Every injection-molding cycle produces not just parts but a runner system and sprue — often 20–50% of the shot weight. A beside-the-press plastic granulator turns that runner into clean regrind within seconds so it can be fed back into the same machine. This article explains how the setup works, how much regrind is safe to reuse, and how to select and run the machine correctly.

Why injection molders granulate runners immediately

Without immediate recycling, runners are collected in bins, dragged across the floor, and granulated centrally later. Along the way they can pick up dust, mix with other resins or colors, and absorb moisture. Immediate beside-the-press granulation removes those problems and shrinks waste handling to a single automated step.

• Lower material cost: runners that can be 20–50% of shot weight become reusable regrind
• No contamination: the same resin and color return to the same machine
• Less handling: no bins, no transport, no central sorting labor
• Cleaner regrind: granulated hot, before dust and moisture exposure
• Floor-friendly: low-speed units are quiet enough to run beside operators

How a beside-the-press setup works

• The injection machine opens and a robot (or chute) separates parts from the runner system.
• The sprue and runner drop directly into the low-speed granulator's hopper.
• Slow rotating knives cut the runner against fixed bed knives with minimal heat and dust.
• Regrind passes through a screen (commonly 6–10 mm) into a collector or blower.
• An optional proportional valve feeds clean regrind back into the machine hopper with virgin material.

The key engineering choice is a slow rotor (low-speed or medium-low-speed design). A high-speed granulator beside a press generates noise, dust, and heat that degrade regrind and disturb operators; low-speed designs cut quietly with high torque.

How much regrind can safely be reused?

There is no single universal percentage — it depends on resin, part requirements, and how clean and correctly sized the regrind is. Common industrial practice:

Regrind should be dry, dust-free, and properly flaked. Excess fines change melting behavior and can cause streaks or dimensional drift. Keep each generation of regrind traceable and avoid endlessly recycling already-recycled material.

Choosing a granulator for injection molding

• Match the feed to the runner: the largest runner and sprue must drop in without bridging
• Prefer low / medium-low speed: quieter, fewer fines, cooler regrind
• Check the screen: 6–10 mm for general reuse; smaller mesh only when required
• Insist on interlocks: hopper and cradle safety switches, emergency stop
• Plan blade access: frequent color/material changes need quick-open cleaning
• Confirm blade steel: SKD for abrasive/engineered plastics; 9CrSi for general PP/PE/ABS/PS
• Automate the return: proportional feeder or blower-regrind system for true closed loop

Common mistakes that ruin regrind quality

• Running dull blades — causes fines, drag tails, overheating, and motor overload
• Using a screen that is too small — throughput collapses and powder content rises
• Mixing resins or colors in one collector — contaminates production and wastes regrind
• Skipping cleaning between materials — specks and streaks in the next run
• Granulating metal-contaminated scrap — chipped knives and damaged rotors
• Letting regrind sit open — moisture uptake hurts hygroscopic materials like PA, PET, PC

Frequently Asked Questions

Q: What percentage of an injection shot is typically the runner?

A: It varies widely with mold design, but cold-runner systems commonly make the sprue and runner 20–50% of the total shot weight. Hot-runner molds eliminate most of the runner but still leave a sprue or occasional rejects to granulate.

Q: Is regrind from beside-the-press granulation as good as virgin material?

A: Clean, dry, correctly sized same-material regrind performs very close to virgin for most non-critical and semi-critical parts at controlled ratios, commonly 10–30%. Each regrind cycle can slightly shorten polymer chains, so appearance and engineering parts use lower ratios and controlled, traceable regrind.

Q: Do I need a hot runner or a granulator?

A: They solve different problems. Hot runners reduce runner material in the mold; granulators recycle whatever runner, sprue, or rejected parts still exist. Many plants use both: hot-runner molds plus a small beside-the-press granulator for sprues and rejects.

Q: Can the granulator handle hot runners straight from the mold?

A: Yes — low-speed beside-the-press granulators are designed to accept runners directly after demolding. Their slow cutting action avoids the heat buildup and fiber/fines problems high-speed machines can create with hot, soft runners.

Q: Which blades should an injection-molding granulator use?

A: For general PP, PE, ABS, and PS runners, 9CrSi blades offer a sharp, durable, cost-effective edge. For glass-filled or abrasive engineering plastics, tougher SKD blades resist chipping. Lesintor supplies and customizes both.

The business case is simple: runners are money that has already been melted once. Quiet, low-speed, closed-loop granulation beside the press recovers that money with the least degradation, labor, and contamination — which is why it is standard practice in well-run injection plants.

About Lesintor

Suzhou Lesintor Import and Export Co., Ltd. (Lesintor) is a Chinese manufacturer with 20 years of experience in injection-molding auxiliary equipment. Its granulator range covers powerful granulators, low-speed beside-the-press granulators, medium-low-speed models, and large-hollow / large-diameter heavy-duty granulators, with SKD or 9CrSi blades customized to the application. Every machine carries a one-year warranty and is built to ISO and CE standards.

Website: www.lesintormachine.com     |   WhatsApp: +1 628 488 4101