Troubleshooting Guide

Poor screw or nozzle design

Pull and clean the screw and barrel. Check for carbon buildup behind the screw threads in the screw conveying zone and metering zone. Most general - purpose screws have this problem because there are dead zones behind the screw threads. General - purpose screws are not very suitable for injection molding of acrylic, polycarbonate, and SAN lenses. Special - designed dead - angle - free screws should be used to make the melt uniform. Check the check valve, nozzle tip cover, and nozzle tip to prevent sudden changes in the melt flow path. See "Screw Design".

Nozzle is too long

Determine that the nozzle length is as short as possible, which is controlled by a PID temperature controller.

Incorrect or poor hot runner design

Check the open or incorrect thermocouple values. Repair and replace those percentage - type thermocouples or autotransformers. Redesign the hot runner. Check the watt power density and heat source location. Check the position of the thermocouple.

High melt temperature: hot zones in the screw or barrel

Use a thermal probe technique or an appropriate IR (infrared) sensor to check the melt temperature. IR sensors are generally not very accurate. Calibrate the temperature only when necessary. Check the hot zones of the screw and barrel. Are there any areas that exceed the set temperature? If so, adjust that area or the area before it. Reduce backpressure. Slow down the screw RPM.

Impurities in the virgin resin

Use a whiteboard to check the virgin resin. Spread 1.5 kg of virgin resin over an area of 1500 cm² and examine it for 5 minutes under appropriate lighting. If there are black spots, note whether they are on the surface or inside the pellets, and inform the virgin resin supplier.

Impurities in reprocessed or reground resin

Check the screw design, material grinding, and processing procedures. Discard or resell this batch of material.

Excessive temperature or excessive residence time at melt temperature

Utilize 25% - 65% of the barrel capacity. In a space of less than 25%, the resin and additives are exposed to a long residence time, causing degradation. If possible, use a smaller barrel. Check whether the heating power is normal. The nozzle should be controlled by a PID controller. Check the high - shear dispersive mixing elements of the screw. Repair/replace.

Excessively fine

Remove overly fine granules before processing; this is essential when making lenses.

Special attention

If the black spot problem is caused by screw design, even the purest blend cannot solve the black spot issue. In 80% of cases, pulling and cleaning the screw is a wiser approach. The screw should be highly polished.

Exhaust barrel

Sharp angles or corners at the gate

Polish the gate area, round off sharp corners, change the gate type, or expand the gate.

Hot runner

Check for proper temperature control, burnt - out heating elements, and open thermocouples. Check the depth of the probe relative to the gate surface and calculate thermal expansion. Check for material - hanging areas or dead corners.

Poor nozzle temperature control

The temperature of the nozzle is extremely difficult to control. If the nozzle length exceeds 3 in (75 mm), the thermocouple should be positioned at 1/3 of the nozzle length from the nozzle tip. For nozzles 3 in (75 mm) or shorter, it is better to embed the thermocouple within the hexagon on the nozzle body. For longer nozzle bodies, the thermocouple should be embedded below the heating band in a butterfly - shaped thermocouple. The nozzle should be controlled by a PID controller; percentage - type thermocouples or autotransformers should not be used. Do not fix the thermocouple to the heating band clip with screws. Clean the nozzle and check for obstructions.

Sharp corners at the screw tip or broken check valve

Check the entire melt flow path for any adhering substances, sharp corners, grooves, etc. Check all plasticizing components: screw, screw tip, check ring, screw threads, etc.

Impurities

Check for foreign material resins.