Overview: Why high toughness glass fiber reinforced plastic matters
Kerke, a Wanplas factory specializing in co-rotating parallel twin-screw compounding extruders, develops and supplies both the machinery and the process know-how required to produce high toughness glass fiber reinforced plastic on an industrial scale. As engineering plastics continue to replace metal in automotive, electrical, and consumer-product structures, the single property that most often limits wider adoption is brittleness at the notch and under impact. A standard glass fiber reinforced compound gains stiffness and strength, but the glass fibers themselves act as stress concentrators that can lower impact resistance. The discipline of “high toughness” formulation is therefore the art and science of adding strength without sacrificing the energy-absorbing character that keeps a part from cracking in service.
This article explains, in practical compounding terms, how to design a high toughness glass fiber reinforced plastic formula and how to manufacture it reliably on a co-rotating parallel twin-screw extruder. We cover base resin selection, the role of coupling agents and elastomeric impact modifiers, glass fiber loading and sizing, the mechanical-property trade-offs, the extruder screw configuration and temperature profile, and the specific Kerke KTE series machines that are well suited to this class of material. Whether you run a color and additive masterbatch plant, an engineering-plastic compounder, or an in-house molding operation, the guidance below is written to be directly usable on the production floor.
Glass fiber reinforced plastic, often abbreviated as GFR or GFRP in technical literature, is a short-fiber composite in which discontinuous glass fibers are dispersed in a thermoplastic matrix. In a twin-screw extruder the fibers are introduced late in the process through a side feeder so that they are gently wetted and only mildly sheared, preserving fiber length. The resulting pellet, when injection molded, yields parts with tensile strength and flexural modulus far above the neat resin, at a modest cost increase. The challenge addressed throughout this article is to retain that stiffness while pushing notched impact strength upward through a deliberate toughening system.
Key point. A “high toughness” glass fiber reinforced formula is not achieved by adding more glass fiber. It is achieved by matching the right base resin, treating the fiber surface with a coupling agent, and blending a compatible elastomeric impact modifier so the matrix can absorb energy before a crack propagates along a fiber.
The market context in 2026 favors these materials. Lightweighting mandates in transportation, the spread of electrified vehicles that need dimensionally stable under-hood and structural components, and the demand for thin-wall durable housings in electronics all push compounders toward glass fiber reinforced polyamide and polypropylene with elevated impact performance. A correctly designed formula can cut part weight by 30 percent or more versus aluminum while meeting the same load and impact specifications. The rest of this article gives you the formula building blocks and the extrusion conditions to get there.
What defines “high toughness” in glass fiber reinforced plastic
Toughness in a composite is the ability to absorb energy and deform plastically before failure. In short-fiber thermoplastic compounds it is governed by a competition between the reinforcing fibers, which raise strength and stiffness but concentrate stress, and the matrix, which must yield and bridge cracks. A “high toughness” grade is one in which the matrix and the fiber-matrix interface cooperate to blunt cracks rather than let them run.
Key toughness metrics: notched impact and elongation
The two metrics compounders watch most closely are notched Izod impact strength (reported in kJ/m² or J/m) and elongation at break. Notched Izod, measured on a specimen with a machined notch, simulates the worst case a molded part experiences at a gate, weld line, or sharp corner. A neat polyamide such as PA66 may show around 4 to 5 kJ/m² notched; a well-toughened PA66 with 30 percent glass fiber and a coupled elastomer system can reach 11 to 15 kJ/m². Elongation at break tells you how much deformation the material tolerates before fracture and is a leading indicator of resistance to brittle failure in service.
Charpy impact (unnotched and notched) is the European counterpart and is useful for comparing grades on the same basis. For structural parts, instrumented falling-dart or Gardner impact at the part level is the ultimate validation, but the laboratory notched impact value is the formulation trigger you tune on the twin-screw line.
Failure modes in glass fiber reinforced compounds
Understanding how a glass fiber reinforced part fails tells you where to add toughness. The dominant failure modes are fiber pull-out, matrix cracking, debonding at the fiber-matrix interface, and crack propagation along weld lines. When the interface is weak, fibers simply pull out and contribute little; the part looks reinforced on paper but fails early. A coupling agent chemically links the polar matrix to the silane-sized glass surface, raising the interfacial shear strength so the fiber carries load. An impact modifier, by contrast, works in the matrix: it creates a second rubbery phase that cavitates and relieves stress around a growing crack.
