Material Hardness vs. Mixer Selection Guide

Why Do Customers Always Complain About Caking & Burning? – The Critical Role of Hardness

“Last month, a client processed Shore A50 TPE in a vertical silo mixer – it fused into basketball-sized lumps in under 10 minutes, destroying 5 tons of material…” With over 20 years in this industry, we at Rumtoo Machine have learned one undeniable truth: matching mixer speed and type to Shore hardness isn’t optional—it’s essential for operational success and cost efficiency.

Understanding Material Hardness: The Foundation of Proper Mixer Selection

Material hardness, typically measured on the Shore scale, dictates how a polymer will behave during mixing. Misjudging this can lead to thermal degradation, material waste, and equipment damage.

• Shore A Scale: Used for elastomers and soft plastics (0A-100A), where lower values indicate softer materials.
• Shore D Scale: Applied to rigid plastics (0D-100D), with higher values representing harder materials.
• Critical Threshold: Shore A80 serves as the crucial dividing line in equipment selection:
  • Materials >A80: May be processed in vertical mixers with caution
  • Materials ≤A80: Require horizontal mixers for proper processing

Why Vertical Silo Mixers Become Destructive for Soft Materials

The Physics of Material Destruction:

Vertical silo mixers operate at high speeds that generate excessive friction and heat when processing softer materials:

• Blade tip speed ≈ 18m/s (calculated from 350rpm rotation)
• Localized temperature >120°C at contact points
• Centrifugal force > 200G at wall surface

Three Primary Risks for Soft Materials (≤A80)

Risk Factor	Physics Mechanism	Real-World Consequence
Thermal Degradation	Friction heat > melting point	TPU-A65 yellowed at 110°C
Centrifugal Wall-Adhesion	350rpm G-force > material adhesion	80% silicone A30 stuck to walls
Molecular Fracture	Shear rate > polymer chain strength	SEBS gel A10 disintegrated

Costly Lesson: An auto sealant manufacturer lost ¥800,000 when EPDM-A65 pre-cured in a vertical silo mixer. This demonstrates why vertical mixing machines should never be used for elastomers and soft plastics below Shore A80.

How Horizontal Mixers and Ribbon Blenders Preserve Material Integrity

Core Design Advantages:

Horizontal mixers from Rumtoo Machine utilize a gentle, efficient material flow process: Bottom → Ribbon lift → Top cascading → Axial dispersion → Counter-flow mixing → Repeat.

Key Preservation Mechanisms

1. Gentle Processing:

   • Maximum temperature rise: <40°C (vs. 115°C in vertical mixers)
   • Shear force reduction: 92% compared to vertical systems

2. Superior Material Flow Efficiency:

   • 8-10 complete turnovers/minute
   • Zero dead zones guaranteed for consistent mixing

3. Material Structure Preservation:

   • Fiber retention: >1.2mm in 30% GF-TPV
   • Foam integrity: 99.3% pore retention in EVA-A28

The Hardness-Equipment Compatibility Matrix

Materials to Avoid in High-Speed Vertical Silo Mixers

Material Type	Hardness Range	Risk Assessment
Silicone Rubber	A10-A40	Permanent hardening → material scrap
TPE Ultra-Soft	A0-A30	Fusion into solid mass
EVA Foam	A25-A50	Cell collapse → elasticity loss
Flexible PVC	A50-A70	Plasticizer evaporation → brittleness

Vertical Mixer Compatible Materials (With Strict Temperature Control)

Material Type	Hardness	Maximum Batch Time
30% GF-PP	D85	8 minutes
Rigid TPU	95A	6 minutes
PC/ABS Alloy	R115	10 minutes

Essential Questions for Proper Mixer Selection

1. Which machine suits my specific material?

Decision Protocol:

• Precisely measure material hardness using a Shore durometer
• If hardness ≤80 Shore A: Select Ribbon Blender or Horizontal Mixer
• If hardness ≥85 Shore A/D: Consider Vertical Silo Mixer with temperature monitoring
• Critical transition zone (A75-A85): Conduct thorough material testing before decision

2. Aren’t horizontal blenders too slow for production needs?

Homogenization Time Comparison:

Material Type	Horizontal Mixer Time	Vertical Silo Mixer Result
TPV-A60	8 minutes	FAILED (material degradation)
HIPS R110	15 minutes	6 minutes (acceptable)
LDPE-A65	10 minutes	4 minutes (with degradation)

⚠️ Critical reminder: Soft materials and certain powders cannot properly homogenize in vertical silo mixers without risking material damage.

3. What if I’ve already purchased the wrong equipment?

Damage Control Solutions:

Solution	Cost Estimate	Effectiveness	Drawbacks
Liquid Nitrogen Cooling	$20,000+	Medium	30% capacity reduction
Batch Size Limitation (30%)	$0	Low	50% throughput loss
Conversion to Horizontal System	$50,000-$150,000	High	14-month ROI typically

Frequently Asked Questions

Q: How often should I test material hardness for mixing processes?
A: Test hardness with every new material batch and whenever you change suppliers. Material properties can vary significantly between batches, affecting mixer performance.

Q: Can I modify a vertical mixer to handle softer materials?
A: While limited modifications like reduced blade speed or cooling jackets can help, they rarely provide complete solutions. The fundamental design of vertical mixers creates inherent challenges for soft materials.

Q: What’s the typical ROI when upgrading to the proper mixer type?
A: Most operations see ROI within 12-18 months through reduced material waste, lower energy consumption, and decreased downtime. Rumtoo Machine has documented cases where companies recovered costs in under a year.

Q: How does ambient temperature affect mixer selection?
A: Higher ambient temperatures exacerbate heat buildup in vertical mixers. In warm climates or non-climate-controlled facilities, the threshold for switching to horizontal mixers may be as high as Shore A85.

Q: Are there hybrid systems that handle both soft and hard materials effectively?
A: Some advanced systems from Rumtoo Machine offer variable speed controls and cooling options that provide broader material compatibility, but specialized equipment typically delivers superior results for specific material ranges.

Conclusion: Lessons Learned Through Experience

“After witnessing Shore A35 silicone carbonize in a vertical mixer, we at Rumtoo Machine reached a fundamental understanding:

MATERIAL HARDNESS = EQUIPMENT SELECTION LAW
MIXER RPM = MATERIAL PRESERVATION OR DESTRUCTION

This knowledge now drives our protocol: we require hardness reports before shipping any mixer – not as bureaucracy, but as ethical responsibility to our clients’ success and bottom line.”

The relationship between material hardness and mixer selection represents one of the most critical decisions in plastics processing. By understanding the Shore scale, recognizing the limitations of different mixer types, and implementing proper selection protocols, manufacturers can avoid costly mistakes, improve product quality, and enhance operational efficiency.