Views: 0 Author: Site Editor Publish Time: 2026-08-25 Origin: Site
Sheet Molding Compound (SMC) compression molding is widely used to manufacture strong, lightweight and dimensionally stable composite parts. Typical applications include electrical enclosures, automotive components, battery housings, sanitary products, access covers and structural panels.
Although SMC is suitable for high-volume production, molding defects can still occur when the material formulation, mold design, charge pattern or processing parameters are not properly matched. Common problems include surface porosity, blisters, cracks, short shots, fiber exposure, warpage and dimensional instability.
This guide explains the most common SMC compression molding defects, their possible causes and practical solutions. It can help molders troubleshoot production problems and select a more suitable SMC material for each application.
Table of Contents
Sheet Molding Compound is a ready-to-mold composite material consisting mainly of:
· Thermosetting resin, usually unsaturated polyester or vinyl ester
· Chopped glass fiber reinforcement
· Mineral fillers
· Low-shrink or low-profile additives
· Thickening agents
· Mold release agents
· Pigments
· Initiators and other functional additives
During compression molding, a pre-weighed SMC charge is placed in a heated mold. The press closes and applies pressure, causing the material to flow and fill the mold cavity. Heat activates the curing reaction, and the material solidifies into the required shape.
The quality of the finished SMC molded part depends on several interacting factors:
1. SMC formulation and material maturity
2. Material storage and handling
3. Charge weight, shape and placement
4. Mold temperature and temperature uniformity
5. Pressing pressure and closing speed
6. Mold design and venting
7. Curing time
8. Part design and thickness distribution
A defect is rarely caused by only one factor. Effective troubleshooting requires checking the entire molding process instead of changing one parameter repeatedly without identifying the underlying cause.
Surface porosity appears as small holes, pits or cavities on the molded surface. The defect may become more visible after sanding, painting or applying a glossy coating.
· Air trapped inside the SMC charge
· Insufficient mold venting
· Moisture or volatile substances in the material
· Poor charge placement
· Excessively fast mold closing
· Inadequate molding pressure
· Material flow over an excessively long distance
· Improper resin or additive formulation
· Contamination on the material or mold surface
· Store SMC in sealed packaging under the recommended conditions.
· Allow refrigerated SMC to reach the processing temperature before opening the packaging to prevent condensation.
· Optimize the SMC charge pattern to reduce folded material and trapped air.
· Reduce the initial mold closing speed so air has enough time to escape.
· Check and clean vents regularly.
· Adjust molding pressure to achieve complete material consolidation.
· Reduce unnecessary material flow by increasing charge coverage.
· Review the formulation if porosity occurs consistently across different molds.
· Keep the mold and work area free from dust, oil and moisture.
For Class A or other appearance-sensitive components, porosity control must begin with both the SMC formulation and the mold design. Processing adjustments alone may not completely solve a formulation-related surface problem.
Blisters are raised areas on the surface of the molded component. They may appear immediately after demolding or during painting, post-curing or exposure to elevated temperatures.
· Entrapped air or gas
· Moisture in the SMC
· Volatile substances released during curing
· Premature curing of the surface
· Insufficient curing time
· Uneven mold temperature
· Excessive part thickness
· Internal delamination
· Excessively rapid mold closing
· Check the storage history and remaining shelf life of the SMC.
· Prevent condensation when moving material from cold storage into the production area.
· Improve venting in areas where air is likely to accumulate.
· Reduce closing speed during the air-release stage.
· Verify that the upper and lower mold temperatures are uniform.
· Increase the curing time when internal curing is incomplete.
· Review thick sections and sudden thickness transitions in the part design.
· Avoid charge layouts that create multiple converging flow fronts.
· Select an SMC formulation with a curing system suitable for the part thickness and molding temperature.
If blisters appear only after painting or baking, the molded part may contain hidden porosity or residual volatiles. It is therefore important to inspect both the molding process and the subsequent coating temperature.
A short shot occurs when the SMC fails to fill the entire mold cavity. Corners, ribs, bosses or thin sections may be incomplete.