The practical rule is that interfacial strength and matrix ductility must be improved together. Toughening only the matrix while leaving a weak interface gives a soft part that still breaks at the fiber. Strengthening only the interface with a brittle matrix gives a strong part that shatters. High toughness requires both, which is why the formula in the next sections combines a coupling agent with an elastomeric modifier.
Base resin selection for high toughness GFR compounds
The base resin sets the ceiling on toughness. Some matrices are intrinsically ductile and respond well to glass fiber plus coupling; others are already stiff and brittle and need aggressive modification. The table below compares the common engineering and commodity matrices used for glass fiber reinforcement, with the realistic toughness tier after a full coupling plus impact-modifier system is applied.
| Base resin | Density (g/cm³) | Neat tensile strength (MPa) | Neat notched Izod (kJ/m²) | Heat deflection temp at 1.8 MPa (°C) | Toughness tier after 30% GF + coupling + modifier |
|---|---|---|---|---|---|
| PA66 (polyamide 66) | 1.14 | 80 | 4.5 | 70 | Very High |
| PA6 (polyamide 6) | 1.13 | 75 | 5.0 | 60 | Very High |
| PP (polypropylene, copolymer) | 0.91 | 30 | 4.0 | 55 | High (with coupling) |
| PBT (polybutylene terephthalate) | 1.31 | 55 | 4.5 | 55 | High |
| PET (polyethylene terephthalate) | 1.38 | 55 | 4.0 | 70 | High |
| ABS (acrylonitrile butadiene styrene) | 1.05 | 45 | 18 | 95 | Medium |
| PC (polycarbonate) | 1.20 | 62 | 45 | 125 | Premium (intrinsic) |
Polyamide (PA6 and PA66) matrix
Polyamide is the workhorse of high toughness glass fiber reinforced compounds. PA66 and PA6 are polar, crystalline, and accept maleic anhydride grafted coupling agents extremely well, which is why they dominate under-hood automotive and power-tool housings. PA66 brings higher heat deflection temperature and modulus; PA6 brings slightly better impact and easier processing. Both are moisture sensitive: conditioned (equilibrated) properties differ from dry-as-molded properties, and a toughened grade should be specified on conditioned samples for realistic service data.
For a high toughness PA66 grade the typical target is 30 percent glass fiber with 2 to 4 percent of a maleic anhydride grafted polyolefin coupling agent and 4 to 10 percent of a maleic anhydride grafted elastomer such as POE-g-MAH. The result is a compound with tensile strength near 180 MPa, flexural modulus near 8.5 GPa, and notched Izod near 12 kJ/m², which is the profile most structural molders request.
Polypropylene (PP) matrix
Polypropylene is the low-cost, low-density option. Neat PP is soft, but with glass fiber and a coupling agent it becomes a credible structural material at roughly half the raw-material cost index of polyamide. The catch is that PP is non-polar, so a maleic anhydride grafted polypropylene (often called MAPP) coupling agent is mandatory; without it the glass fiber simply acts as inert filler and impact actually drops. A copolymer PP base gives better low-temperature impact than homopolymer. PP-GF compounds are common in appliance structures, fan blades, and battery modules where heat resistance above 100°C is not required.
PBT and PET matrix
Polyester matrices such as PBT and PET offer excellent dimensional stability, low moisture uptake, and good electrical properties, which makes them attractive for connectors, bobbins, and sensor housings. They need a coupling agent and an impact modifier tailored to the polyester; a glycidyl methacrylate or maleic anhydride functionalized modifier is typical. PBT-GF30 with toughening reaches notched Izod around 9 to 12 kJ/m² while keeping a heat deflection temperature near 200°C at 1.8 MPa, which polyamide cannot match without reinforcement and mineral filler.
