Additives Fillers Plastic Injection Molding The base polymer is just the starting point.
Additives and fillers can transform a commodity plastic into an engineering material, or completely destroy its processability. I’ve seen projects succeed with the right additive package, and I’ve seen million-dollar tools fail because someone didn’t understand what happens when you add 30% glass fiber to ABS. Let me break down what additives actually do and how they affect your part.
Understanding Additive Categories
Types of Additives
| Category |
Function |
Typical Load |
Cost Impact |
| Reinforcements |
Increase strength, stiffness |
15-60% |
+30-200% |
| Fillers |
Reduce cost, modify properties |
10-50% |
-10-40% |
| Flame retardants |
Fire resistance |
5-30% |
+20-80% |
| Stabilizers |
Heat/UV protection |
0.5-5% |
+5-30% |
| Plasticizers |
Flexibility |
5-50% |
+10-50% |
| Impact modifiers |
Toughness |
5-30% |
+20-100% |
| Processing aids |
Flow, release |
0.5-5% |
+5-15% |
| Colorants |
Appearance |
1-5% |
+5-50% |
Common Additive Combinations
| Application |
Base Material |
Typical Additive Package |
| Structural bracket |
Nylon 6/6 |
30% glass fiber |
| Flame-retardant housing |
ABS |
15% glass, 12% FR |
| UV-stable outdoor |
PP |
UV stabilizer, 20% talc |
| Tough medical |
PC |
Impact modifier, ISO 10993 |
| High-slip packaging |
PP |
Slip agent, antiblock |
Reinforcements
Glass Fiber The most common reinforcement.
increases strength and stiffness.
| Property |
Unfilled |
15% GF |
30% GF |
45% GF |
| Tensile Strength |
8,000 psi |
11,000 psi |
18,000 psi |
22,000 psi |
| Flexural Modulus |
350K psi |
550K psi |
1,200K psi |
1,800K psi |
| Impact Strength |
1.0 ft-lb |
1.2 ft-lb |
1.5 ft-lb |
1.8 ft-lb |
| Shrinkage |
0.7% |
0.4% |
0.2% |
0.1% |
| HDT @ 264 psi |
180°F |
220°F |
250°F |
280°F |
Expensive but exceptional properties.
| Property |
10% CF |
30% CF |
| Tensile Strength |
12,000 psi |
28,000 psi |
| Flexural Modulus |
800K psi |
2,500K psi |
| Conductivity |
Conductive |
Conductive |
| Weight |
10% lighter |
15% lighter |
| Cost Index |
3-4× |
8-12× |
Mineral Fillers Cost reduction with moderate property enhancement.
| Filler |
Cost Index |
Property Effects |
| Talc |
0.3-0.5× |
Stiffness, warpage reduction |
| Calcium carbonate |
0.2-0.4× |
Stiffness, surface finish |
| Mica |
0.4-0.6× |
Stiffness, dimensional stability |
| Glass beads |
0.6-0.8× |
Isotropic shrinkage, rigidity |
| Wollastonite |
0.5-0.7× |
Stiffness, surface finish |
| Reinforcement |
Strength Increase |
Stiffness Increase |
Cost Increase |
Processing Difficulty |
| Glass fiber (30%) |
+100-150% |
+200-300% |
+50-80% |
Moderate |
| Carbon fiber (30%) |
+200-300% |
+400-600% |
+400-800% |
High |
| Talc (40%) |
+20-30% |
+80-120% |
-20-40% |
Low |
| Mineral (40%) |
+10-20% |
+50-80% |
-10-30% |
Low |
| Long fiber (30%) |
+150-200% |
+250-350% |
+80-120% |
High |
Flame Retardants
FR Mechanism Types
| Type |
Mechanism |
Typical Load |
Effectiveness |
| Halogenated |
Radical scavenger |
10-25% |
Excellent |
| Phosphorus |
Char formation |
5-15% |
Good |
| Nitrogen (MCA) |
Gas dilution |
10-20% |
Moderate |
| Mineral (ATH, MDH) |
Endothermic cooling |
40-60% |
Good |
| Nanoclays |
Barrier/char |
2-10% |
Synergistic |
Flame Rating Comparison
| Material |
UL 94 Rating |
LOI (%) |
Comments |
| ABS (unfilled) |
HB |
18 |
Burns |
| ABS + 15% FR |
V-0 |
28 |
Self-extinguishing |
| PC (unfilled) |
V-2 |
25 |
Drips burning |
| PC + 10% FR |
V-0 |
32 |
Self-extinguishing |
| Nylon 6/6 (unfilled) |
V-2 |
23 |
Drips |
| Nylon + 30% FR |
V-0 |
32+ |
Self-extinguishing |
| PP (unfilled) |
HB |
17 |
Burns |
| PP + 30% ATH |
V-0 |
28 |
Self-extinguishing |