· Insufficient charge weight
· Poor charge positioning
· Low mold temperature
· Insufficient molding pressure
· Material curing before the cavity is completely filled
· Excessively slow mold closing
· Long or restricted flow paths
· SMC viscosity that is too high
· Inadequate press capacity
· Poor venting that creates trapped-air resistance
· Confirm the required charge weight and maintain consistent weighing accuracy.
· Place the charge near the center of the required flow path.
· Increase the charge coverage when possible.
· Check whether the mold temperature is within the recommended processing range.
· Increase pressure if the press and mold are designed for it.
· Optimize closing speed to complete flow before significant curing begins.
· Clean blocked vents and add vents to difficult filling areas if necessary.
· Review thin walls, deep ribs and sudden geometry changes.
· Use an SMC grade with better flowability for complex components.
· Confirm that the press can provide sufficient force across the projected area of the part.
Adding more material is not always the best solution. Excessive charge weight may create flash, fiber orientation problems or internal stress. Charge shape and placement should be optimized together with charge weight.
Flash is a thin layer of cured SMC extending beyond the intended edge of the molded part. A small amount may be acceptable, but excessive flash increases trimming costs and may indicate a molding problem.
· Excessive charge weight
· Molding pressure that is too high
· Worn or damaged mold parting surfaces
· Incorrect mold alignment
· Uneven charge placement
· Material with excessively low viscosity
· Improper press parallelism
· Foreign material on the parting line
· Reduce and control the charge weight.
· Optimize charge placement to avoid concentrating material near the mold edges.
· Adjust molding pressure within the qualified process window.
· Inspect the mold parting line for wear or damage.
· Check mold alignment and press platen parallelism.
· Clean the parting surfaces before molding.
· Review SMC viscosity and maturity.
· Repair the mold if the shut-off surfaces can no longer maintain sufficient sealing.
Repeated flash in one specific area usually indicates a mold alignment, parting-line or charge-distribution problem rather than a general pressure problem.
Glass fibers are visible on the surface, or their pattern can be seen through the resin-rich surface layer. This defect is especially undesirable for visible automotive, sanitary or painted parts.
· Insufficient resin-rich surface
· Excessive material flow
· Poor fiber wet-out
· High glass fiber content
· Improper low-shrink or low-profile system
· Excessive molding pressure
· Uneven mold temperature
· Inadequate charge coverage
· Fiber bundles separating during flow
· Increase charge coverage to shorten the flow distance.
· Optimize the charge pattern to avoid excessive shear.
· Review the glass fiber content and fiber length.
· Improve fiber wet-out during SMC production.
· Adjust the resin, filler and low-profile additive system.
· Check mold temperature uniformity.
· Use an SMC grade developed for appearance-sensitive components.
· Consider an in-mold coating or suitable surface treatment when very high surface quality is required.
High mechanical strength and excellent surface appearance do not always require the same formulation. The glass content, fiber architecture and shrink-control system must be balanced according to the final application.
Cracks may occur around inserts, ribs, corners, holes or thick-to-thin transitions. They may appear during demolding, machining, assembly or service.
· Incomplete curing
· Excessive internal stress
· Sharp corners or poor part geometry
· Uneven wall thickness
· Excessive ejection force
· Part removal before sufficient curing
· Incorrect insert temperature or design
· Poor material flow around inserts
· Excessive shrinkage
· Inappropriate glass fiber orientation
· Mechanical damage during trimming
· Increase curing time or verify the actual mold temperature.
· Add suitable radii to sharp corners.
· Avoid abrupt thickness changes whenever possible.
· Improve material flow around ribs, bosses and inserts.
· Ensure ejector pins are properly positioned and operate uniformly.
· Delay demolding if the part is still too weak or too hot.
· Preheat inserts when required by the qualified process.
· Use an SMC formulation with improved toughness or lower shrinkage.
· Optimize trimming and drilling parameters.
· Confirm that the component design is suitable for compression-molded SMC.
Cracks around metal inserts are often caused by the difference in thermal expansion and shrinkage between the metal and the composite. Insert geometry, surface condition, temperature and surrounding laminate thickness should be evaluated together.