The toughening system: coupling agents, impact modifiers, compatibilizers
A high toughness glass fiber reinforced formula is built from three additive families working in concert: a coupling agent at the fiber surface, an elastomeric impact modifier in the matrix, and supporting stabilizers. Getting the balance right is the difference between a commercial grade and a laboratory curiosity.
Maleic anhydride grafted coupling agents
Coupling agents bridge the polar matrix and the silane-sized glass surface. For polyamide the standard is a maleic anhydride grafted polyolefin or a reactive polyamide-based compatibilizer that forms hydrogen bonds or covalent links with both phases. The graft level (the percentage of maleic anhydride groups) matters: too low and coupling is incomplete, too high and the agent plasticizes the matrix and hurts heat resistance. A graft level around 0.8 to 1.2 percent is the practical sweet spot for PA66 with 30 percent glass fiber.
For polypropylene the coupling agent is maleic anhydride grafted polypropylene, and the dosage is higher because the matrix is fully non-polar. A rule of thumb is 3 to 5 percent MAPP based on the glass fiber weight, so a 20 percent glass fiber PP compound uses roughly 0.6 to 1.0 percent of total formulation as coupling agent. The visual proof of good coupling is fiber fracture rather than pull-out in a fractured sample observed under a scanning electron microscope.
Elastomer impact modifiers (POE, EPDM, SEBS)
The impact modifier is a dispersed rubbery phase. The most common choices are polyolefin elastomer grafted with maleic anhydride (POE-g-MAH) for polyamide and polypropylene, ethylene propylene diene monomer rubber (EPDM) for polyolefins, and styrene ethylene butylene styrene block copolymer (SEBS) for less polar systems. The modifier must be compatible enough to disperse as fine particles of 0.2 to 1.0 micrometer, yet phase-separated enough to cavitate under impact. Too fine a dispersion gives no toughening; too coarse weakens the matrix.
Dosage is the key tuning knob. Adding 5 percent POE-g-MAH to a PA66-GF30 compound typically lifts notched Izod from about 9 to 13 kJ/m². Adding 15 percent lifts it further but erodes tensile strength and heat deflection temperature. The compounder trades a small loss of stiffness for a large gain in impact, and the twin-screw extruder must distribute the rubber uniformly to avoid agglomerates that become internal flaws.
Glass fiber sizing and surface treatment
The glass fiber itself is rarely used bare. It carries a sizing (a thin surface coating) optimized for the target resin: an aminosilane sizing for polyamide, a silane or proprietary sizing for polyester, and an adhesion promoter for polypropylene. The chopped strand length and the sizing integrity determine how much fiber length survives the extruder. Longer initial strand (4.5 mm chopped strand is common) and a robust sizing preserve length through the side-feeding zone. Fiber diameter is typically 10 to 17 micrometers; finer fiber gives smoother surfaces but slightly lower absolute reinforcement per weight.
The table below places the additive families on a cost index scale where the polyamide resin is set to a baseline of 100. This lets a buyer compare formula economics without referencing absolute currency, and it is the same convention used in the selection guide later in this article.
| Formula component | Typical use level (wt%) | Cost index (base resin = 100) | Cost tier |
|---|---|---|---|
| PA66 base resin | 60 to 67 | 100 | High |
| PA6 base resin | 60 to 67 | 82 | Medium |
| PP copolymer resin | 70 to 78 | 55 | Low |
| E-glass fiber (chopped strand) | 10 to 50 | 40 | Low |
| MAPP / maleic anhydride coupling agent | 1 to 5 | 70 | Medium |
| POE-g-MAH impact modifier | 4 to 12 | 120 | Very High |
| Antioxidant and stabilizer package | 0.3 to 0.6 | 90 | Medium |
Glass fiber content and its effect on mechanical performance
Glass fiber content is the single biggest lever on stiffness and strength, but its effect on toughness is non-linear. As loading rises, tensile strength and flexural modulus climb steeply while notched impact first rises (because coupling improves stress transfer) then plateaus or falls (because the matrix volume that can yield shrinks). The table below shows representative values for a coupled PA66 compound with increasing glass fiber, holding the coupling and impact-modifier levels proportional.