FR material properties Guide
| Application |
Required Rating |
Recommended FR Type |
| Consumer electronics |
V-0, 5VB |
Halogenated or phosphorus |
| Electrical enclosures |
V-0 |
Halogenated (best flow) |
| Automotive interior |
V-0, 5VB |
Phosphorus (low smoke) |
| Building/construction |
V-0 |
Mineral (ATH/MDH) |
| Wire and cable |
V-0, VW-1 |
Halogenated (wire grades) |
| Medical devices |
V-0 |
Halogen-free preferred |
Stabilizers
Heat Stabilizers
| Type |
Application |
Effectiveness |
Cost |
| Hindered phenolics |
General purpose |
Good |
$ |
| Phosphites |
Processing stability |
Good |
$ |
| Thioesters |
Long-term heat |
Better |
$$ |
| Quinone methides |
High-temp, long life |
Excellent |
$$$ |
| Copper-based |
Wire, electrical |
Excellent |
$$ |
UV Stabilizers
| Type |
Mechanism |
Effectiveness |
Durability |
Cost |
| HALS (light) |
Hinder free radicals |
Excellent |
Longest |
$$$ |
| UV absorbers |
Absorb UV |
Good |
Medium |
$$ |
| Quenchers |
Energy dissipation |
Moderate |
Medium |
$$ |
| Carbon black |
UV absorption |
Excellent |
Longest |
$ |
| Pigments |
Variable |
Variable |
Variable |
$$ |
Stabilizer Effectiveness Comparison
| Stabilizer Package |
Heat Resistance |
UV Resistance |
Cost Index |
| None |
Baseline |
Baseline |
1.0 |
| Basic antioxidant |
2× |
No change |
1.05 |
| Heat stabilizer (basic) |
3-5× |
No change |
1.1 |
| UV package (basic) |
No change |
3-5× |
1.15 |
| Heat + UV (premium) |
5-10× |
5-10× |
1.25 |
| Long-life package |
10-20× |
10-20× |
1.5 |
Impact Modifiers
Types of Impact Modifiers
| Modifier Type |
Base Materials |
Toughness Increase |
Temp Performance |
| MBS |
PC, PMMA, PVC |
High |
Good |
| ABS rubber |
PS, PVC |
Medium-High |
Moderate |
| EPDM rubber |
PP, PE |
High |
Excellent |
| Maleated elastomers |
Nylon, PET |
High |
Good |
| Ionomers |
PE, EVA |
Very High |
Excellent |
Impact Modifier Effectiveness
| Material |
Notched Izod (RT) |
Notched Izod (-20°C) |
| PP homopolymer |
0.5 ft-lb/in |
0.3 ft-lb/in |
| PP + 20% EPDM |
3.0 ft-lb/in |
1.2 ft-lb/in |
| PP + 30% EPDM |
6.0+ ft-lb/in |
2.5 ft-lb/in |
| Nylon 6/6 |
1.0 ft-lb/in |
0.6 ft-lb/in |
| Nylon + 20% rubber |
3.5 ft-lb/in |
1.5 ft-lb/in |
| Nylon + 30% GF + modifier |
2.0 ft-lb/in |
1.2 ft-lb/in |
Processing Aids
Internal Lubricants
| Type |
Function |
Typical Load |
Effect on Properties |
| Fatty acids |
Mold release |
0.5-2% |
Slight plasticization |
| Amides |
Slip, anti-block |
0.5-2% |
Minimal |
| Metallic soaps |
Lubrication |
0.5-2% |
May affect adhesion |
| Fluoropolymers |
Process aid |
0.1-0.5% |
Minimal |
Flow Modifiers
| Modifier |
Function |
Typical Load |
Result |
| Viscosity modifiers |
Adjust flow |
1-5% |
Lower or higher viscosity |
| Nucleating agents |
Crystallinity |
0.1-1% |
Faster cycle, clearer parts |
| Chain extenders |
MW restoration |
0.1-0.5% |
Recover MW from degradation |
Mold Release Agents
| Type |
Application |
Frequency |
Notes |
| Internal (additive) |
Every shot |
Automatic |
May affect painting |
| External (spray) |
Every shot |
Operator applied |
Residue concern |
| Permanent coating |
Weekly-monthly |
Applied to mold |
Non-transfer |
Additive Interactions and Compatibilities
Common Interactions
| Additive Pair |
Interaction |
Effect |
| Glass fiber + flame retardant |
May be incompatible |
Reduced FR effectiveness |
| Stabilizers + flame retardants |
Synergistic |
Better performance |
| Impact modifiers + glass fiber |
Synergistic |
Better toughness |
| Pigments + heat stabilizers |
Variable |
Test required |
| UV absorbers + HALS |
Synergistic |
Better UV protection |
Additive Load Limits
| Additive |
Max Practical Load |
Beyond This… |
| Glass fiber |
50-60% |
