Compression molding remains a cornerstone process for manufacturing durable plastic and rubber components, yet surface defects like sink marks and internal voids can compromise both mechanical integrity and aesthetic quality. Eliminating these imperfections demands a systematic understanding of material behavior, mold design, and processing parameters. This article provides a comprehensive technical guide to identifying root causes and implementing proven countermeasures to produce defect-free compression molded parts.

Understanding Sink Marks and Voids

Sink marks appear as shallow depressions on the surface of a molded part, typically occurring at thick sections or behind ribs and bosses. They result from differential shrinkage between the outer surface layer, which cools and solidifies first, and the inner core that remains molten longer. As the core cools and shrinks, it pulls the already rigid surface inward, creating a visible dip. The severity of sink marks depends on the magnitude of shrinkage, the stiffness of the surface layer, and the pressure applied during the holding phase.

Voids, by contrast, are empty cavities trapped within the material—not visible on the surface but detectable through sectioning or non-destructive testing. They form when air, moisture vapor, or volatiles become encapsulated during the compression stroke, or when the material’s volumetric shrinkage is so large that it creates a vacuum pocket in the thickest region. While sink marks affect appearance and may indicate structural weakness, voids directly reduce load-bearing capacity and can lead to premature failure under stress. Both defects share common precursors: improper material preparation, inadequate mold filling, insufficient pressure, or non‑uniform cooling.

Root Causes of Sink Marks

Non‑uniform Wall Thickness

Abrupt transitions from thin to thick sections create areas where the core remains molten long after the surrounding material has solidified. The thicker region shrinks more and is not constrained by neighboring solid material, resulting in a sink mark. This is the single most common design cause.

Insufficient Packing Pressure

After the mold cavity is filled, additional pressure must be applied to densify the material and compensate for thermal shrinkage. If the packing pressure is too low or applied for too short a duration, the core can contract without being replenished by molten material, leaving a depression.

High Material Shrinkage

Some resins shrink more than others. Semi‑crystalline polymers, for example, undergo significant volumetric reduction as they crystallize. Without proper process compensation, they are prone to sink marks even in well‑designed molds.

Slow Surface Cooling

If the mold surface temperature is too high, the skin layer remains semi‑molten, allowing the core to pull it inward as it shrinks. Conversely, if cooling is too rapid on one side, the part may warp but not necessarily sink. Balanced cooling is key.

Preventing Sink Marks: Design and Process Strategies

Mold Geometry Rules

  • Maintain uniform wall thickness throughout the part. Where changes are unavoidable, use gradual tapers rather than steps.
  • Limit maximum thickness to the minimum necessary for structural requirements. A typical rule of thumb: keep thickness under 4 mm for many thermoplastics.
  • Locate ribs and bosses in areas that can be filled with a separate core or use hollow designs. When solid ribs are required, keep rib thickness at 50–60% of the nominal wall.
  • Add radius to internal corners to reduce stress concentrations and promote even flow.

Optimizing Packing Pressure and Time

Compression molding presses must be capable of delivering a holding pressure that is 1.5–2 times the injection pressure. The packing phase should continue until the gate or parting line freezes; otherwise, backflow can occur. Use pressure sensors in the cavity to verify that the packing profile is sufficient. For materials with high shrinkage, consider multi‑stage packing—starting high and then stepping down gradually to avoid over‑packing.

Controlling Cooling Uniformity

Design cooling channels to follow the part contour as closely as possible. Use turbulent flow in channels to maximize heat transfer. If the mold has both steel and copper alloys, balance thermal conductivity—aluminum or beryllium‑copper inserts in thick sections can accelerate cooling. Monitor coolant temperature and flow rate to achieve a stable thermal cycle. Plastics Technology’s guide on sink marks offers practical troubleshooting steps.

Material Selection

Choose resins with low shrinkage if dimensional stability is critical. Amorphous polymers (e.g., ABS, PC) generally shrink less than semi‑crystalline ones (e.g., PP, PA). For existing molds, consider switching to a material with added mineral fillers or glass fibers, which reduce overall shrinkage. Verify material data sheets for shrinkage values and molding conditions.

Preventing Voids in Compression Molded Parts

Moisture and Volatiles Management

Hygroscopic materials such as nylon, PET, and PC must be dried thoroughly before molding. Undried resin releases steam during melting, which gets trapped inside the part as bubbles. Follow the supplier’s recommended drying temperature and time; use a moisture analyzer to confirm that the material is below the critical threshold (typically 0.02–0.05%). This article on drying polymers explains the risks of insufficient pre‑conditioning.

Air Entrapment and Mold Venting

During compression, air must escape from the cavity. Inadequate venting causes air to be compressed into the material, forming voids. For compression molds, vents can be machined as shallow channels (0.01–0.05 mm deep) at the parting line or around cavity inserts. Place vents at the last area to fill, which is typically opposite the charge location. Also consider vacuum venting systems for critical applications where trapped air is persistent.

Proper Material Flow and Degassing

If the charge is too large or placed asymmetrically, the material may fold over and trap air. Use a charge that is slightly smaller than the cavity volume and allow it to flow uniformly. Some processes benefit from a degassing step—opening the mold slightly during the initial compression to release trapped air, then closing fully for final forming. Compression speed should also be controlled: too fast and air cannot escape; too slow and the material may prematurely cure or cool.

