thickness of front end of the product, so that weld line appears in larger cross-section, eliminates undercurrent effect, and solves problem of cracking at the front end of product.

Scorch marks refer to fact that gas in cavity cannot be discharged in time, resulting in phenomenon of burning black at the end of flow. Causes of burn marks:
1. Air in cavity cannot be discharged in time
2. Material degradation: too high melt temperature; too fast screw speed; improper flow system design

 

Scorch-Solution

 

Temperature at flow front indicates flow front temperature. Generally, flow front temperature should be within ±20℃ of material temperature. If filling speed is too fast, shear heat may cause flow front temperature to reach material degradation temperature, and product will burn.

Flash refers to excess plastic on mold parting surface or ejector pin.

 

Causes of flash:

1. Insufficient clamping force

2. Defective mold
3. Unreasonable molding conditions
4. Improper exhaust system design
Flash-Solution

 

When pressure in cavity is greater than 80MPa, or volume shrinkage is negative, product is more likely to have flash.

Delamination and peeling means that surface of product can be peeled off layer by layer. Analysis of causes of delamination and peeling:
1. Mixing incompatible other polymers
2. Using too much release agent during molding
3. Inconsistent resin temperature
4. Excessive moisture
5. Sharp angles in gates and runners

 

Delamination and peeling-solution

Flow marks refer to wavy molding defects on the surface of product.
Analysis of causes of flow marks:
1. Mold temperature and material temperature are too low
2. Injection speed and pressure are too low
3. Runner and gate size are too small
4. Due to structure of product, acceleration is too large during filling flow

 

Flow marks-solutions

 

Temperature of material flow front in local area is too low, and stagnation occurs during filling, which easily forms flow marks on the surface of product.

 

Silver streaks refer to radiating distribution of water, air or carbonized materials on the surface of workpiece along flow direction. Causes of silver streaks:
1. Moisture content in raw material is too high
2. Air is trapped in raw material
3. Polymer degradation: material is contaminated; barrel temperature is too high; injection volume is insufficient

 

Silver streaks-solution

Dents refer to phenomenon that surface of part is concave at wall thickness.
Analysis of causes of dents:
1. Injection pressure or holding pressure is too low
2. Holding time or cooling time is too short
3. Melt temperature or mold temperature is too high
4. Improper part structure design

 

Dents-Solution

 

Dents are more likely to appear in red area due to its thicker wall and larger volume shrinkage.
Generally, when volume shrinkage value is >5% during demoulding, and it is very different from volume shrinkage in adjacent area, dents are more likely to appear on product surface. Volume shrinkage can be reduced by optimizing product wall thickness, placing gate in wall thickness area, and increasing pressure holding.

Welding line refers to surface defect caused by welding of two streams of material.

 

Analysis of cause of welding line: If there are holes, inserts or multi-gate injection molding mode in part or wall thickness of part is uneven, welding line may be generated.
Welding line-solution

When butt angle of weld line is greater than 75℃, temperature of material flow front is high, and there is no trapped air in weld line area, weld line is less obvious.

 

Using Moldflow software, you can accurately predict filling time, holding time, cooling time, time to open and close mold, which is molding cycle. You can also shorten molding cycle and improve productivity by optimizing product wall thickness or mold structure.

 

 

The most common and difficult problem to solve in design and production of plastic parts is warpage.
Main reasons for product warpage include:
1. Mold structure: pouring system, cooling system and ejection system, etc.
2. Product structure: changes in wall thickness of plastic parts, curved or asymmetric geometric shapes, unreasonable design of ribs and BOSS columns, etc.
3. Production process: plastic parts are ejected before they are completely cooled, injection and holding pressure curves are unreasonable.
4. Plastic material: difference between plastic material with and without added fillers, size of shrinkage, etc.

Moldflow summarizes product warpage into four main factors:
1. Uneven cooling: Unreasonable design of cooling water channel makes product unable to obtain uniform cooling.
Solution: Optimize cooling water channel
2. Uneven shrinkage: Shrinkage of product is inconsistent.
Solution: Change material, product structure, number and position of gates, and pressure holding curve fiber
3. Uneven orientation: When fiber orientation is uneven, it causes large warpage of product.
Solution: Number and position of gates, product structure
4. Corner effect: Heat is concentrated at the corners of deep box-shaped products, and shrinkage is large, which brings bending deformation.
Solution: Strengthen cooling at corners

 

Main reason for product deformation in Z direction is large shrinkage of material. After changing material, deformation amount is reduced from 2mm to almost nothing, which is basically natural shrinkage.

 

Main reason for product deformation in Z direction is that ribs are too thick. After thinning ribs, deformation amount: 1.68mm is reduced to 0.56mm.

 

Main reason for product deformation in Y direction is uneven shrinkage. Change three inner submerged gates to four outer submerged gates, and maximum deformation is reduced from 0.44mm to 0.09mm

 

Main reason for product deformation in Y direction is uneven cooling. After optimizing cooling water channel, deformation amount: 0.98mm reduced to 0.47mm

 

Main reason for product deformation in Z direction is over-pressure holding. After reducing pressure holding, maximum deformation: 0.49mm is reduced to 0.15mm.