The most troublesome part of structural design is mold trial molding—after several sleepless nights drawing and repeatedly checking structural details, all sorts of defects pop up during trial molding: insufficient glue, shrinkage, air trapping, and flash. At best, mold needs repair and rework; at worst, it's scrapped. This not only delays project cycle but also leads to complaints from both mold and production teams. In the end, structural engineers have to take blame—it's truly heartbreaking!
Actually, defects appearing during trial molding aren't scary part; scary part is not finding root cause of problem, blindly letting mold factory repair mold, making things worse, and ultimately, we have to take blame. Today, based on our past experience in following up on trial molding, coordinating with mold manufacturers, and rectifying defects, I've compiled eight of the most common trial molding defects for our fellow structural engineers. I've included detailed causes and readily implementable rectification methods (covering both structural optimization and mold adjustment). Even beginners can follow these steps to troubleshoot and coordinate, avoiding unnecessary detours!
Important Reminder: When troubleshooting trial molding defects, first observe defect's morphology on product, then identify corresponding structural design, mold, or process issues. Don't immediately ask mold manufacturer to modify mold, and don't blindly modify structure—it's a complete waste of time. This is a lesson we've learned from countless mistakes!

Defect Symptoms: Product is not fully filled in certain areas; there are gaps at corners, ribs, or even broken glue lines. It's easily visible to naked eye.
Core Causes: Either mold structure design has insufficient runners or gates, hindering plastic flow; or injection pressure and temperature are insufficient, resulting in poor plastic flow; or there are too many dead angles in structure, leading to poor mold venting and trapped air blocking plastic filling.
Corrective Measures: First, optimize structure, appropriately increasing gate and runner dimensions to ensure smooth plastic flow to all corners. Second, coordinate with injection molding plant to increase injection pressure and material temperature, improving plastic fluidity. Finally, instruct mold factory to add venting channels in structural dead corners and areas with insufficient material, controlling depth to 0.01-0.02mm, avoiding excessive depth to prevent flash.

 

Defect Manifestations: Dents and shrinkage marks appear on product's exterior, especially near uneven wall thickness, screw pillars, and ribs, severely affecting aesthetics, leading to direct customer rejection.
Core Causes: Primarily, uneven wall thickness in our structural design, with some areas being too thick, causing inconsistent plastic cooling and shrinkage. Secondly, insufficient holding time and pressure result in inadequate shrinkage compensation. Another possibility is an unreasonable gate placement in mold design, leading to insufficient shrinkage compensation.
Corrective Measures: First, optimize product structure, ensuring uniform wall thickness, with rib thickness controlled at 1/2 to 2/3 of main body wall thickness; then, coordinate with injection molding plant to increase holding pressure and holding time to allow for sufficient shrinkage compensation; if gate location is unreasonable, adjust pre-reserved gate position in structure, moving it to a location with thicker walls to facilitate shrinkage compensation and avoid shrinkage marks.

 

Defect Manifestations: Product surface shows charred blackening, bubbles, and even burn marks, often appearing in dead corners, ribs, and opposite gate. This not only affects appearance but also reduces product's strength.
Core Causes: Structural design did not consider venting, leading to poor mold venting. During plastic filling, air in cavity cannot escape, and compressed air generates high temperatures, causing plastic to burn; it could also be due to excessively high material temperature, too high injection speed, or excessively heated plastic; or too many dead corners in structure, preventing air from escaping.
Corrective Measures: Focus on communicating with mold manufacturer to add venting channels in structural air traps and dead corners, especially around sliders and ejector pins; appropriately instruct injection molding plant to reduce material temperature and injection speed to minimize plastic overheating; simultaneously optimize structure to reduce unnecessary dead corners and facilitate venting.

 

Defect Manifestation: Excess plastic flash appears on parting surface, sliders, and ejector pins, requiring manual trimming, increasing labor costs and easily scratching product's appearance.
Core Causes: Inadequate parting surface design during structural design leads to loose parting surface fit and gaps; or excessive injection pressure and insufficient clamping force cause plastic to overflow from gaps; it could also be due to insufficient mold machining precision, resulting in burrs on parting surface (requires communication with mold manufacturer for rectification).
Rectification Methods: First, optimize parting line design to ensure a tight fit and no unreasonable gaps; second, coordinate with injection molding plant to increase clamping force and reduce injection pressure to prevent plastic overflow; if flash appears at slider or ejector pin, coordinate with mold manufacturer to adjust clearance between slider and ejector pin to 0.01~0.02mm, simultaneously check for interference at corresponding structural positions.