| Glass fiber content (wt%) | Tensile strength (MPa) | Flexural modulus (GPa) | Notched Izod impact (kJ/m²) | Elongation at break (%) | Relative melt flow rate |
|---|---|---|---|---|---|
| 0 (neat PA66) | 80 | 2.8 | 4.5 | 45 | Very High |
| 10 | 110 | 4.5 | 6.0 | 6 | High |
| 20 | 150 | 6.5 | 8.0 | 4 | Medium |
| 30 | 185 | 8.5 | 11.0 | 3 | Medium |
| 40 | 210 | 11.0 | 13.0 | 2.5 | Low |
| 50 | 230 | 13.5 | 14.0 | 2.0 | Very Low |
Two practical conclusions follow. First, 30 percent glass fiber is the most common production target because it delivers the best balance of stiffness, strength, and processability for injection molding; melt flow is still acceptable and fiber breakage in the screw is manageable. Second, pushing past 40 percent glass fiber mostly buys modulus, not toughness, and it sharply reduces melt flow rate, which can cause short shots in thin-wall tools. A high toughness formula therefore usually stays in the 20 to 35 percent glass fiber band and reaches its impact target through the modifier rather than through more fiber.
Fiber length retention is as important as fiber content. A compound that starts with 4.5 mm chopped strand but is over-sheared in the extruder may end up with an average fiber length below 0.3 mm, losing much of the reinforcement. This is why glass fiber is side-fed downstream of the main melting zone and why the screw element layout uses forward-conveying and mild kneading rather than aggressive reverse elements in the reinforcement zone. The process design section expands on this.
Twin screw extruder process design for GFR compounds
The twin-screw extruder is where a formula becomes a consistent pellet. For glass fiber reinforced compounds the machine is almost always a co-rotating parallel twin-screw extruder because its modular screw and barrel elements let you place the fiber side feeder exactly where the melt is homogeneous and the shear is low. Counter-rotating and conical twins are used for PVC and rigid profiles, but for engineering-plastic compounding the co-rotating parallel design is the standard.
Screw configuration and L/D ratio
The screw is assembled from individual elements: conveying, kneading blocks (bilobal or trilobal), and reverse elements. For a high toughness GFR compound the layout is deliberate. The first third melts and homogenizes the resin plus coupling agent and impact modifier. The kneading blocks in this zone must be enough to disperse the rubber phase but not so intense that they degrade the polymer. The glass fiber is added through a side feeder after this zone, into a section of forward-conveying elements with one or two short, low-intensity kneading blocks to wet the fiber without shredding it. A venting zone follows to remove moisture and volatiles, then a final metering section builds pressure for the die.
The length-to-diameter ratio, written as L/D, is typically 40 to 48 for glass fiber compounds. An L/D of 40 is adequate for a 30 percent glass fiber PA66 with a single side feeder. Higher L/D (44 to 48) helps when you need two side feeders (for example, long glass fiber plus mineral filler) or a devolatilization step under vacuum. Kerke KTE series machines offer L/D options across this range and the barrel is built in blocks so the side feeder and vent positions are configurable.
Side feeding and gentle fiber incorporation
Side feeding is the defining step. If glass fiber is dumped into the main hopper with the resin, it is ground to dust before it is wetted, producing a brittle compound. Instead, the resin and additives are melted and homogenized in the first barrels, then the chopped glass fiber strand is metered into a downstream barrel through a side feeder (a twin-screw side stuffer or a crammer feeder). At that point the melt is viscous enough to wet the fiber and the screw only needs to fold it in. A loss-in-weight feeder on the side stuffer holds the glass fiber dosage to within a fraction of a percent, which is what keeps the mechanical properties on spec batch after batch.
For very high toughness with long fiber, a secondary side feeder can introduce a portion of the fiber even later, or a long-glass-fiber roving can be pulled directly into the melt. The machine and the formula must be matched: a higher fiber content demands a longer L/D and a more powerful drive to handle the elevated torque of a filled melt.