The finished SMC part bends, twists or fails to remain flat after demolding and cooling. Warpage may make assembly difficult or cause dimensional rejection.
· Uneven shrinkage
· Non-uniform mold temperature
· Unbalanced fiber orientation
· Uneven charge placement
· Large differences in wall thickness
· Part ejected too early
· Uneven cooling
· Poor rib design
· Incorrect stacking or handling of hot parts
· Unsuitable low-shrink or low-profile formulation
· Check temperature uniformity across both mold halves.
· Use a balanced and repeatable charge layout.
· Reduce excessive material flow that may align fibers in one direction.
· Improve the symmetry of the part and rib structure.
· Minimize abrupt wall-thickness changes.
· Increase curing time before ejection.
· Use cooling fixtures when necessary.
· Stack or support parts correctly during cooling.
· Select a low-shrink or low-profile SMC grade for tight dimensional requirements.
· Measure parts only after they have reached a stable temperature.
The direction of warpage can provide useful diagnostic information. If the distortion changes when the charge orientation changes, fiber orientation and flow are likely to be major contributing factors.
Sink marks are shallow depressions on the visible surface, often opposite ribs, bosses or other thick sections.
· Localized thick areas
· Uneven shrinkage
· Insufficient pressure during curing
· Inadequate low-shrink performance
· Non-uniform temperature
· Poor rib-to-wall thickness ratio
· Insufficient curing
· Reduce unnecessary thickness behind the visible surface.
· Optimize rib and boss dimensions.
· Maintain sufficient pressure during the curing stage.
· Check mold temperature uniformity.
· Increase curing time if required.
· Use a low-shrink or low-profile SMC formulation.
· Consider relocating heavy ribs away from critical appearance surfaces.
Part design has a major effect on sink marks. A material change may reduce the problem, but it cannot always compensate for excessively thick ribs or bosses.
Layers within the molded SMC separate, creating internal cracks, weak regions or blister-like surface defects.
· Air trapped between charge layers
· Contamination between SMC sheets
· Dried or aged material
· Poor bonding between separate flow fronts
· Insufficient pressure
· Incomplete curing
· Moisture
· Incorrect charge stacking
· Avoid trapping air when stacking multiple SMC sheets.
· Keep protective films, dust, oil and other contaminants out of the charge.
· Use material within its recommended shelf life.
· Store and condition the SMC correctly.
· Increase pressure where appropriate.
· Improve charge placement to reduce cold weld lines.
· Verify curing time and mold temperature.
· Review the formulation if interlaminar bonding remains weak.
Operators should not fold or stack charges casually. A repeatable charge-preparation procedure is essential for stable SMC molding quality.
Visible lines or changes in color and texture appear where two or more material flow fronts meet. These regions may also have lower mechanical strength.
· Poor charge positioning
· Multiple separated charges
· Complex cavity geometry
· Low mold temperature
· Premature curing
· Restricted flow around inserts or ribs
· Pigment separation during long flow
· Inadequate pressure
· Redesign the charge pattern so critical flow fronts do not meet in highly stressed or visible areas.
· Increase charge coverage.
· Optimize the mold temperature and closing speed.
· Improve venting near the meeting point.
· Adjust the position of inserts, ribs or gates in the component design where possible.
· Use an SMC material with appropriate flow and curing characteristics.
· Evaluate the strength of weld-line areas during product validation.
A weld line is not only a cosmetic issue. For structural SMC components, its position and mechanical performance should be considered during mold-flow planning and testing.
The part adheres to the mold, requires excessive ejection force or suffers surface damage during removal.
· Insufficient internal or external mold release
· Dirty or damaged mold surface
· Incomplete curing
· Incorrect mold temperature
· Poor draft angle
· Undercuts or unsuitable part geometry
· Uneven ejector distribution
· Excessive surface roughness
· Check the internal mold-release level in the SMC formulation.
· Clean and maintain the mold surface.
· Apply a compatible external release agent when required.
