LNP Engineered Composites Injection Molding Troubleshooting Guide

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.

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

To get a well-formed part, part geometry is key. Factors to consider include:

It is important to ensure that wall thickness is uniform throughout part and to pay attention to nominal thickness. (See diagram below)

 

 

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.

 

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.

 

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 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.

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.