Temperature profile and melt temperature
The barrel temperature profile must melt the resin before the fiber arrives but stay below the degradation point of the impact modifier and the antioxidant. For PA66 the profile below is representative; the side-feed barrel is set a little lower than the melt zone to keep the fiber from overheating at the wall, and the die is set near the melt temperature to avoid freeze-off.
| Barrel zone | Function | Set temperature (°C) |
|---|---|---|
| Barrel 1 (feed) | Resin melting start | 240 |
| Barrel 2 | Plasticizing and mixing | 255 |
| Barrel 3 (side feed) | Glass fiber incorporation | 265 |
| Barrel 4 (kneading) | Fiber wetting, mild shear | 275 |
| Barrel 5 | Homogenization | 280 |
| Barrel 6 (vent) | Devolatilization | 282 |
| Die | Pressure build, strand formation | 285 |
Melt temperature, measured at the die, should be held in the 280 to 290°C window for PA66-GF30. Exceeding about 300°C risks oxidative degradation of the polyamide and yellowing; dropping below about 270°C raises viscosity, increases shear heating, and can over-shorten the fiber. Polypropylene runs cooler, around 210 to 230°C melt, and polyester around 250 to 270°C. The exact profile is tuned on the plant floor against actual melt-pressure and motor-load readings.
Screw speed, torque, and residence time
Screw speed sets the residence time and the shear the fiber sees. Higher speed shortens residence, raises output, and increases fiber breakage; lower speed protects fiber length but reduces throughput. For a 30 percent glass fiber PA66 a screw speed of 300 to 450 rpm on a mid-size machine is typical. The drive torque must be sufficient: a highly filled melt draws more torque per rpm, so a high-torque gearbox protects the line from stalling and lets you run a longer, more reinforced formulation. Kerke KTE machines use a computer-aided screw assembly with a kneading co-type geometry that has excellent self-cleaning, which prevents dead spots where degraded material would accumulate.
Residence time also governs degradation. A co-rotating twin-screw has short, controllable residence (often under two minutes), which is why it suits heat-sensitive toughened compounds far better than a single-screw or a batch mixer. The vent zone under vacuum strips moisture and residual monomer, which matters for polyamide because water plasticizes the melt and hydrolyzes the chain at high temperature.
Recommended high toughness formula examples
The following three formulas are starting points validated in practice for the stated resin systems. They assume a co-rotating parallel twin-screw line with a side feeder and a vacuum vent, processed on the temperature and speed windows described above. Exact dosages should be confirmed by a trial run on your own resin grade, because lot-to-lot variation in base resin and glass fiber sizing shifts the optimum.
High toughness GFR-PA66 (30% GF) formula
| Ingredient | Role | Weight percent |
|---|---|---|
| PA66, heat-stabilized grade | Matrix | 62.5 |
| E-glass fiber, 4.5 mm chopped, aminosilane sizing | Reinforcement | 30.0 |
| MAPP or maleic anhydride grafted PA coupling agent (1.0% graft) | Fiber-matrix coupling | 3.0 |
| POE-g-MAH elastomer impact modifier | Toughening phase | 4.0 |
| Antioxidant blend (phenolic plus phosphite) | Thermal stability | 0.3 |
| UV stabilizer (optional, for outdoor parts) | Weathering resistance | 0.2 |
This formula targets tensile strength about 185 MPa, flexural modulus about 8.5 GPa, and notched Izod about 12 kJ/m² after conditioning. If the part sees low-temperature impact, raise the POE-g-MAH to 6 percent and accept a small drop in modulus. If stiffness is paramount and impact is secondary, drop the modifier to 2 percent.
High toughness GFR-PP (20% GF) formula
| Ingredient | Role | Weight percent |
|---|---|---|
| PP copolymer, high-impact grade | Matrix | 74.0 |
| E-glass fiber, 4.5 mm chopped, adhesion-promoted sizing | Reinforcement | 20.0 |
| MAPP coupling agent (1.0 to 1.2% maleic anhydride) | Fiber-matrix coupling | 4.0 |
| EPDM or POE-g-MAH impact modifier | Toughening phase | 1.5 |
| Antioxidant (phenolic) | Thermal stability | 0.3 |
| Acid scavenger (optional) | Processing stability | 0.2 |
With MAPP doing the heavy lifting on coupling, this PP compound reaches tensile strength near 90 MPa and notched Izod near 8 kJ/m², at a raw-material cost index roughly half that of the PA66 grade. It is the value choice for appliance and battery-structure parts that do not exceed about 100°C in service.
