Injection molding process for protective cases — 2026 B2B guide

Injection molding is the dominant production process for plastic protective cases (PP, ABS, HD-PE). This B2B guide covers machine types, mold design, process parameters, cycle time, and quality control for buyers evaluating suppliers.

Machine types by tonnage

Small cases (< 400×300×200mm)

• Machine: 50-200 ton clamping force
• Shot weight: 100-500 g
• Cycle time: 25-45 seconds
• Kexin uses: Haitian Mars II 90T, 150T, 200T
• Cavitation: 1-2 per mold (for low MOQ flexibility)

Medium cases (400-600mm)

• Machine: 250-500 ton
• Shot weight: 500-1,500 g
• Cycle time: 45-90 seconds
• Kexin uses: Haitian 380T, 500T
• Cavitation: 1 per mold (large parts)

Large cases (> 600mm, e.g. 9-series HD trolley)

• Machine: 800-1,600 ton
• Shot weight: 2,000-5,000 g
• Cycle time: 90-180 seconds
• Kexin uses: Haitian 1,200T, 1,600T
• Special: 2-shot molding for hard-soft combinations

Mold design basics

Cavity count trade-off

• 1 cavity: lowest mold cost (USD 3,000-5,000), slowest production (5-10 sec per shot, total cycle = single cycle)
• 2 cavities: balanced, USD 5,000-8,000, 50% faster than 1-cavity
• 4 cavities: USD 10,000-15,000, 75% faster but requires precise flow balance
• 8+ cavities: high-volume only (10,000+ pieces), USD 25,000+, requires hot runner system

Kexin default: 1 cavity for OEM (low MOQ flexibility), 2 cavities for ODM/White-label

Mold material

• P20 steel: standard, 200,000-500,000 shots lifetime
• H13 steel: pre-hardened, 1M+ shots, higher cost
• S136 stainless: medical/food grade, corrosion-resistant
• Aluminum: prototype only, 5,000-10,000 shots

Cooling channels

• Conformal cooling (3D printed): -30% cycle time, +50% mold cost
• Standard drilled channels: baseline
• Beryllium copper inserts: for hot spots, +USD 2,000-5,000 per insert

Process parameters

Temperature

• PP: melt 200-260°C, mold 20-60°C
• ABS: melt 220-260°C, mold 40-70°C
• HDPE: melt 200-240°C, mold 20-50°C
• Mold temperature: critical for surface finish, warpage, cycle time

Pressure

• Injection pressure: 800-1,500 bar (typical)
• Holding pressure: 30-60% of injection, 5-15 seconds
• Back pressure: 5-30 bar (controls melt homogeneity)

Speed

• Injection speed: 30-100 mm/s (faster = better surface, more flash risk)
• Screw speed: 50-150 rpm (plasticizing)

Cycle time breakdown

Phase	Duration	Variables
Mold close	2-5 s	Clamp speed
Injection	1-3 s	Shot size, pressure
Holding	5-15 s	Part thickness
Cooling	15-60 s	Wall thickness, mold temp
Plasticizing	5-15 s	Screw design, melt temp
Mold open	2-3 s	Ejector design
Part ejection	1-2 s	Automation
Total	30-100 s

Quality control checkpoints

IPQC (in-process)

• Every 50 shots: visual + dimensional check (3-5 key dimensions)
• Every 200 shots: full AQL 2.5 sample (50 pieces)
• Every shift: Cpk verification on critical dimensions

Critical dimensions to monitor

• Wall thickness: ±0.3mm typical tolerance
• Latch force: 8-15 N (depending on series)
• Hinge torque: 5-10 N (depending on series)
• Color: ΔE ≤ 3.0 vs. Pantone reference
• Surface finish: SPI-A2 to SPI-B3 (Ra 0.2-1.6 μm)

Common defects and solutions

Sink marks (depressions on thick sections)

• Cause: insufficient packing pressure, too short hold time
• Solution: increase holding pressure +20%, add cooling time

Flash (excess material at parting line)

• Cause: clamp force too low, worn mold
• Solution: reduce injection speed, check clamp tonnage, polish mold surfaces

Weld lines (visible flow fronts meeting)

• Cause: low melt temperature, slow injection
• Solution: increase melt temp +10°C, faster injection

Warpage

• Cause: uneven cooling, residual stress
• Solution: longer cooling time, balanced mold temperature, annealing

Short shots (incomplete filling)

• Cause: insufficient material, low pressure, blocked gates
• Solution: increase shot size, check gate cleanliness

Burn marks (dark spots)

• Cause: trapped air, excessive injection speed
• Solution: add venting, reduce speed

Cycle time optimization

Kexin 2026 best practices:

• Conformal cooling on high-volume molds: -25% cycle time
• Hot runner system: -15% cycle time + no runner waste
• Servo-electric machines: -20% energy + faster response
• Parallel operations: insert + eject during closing

Material flow analysis

Kexin uses Moldflow simulation for:

• Gate location optimization
• Weld line prediction
• Cooling time estimation
• Warpage prediction
• Pressure drop calculation

Recycled material processing

• 35% rPP in 6-series: requires melt filtration (60-100 mesh), cycle time +10%
• 50% r-aluminum in 8-series: aluminum is machined, no melt processing
• Bio-PP: slightly higher melt viscosity, +5°C melt temp

Industry 4.0 / smart manufacturing

• Real-time process monitoring (pressure, temp, position)
• Statistical process control (SPC) dashboards
• AI defect detection via vision system (5 cameras per line)
• Predictive maintenance (vibration analysis on tie bars)
• MES (Manufacturing Execution System) integration

Kexin: 4 of 6 injection lines have Industry 4.0 monitoring (2026 plan: 6 of 6)

Capacity calculation example

500 pieces of 8-series 5023 (500×350×230mm, 1.2kg shot weight):

• Cycle time: 75 seconds (with conformal cooling)
• Available hours: 16h/day × 2 shifts × 30 days = 960h/month = 57,600 min/month
• 1-cavity mold: 57,600/75 = 768 shots/month = 768 pieces
• 2-cavity mold: 1,536 pieces/month
• For 500 pieces delivery: 1-cavity works (less than 1 month)

B2B buyer checklist

When visiting a factory, ask:

• How many injection lines? (5+ for serious factory)
• Machine age? (5-10 years is fine; > 15 years is concerning)
• Clamp tonnage range? (covers all series you need?)
• Hot runner availability? (essential for high-volume)
• In-house mold maintenance? (vs. outsourced)
• Mold storage capacity? (5,000+ molds for serious OEM)
• Cpk data on critical dimensions? (target Cpk ≥ 1.33)
• First pass yield (FPY)? (target ≥ 95%)
• Internal scrap rate? (target < 2%)
• Industry 4.0 monitoring? (best practice but not required)

Bottom line

Injection molding is a mature process; the differentiator for B2B buyers is:

1. Cycle time (drives unit cost)
2. Cpk (drives quality)
3. Mold maintenance (drives delivery reliability)
4. Capacity (drives MOQ flexibility)

Kexin offers 6 injection lines (90T-1,600T), 95% FPY, 1.8% scrap, in-house mold maintenance, and 4/6 lines with Industry 4.0 monitoring as of 2026.