Which materials are most easily degraded during hot runner heating?

I. Materials with High Degradability

Polyvinyl chloride (PVC)

Degradation Trigger Temperature Above 180℃, rapid breakdown. If the temperature of a local section of the hot runner is above 200℃, a substantial volume of corrosive hydrogen chloride gas is emitted within minutes.

Decomposition Hazards: Decomposition products damage the inner wall of conventional stainless steel runners in hot runners, and cause yellow streaks and black spots on the injection molded parts. In extreme circumstances this will generate enormous amounts of smoke.

POM (Polyoxymethylene)

Degradation Trigger Temperature: If the processing temperature is above 240℃, or the temperature is kept above 220℃ for 30 minutes or more, chain depolymerization will begin.

Degradation hazards: Rapid decomposition leads to the formation of formaldehyde gas, a rapid pressure rise in the runner may cause material to spray out, and injection-molded parts may have noticeable silver streaks and bubbles.

 

II. Medium-High Degradable Materials

PET (Poly Ethylene Terephthalate)

Degradation Triggering Conditions: When the material moisture content is higher than 50ppm during heating process, and the temperature is higher than 280℃, high temperature hydrolytic degradation will occur.

Hazards of Degradation The breaking of the molecular chains leads to a great reduction of the melt strength . This leads to insufficient transparency, haziness of surface, and rapid reduction of the mechanical characteristics in injection molded parts.

TPU (Thermoplastic Polyurethane)

Degradation Triggering Temperature: Molecular chain breakdown degradation occurs when the temperature is above 230℃ and residence duration is above 20 minutes.

Degradation Hazards Yellowing of melt. Bubbles and pitting of surface of injection molded items. Loss of elasticity.

 

III. Medium Readily Decomposable Materials

PC (Polycarbonate)

deterioration Triggering Conditions: When the material is not completely dried after heating, and the moisture content is too high and the temperature is higher than 330℃, hydrolytic deterioration will occur.

Degradation Hazards Silver streaking and yellowing on surface of injection molded parts, loss of transparency, and in extreme situations black patches. PA66 (Nylons 66)

Degradation Triggering Conditions : Thermal oxidative degradation will happen if the temperature during the heating process is more than 320℃ or the material is maintained at high temperature for a long length of time .

Degradation Hazards: Embrittlement, decline in mechanical strength of the injection molded parts because of abrupt decrease in melt viscosity.

 

IV. Common Features of Degradation Risks

If it is this type of easily degradable material , the deterioration trigger time will be considerably accelerated if localized hot spots happen during the hot runner heating process , if the melt stays in dead zones for long periods of time , or if the material is not dried sufficiently . The heating curve and holding time should be strictly controlled.