High toughness GFR-PBT (30% GF) formula
| Ingredient | Role | Weight percent |
|---|---|---|
| PBT, fast-crystallizing grade | Matrix | 64.0 |
| E-glass fiber, 4.5 mm chopped, polyester-compatible sizing | Reinforcement | 30.0 |
| Glycidyl or maleic anhydride functionalized compatibilizer | Coupling | 3.0 |
| Core-shell acrylic or elastomer impact modifier | Toughening phase | 2.5 |
| Antioxidant and hydrolysis stabilizer | Long-term stability | 0.5 |
This PBT grade targets notched Izod around 10 kJ/m² with a heat deflection temperature above 200°C, which suits electrical connectors and sensor housings where dimensional stability under heat matters more than absolute impact.
Kerke KTE series twin screw extruders for GFR compounding
Kerke, a Wanplas factory, builds the KTE series of parallel co-rotating twin-screw extruders specifically for compounding tasks like the glass fiber reinforced formulas above. The range runs from the KTE-16B bench unit for formulation trials up to the KTE-135D for full-scale production, all sharing the same modular barrel-block and screw-element philosophy so a formula developed on a small machine scales predictably to a large one.
| Model | Screw diameter (mm) | L/D ratio | Max screw speed (rpm) | Drive power (kW) | Typical output for GFR-PP (kg/h) |
|---|---|---|---|---|---|
| KTE-16B | 16 | 40 | 600 | 4 | 30 |
| KTE-26 | 26 | 40 | 500 | 11 | 120 |
| KTE-36 | 36 | 40 | 500 | 22 | 300 |
| KTE-50 | 50 | 44 | 400 | 55 | 600 |
| KTE-65B | 65 | 44 | 400 | 110 | 1000 |
| KTE-75 | 75 | 44 | 350 | 160 | 1500 |
| KTE-95 | 95 | 48 | 300 | 250 | 2200 |
| KTE-135D | 135 | 48 | 250 | 500 | 3500 |
The values above are representative for glass fiber reinforced polypropylene with a side feeder and vacuum vent; output for polyamide is comparable, while highly filled or long-glass grades run lower. Every model is built from computer-aided designed screw assemblies with a kneading co-type geometry that gives excellent self-cleaning, good interchangeability of elements, and the conveying, plasticizing, shearing, dispersion, homogenization, venting, and pressure-build functions a toughened compound needs. The barrel is sectioned so the side feeder, vent, and liquid-feed positions are placed exactly where the process demands.
KTE-135D high capacity compounding line
The KTE-135D is the flagship for large compounders running thousands of tonnes per year of glass fiber reinforced engineering plastic. Its 135 mm screws and 500 kW drive handle 30 to 50 percent glass fiber loads with margin, and the 48 L/D barrel accommodates two side feeders plus a vacuum vent for demanding formulations. For a high toughness PA66-GF30 line this machine delivers up to about 3500 kg/h of pellet, with the loss-in-weight side feeder holding glass fiber dosage tight enough that notched Izod stays within specification across the campaign.
KTE-65B mid-range co-rotating line
The KTE-65B is the most common production workhorse, with a 65 mm screw and 110 kW drive giving around 1000 kg/h on GFR-PP. It is the natural scale-up target from a pilot line and is flexible enough to run masterbatch, filled compounds, and toughened glass fiber grades on the same frame by swapping screw elements and barrel inserts. Most compounders buy a KTE-65B as their first dedicated glass fiber line because it balances output, floor space, and capital cost index in the Medium tier.
KTE-16B laboratory twin screw extruder
The KTE-16B is the bench and pilot unit used to develop the formulas in this article. With a 16 mm screw and 4 kW drive it produces 30 kg/h, enough for hundreds of molded plaques and impact specimens from a single small batch. Because it shares the KTE geometry and control philosophy, a formula tuned on the KTE-16B transfers to the KTE-65B and KTE-135D with predictable screw-speed and temperature mapping, which removes most of the scale-up risk that plagues compound development.