Temperature and Pressure Balance

High melt temperatures reduce viscosity and help collapse voids, but they also increase shrinkage. Conversely, low mold temperatures may cause the surface to freeze before air escapes, sealing voids inside. Find the processing window where viscosity is low enough for complete material filling and air evacuation, yet cooling is controlled to prevent sink marks. Pressure must be sufficient to compress any remaining gas bubbles to a negligible size; typically, holding pressures above 100 bar are required for void elimination in thermoplastics.

Mold Design Principles for Defect‑Free Parts

A well‑designed mold prevents both sink marks and voids simultaneously. Key principles include:

  • Balanced filling – The charge location should ensure that material reaches all cavity extremities at the same time. Use flow simulation software to predict filling patterns.
  • Consistent cooling – Cooling channels must be sized and routed to extract heat evenly. For complex geometries, consider conformal cooling produced by additive manufacturing.
  • Sufficient venting – Even with a good charge placement, trapped air can occur. Include multiple venting slots or a vacuum port.
  • Rigid construction – The mold must withstand high clamping pressures without deflection, which could create thin sections that cool too fast and thick sections that sink.

Processing Parameter Optimization

Temperature Control

Set the mold temperature according to the material supplier’s recommended range. For semi‑crystalline polymers, a mold temperature near the crystallization temperature promotes uniform shrinkage. A rule of thumb: higher mold temperature reduces sink marks but may increase cycle time and voids. Lower mold temperature accelerates surface freezing, increasing sink mark risk but potentially reducing void formation if venting is adequate. Each material requires careful tuning.

Pressure Profiles

Compression molding often uses a two‑stage pressure profile: a high initial pressure to force material into the cavity, followed by a lower hold pressure. The transition timing is critical. For void‑prone materials, a short burst of high pressure during the degassing step can collapse bubbles. Pressure should be maintained until the material temperature drops below its solidification point.

Cooling Rate and Cycle Time

Rapid cooling increases residual stress and sink marks; slow cooling improves dimensional stability but lengthens cycles. Use a controlled cooling rate—for example, 10–20 °C per minute for glass‑filled nylon—and monitor core temperature with thermocouples. Adjust cycle time based on part geometry; thicker sections require longer cooling. Over‑cooling wastes time, while under‑cooling leads to post‑mold shrinkage.

Material Considerations and Pre‑conditioning

Material selection is the first line of defense. Low‑shrinkage grades, impact‑modified resins, or materials with nucleating agents for finer crystallinity can reduce sink marks. Pre‑conditioning is equally critical: drying, preheating, and sometimes blending with additives (e.g., lubricants to improve flow) must be performed to spec. For rubber compounds, ensure that the batch is homogeneous and that vulcanization accelerators are well dispersed to avoid localized gassing.

Semi‑crystalline polymers are particularly sensitive to cooling rate because crystallization continues after the part is removed from the mold. To stabilize dimensions, consider annealing parts after demolding at a temperature just below the crystalline melting point. This allows slow crystallization to proceed without causing voids.

Troubleshooting Common Defects

IssuePossible CauseSolution
Sink marks at ribsRib too thick or too close to surfaceReduce rib thickness to 50% of nominal wall; add draft for ejection
Sink marks opposite gatesPacking pressure lost before gate freezesIncrease hold pressure or extend hold time; reduce mold temp near gate
Voids in thick sectionsThermal shrinkage overwhelms packingUse a longer packing stage; consider using a material with lower shrinkage; add internal cooling channels
Small surface bubblesMoisture or volatilesDry material thoroughly; check regrind contamination
Large internal cavitiesAir entrapment or severe shrinkageImprove venting; reduce compression speed; degas

For a more extensive troubleshooting matrix, consult ScienceDirect’s overview of compression molding defects.

Quality Assurance and Process Control

Preventing sink marks and voids requires continuous monitoring. Key quality metrics include:

  • Part weight – A consistent weight indicates stable material density and minimal voids.
  • Visual inspection – Trained operators can spot surface sink marks under directional lighting.
  • Sectional analysis – Cross‑sectioning representative parts reveals internal voids and allows measurement of wall thickness uniformity.
  • Non‑destructive testing – X‑ray or ultrasonic scanning can detect subsurface voids without destroying parts.

Process parameters should be recorded for every cycle using a data acquisition system. When deviations occur—such as a sudden increase in sink marks—engineers can review the logged pressures, temperatures, and cycle times to identify the root cause. Implementing statistical process control (SPC) charts for part weight and thickness is a proven method to catch trends before defects become scrap.

Additionally, periodic mold maintenance is critical. Worn vents, blocked cooling channels, or damaged parting lines all contribute to defect formation. Perform regular cleaning of vent slots and check cooling channel flow rates. MoldMaking Technology’s article on defect prevention provides further maintenance best practices.

Conclusion

Sink marks and voids in compression molding stem from three interrelated factors: part and mold design, material behavior, and processing conditions. By methodically addressing each area—optimizing wall thickness, ensuring uniform cooling, proper material drying, sufficient venting, and accurate pressure profiles—manufacturers can produce parts that meet both structural and cosmetic requirements. Investing in mold flow simulation, process monitoring, and quality inspection tools reduces scrap rates and boosts overall equipment effectiveness. With a disciplined approach to defect prevention, compression molding remains a reliable, high‑quality production method for demanding applications.