PLASTIC INJECTION MOLDING TECHNIQUE 

Guide to setting parameters, starting, operating, and maintaining in thermoplastic injection molding 

Quick Reference Table 

Common synthetic plastics can be processed without pre-drying. However, in high humidity conditions, moisture can condense on the surface of plastic pellets, causing defects such as excessive splay, air bubbles, low weld line strength, or surface imperfections.

If moisture in the material is suspected, it can be dried using a hopper dryer with a dehumidifying air system. If a suitable hopper dryer is not available, a tray dryer can be used, ensuring the plastic pellet layer in each tray does not exceed 5 cm (2 in.). Avoid overnight drying or drying at excessively high temperatures.

Setting the barrel temperature largely depends on the machine size, screw design, and other operating parameters such as back pressure and screw speed. Since most heat is generated from friction, the actual plasticizing temperature in the injection machine is often higher than the set temperature on the barrel.

For initial trials on machines with a clamping force of 450 tons, a starting barrel temperature configuration of 176 °C (350 °F) and a nozzle temperature of 193 °C (380 °F) is recommended. For machines with a clamping force below 450 tons, a barrel temperature configuration of 182 °C (360 °F) and a nozzle temperature of 193 °C (380 °F) is suitable. These settings should be adjusted so that the measured plasticizing temperature during each purge is about 5–10 °C (10–20 °F) lower than the desired temperature, as additional heat will be generated when the machine operates continuously in cycles.

Since heat is also generated by the mechanical action of the screw on the material, it is normal for the temperature in the middle and front barrel zones to rise slightly above the set point; however, excessive increases can cause material degradation. Once the machine is operating stably (regular cycles), these settings do not need further adjustment. The melt temperature should be monitored with a pyrometer during the initial startup and after each change in operating conditions.

With the recommended screw design (see Screw Design section), a screw speed of 40–50 RPM is appropriate. Due to the larger diameter, larger screws have a higher surface speed than smaller screws at the same RPM, generating more frictional heat. Therefore, larger machines typically require lower RPM speeds to achieve optimal operating conditions.

The optimal back pressure varies depending on the machine, but generally falls within the range of 0.35–0.70 MPa (50–100 psi). Screws with a low compression ratio often require higher back pressure to ensure the plastic is mixed and melted uniformly.

The melt temperature of the material is controlled through a suitable combination of barrel temperature settings, back pressure, and screw speed. To achieve optimal mechanical and physical properties, it is essential to strictly adhere to the manufacturer's recommendations for melt temperature when molding plastic compounds.

To measure the melt temperature during machine startup, use a handheld thermometer that has been accurately calibrated. When measuring with a needle probe, the molten plastic should be injected directly from the nozzle onto a piece of cardboard or other insulating material to prevent rapid heat dissipation. Since the injection pressure, injection speed, and back pressure during sampling are usually set lower than in normal production cycles, an initial reasonable target is for the measured melt temperature to be about 5–10 °C (10–20 °F) lower than the recommended range. The probe should be inserted into the molten plastic mass 4–5 times at different locations before recording the actual measurement result. After the processing has stabilized, the barrel temperature settings should be adjusted to achieve the appropriate melt temperature in production as necessary.

Occasionally cleaning the probe with a mold cleaner solution will help prevent hardened plastic from adhering to the probe tip. If the plastic hardens immediately on the first probe insertion, this layer of plastic will act as an insulating layer on the probe surface, causing the measurement result to be inaccurately lower than the actual value. If smoke or air bubbles are observed in the molten plastic during purging, it is often a sign that the actual melt temperature is higher than the set temperature, and it may also indicate moisture in the material — the plasticizing temperature should be rechecked. The extruded plastic stream will have a smooth surface and reasonable gloss if the melt temperature is close to the optimal level (when assessing gloss, surface light reflection should be disregarded). Voids on the surface or within the core of the extruded plastic stream can also indicate moisture in the material.

Injection should start at a slow to medium speed, then gradually increase until the product is completely filled and there are no signs of weld lines or sink marks. If the injection speed is too fast, excessive frictional heat can cause jetting when the plastic flow passes through obstacles or sharp edges in the mold, leading to surface defects, brittleness, or even material degradation. The injection speed in the final stage should be relatively slow to reduce the flow resistance of the material.

The first stage injection pressure (filling pressure) necessary to fill the mold cavity depends on the material temperature, injection speed, mold temperature, and mold design. Generally, pressure in the range of 50–70% of the machine's maximum provides the best stability and scalability of the process. Start with low pressure and gradually increase to the desired level to avoid flash. The duration of the first injection pressure stage should be set so that the transition to holding pressure occurs just as the product is fully filled — this timing should coincide with the screw nearing the end of its retraction stroke, leaving only a cushion of about 0.3–0.6 cm (0.125–0.25 in.). The screw tip usually continues to move slowly forward and may add a small amount of material to the cushion.

The second stage injection pressure (holding pressure) only needs to be sufficient to maintain the product in the mold as it cools and shrinks, typically 1/2 to 2/3 of the first stage injection pressure. Products with thicker sections usually require higher holding pressure.

Excessive product weight due to too high holding pressure or too long first injection pressure time will increase molding stress, adversely affecting product quality. Generally, sink marks appearing far from the gate indicate the need to increase injection pressure/time, while sink marks near the gate indicate the need to increase holding pressure/time. Once the gate has solidified, holding pressure should be reduced to save energy.

A small cushion of plastic should be maintained in front of the screw to compensate for the product's shrinkage as it cools under holding pressure, thereby preventing sink marks. Ideally, the screw should only reach its maximum retraction position just after the material flow into the mold has stopped.

Accurate mold temperature control is essential to optimize cycle time and product quality. Synthetic plastics are typically processed with mold cooling water temperatures ranging from 20–65 °C (70–150 °F). Higher mold temperatures generally improve surface gloss, material flowability, weld line strength, and reduce internal stress; lower mold temperatures allow for shorter cycle times.

High coolant flow rates, combined with optimally designed water channels and fittings (with minimal flow restrictions), will improve mold temperature control and optimize the balance between product quality and cycle time. Operating with the "B" half of the mold cooler than the "A" half often helps the product release and eject more easily.

Plastic from runners and trimmed flash can often be reused. Recycled material should be kept clean; the blades of the grinder should be kept sharp and clean to minimize heat generation during grinding. To avoid thermal degradation, regrind should be cooled and stored at temperatures below 60 °C (150 °F).

The regrind ratio is typically used at 10–30%, mixed with virgin material. Recycled material that has degraded or is contaminated should not be used, as it will degrade the quality of the good material it is mixed with.