Glass fiber reinforced plastics are widely used in the injection molding industry due to high strength, good rigidity and stable dimensional performance. Nevertheless, glass fibers feature high hardness and strong abrasiveness. During injection molding, glass fibers continuously scour and rub mold cavities, runners, ejector pins and other components. Long-term production will easily cause surface scratches, dimensional deviation, surface fogging and increased flash on molds. These problems not only shorten mold service life, but also impair product quality and production efficiency. Combining practical production experience, this paper sorts out comprehensive mold wear prevention solutions from the aspects of mold steel selection, structural design, surface treatment, process parameters and daily management.
Selection of Mold Steel to Improve Wear ResistanceThe hardness and wear resistance of mold steel act as the first line of defense against wear caused by glass fibers. Ordinary carbon steel and pre-treated steel are not allowed for molds used in mass production of glass fiber reinforced plastics. High-hardness and wear-resistant mold steel such as SKD11, DC53 and Cr12MoV are preferred for core components including cavities, cores and runners. After quenching and tempering, the hardness of these steel grades reaches HRC58~62, with excellent anti-scour and anti-friction performance, which effectively slow down mold wear caused by glass fibers.
For small-batch production and simple-structured molds, pre-hardened steel such as P20 and 718H can be adopted after reinforcement treatment. Moving accessories including guide pins, guide bushes, ejector pins and inserts must be made of bearing steel and high-speed steel with high hardness, to prevent rapid wear and failure of moving parts rubbed by glass fiber particles. Different materials are selected for different areas: forming surfaces prioritize wear resistance, while structural components balance strength and toughness, so as to control costs and ensure overall service life.
Optimization of Mold Structural Design to Reduce Direct Scour of Glass FibersUnreasonable runner and gate structures accelerate high-speed material scour on molds, which is the main cause of local rapid wear. Optimize gate types and positions in the design phase. Fan gates, film gates and ring gates are preferred with enlarged cross-sectional areas to reduce melt flow velocity. Pin-point gates and small side gates are avoided, as they make glass fibers shoot directly at cavity walls at high speed. Material flow is prohibited from impacting cavity side walls and core surfaces directly. Buffering areas and diversion slopes can be added to change flow directions and disperse scouring force.
Runners are designed with smooth arc transitions without sharp corners and dead angles. This not only reduces material retention and decomposition, but also lowers friction between glass fiber particles and runner walls. The depth and width of mold vent grooves shall be set properly. Poor ventilation leads to local heat accumulation and turbulent material flow, which aggravates wear and corrosion. Smooth ventilation ensures stable melt filling. Adopt large-area ejector pins, ejector sleeves or ejector plates for ejection systems, and minimize the use of tiny ejector pins that are easily worn thin and broken by glass fibers. The fitting clearance of ejector pins is precisely controlled to prevent glass fibers from entering gaps and causing abrasion.
Mold Surface Strengthening Treatment to Improve Wear and Scratch ResistanceEven if high-hardness steel is used, mold surfaces will still wear after long-term contact with glass fiber melt. Surface strengthening treatment is a key method to extend mold life. Various coatings and platings are the mainstream treatment methods. Nitriding treatment including gas nitriding and ion nitriding is widely applied. A high-hardness nitride layer forms on mold surfaces after treatment, featuring low friction coefficient, excellent anti-friction, anti-seizure and anti-corrosion performance. It is suitable for cavities, runners, ejector pins and other full-area treatment.
For severely worn areas such as gates and flow scouring zones, hard chrome plating, TD coating and PVD coating can be adopted. These coatings have much higher hardness than base steel and ultra-smooth surfaces, which greatly reduce adhesion and friction loss of glass fibers. After surface treatment, forming surfaces and runners are finely polished to mirror finish, as smooth surfaces decrease friction between melt and molds and mitigate scratch caused by glass fibers. Hard collision and grinding damage to coatings shall be avoided after treatment, for damaged areas will become wear sources and expand rapidly.
Adjustment of Injection Molding Process Parameters to Reduce Wear LoadProcess parameters directly affect melt state and filling velocity, and reasonable parameter adjustment effectively relieves mold wear. The melt fluidity of glass fiber reinforced plastics is poor. Some operators blindly increase injection pressure and speed. Under high pressure and high velocity, glass fiber particles act like abrasive sand and polish mold surfaces continuously, accelerating wear. On the premise of ensuring product forming, properly reduce injection speed and pressure in production. Apply multi-stage injection: low speed for flow guidance in the initial stage, constant speed for filling in the middle stage, and low speed for pressure holding in the final stage, to slow down impact and friction of material flow on molds.
Set barrel, nozzle and mold temperature in accordance with material standards. Excessively low temperature increases melt viscosity and friction resistance between glass fibers and molds. Excessively high temperature causes resin decomposition and carbonization. Mixture of carbonized materials and glass fibers forms composite abrasives and further aggravates wear. Strictly control the proportion of recycled materials. Broken glass fibers in recycled scraps have sharp edges and enhance abrasiveness. The proportion of recycled materials is suggested to be less than 20%, and uniform mixing shall be guaranteed. In addition, regularly clean carbon deposits in barrels and runners to prevent hard impurities from forming wear points on mold surfaces.
Standardized Daily Use and Maintenance to Eliminate Hidden Dangers in TimeSound daily management delays mold aging and prevents minor wear from developing into large-area damage. Maintain cooling systems during continuous production to ensure unobstructed water circuits and stable temperature control. Frequent temperature alternation causes steel fatigue and surface peeling. Clean residual materials and carbon deposits on mold parting lines and cavity edges regularly during production. Use special tools such as copper scrapers and soft brushes for cleaning. Hard iron tools and steel wire brushes are forbidden to scratch forming surfaces and wear-resistant coatings. Increase cleaning frequency for molds processing glass fiber and modified plastics. Monitor the operation of ejection systems all the time, and stop production for troubleshooting once stalling and abnormal noise occur.
ConclusionMold wear caused by glass fiber reinforced plastics is affected by material characteristics, mold design, surface treatment, production technology and daily management. Single measure cannot solve the problem fundamentally, and comprehensive control is required. Select wear-resistant mold steel at the source, optimize structural design to reduce material scour, improve surface performance via strengthening treatment, lower friction load with reasonable injection parameters, and implement standardized daily inspection and maintenance. The integrated protection system effectively slows down mold wear, extends mold service cycle, reduces mold repair frequency, and guarantees long-term stable operation of injection molding production for glass fiber reinforced plastics.