Application industries for high toughness GFR compounds
High toughness glass fiber reinforced plastic serves industries where a part must be light, stiff, and resistant to shock and vibration. As a Wanplas factory focused on twin-screw compounding, Kerke supplies the lines that produce these materials for the following sectors.
- Automotive and transportation. Under-hood covers, intake manifolds, fan shrouds, door modules, and battery structural housings use toughened PA66-GF30 and PP-GF for weight savings versus die-cast metal while surviving engine-bay thermal cycling and stone impact.
- Electrical and electronics. Connectors, relay housings, and sensor bodies use PBT-GF and PA-GF for dimensional stability, flame behavior, and tracking resistance, with toughness added so thin walls do not crack during assembly.
- Power tools and appliances. Gear housings, handles, and bases use toughened glass fiber compounds to absorb drop impact and repeated load without fracturing.
- Consumer and sporting goods. Structurally loaded frames, brackets, and shells use the high stiffness-to-weight ratio of GFR compounds with the impact margin that keeps a dropped product intact.
- Industrial and building components. Pumps, valves, and profile structures use glass fiber reinforced thermoplastic for corrosion resistance plus the toughness to withstand pressure surges.
In every one of these applications the compounder’s job is the same: hold the fiber length, couple the interface, and disperse the impact modifier. The twin-screw line is the instrument that delivers all three at volume.
Formula-to-machine selection guide
Matching a formula to a machine prevents both under-powered lines that stall on filled melt and oversized lines that waste energy. The guide below maps common development and production scenarios to the Kerke KTE models introduced above. Use it together with the cost index table earlier: a higher glass fiber content and a higher impact-modifier level both raise torque demand, so step up a model when you combine them.
| Development or production scenario | Recommended Kerke model | Screw diameter (mm) | Side feeders needed | Capacity tier |
|---|---|---|---|---|
| Bench formula trial, under 5 kg/h | KTE-16B | 16 | 0 to 1 | Laboratory |
| Pilot batch, 30 to 150 kg/h | KTE-26 / KTE-36 | 26 to 36 | 1 | Pilot |
| Small production, 300 to 600 kg/h | KTE-50 | 50 | 1 | Small |
| Standard production, 600 to 1000 kg/h | KTE-65B | 65 | 1 to 2 | Medium |
| High volume, 1500 to 2200 kg/h | KTE-75 / KTE-95 | 75 to 95 | 2 | High |
| Full scale, 2200 to 3500 kg/h | KTE-135D | 135 | 2 | Very High |
As a rule, choose the smallest model that meets your required output with at least 15 percent torque headroom, because toughened glass fiber compounds thicken the melt and you will want reserve for start-up and grade changes. The KTE series lets you reconfigure barrel sections and screw elements rather than buy a new frame, so moving from a 20 percent to a 40 percent glass fiber formula is often a component change, not a capital purchase.
Service and support
Kerke, a Wanplas factory, backs every KTE line with the shared Wanplas group service commitments. Before shipment each machine is run on a representative formula and tested for continuous operation so that what arrives at your plant is a validated line, not a promise. On-site installation and commissioning are performed by engineers who set the temperature profile, side-feeder calibration, and screw mapping to your specific resin and glass fiber combination.
The Wanplas group policy provides USD 500 free parts every year for the life of the equipment, plus free replacement of parts damaged within the warranty window. Training covers formula development on the KTE-16B, scale-up mapping to production models, screw-element changeover, and routine maintenance of the gearbox, side feeder, and screen changer. The open-factory policy welcomes customers to witness a trial run of their own formula before purchase, and remote monitoring lets the service team read PLC data from the factory to diagnose process drift without a site visit.
These supports matter most for toughened glass fiber compounds, where a small change in side-feeder calibration or barrel temperature moves notched Izod by several points. Having the original process recipe archived and the same screw geometry available as spare parts keeps a qualified compound in spec long after the first campaign.
Frequently asked questions
Why does adding more glass fiber sometimes lower impact strength?
