In injection molding plus process, it may be due to poor processing of raw materials, unreasonable design of plastic parts or molds, operators not mastering suitable process operating conditions, or due to mechanical reasons, often short shots and dents in plastic products. , flash, trapped air, cracking, warping and other forming defects.

 

This article aims at various injection molding defects in molding process of plastic parts, mainly: short shot, trapped air, brittleness, scorching, flash, layered peeling, spray marks, flow marks, fog spots (gate Halo), silver streak (water pattern), dents, weld lines, too long molding cycle, warping deformation, analyze possible causes of problem, and propose solutions from raw materials, plastic parts or mold design, molding process and other aspects program.

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 liquidity of raw materials.
5. Part is too thin or the gate size is too small.
6. Premature solidification of polymer melt due to unreasonable structural design.
Short Shot Solutions:
Material: Choose a material with better fluidity.
Mould Design:
1. Fill thick walls before filling thin walls to avoid retention.
2. Increase number of gates and size of runner, reduce flow ratio and flow resistance.
3. Position and size of exhaust port are set properly to avoid phenomenon of poor exhaust.
Injection molding machine:
1. Check whether check valve and inner wall of barrel are seriously worn.
2. Check whether feeding port has material or whether it is bridged.
Process conditions:
1. Increase injection pressure and injection speed, and enhance shear heat.
2. Increase injection volume.
3. Increase barrel temperature and mold temperature.

Trapped air refers to fact that air is trapped in cavity and causes air bubbles in part.
Causes of suffocation:
It is caused by the inability of gas to escape from parting surface, ejector or vent when the two melt fronts meet. Gas trapped in cavity cannot be discharged in time, which can easily lead to surface blistering, air inclusion in part, and injection molding dissatisfaction.
Breathless Solutions:
Structural design: reduce inconsistency of thickness and try to ensure uniform wall thickness.
Mould Design:
1. Add an exhaust port at the last filling place.
2. Redesign gate and runner system.
Process conditions:
1. Reduce injection speed of last stage.
2. Increase mold temperature.

 

Brittleness of plastic parts means that parts are easily cracked or broken in some parts.
Brittle Reasons:
1. Unsuitable drying conditions; use too much recycled material.
2. Injection temperature is not set correctly.
3. Inappropriate settings of gate and runner system.
4. Strength of melting marks is not high.
Crispy Solutions:
Material:
1. Set proper drying conditions before injection molding.
2. Reduce use of recycled materials and increase proportion of raw materials.
3. Use high-strength plastic.
Mold design: increase size of main runner, runner and gate
Injection molding machine: choose a well-designed screw to make the temperature distribution more uniform during plasticization.
Process conditions:
1. Reduce temperature of barrel and nozzle.
2. Reduce back pressure, screw speed and injection speed.
3. By increasing material temperature and increasing injection pressure, strength of melting mark is improved.

Scorch marks refer to fact that gas in cavity cannot be discharged in time, resulting in black burning at the end of flow.
Burning Reasons:
1. Cavity air cannot be discharged in time.
2. Material degradation: too high melt temperature; too fast screw speed; improper design of runner system.
Scorched solution:
Mould Design:
1. Add an exhaust system in places that are prone to poor exhaust.
2. Increase size of runner system.
Process conditions:
1. Reduce injection pressure and speed
2. Reduce barrel temperature
3. Check whether heater and thermocouple are working properly

Flash refers to appearance of excess plastic on parting surface of mold or ejector pin.
Causes of flashing:
1. Insufficient clamping force.
2. Mold is defective.
3. Unreasonable molding conditions.
4. Improper design of exhaust system.
Flashing Solutions
Mould Design:
1. Reasonably design mold to ensure that mold can be tightly closed when mold is closed.
2. Check size of exhaust port.
3. Clean mold surface.
Injection molding machine: set appropriate size tonnage injection molding machine
Molding:
1. Increase injection time and reduce injection speed.
2. Reduce barrel temperature and nozzle temperature.
3. Reduce injection pressure and holding pressure.

Delaminated peeling means that surface of workpiece can be peeled off layer by layer.
Analysis of reasons for peeling peeling:
1. Mixed with other incompatible polymers.
2. Excessive release agent is used during molding.
3. Resin temperature is inconsistent.
4. Too much moisture.
5. There are sharp corners in gates and runners
Layered peeling solution
Materials: Avoid mixing of incompatible impurities or contaminated recyclate into raw materials.
Mold Design: Chamfer all runners or gates with sharp angles.
Process conditions:
1. Increase barrel and mold temperature.
2. Properly dry material before molding.
3. Avoid using too much release agent.

