Injection Molding Processing Guidelines | Part Design
Time:2024-07-19 08:28:59 / Popularity: / Source:
Injection Molding Troubleshooting Guide
LNP Engineered Composites Injection Molding Troubleshooting Guide
Question | Reason | Solution |
Fragile | Material gets damp | Check drying procedures |
Overheat | Reduce barrel/nozzle temperature | |
In-mold stress | Increase barrel/nozzle temperature | |
Improper component design | Remove sharp edges | |
Weld line | Increase injection pressure Increase solution temperature |
|
Part warping | Part temperature difference | Check mold cooling system* |
Excessive contraction | Increase amount of part filler | |
Material orientation | Change gate location | |
Improper component design | Add stiffeners or increase part thickness to increase stiffness Check whether wall thickness is uniform |
|
Demoulding problem | Increase cooldown time Reduce mold temperature Increase thimble area |
|
Spill | Insufficient mold clamping force | Use larger machines |
Injection pressure is too high | Reduce injection pressure | |
Mold plate is not aligned | Align platen | |
Exhaust groove too deep | Check mold exhaust | |
Scorched mark | Air trapped | Improve mold exhaust situation |
Barrel or nozzle overheated | Check heater controls | |
Shear heat generation | Reduce injection speed | |
Pollute | Clean barrel Clean hopper dryer |
|
Molding machine clogged | Remove screw and clean it | |
Weld line strength is low | Insufficient exhaust | Improve mold cavity exhaust |
Injection speed or mold temperature is too low | Increase injection speed and mold temperature | |
Gate location is incorrect | Adjust gate position or add overflow piece | |
Mold stuck | Too much filler | Reduce injection pressure Reduce injection speed |
Mold design issues | Check whether there is chamfering Check ejection system Increase draft angle in mold |
|
Surface defects (boxing or white spots on the surface) | Injection speed too low | Increase injection speed |
Melt supercooling/mold supercooling | Increase barrel temperature/mold temperature | |
Material gets damp | Check drying procedures | |
Pit or blowhole | Holding pressure is too low or time is too short | Increase holding pressure or extend time |
Insufficient feed | Increase injection volume | |
Gate is frozen or improperly positioned | Check gate size and location | |
Gate discoloration | Material is too cold | Add cold slug well to runner Increase melt temperature |
Melting | Reduce injection speed Increase gate Increase gate radius |
|
Dimensional instability | Uneven injection volume | Maintain appropriate buffering Check whether stop ring is worn |
Melt temperature difference | Check heating belt/controller | |
Insufficient amount of filler | Extend holding time Enlarge orifice to prevent premature solidification |
Part design
Performance of parts made from LNP composites depends on properties of composite, part design and molding process. Excellent part design is critical to meeting structural requirements and molding productivity of your application. Assembly requirements also need to be considered during design phase.
Designing process
Design process can be simplified into three stages involving material design and processing. GE Plastics engineers should be consulted early in design process. Additional information on each of these stages can be found in LNP Specialty Composites Design Guide. Following guidelines are intended to remind readers of good design practices to produce high-quality injection molded parts.
Preparatory stage
• Clarify needs
• Determine the approximate geometry
• Choose materials
• Select processing method
• Conduct feasibility analysis
• Decision before next stage
Engineering stage
• Complete detailed component design
• Processing decisions
• Material decisions
• Sample testing
• Evaluate and redesign
Manufacturing stage
• Mold design, production and evaluation
• Cavity filling analysis
• Select manufacturing equipment
• Component testing
• Customer assessment
Preparatory stage
• Clarify needs
• Determine the approximate geometry
• Choose materials
• Select processing method
• Conduct feasibility analysis
• Decision before next stage
Engineering stage
• Complete detailed component design
• Processing decisions
• Material decisions
• Sample testing
• Evaluate and redesign
Manufacturing stage
• Mold design, production and evaluation
• Cavity filling analysis
• Select manufacturing equipment
• Component testing
• Customer assessment
Injection molding design
To get a well-formed part, part geometry is key. Factors to consider include:
Wall thickness
It is important to ensure that wall thickness is uniform throughout part and to pay attention to nominal thickness. (See diagram below)
Core digging
Make thicker parts of part hollow to maintain uniform wall thickness and cooling.
