Sinterable mass of polymer powder having resistance to caking and method of preparing the mass

A laser sinterable mass of partially coated particles of substantially amorphous poly(vinyl chloride) PVC particles comprising,

• PVC particles no greater than 177 .mu., having a bulk density in the range from 0.35 to 0.6 gm/cm.sup.3, an initial T.sub.g occurring at about 65.degree. C. and a final T.sub.g occurring at about 88.degree. C.;
• and,
• a coating powder having coating particles which coat said PVC particles leaving at least some portion thereof uncoated;
• said coating powder is selected from the group consisting of a substantially amorphous coating powder and a crystalline coating powder each having an average diameter in the range from 0.02 --44 .mu. but smaller than said PVC particles, a bulk density in the range from 0.15 to 0.35, and, an initial T.sub.g higher than 100.degree. C.;
• said amorphous coating powder having an initial T.sub.g and sintering temperature higher than said final T.sub.g of said PVC particles, and said crystalline coating powder having a crystalline melting point and sintering temperature higher than said final T.sub.g of said PVC particles;
• said partially coated particles of PVC having an increased bulk density at least 10% higher than that of said host PVC powder, being essentially free from electric charge, and flowable under applied force in the range from about 10 g-force to about 100 g-force, without caking at a storage temperature above 65.degree. C. but below 130.degree. C.;
• and,
• said increased bulk density is obtained by blending said PVC powder with said coating powder with an energy input greater than 30 watt-hr/lb of said PVC powder, so as to discharge said partially coated PVC powder at a drop temperature above 65.degree. C. , but no higher than 88.degree. C.

A laser sinterable mass of partially coated particles of substantially amorphous polycarbonate (PC) particles comprising,

• host PC particles no greater than 177 .mu., having a bulk density in the range from 0.33 to 0.55 gm/cm.sup.3, an initial T.sub.g occurring at about 130.degree. C. and a final T.sub.g occurring at about 171.degree. C.;
• and
• a coating powder having coating particles which coat said PC particles leaving at least some portion thereof uncoated;
• said coating powder is selected from the group consisting of a substantially amorphous coating powder having an initial T.sub.g higher than 200.degree. C., and a crystalline coating powder each having an average diameter in the range from 0.02 .mu.-44 .mu. but smaller than said PC particles, a bulk density in the range from 0.15 to 0.35 gm/cm.sup.3, and, said partially coated particles of PC having an increased bulk density at least 10% higher than that of said host PC powder, being essentially free from electric charge, and flowable under applied force in the range from about 10 g-force to about 100 g-force, without caking at a storage temperature above 130.degree. C. but below 171.degree. C.;
• and, said increased bulk density is obtained by blending said PC powder with said coating powder with an energy input greater than 30 watt-hr/lb of said PC powder, so as to discharge said partially coated PC powder at a drop temperature above 130.degree. C., but below 171.degree. C.

A process for preparing a mass of sinterable particles comprising,

• (a) charging a mixture of powder particles to a blending zone, said mixture consisting essentially of
• (i) a thermoplastic synthetic resinous host powder having a glass transition temperature (T.sub.g) occurring over a range of at least 3+ C., and an average host particle diameter no greater than about 177 .mu., and,
• (ii) a coating powder selected from the group consisting of a substantially amorphous coating powder and a crystalline coating powder each having an average particle diameter in the range from 0.02 .mu.-44 .mu. but smaller than host particles of said host powder, and a bulk density in the range from 0.15 to 0.35 gm/cm.sup.3.
• (b) blending said mixture of powder particles at high shear in said blending zone with energy in excess of about 30 watt-hr per lb of host powder until said mixture of powder particles reaches a chosen-drop temperature above (initial T.sub.g -20.degree. C.) but below said host powder's final T.sub.g ;
• and, (c) discharging a blended mixture of partially coated host particles having at least some portion of their surfaces left uncoated, and the remaining portion coated with said coating particles, a bulk density at least 10% higher than that of said host powder, said blended mixture being flowable and storable without caking at a storage temperature above the initial T.sub.g of said host powder but below its final T.sub.g at which storage temperature said host powder would not be flowable as an uncoated powder.