Screw and Barrel Selection for Thin-Wall Molding

Thin-wall injection molding demands screws with specific geometric characteristics that differ significantly from general-purpose designs. The critical parameters are the length-to-diameter (L/D) ratio, compression ratio, and flight depth. For processing polypropylene at melt flow indices of 25-40 g/10min (230 degrees C/2.16kg), an L/D ratio of 22:1 to 25:1 provides sufficient residence time for complete melting without thermal degradation. The HWAMDA SPV5 screw designs use a 24:1 to 25:1 L/D ratio across the range -- the HMD 380M8-SPV uses a 55mm screw with 24:1 L/D, while the HMD 470M8-SPV steps up to a 60mm diameter at 24.5:1 L/D. Compression ratios of 2.5:1 to 3.0:1 are standard for PP thin-wall work, balancing melt homogeneity against excessive shear heating. Feed zone depth is typically 1.5-1.8 times the metering zone depth, ensuring consistent pellet intake. The metering zone should occupy 40-50 percent of the total screw length to maximize melt uniformity, which is essential when filling 8 or 16-cavity molds where cavity-to-cavity weight variation must remain below 2 percent.

Different thin-wall products require different screw specifications based on shot weight and cycle time demands. For yogurt cups (4-8g, 0.45-0.6mm wall), the HMD 380M8-SPV with a 55mm screw provides an injection volume of 640 cm3 and plasticizing rate of 38 g/s PP, sufficient for 8-cavity molds at 4.0-4.5s cycle times. For food containers (15-25g, 0.5-0.7mm wall), the HMD 470M8-SPV with a 60mm screw delivers 760 cm3 injection volume and 48 g/s plasticizing rate, supporting 4-cavity production at 5.0-6.0s cycles. Sauce cup production (2-4g, 0.35-0.5mm wall) on 16-cavity molds requires the HMD 330M8-SPV with a 52mm screw delivering 568 cm3 injection volume. The general rule is to size the shot weight at 30-70 percent of the barrel capacity to avoid excessive residence time, which causes PP degradation above 280 degrees C. Running a 6g x 8-cavity yogurt cup mold (48g shot plus 15g runner = 63g total) on a 640 cm3 barrel means approximately 56 percent utilization -- well within the optimal range.

The non-return valve (check ring) at the screw tip is arguably the most critical component for thin-wall shot consistency. During injection at speeds of 368-422 mm/s, the valve must seal instantaneously to prevent melt backflow. A leaking check ring causes shot weight variation exceeding plus or minus 0.3g, which translates to visible wall thickness inconsistency on 0.5mm-wall containers. HWAMDA SPV5 machines use a 3-piece sliding ring check valve with lapped seating surfaces ground to a surface finish of Ra 0.2 micrometers. The ring-to-barrel clearance is held at 0.03-0.05mm on diameter. Valve response time is under 0.02 seconds at maximum injection speed. For high-speed yogurt cup production where cycle times are 3.5-4.5s, check ring wear should be inspected every 2 million shots and replaced when clearance exceeds 0.10mm. Worn check rings are the number one cause of weight scatter in thin-wall production, often misdiagnosed as process instability. A quick diagnostic is to run a purge shot in air and measure cushion consistency -- variation above 1.0mm indicates valve replacement is needed.

Plasticizing efficiency directly impacts cycle time in thin-wall production because screw recovery must complete within the cooling phase. On the HWAMDA HMD 380M8-SPV producing 8-cavity yogurt cups at 4.0s cycle time, the available plasticizing window is approximately 2.0-2.5s (during mold cooling and part ejection). With a 63g total shot, the required plasticizing rate is 25-32 g/s, well within the 38 g/s capacity of the 55mm screw. Back pressure should be set at 8-12 MPa for standard PP to ensure melt homogeneity without extending recovery time. Screw RPM is typically 180-220 for PP with MFI 30-40, producing a melt temperature of 230-245 degrees C. Higher RPM generates more shear heat, which can be beneficial for rapid melting but risks thermal degradation if melt temperature exceeds 260 degrees C. The INOVA controller's charging-on-the-fly function begins screw recovery during mold opening, overlapping plasticizing with mold movement to recover an additional 0.3-0.5s per cycle. This feature alone can increase hourly output by 8-12 percent on yogurt cup production lines, translating to approximately 800-1,200 additional cups per hour on a 16-cavity setup.