Underdrying usually means that moisture has not been sufficiently removed from part of the rubber. Opaque white spots may instead be non-rubber foreign matter, while tackiness can be associated with changed raw-material condition, excessive or uneven heating, abnormal stoppage or inadequate blending. Classify the defect first, preserve the lot and location records, and then investigate the process from raw-material acceptance through wet processing, trolley loading and drying.
Separate the three conditions
Underdrying
Underdried areas may appear as internal lumps or crumb-shaped regions whose colour, transparency, texture or moisture condition differs from the surrounding rubber. Product disposition should follow the applicable sampling, testing and nonconforming-product procedure rather than visual appearance alone.
Opaque white spots
Some opaque white spots are not wet rubber. Starch, talc, coating residue or other non-rubber matter can enter with poorly controlled raw material. If laboratory and traceability evidence identifies foreign matter, the corrective action belongs in supplier control, collection, acceptance and segregation rather than dryer temperature.
Tackiness
Tackiness may be local or widespread and may or may not accompany other property changes. Possible contributors include prolonged storage or sun exposure before processing, excessive or non-uniform heating, abnormal high-temperature residence during a stoppage, and inadequate blending of materials with different behaviour.
Common causes of underdrying
- Agglomerated crumb: stuck-together wet crumb restricts airflow and creates a longer path for internal moisture migration.
- Wide variation in crumb size or moisture: large and small crumb loaded together may require different drying conditions.
- Uneven or dense trolley loading: local overloading raises airflow resistance and can produce repeatable defect locations.
- Uneven airflow: fan condition, damper position, leakage, cabinet sealing, blocked screens and deposits can reduce or redirect air.
- Insufficient heat or effective residence time: heat-source output, actual airflow or trolley movement may not support the current load.
Set-point values on the operator screen are not enough. Compare the physical crumb and loading condition, equipment state, measured process data and defect location.
Common causes of tackiness
Raw material can change during long or unsuitable storage. Oxidative deterioration, high surface temperature or uncontrolled exposure may alter how the material behaves during drying. Within the dryer, excessive local heat, long exposure or non-uniform airflow can increase the risk of sticky rubber.
Stoppages require special investigation. If trolley movement stops while residual heat remains, one part of the load may experience abnormal residence. Restart procedures should identify affected trolleys and preserve the event record. A simple instruction to lower all temperatures may hide the actual cause.
When defects occur together
Underdrying and tackiness in the same lot can indicate a system-level variation rather than one temperature problem. Check whether materials of different origin, hardness or storage condition were blended unevenly; whether crumb, retained water or loading density varied across the trolley; and whether airflow, exhaust or the staged drying profile produced different conditions in different positions.
Associate each observation with the raw-material lot, trolley number, compartment, dryer position and operating record. Avoid changing several major parameters at once, because that makes the real cause difficult to identify.
Why white spots need separate investigation
First determine whether the white area is insufficiently dried rubber or foreign matter. Its appearance, position, raw-material history and appropriate laboratory examination can help make that distinction. Non-rubber material will not be removed by higher temperature or longer drying, and unnecessary heating can introduce an additional quality risk.
Recommended troubleshooting sequence
- Isolate and identify: hold the affected lot and preserve time, trolley, compartment and batch information.
- Describe the condition: record whether it is crumb-shaped, lump-shaped, local or widespread and where it appears in the bale.
- Sample and test: use the applicable sampling, preparation and test procedures.
- Review raw material: check origin, storage, acceptance, contamination and blending records.
- Review the wet line: check creping, granulation, agglomeration, retained water and trolley filling.
- Review the dryer: check sensors, heat supply, fans, ducts, exhaust, sealing, trolley interval and stoppage events.
- Verify corrective action: define the changed factor, preserve a comparison lot and evaluate finished-product results over continued operation.
Frequently asked questions
Why can the same bale contain both underdried and tacky areas?
Material or airflow may not be uniform. Different raw-material conditions, crumb sizes, loading densities and local heat or airflow can create different results within one load.
Should the entire dryer temperature be raised as soon as underdrying is found?
Not before diagnosis. Higher temperature may leave a local loading or airflow problem unresolved while overheating the rest of the rubber.
Can an opaque white spot be dried out?
Not if it is non-rubber foreign matter. Confirm the nature of the spot and then correct the relevant raw-material or process-control source.
Technical reference: Introduction to Natural Rubber Primary Processing Technology, Chapter 4 and the drying-technology sections. This article does not replace a plant's inspection and nonconforming-product procedures. Content prepared by the Sinyoung technical team.