Brittleness, wear |
| Mineral fillers |
50-60% |
Poor impact |
| Flame retardants |
30-40% |
Property degradation |
| Impact modifiers |
30-40% |
Softening, creep |
| Plasticizers |
40-50% |
Plasticization issues |
Processing Considerations
Effects on Processing Parameters
| Additive |
Melt Temp |
Injection Pressure |
Cooling Time |
Screw Wear |
| Glass fiber |
+10-30°F |
+10-30% |
Similar |
High wear |
| Minerals |
Similar |
+5-15% |
Similar |
Medium wear |
| Flame retardants |
Similar or -20°F |
+5-15% |
Similar |
Low-medium |
| Plasticizers |
-20-50°F |
-10-30% |
Similar |
Low |
| Impact modifiers |
Similar |
+5-10% |
+10-20% |
Low |
Screw and Barrel Wear
| Material |
Wear Level |
Screw Tip Protection |
| Unfilled |
Low |
Standard |
| Glass filled (30%) |
High |
Bimetallic or tool steel |
| Mineral filled (40%) |
Medium-High |
Hardened or coated |
| Carbon fiber |
Very High |
Special coatings required |
| Flame retardants |
Low-Medium |
Standard |
Drying Considerations
| Material |
Standard Dry |
With Additives |
| Nylon 6/6 |
180°F, 4 hr |
May need longer |
| PC |
250°F, 4 hr |
May need higher temp |
| PET |
250°F, 4-6 hr |
Similar |
| ABS |
180°F, 3-4 hr |
Similar |
Cost-Impact Analysis
Additive Cost Index (Base Material = 1.0)
| Additive Type |
Low Load |
Medium Load |
High Load |
| Glass fiber (15/30/45%) |
1.2 |
1.5 |
2.0 |
| Mineral filler |
0.8 |
0.7 |
0.6 |
| Flame retardant |
1.3 |
1.5 |
1.8 |
| Impact modifier |
1.3 |
1.6 |
2.0 |
| UV stabilizer |
1.1 |
1.2 |
1.4 |
| Heat stabilizer |
1.05 |
1.1 |
1.2 |
| Colorant (standard) |
1.05 |
1.1 |
1.15 |
| Colorant (premium) |
1.2 |
1.4 |
1.6 |
| Goal |
Material Option |
Cost Index |
Performance Gain |
| Stiffness |
PP + 40% talc |
0.7 |
2× stiffness |
| Stiffness |
PP + 30% GF |
1.4 |
4× stiffness |
| Toughness |
PP + 20% EPDM |
1.4 |
6× impact |
| Toughness |
Nylon + 30% EPDM |
1.8 |
3× impact |
| Heat resistance |
Nylon + 30% GF |
1.8 |
+50°C HDT |
| Flame resistance |
ABS + FR |
1.6 |
V-0 rating |
Selection Framework
Decision Process
Step 1: Define Requirements
- Mechanical needs (strength, stiffness, toughness)
- Environmental needs (heat, UV, chemicals)
- Regulatory needs (flame, food contact)
- Cosmetic needs (color, surface finish)
Step 2: Select Base Material
- Match to primary requirements
- Consider processing compatibility
- Evaluate cost baseline
Step 3: Select Additive Package
- Reinforcement if strength/stiffness needed
- Flame retardant if required
- Stabilizers for environment
- Impact modifier if toughness needed
- Processing aids if flow issues
Step 4: Validate Compatibility
- Additive-additive interactions
- Processing parameters
- Final part properties
- Regulatory compliance
Common Mistakes
Mistake 1
Ignoring Interactions Combining incompatible additives reduces effectiveness or causes failures.
Mistake 2: Overloading More isn’t better.
Additives have optimal loading levels. Excessive loading causes brittleness, poor surface, processing issues.
Mistake 3: Assuming Additive Replacement Not all glass-filled materials are equal.
Different fiber types, treatments, and loadings behave differently.
Mistake 4
Ignoring Regulatory Some additives are restricted in certain applications (food contact, medical, children’s products).
Mistake 5
Forgetting Processing Impact Additives affect processing, screw wear, mold wear, cycle time, energy use.
Factor these into cost analysis.
The key is understanding what each additive does, how it interacts with others, and what trade-offs you’re making. The data is your friend. Use it. Test thoroughly. And remember: the most expensive additive package isn’t always the best—just the one that meets your specific requirements.