Glass fiber raises stiffness and tensile strength, but each fiber end is a stress concentrator. Below about 30 percent loading, good coupling lets the fiber carry load and impact rises; past that point the shrinking matrix volume has less material to yield, and fiber-fiber contact creates easy crack paths, so notched impact plateaus or falls. High toughness is therefore reached with moderate fiber (20 to 35 percent) plus an impact modifier, not by maximizing fiber alone.
Can I use a single-screw extruder instead of a twin-screw for glass fiber compounds?
A single-screw extruder cannot place a side feeder precisely in the melt or control residence and shear as tightly as a co-rotating parallel twin-screw, so fiber length retention and additive dispersion suffer. For a high toughness grade with an elastomeric modifier, the twin-screw’s modular screws and downstream side feeding are effectively required to keep the properties on spec.
How do I stop the glass fiber from turning into dust inside the extruder?
Side-feed the chopped strand into a downstream barrel after the resin is melted, use forward-conveying and only mild kneading elements in the reinforcement zone, and keep screw speed in the moderate range (300 to 450 rpm for PA66-GF30). A loss-in-weight side feeder holding dosage steady also reduces the temptation to over-shear. The Kerke KTE self-cleaning kneading geometry protects fiber length while still wetting it.
What is the difference between a coupling agent and an impact modifier?
A coupling agent (such as maleic anhydride grafted polypropylene or a grafted polyamide compatibilizer) bonds the glass fiber to the matrix at the interface, raising interfacial strength so the fiber carries load. An impact modifier (such as POE-g-MAH or EPDM) is a rubbery phase dispersed in the matrix that cavitates and absorbs energy under impact. You need both: coupling alone leaves a brittle matrix, while a modifier alone leaves a weak interface.
How should I specify properties so my molder gets the right grade?
Specify on conditioned samples for polyamide (because moisture changes the numbers), give the notched Izod target, the flexural modulus, the tensile strength, and the heat deflection temperature, and state the glass fiber content and any flame or electrical requirement. Ask for a representative injection-molded plaque so impact is measured on molded stock rather than only on extruded strand, which can read higher than the real part.
Does a higher impact modifier level always improve toughness?
No. Beyond a point, more elastomer lowers tensile strength, flexural modulus, and heat deflection temperature, and can hurt surface appearance and creep resistance. The gain in notched Izod also flattens above about 10 to 12 percent POE-g-MAH in PA66-GF30. Tune the modifier against your stiffness floor; a 4 to 6 percent level is the usual starting window for a high toughness grade.
How do I scale a formula from the lab KTE-16B to the production KTE-135D?
Because the KTE series shares screw geometry and control logic, keep the same element sequence per barrel length, hold melt temperature and specific mechanical energy per kilogram constant, and map screw speed to the larger diameter using the same residence-time target. The side-feeder calibration transfers as a weight percent, so glass fiber dosage stays identical. A trial run on the production model confirms the notched Izod before a full campaign.
Conclusion and next step
A high toughness glass fiber reinforced plastic formula is an exercise in balance: pick a ductile-enough base resin, couple the glass fiber at the interface, disperse an elastomeric impact modifier in the matrix, and manufacture it on a co-rotating parallel twin-screw extruder that side-feeds the fiber and protects its length. The reward is a compound that is stiff like metal, light like plastic, and tough enough to survive real-world impact, opening metal-replacement applications across automotive, electrical, appliance, and industrial sectors.
Kerke, a Wanplas factory, supplies the full path from formula to production through the KTE series of twin-screw extruders, from the KTE-16B laboratory unit that develops your grade to the KTE-135D that compounds it by the tonne, all on a shared modular platform that makes scale-up predictable. The Wanplas group’s USD 500 free parts per year policy, shipment testing, on-site commissioning, and open-factory trial runs keep your qualified compound running on spec.
To take the next step, send your target properties and base resin to Kerke for a tailored screw configuration and a trial run of your exact formula on a KTE line. A bench trial on the KTE-16B will confirm the notched Izod, flexural modulus, and fiber length retention you need, and from there the same recipe maps cleanly to the production model that matches your volume. Visit the factory to watch your compound run, review the process recipe, and plan the scale-up with the engineering team.
