A complete guide to plastic materials: from performance defects to mold design taboos, this article

Time：2025-03-07 08:10:23 / Popularity： / Source：

1. Core rules of material selection

"Underlying code of mold design = material properties × molding process"
This article systematically disassembles performance map of 6 major engineering plastic families and directly hits pain points of selection.

2. Full analysis of performance matrix of six major plastics

Plastic type	Performance advantages	Fatal defects	Key points of mold design	Typical application scenarios
LDPE	Good flexibility, chemical corrosion resistance	Low mechanical strength, thermal deformation only 70℃	Gate avoids stress area, exhaust groove depth ≤0.03mm.	Packaging film/hose
HDPE	Strong impact resistance, environmental stress cracking resistance	Shrinkage rate 2-5%, easy processing warping	Asymmetric layout of water channels (thick area density +40%), ejection system reinforcement	Chemical storage tank/automobile fuel tank
PP	High temperature resistance 120℃, food grade certification	Embrittlement at low temperature, obvious crystallization difference	Temperature difference between moving and fixed mold ±5℃, draft angle ≥3°, push plate SKD61 material (hardness HRC52↑)	Medical equipment/food container
ABS	High impact strength, excellent surface gloss	Sensitive weld mark, viscosity fluctuation ±20%	Multi-gate balanced layout, mirror polishing direction consistent with material flow (Ra≤0.01μm)	Electronic product housing/automotive interior
PC	Transmittance 90%↑, impact resistance TOP grade	Poor fluidity, high risk of hydrolysis	Mold temperature fluctuation ≤±1℃	Optical lens/bulletproof glass
POM	Excellent self-lubrication, strong dimensional stability	Strong formaldehyde smell, large shrinkage difference in all directions	Vacuum exhaust (residual pressure ≤5kPa), gate electrolytic polishing (Ra0.2→0.08)	Gear/precision transmission parts

3. Deadly forbidden areas and solutions for mold design

LDPE/HDPE
▌Core pain points:
Differences in shrinkage lead to stress concentration (typical value 1.5-5%)
Low melt strength causes flow distortion
��Mold design tips:
①Force demoulding slope ≥2°, and set boss reinforcement in thin-walled areas
②Asymmetric layout of cooling water channels (40% increase in water channel density in thick areas)
③Gate adopts a fan-shaped design (expansion angle 15-30°)
PP
▌Typical defects:
Low-temperature impact brittle fracture (-20℃ elongation at break <5%)
Weld mark strength attenuation reaches 30% of substrate
��Solutions:
①Control temperature difference between moving and fixed molds within ±5℃
②Open gate at geometric center of product (distance from R angle>3t)
③ Wear-resistant coating (hardness HRC58+) is added to ejection system
ABS
▌The most vulnerable areas for rollover:
Obvious color difference at weld line (VL＜85)
Floating fibers appear on high-gloss surface (frequent when GF content＞30%)
�� Ultimate countermeasures:
① Use hot runner timing valve control (delayed opening＜0.3s)
② Mirror polished cavity (Ra≤0.01μm)
③ Mold temperature gradient is controlled at ±3℃/100mm
PC
▌High-pressure minefield:
Internal stress causes silver streaks (shear rate＞5000 1/s)
Cold runner backflow causes black spots
�� High-level solution:
① Rubber ring is made of titanium alloy (thermal conductivity 5.8W/m·K)
② Precise design of gate sleeve taper (1°30'±10')
③ Mold temperature control module is equipped with PID fuzzy algorithm (fluctuation ≤ ±1℃)
POM
▌Industry pain points:
Gas entrapment leads to burning (precipitated formaldehyde concentration＞0.1ppm)
Uneven crystallization shrinkage causes dimensional deviation
��Innovative process:
① Vacuum-assisted exhaust (air pressure ≤5kPa)
② Use copper-beryllium alloy inserts (thermal conductivity 105W/m·K)
③ Electrolytic polishing of casting system (surface roughness Ra0.8→0.2)

4. Golden parameter quick check table

Parameters/materials	LDPE	HDPE	PP	ABS	PC	POM
Melt index (g/10min)	0.3-20	3-50	4-30	1-25	3-25	2-30
Mold temperature (℃)	20-50	40-80	40-85	50-80	80-120	80-110
Dwelling time (s/mm)	0.6-0.8	0.8-1.2	0.8-1.5	1.0-1.5	1.5-2.0	0.8-1.5
Upper limit of shear rate (1/s)	10^4	8x10^3	6x10^3	5x10^3	3x10^3	2x10^3
Ejection temperature (℃)	40-60	55-75	60-85	70-90	95-120	85-105

5. Strategic decision framework

"Material selection ≠ simple parameter comparison, but a comprehensive game of supply chain flexibility/process feasibility/life cycle cost! "
Three elements of optimal solution:
① Economical: PC material cost is 3-4 times that of ABS, but life cycle is extended by 8-10 times
② Manufacturability: POM ejection slope needs to be increased by 20% compared to theoretical value
③ Environmental compliance: EU REACH regulations limit BPA content in PC to <0.1%

Gud Mould Industry Co., Ltd