Uniformity
• Residual stress (warpage, pitting, chemical resistance)
• Mechanical properties (strength, impact resistance)
Nominal thickness
• Agency approval (flammability)
• Processing performance (flow, length, production cycle)
• Maximum thickness based on polymer systems
Target
Uniform wall thickness maintains uniform pressure distribution and cooling
Radius
Sharp corners can cause stress concentrations and should be avoided. See diagram below to determine appropriate corner radius.
Uniformity
• Residual stress (warpage, pitting, chemical resistance)
• Mechanical properties (strength, impact resistance)
Nominal thickness
• Agency approval (flammability)
• Processing performance (flow, length, production cycle)
• Maximum thickness based on polymer systems
Target
Uniform wall thickness maintains uniform pressure distribution and cooling
Radius
Sharp corners can cause stress concentrations and should be avoided. See diagram below to determine appropriate corner radius.
Reinforcing ribs/convex sleeves/gussets
Improperly designed ribs, bosses or gussets can cause demolding problems or excessively thick sections (pits). Long mandrels can cause overheating or deflection if not designed properly. Length-to-diameter ratio of unsupported mandrels should be less than 5/1. Copper alloys can be used to improve cooling of long mandrels.
Draft angle
Since reinforced composites shrink less than pure resin, all part sidewalls must maintain a draft angle of 2°–3° where possible, but no less than 1°. Unfilled composites shall have a draft angle of not less than 1/2° on each side. For textured surfaces, increase draft angle by 1° on each side for every 0.001" increase in texture depth.
Shrinkage and tolerances
Shrinkage of fiberglass-reinforced composites is typically one-third to one-half that of unreinforced resins. LNP recommends first determining exact shrinkage rate through a prototype mold, especially for parts with complex shapes or large wall thickness fluctuations. Parts molded from composites with anisotropic shrinkage characteristics (reinforced, crystalline resins) should also be molded first using a prototype mold or using a "surrogate" mold to infer significant shrinkage results.
Typically, reinforced composites can be molded to more precise dimensions than unfilled materials. Maintaining tight tolerances can significantly increase cost of molded parts because designing to tighter tolerances would add steps to manufacturing process or incur higher tooling costs than rougher tolerances.
Typically, reinforced composites can be molded to more precise dimensions than unfilled materials. Maintaining tight tolerances can significantly increase cost of molded parts because designing to tighter tolerances would add steps to manufacturing process or incur higher tooling costs than rougher tolerances.
Weld line
Proper mold design (gate location, etc.) can help inhibit formation of weld lines. If weld lines cannot be avoided, they should be located on part where the least stress is expected.
Experience with LNP composites and weld lines shows
• Tensile strength of a filled or reinforced composite weld line depends in part on weld line integrity of matrix resin itself.
• If fiber reinforcement is kept parallel to weld line, strength of weld line area will be greatly reduced. Degree of reduction is directly related to amount of reinforcement used. Granular fillers do not have such a large impact on strength of weld line.
• Although increased wall thickness generally reduces stress, part thickness has little effect on tensile strength of weld lines.
• Molding variables do not have much impact on weld line strength except dwell time.
• Exhaust conditions at weld line should be optimized to maximize strength of weld line.
• Relying on an overflow piece to improve strength of weld line is not worth gain in terms of cost.
Experience with LNP composites and weld lines shows
• Tensile strength of a filled or reinforced composite weld line depends in part on weld line integrity of matrix resin itself.
• If fiber reinforcement is kept parallel to weld line, strength of weld line area will be greatly reduced. Degree of reduction is directly related to amount of reinforcement used. Granular fillers do not have such a large impact on strength of weld line.
• Although increased wall thickness generally reduces stress, part thickness has little effect on tensile strength of weld lines.
• Molding variables do not have much impact on weld line strength except dwell time.
• Exhaust conditions at weld line should be optimized to maximize strength of weld line.
• Relying on an overflow piece to improve strength of weld line is not worth gain in terms of cost.
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