· Increase curing time if the part is insufficiently cured.
· Verify the mold temperature.
· Add adequate draft angles.
· Improve ejector pin placement and synchronization.
· Repair damaged mold surfaces.
Excessive use of external mold release may contaminate parts and interfere with painting or bonding. The release system should therefore be optimized rather than applied without control.
The molded component shows uneven color, dark spots, streaks, yellowing or visible contamination.
· Uneven pigment dispersion
· Material from different batches mixed together
· Excessive mold temperature
· Excessive curing time
· Contamination
· Degraded or expired SMC
· Poor mold cleaning
· Uneven material flow
· Incompatible additives
· Use consistent production batches for appearance-critical components.
· Check pigment dispersion during SMC production.
· Control mold temperature and curing time.
· Protect the material from dust, oil and moisture.
· Follow first-in, first-out inventory management.
· Avoid mixing aged and fresh materials without validation.
· Clean the mold and handling equipment.
· Review additive compatibility if staining continues.
When a molding defect occurs, random parameter changes can create additional variation. A structured troubleshooting process is more effective:
1. Record the defect location, frequency and production time.
2. Determine whether the problem affects one cavity, one mold or all production.
3. Check the SMC batch number, storage time and material condition.
4. Verify charge weight, dimensions, stacking and placement.
5. Record actual mold temperatures rather than relying only on controller settings.
6. Check pressure, closing speed and curing time.
7. Inspect vents, ejectors and parting surfaces.
8. Compare defective parts with approved production samples.
9. Change only one major variable at a time.
10. Document the result and establish a validated processing window.
This approach helps determine whether the defect comes primarily from the SMC material, molding equipment, mold design or operating method.
The best defect-control strategy begins before mass production. SMC material should be selected according to the component’s actual requirements, including:
· Mechanical strength
· Glass fiber content
· Surface quality
· Shrinkage control
· Flow distance
· Flame resistance
· Electrical insulation
· Water absorption
· Chemical resistance
· Heat resistance
· Color and UV stability
· Dimensional tolerances
· Final molding cycle
A standard SMC formulation cannot deliver the best performance for every application. Electrical enclosures, automotive parts, sanitary products and structural covers may require very different combinations of resin, reinforcement, filler and additives.
JLON supplies Sheet Molding Compound for a wide range of compression-molded composite products. Instead of offering only one fixed SMC grade, JLON can work with customers to adjust the formulation according to the part design, molding conditions and end-use requirements.
Available options can include:
· General-purpose SMC
· Low-shrink SMC
· Low-profile SMC
· Flame-retardant SMC
· Electrical insulation SMC
· High-strength SMC
· Water-resistant SMC
· Heat-resistant SMC
· Chemical-resistant SMC
· SMC for automotive and transportation components
· SMC for electrical enclosures and meter boxes
· SMC for battery and energy-storage housings
· SMC for sanitary and construction products
· SMC for access covers and structural panels
To recommend a suitable JLON SMC material, customers should provide as much of the following information as possible:
· Finished part drawing
· Part dimensions and weight
· Wall thickness
· Required color and surface quality
· Mechanical properties
· Flame-retardant standard
· Electrical requirements
· Operating temperature
· Chemical or weather exposure
· Press capacity
· Mold temperature
· Expected molding cycle
· Existing defects or rejected-part photographs
The more complete the application information is, the easier it is to match the SMC formulation with the mold and production process.
Most SMC compression molding defects result from an interaction between material properties, charge design, mold condition and processing parameters. Surface porosity may be related to moisture or poor venting, while short shots may result from insufficient charge coverage, low flowability or premature curing. Warpage and cracks often involve both part design and shrinkage control.
For reliable production, molders should establish a controlled process window and use an SMC grade developed for the specific application. JLON provides SMC materials and formulation support for electrical, automotive, energy, sanitary, construction and industrial composite components.
If you are experiencing SMC molding defects or developing a new compression-molded part, contact JLON with your drawings, performance requirements and processing conditions. Our team can help evaluate the application and recommend a suitable Sheet Molding Compound solution.
