Laboratory small powder tablet presses are widely used for sample preparation in materials, chemical, pharmaceutical and powder research fields to prepare standard pellets. During sample compression, powder tends to adhere to the punch surface, resulting in pits, scratches and defects on the surface of pressed samples. In severe cases, samples stick tightly to upper or lower punches and break directly during demolding, so intact samples cannot be obtained. For laboratory sample preparation, the sample amount per run is small with fine powder. Many researchers simply increase compression pressure when punch sticking occurs. This method fails to solve the problem and may even scratch punches and jam molds. Punch sticking is affected by powder characteristics, mold status, compression process and sample storage environment. Four practical solutions are listed below.

Solution 1: Preprocess Powder Samples

Powder property is the primary cause of sticking in laboratory tests. Fine powder, moist samples and low-melting-point powder easily stick to molds. First, test the moisture content of powder. Moist samples show high plasticity and adhere to punches after compression. Damp samples can be dried in an oven at low temperature, then cooled and sealed in a desiccator. Avoid moisture absorption from air before pressing. Second, add a small amount of release agent such as stearic acid or magnesium stearate and mix fully with powder. The additive forms an isolation layer on powder surface and reduce adhesion between powder and metal punches. Keep additive dosage low to avoid affecting sample performance. Third, pre-granulate ultrafine powder. Ultrafine powder has large specific surface area and tends to stick. Mild granulation and screening can reduce the proportion of ultrafine powder and improve demolding performance.

Solution 2: Inspect, Clean and Maintain Punches & Molds

Small tablet presses have simple molds. Surface finish of punch tips directly affects demolding effect. First, take out molds after shutdown and inspect upper and lower punch surfaces. If scratches, corrosion spots or caked residual samples exist, polish gently with polishing cloth to restore mirror finish. Replace deformed or cracked punches immediately and discontinue use. Second, clean mold cavity thoroughly. Caked residual powder from previous tests will scratch samples and cause repeated sticking. Clean the cavity with brush and dust-free paper after each test. Finally, check fitting clearance between punches and die sleeve before use. Too small clearance traps powder, while excessive clearance leads to powder overflow. Stop using mismatched molds.

Solution 3: Adjust Compression Process Parameters

With qualified samples and molds, adjust compression parameters to reduce sticking risk. First, appropriately reduce compression pressure. Excess pressure leads to plastic deformation of powder, making powder tightly attach to punches and hard to demold. Use the minimum forming pressure that can keep samples intact. Second, adjust holding time. Maintain pressure for several seconds before slow pressure release, to fully release powder stress and reduce rebound adhesion. Do not release pressure and take samples immediately after compression. Third, control powder filling amount. Avoid overfilling, because overflow fine powder adheres to punch tips and worsens sticking. Keep consistent filling quantity for each test.

Solution 4: Control Laboratory Environment

Laboratory temperature and humidity are often overlooked. Powder absorbs moisture rapidly under high humidity, greatly increasing sticking risk. The relative humidity of laboratory environment is recommended at 45%~60%. Turn on dehumidifier when humidity rises. Store powder samples in desiccators. Load powder into mold and press quickly after sampling to shorten air exposure time. Keep the lab bench clean. Floating fine powder accumulates on punch surfaces and worsens sticking. Clean residual powder on molds and punches after every test.

Summary

When punch sticking appears on small laboratory powder press, do not blindly raise pressure. The recommended troubleshooting sequence: powder pre-treatment → mold cleaning and inspection → compression pressure & holding time → laboratory environment. Run small trial tests before switching samples to predict sticking risk in advance. It reduces sample waste, protects punches and molds, and improves the success rate of sample preparation.