 

Defect Manifestations: Whitening and scratches appear at ejection point of product, and in severe cases, cracking occurs directly. This often occurs at thin-walled areas, sharp corners, and ribs; once these appear, product is essentially scrapped.
Core Causes: Inadequate ejector pin placement during structural design leads to uneven force on ejector pins; excessive ejection speed and force; or insufficient draft angle in structural design results in high demolding resistance. These are points most easily overlooked by structural engineers.
Corrective Measures: Optimize structural design; adjust ejector pin positions to distribute them evenly across areas of high stress, avoiding concentration at thin walls and sharp corners; coordinate with mold manufacturer to reduce ejection speed and force, using segmented ejection; increase draft angle of product structure, ≥1.5° for exterior surfaces and ≥0.5° for non-exterior surfaces, reducing demolding resistance.

 

Defect Manifestations: Products bend and deform after cooling, making normal assembly impossible, especially thin-walled and elongated products.
Core Causes: Asymmetrical product structure during design, resulting in unbalanced shrinkage stress; secondly, an unreasonable mold cooling system design leading to uneven cooling and inconsistent shrinkage across product parts; insufficient injection molding cooling time (coordinate with injection molding plant for adjustment).
Corrective Measures: Prioritize optimizing product structure to achieve symmetry and reduce shrinkage stress; coordinate with mold manufacturer to optimize cooling system, adding cooling water channels to ensure uniform cooling across product parts; request injection molding plant to extend cooling time, allowing product to fully cool and solidify before ejection.

 

Defect Symptoms: Obvious weld lines appear on product surface, affecting not only appearance but also reducing product strength, making it prone to breakage under stress. This is most common in multi-gate, complex structures. It is most noticeable on black products, often appearing at the edges of holes!
Core Causes: An unreasonable multi-gate layout in structural design leads to plastic filling from different directions, resulting in incomplete fusion at cavity junction; or material temperature and mold temperature are too low, resulting in poor plastic flow and insufficient fusion; gate position is unreasonable, with weld lines located precisely at stress points.
Corrective Measures: Optimize structural gate layout and adjust gate position to avoid weld lines at stress points; coordinate with injection molding plant to increase material temperature and mold temperature to improve plastic fusion; have mold manufacturer add venting channels at weld line location to expel air and promote plastic fusion; appropriately increase injection speed and shorten filling time.

 

Defect Manifestations: Scratches and drill marks appear on product's exterior surface, often along demolding direction, especially on high-gloss and matte surfaces. Once these appear, they are irreparable and product must be scrapped.
Core Causes: Insufficient draft angle in structural design, leading to excessive friction between product and mold cavity during demolding; or insufficient surface smoothness of mold cavity, resulting in burrs and scratches (requires coordination with mold manufacturer for rectification); it could also be due to residual plastic causing scratches.
Rectification Methods: Optimize structural design, increase draft angle, reduce friction; coordinate with mold manufacturer to polish mold cavity, improve surface smoothness, remove burrs and scratches; have injection molding plant apply a release agent to cavity surface to reduce adhesion between plastic and cavity; inspect plastic material to avoid any residual impurities.

 

Core of trial molding is not "blindly modifying mold," but rather "finding root cause of problem," distinguishing whether it is a structural issue, a mold issue, or a process issue. Above eight defects are the most common ones we encounter in our daily mold trials. By identifying root causes and addressing them in conjunction with mold and injection molding plants using the corrective measures, we can avoid many pitfalls.
Finally, a reminder to all structural engineers: Before mold trials, carefully verify structural design, paying particular attention to details such as wall thickness, draft angle, gates, and venting provisions to mitigate risks in advance. After mold trials, do not blindly have mold factory repair mold. First, observe defect morphology, accurately pinpoint root cause, and clarify responsibilities to ensure thorough rectification in one go, minimizing rework and avoiding unnecessary blame, thus making you a worry-free structural engineer.