Jetting: A jetting mark caused by melt flowing too fast, generally in the shape of a serpentine.
Cause analysis of jet marks:
1. Gate size is too small, and it is facing the product surface with a large cross-sectional area;
2. Filling speed is too fast
Jet Trace Solutions
Mould Design:
1. Increase gate size.
2. Change side gate to lap gate.
3. Add a stopper pin directly in front of gate.
Process conditions: reduce filling speed just past gate

Flow marks refer to wavy forming defects on the surface of product, which are a kind of leaping marks caused by slow flow of melt.
Cause analysis of flow marks:
1. Mold temperature and material temperature are too low.
2. Injection speed and pressure are too low.
3. Size of runner and gate is too small.
4. Due to product structure, when filling flow is caused, acceleration is too large.
Flow Mark Solutions
Mould Design:
1. Increase size of cold material well in runner.
2. Increase size of runner and gate.
3. Shorten size of main runner or switch to a hot runner.
Process conditions:
1. Increase injection speed.
2. Increase injection pressure and holding pressure.
3. Extend holding time.
4. Increase mold temperature and material temperature.

Fog: Cloudy discoloration near gate. Reason is melt fracture.
Analysis of causes of fog spots:
Gate is too small or cavity at glue inlet is too thin, melt flow rate is large, cross-sectional area is small, shear rate is large, and shear stress often increases, so that melt glue breaks and fog shifts occur.
Fog spot solution
Moldflow simulations can predict temperature, shear rate, and shear stress of melt as it passes through narrow zone described above. Moldflow generally provides upper limits for various plastic material temperatures, shear rates and shear stresses. Moldflow engineers can make corresponding adjustments based on analysis results to find appropriate gate size and cavity wall thickness at injection site to eliminate fog spots.

Silver streaks refer to radiant distribution of moisture, air or carbon on the surface of workpiece along flow direction.
Causes of silver streaks:
1. Moisture content in raw material is too high.
2. There is air in raw material.
3. Polymer degradation: material is contaminated; barrel temperature is too high; injection volume is insufficient.
Silver Streak Solutions
Material: Dry raw material according to data provided by raw material supplier before injection molding
Die Design: Check for adequate vent position
Molding:
1. Select appropriate injection molding machine and mold.
2. When switching materials, completely clean the old materials from barrel.
3. Improve exhaust system.
4. Reduce melt temperature, injection pressure or injection speed.

 

Dent marks refer to phenomenon that surface of workpiece is concave at wall thickness.
Analysis of reasons for formation 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 structural design of parts
Dent Solutions
Structural Design:
1. Corrugated treatment on the surface that is prone to dents.
2. Reduce size of thick wall of workpiece, try to reduce ratio of thickness to diameter, ratio of adjacent wall thickness should be controlled at 1.5~2, and transition should be as smooth as possible.
3. Redesign thickness of reinforcing ribs, countersunk holes and corner ribs, and their thickness is generally recommended to be 40-80% of basic wall thickness.
Molding:
1. Increase injection pressure and holding pressure.
2. Increase gate size or change gate location.

Weld line refers to surface defect caused by fusion of two streams.
Analysis of causes of weld lines:
Weld lines may occur if there are holes, inserts, or multi-gate injection patterns in part, or if part wall thickness is uneven.
Weld Line Solutions
Material: Increase fluidity of plastic melt.
Product Design: Adjust product structure and wall thickness distribution
Mould Design:
1. Change position of gate.
2. Add an exhaust slot.
Process conditions:
1. Increase melt temperature.
2. Reduce amount of release agent used

Using Moldflow software, you can accurately predict filling time, pressure holding time, cooling time, plus time of opening and closing mold, which is molding cycle. It can shorten molding cycle and improve productivity by optimizing product wall thickness, mold structure and process conditions.

The most difficult and difficult problem to solve in design and production of plastic parts is warpage.
Main reasons for product warpage:
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 stiffeners and BOSS columns, etc.
3. Production process: plastic parts are ejected before they are completely cooled, injection and pressure holding curves are unreasonable and other factors.
4. Plastic material: difference between plastic material with and without addition of fillers, size of shrinkage rate, etc.
Moldflow categorizes product warpage into four main factors:
1. Uneven cooling: Cooling water circuit design is unreasonable, so that product cannot be cooled uniformly.
Solution: Optimizing cooling water circuit.
2. Uneven shrinkage: Product shrinks inconsistently everywhere.
Solution: Change material, product structure, number and location of gates, packing curve
3. Uneven fiber orientation: When fiber orientation is uneven, product will cause large warpage deformation.
Solutions: Number and location of gates, product structure
4. Corner effect: Heat is concentrated at corners of deep box-shaped product, and shrinkage is large, resulting in bending deformation.
Solution: Strengthen cooling at corners, reduce wall thickness at corners.