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7 Key Factors That Kill Your Mold for Injection Molding Machine
Created on 12.09
The mold for injection molding machine is a critical, high-value asset in plastic manufacturing. How long they last depends on how you use them and how well you take care of them. If a mold breaks down too early, you'll have surprise shutdowns and spend more money.
This document identifies seven principal technical and mechanical factors that cause significant degradation of the injection molding machine mold.
1. Corrosion Due to Condensation and Chemical Reaction
Corrosion is a primary cause of surface damage and failure in molds.
Condensation (Sweating)
If a mold's temperature goes below the surrounding air's dew point, water will form on the metal. If you turn off the mold without drying it right away to protect it, that water can cause rust to form quickly. Rust formation compromises highly polished surfaces, leading to reduced part quality and potential sticking.
Resin Decomposition
Some resins release corrosive gases when hot. PVC and flame-retardant materials are examples. They release gases like hydrogen chloride, which creates acids when mixed with moisture. These acids chemically attack standard mold steel.
Mitigation:
- Go for mold steels that don't rust easily, like 420SS or H-13. (Here’s Main Stainless Steel Grades and Grades Chart)
- Put on coatings that protect the surface, such as hard chrome, nickel plating, or PVD/CVD.
- Right after taking the mold out of the machine, clean it, dry it, and spray it with something to prevent rust.
- Keep molds in a place where the temperature and humidity are controlled.
2. Mechanical Misalignment and Uneven Clamping Stress
Molds need uniform and correct force. This prevents wear and breakage.
Insufficient or Uneven Clamping Force
The injection molding machine must apply enough force. If the force is too low, the mold opens slightly. This is called flashing. The hot plastic then erodes the parting line. Platen misalignment causes uneven force that concentrates stress. It speeds up wear on guide pins and causes the mold structure to tire.
Ejector System Failures
Mis-timed or misaligned ejector systems cause localized damage. Ejector pins that extend too early or too forcefully can bend, break, and subsequently damage the cavity steel if a broken pin is crushed between the mold halves during closing. Worn guidance in the ejector plate assembly leads to imprecise pin movement, increasing friction and wear.
Mitigation:
- Perform regular calibration of the injection molding machine clamp unit and platen parallelism.
- Ensure ejector pin stroke and timing are optimized for part release.
- Verify that mold plates are dimensioned adequately to resist deflection under maximum injection pressure.
3. Thermal Fatigue and Cooling System Obstruction
Thermal management is critical, and cooling system efficiency directly impacts mold life.
Thermal Shock and Cycling
The repeated expansion and contraction of the mold steel during operational temperature cycles causes internal stress. Rapid or extreme temperature changes (thermal shock) accelerate the propagation of surface cracks, known as heat checking, especially in mold features created by Electrical Discharge Machining (EDM).
Cooling Channel Fouling
Cooling lines can get clogged up with gunk like scale, rust, and mineral deposits, especially if you're using water that hasn't been treated or has a lot of minerals in it. This buildup makes it harder to transfer heat and slows down the flow. If the cooling isn't working well, you'll get hot spots, longer cycle times, and uneven stress on the mold.
Mitigation:
- Maintain consistent mold temperature control, avoiding rapid shifts.
- Use filtered and chemically treated water in the cooling system.
- Implement a schedule for regularly flushing and chemically cleaning cooling channels to remove accumulated scale and deposits.
4. Poor Material Selection and Heat Treatment
The foundational quality of the mold steel dictates its resistance to operational stresses.
Inadequate steel grade selection for the intended volume or material type results in premature abrasive wear. If the steel isn't hardened correctly, it can end up too soft, which means it wears away faster. Or it could be too brittle, making it more likely to chip or break. So, it's important to check the steel's certification and make sure the heat-treating steps are done right.
5. Abrasion from Glass and Mineral Fillers
Resins containing abrasive reinforcing agents, such as glass fibers, carbon fibers, or mineral fillers, cause severe wear. The flow of the molten, filler-laden plastic erodes the steel surfaces, particularly at high-velocity points like gates and narrow runners. This abrasive action is termed washout.
Mitigation:
- Use tough tool steels that can really take a beating, like D2 or special powder metal steels.
- Coat important surfaces with a super-strong stuff, such as Titanium Nitride (TiN).
- When you plan the mold, try to keep the melted stuff from shooting too fast against sharp edges or small parts.
6. Suboptimal Gate and Runner Design
The geometry and location of the gate system influence localized wear and internal steel stress.
An improperly positioned gate can direct high-velocity plastic flow directly onto a small core pin or thin steel wall, causing rapid erosion. Overly restrictive gates require high injection pressures, inducing excessive shear heat in the plastic and increasing localized thermal and mechanical stress on the gate steel. Optimized gate and runner systems ensure balanced melt distribution and minimize turbulence.
7. Operational and Maintenance Errors
Mold damage often happens because of things people could have prevented.
- Forcing Parts Out: Instead of figuring out why a part is stuck (like not enough cooling or a vacuum issue), people sometimes just force it out. This can damage the mold.
- Putting Things Together Wrong: When putting the mold back together after fixing it, mistakes happen. Core pins might not be seated correctly, or cooling lines might get hooked up wrong. This can cause the mold to fail right away.
- Pushing Too Hard: Running the machine faster or with more pressure than the mold was designed for wears it out quicker.
- No Good Records: If you don't keep track of how many parts the mold has made, what repairs have been done, or what settings have been changed, it's hard to keep up with maintenance and prevent problems.
Conclusion
To get the most life out of your injection molding machine mold, stick to the best ways of doing things when it comes to picking materials, lining things up, managing heat, and keeping good maintenance records. Taking care of these key things helps stop breakdowns and keeps your production quality steady.
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