Scratching (also known as whitening or scratching) of plastic parts due to insufficient draft angle is a typical mold design and process matching problem. Essentially, during demolding, normal pressure on mold surface is too high, causing friction to exceed material's strength, resulting in surface tearing and scratching.

 

Below, we systematically analyze causes from four aspects: mold, equipment, process, materials, and provide a fast and efficient solution.

1. Mold Factors (Root Cause)
Insufficient Draft Angle (Primary Cause): This is "original sin" of design. Insufficient draft angle in deep cavities, deep ribs (reinforcing ribs), bosses, other structures leads to an excessively large contact area between product and mold surface, generating enormous friction.
Poor Mold Surface Polishing: Machining marks, rust, or incorrect polishing direction (polishing should not be done along demolding direction) on cavity and core surfaces will scratch product like a file.
Undercut: Unexpected undercuts exist, such as worn inserts, broken ejector pins, or machining errors, mechanically hooking product.
Mold Structure Issues: Imbalance in ejection system (insufficient number of ejector pins, improper positioning), causing uneven force on product during ejection and resulting in localized tearing; Smooth operation or inaccurate timing of sliding blocks, lifters, and other motion mechanisms.
2. Process Factors (Inducing and Aggravating Factors)
Excessive Holding Pressure/Excessive Holding Time (Main Process Cause): Excessive holding pressure causes over-compacting of product, reducing shrinkage and leading to a sharp increase in its clamping force on core.
Insufficient Cooling Time: Product is not sufficiently cooled and solidified, resulting in insufficient strength. Although it can detach during ejection, surface is soft and easily torn or whitened.
Improper mold temperature settings: Low mold temperature: Product cools too quickly, resulting in high shrinkage and strong clamping force.
High mold temperature: Product surface is too soft, lacks strength, and is prone to tearing.
Excessive injection speed: Generates high molecular orientation and internal stress, increasing clamping force.
3. Material factors
Low material hardness and high viscosity: Soft rubbers such as TPU and TPE, and some PA (nylon), easily adhere to mold surface.
Excessive shrinkage: High material shrinkage results in high clamping force on core.
4. Equipment factors
Ejection system malfunction: Insufficient ejection pressure, excessively slow ejection speed, or unstable ejection stroke.
Uneven clamping force: Causes slight mold deformation, forming micro-undercuts.

Core idea for handling tearing problems is: "Reduce clamping force, reduce friction, and optimize ejection force."
Phase 1: Rapid Response and Process Adjustment (Immediate Implementation)
1. Tear Location: Determine location of tear (e.g., deep ribs, core, boss pillars). This helps determine whether it's a localized or systemic issue.
2. Rapid Process Parameter Optimization (Most Direct and Fastest Action):
Core Strategy: Reduce clamping force at all costs.
Significantly Reduce Holding Pressure and Time (Most Effective Measure): This is the most effective way to reduce clamping force. Gradually reduce holding pressure; even if slight shrinkage occurs, prioritize addressing the tear issue.
Extend Cooling Time: Ensure product has sufficient rigidity to withstand ejection forces.
Adjust Mold Temperature: For tearing caused by sticking to mold, try appropriately lowering mold temperature to increase surface hardness of product; For tearing caused by excessive clamping force, try appropriately increasing mold temperature (especially core temperature) to reduce shrinkage stress and clamping force. Experimentation based on actual conditions is required.
Optimize ejection parameters: Increase ejection speed for rapid demolding with "explosive force"; Employ multiple ejections (vibration ejection) to break static friction in stages; Reduce injection speed to reduce internal stress.
3. External auxiliary measures (emergency): Apply a small amount of high-quality release agent precisely to mold surface corresponding to scratched area. Note: This is only an emergency solution; excessive use may cause stains on product surface or affect secondary processing.
Second stage: Systemic diagnosis and radical treatment (if the first stage is ineffective)
If process adjustments have reached their limits (e.g., holding pressure has been reduced to minimum, resulting in severe product shrinkage) and scratching problem persists, then the only solution is to modify mold.
1. Mold modification (fundamental solution):
Increase draft angle (most fundamental solution): For deep ribs or boss pillars with scratches, perform a reduction (polishing) treatment to increase draft angle. This is the most direct and effective method, but requires a professional mold maker.
Enhanced Polishing (Low-Cost Mold Modification): Mirror polish mold surface in scratched area, strictly following demolding direction.
Optimize the Ejection System: Add or enlarge ejector pins near scratched area. For deep ribs, use sleeve ejector pins.
Check and Repair Undercuts: Carefully inspect mold to eliminate any accidental undercuts.

 

Mold Design
1. Sufficient Draft Angles During Design: - External Surfaces: 1.5° or more - Non-External Surfaces (Ribs, Boss Pillars): 0.5°~1° or more - Deep Cavities/Deep Ribs: Need to be increased as appropriate
2. Good Mold Polishing: Core, Ribs, and other areas must be polished along demolding direction.
3. Robust Ejection System.
Process Development
1. Adopt a "minimum necessary holding pressure" strategy, starting with low holding pressure during debugging.
2. Set sufficient cooling time.
3. Optimize ejection speed and mode.
Material Selection: For products with deep cavities or difficult demolding, prioritize materials with good rigidity and low shrinkage.
Core Solution Approach:
Quick Action: Immediately reduce holding pressure and time, and optimize ejection speed. This is the most effective temporary measure to alleviate tearing issues on-site.
Fundamental Solution: Core issue is draft angle and surface finish of mold. Increasing draft angle through polishing and reducing material is the only permanent solution. Excellent design is cornerstone of successful production.
Important Reminder: When draft angle is absolutely insufficient, attempting to compensate entirely through process is extremely difficult and uneconomical. Process adjustments can only alleviate problem to a certain extent; a complete solution inevitably requires mold modification. Strictly the draft angle during mold design review in the early stages of project is the lowest-cost way to avoid such problems.