Author: Shenye Plastic TechnologyDate：2021-10-12Pageviews：345
In the manufacturing of "drawn parts"—those with deep cavities, long vertical walls, or high draw ratios—cracking is a persistent headache. Unlike flat components, drawn parts undergo immense physical stretching and thermal stress during the molding cycle. If the balance of pressure, temperature, and geometry is off, the result is a brittle part that fails before it even leaves the factory floor.
Here is a technical deep dive into solving the cracking problem for drawn injection components.
One of the most overlooked causes of cracking in deep drawn parts is the vacuum created between the part and the core during ejection.
The Culprit: As the ejector pins push the part, a vacuum forms, literally trying to suck the part back onto the mold. This tension causes the side walls to crack or "stress whiten."
** The Solution:** Install Air Poppet Valves in the core. These valves allow compressed air to enter the cavity at the moment of ejection, breaking the vacuum and allowing the part to slide off effortlessly.
Drawn parts are particularly sensitive to friction.
The Problem: A draft angle that is too shallow (less than 1 degree) for a deep part creates massive drag. If the core surface is slightly rough, it "grabs" the plastic, causing micro-fractures.
The Solution: Aim for a minimum of 1.5 to 2 degrees of draft per side for deep draws. Additionally, ensure the core is polished in the direction of draw (vertical polishing) rather than circular polishing, which reduces friction significantly.
Cracking is often the "explosive" release of internal energy trapped during cooling.
High-Speed Injection Stress: Forcing plastic into a deep cavity at extreme speeds creates highly oriented molecular chains that are "stretched" to their limit.
The Solution:
Increase Mold Temperature: This allows the molecules to relax into a more stable state, reducing "frozen-in" stress.
Slow Down the Final Fill: Use a profile where the injection speed drops as the cavity is 95% full. This prevents "over-packing" near the gate, a common site for radial cracks.
You cannot build a strong drawn part with weak material.
Hydrolysis Warning: For engineering plastics like PC or ABS, even 0.02% moisture can cause hydrolysis. This chemically breaks the polymer chains, making the part so brittle that it cracks under the slightest ejection pressure.
The Solution: Always verify your desiccant dryer's dew point (it should be -40°C or lower). For high-draw parts, avoid using regrind material, as the repeated heat cycles degrade the polymer’s impact resistance.
At Sunye Plastic, we believe that the best way to solve a cracking problem is to prevent it during the DFM (Design for Manufacturing) stage. We utilize advanced FEA (Finite Element Analysis) to identify high-stress zones in deep drawn parts before we ever cut steel. By adjusting wall thickness transitions and optimizing cooling line placement, we ensure that the part shrinks uniformly onto the core, minimizing the force required for ejection.
Solving cracking in drawn parts is a game of millimeters and degrees. By addressing the vacuum effect, optimizing draft angles, and ensuring material purity, manufacturers can dramatically reduce scrap rates and produce parts that stand the test of time.
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