---
title: "Injection Molding Cushion: Process Control Guide for Reliable Parts"
description: "Injection molding cushion control remains a common source of process instability. A practical supplier perspective on setting, measuring, and adjusting screw cushion for consistent part quality."
url: "https://www.hyneu.com/guides/injection-molding-cushion-process-control-guide.html"
language: "en"
type: "Article"
category: "Injection Molding Guide"
datePublished: "2026-10-30"
dateModified: "2026-10-30"
brand: "HYNEU"
manufacturer: "HYNEU"
image: "https://static.hyneu.com/uploads/industries/injection/PYl0kZvxvqSU9.webp"
---

# Injection Molding Cushion: Process Control Guide for Reliable Parts

## The Most Common Misconception About Cushion

Many people new to injection molding assume that the cushion is leftover plastic that should be reduced as much as possible. They treat a larger cushion as scrap, or they try to set the screw to stop exactly at zero. In a production environment, that is the wrong way to think about it. The cushion is not waste. It is an intentional melt reserve that keeps the screw from bottoming out and allows holding pressure to reach the cavity.

![Injection Molding Cushion: Process Control Guide for Reliable Parts](https://static.hyneu.com/uploads/industries/injection/PYl0kZvxvqSU9.webp)
When a molder says the cushion is too small, the problem is usually not wasted material. It is that pressure transfer has become unreliable. Correcting this requires understanding how the cushion is formed, how it is measured, and what changes it during the cycle.

## What Injection Molding Cushion Actually Is

Injection molding cushion, also called screw cushion or injection cushion, is the remaining forward screw position after the injection and hold phases are complete. In practical terms, it represents the small pad of molten material left in front of the screw tip and check ring after the hold pressure has packed the cavity. The machine displays this as a distance, usually in millimeters, between the screw’s final position and its zero reference.

The cushion is not a separate phase of the molding cycle. It is the result of shot size, transfer position, hold pressure, material viscosity, check ring performance, and screw recovery. A stable cushion means the machine is delivering a repeatable amount of melt to the cavity. An unstable cushion means something else in the process is changing.

### Screw Cushion vs. Injection Cushion

These terms are used interchangeably in many process discussions. Screw cushion emphasizes the screw position, while injection cushion emphasizes the melt reserve during injection and packing. In production, some teams also use molding cushion to describe the same final position after hold. The critical point is that cushion setting always refers to a target remaining position or melt pad after hold, not to a pressure value or a timer.

### Confirm How the Machine Displays Cushion

Before changing any parameter, engineers should confirm whether the machine displays the cushion as a positive position from the full-forward zero point or as a distance from another reference. A target of 5.0 mm does not mean the same thing on every controller. This is one of the most common sources of confusion during tool transfer between facilities.

## Why Cushion Control Matters Before Setting Parameters

![Screw Cushion Settings Explained: Avoid Short Shots and Sink Marks](https://static.hyneu.com/uploads/industries/default/NBiPVPUGoT0QX.webp)
Cushion is used for more than checking whether there is extra material. It affects repeatability in several ways:

**Pressure transfer:** If the cushion is too small or reaches zero, the screw may bottom out before holding pressure fully transfers to the cavity.- **Packing consistency:** Shot-to-shot cushion variation often appears as dimensional variation, sink marks, or short shots.
- **Material degradation:** An oversized cushion keeps the same melt in the barrel longer, increasing residence time and the risk of yellowing or black specks in heat-sensitive materials.
- **Check ring diagnosis:** A gradual loss of cushion during production is a practical signal of check ring wear or barrel and screw wear.

## Step-by-Step Cushion Setup and Process Control

The goal is not to find one universal number. The goal is to establish a stable shot that fills the part properly and leaves a controllable cushion window. The following sequence is used for general injection molding process setup, including part tryout and production handover.

### Step 1: Verify Machine and Barrel Condition First

Do not set cushion on a machine with a known feed or check ring problem. Before starting, verify the following:

Check ring or non-return valve moves freely and seats consistently.- Screw and barrel show no severe wear, especially if glass-filled or mineral-filled materials were run previously.
- Heater bands are stable and the barrel temperature profile matches the material supplier’s range.
- Hopper feed throat is not bridged or blocked.
If the check ring leaks, cushion may still be adjusted, but the process will drift. A stable cushion cannot be built on an unstable machine condition.

### Step 2: Establish Shot Size From Part and Runner Weight

Set the metering stroke based on the total shot weight: part weight plus runner or sprue weight plus a reasonable process margin. The shot size should be high enough to fill and pack the cavity, but not so high that the entire barrel capacity is used. For general molding, the shot should normally fall within the machine’s recommended shot capacity range, typically between 20 percent and 80 percent of maximum shot volume. That range protects residence time and melt quality while leaving enough cushion control.

A common mistake is to set shot size too high to be safe. That creates an oversized cushion and increases residence time. An equally serious mistake is to set shot size too tight, leaving no room for packing and causing the cushion to disappear during hold.

### Step 3: Set Transfer Position Using a Fill-Only Trial

Cushion is only meaningful after the transfer point is established. To determine transfer position:

Reduce hold pressure to zero or a very low value.- Adjust shot size and transfer position until the cavity is approximately 95 to 98 percent full at the moment the machine transfers from velocity control to pressure control.
- Record the screw position at that transfer point.
- Restore hold pressure and pack time, then verify the part is fully packed and dimensionally acceptable.
If transfer is set too late, the screw moves too far forward and the cushion drops. If transfer is set too early, the machine enters hold before the cavity is nearly full, which can produce short shots or require excessive packing to complete the part.

### Step 4: Set the Target Cushion and Control Limits

After establishing a stable shot, record the cushion over 10 to 20 consecutive shots without changing parameters. Use that average to set the target. For many general-purpose plastic components, a starting target of **3 to 8 mm** is practical, but small precision parts may run better at **2 to 5 mm**, while larger parts may require **6 to 12 mm**. These are starting ranges, not universal standards.

Set an upper and lower control limit around the target. For general structural parts, a variation of **±1.0 to ±1.5 mm** may be acceptable. For tight-tolerance components, the variation should be kept closer to **±0.5 to ±1.0 mm**. If the cushion reaches **0.0 mm** or falls below the lower limit, the machine should alarm or the operator should stop and investigate.

### Step 5: Monitor Cushion During Sampling and Production

One cushion reading during first article inspection is not enough. Cushion must be monitored over time because it is a leading indicator of process drift. During sampling, record shot size, transfer position, fill time, peak injection pressure, and cushion position together. In production, the interval depends on part risk. For critical dimensions, check cushion every 30 minutes or every few hundred shots. For general parts, checking at process start and after any material lot change is a minimum.

Operators should know the acceptable cushion range before making adjustments. If cushion drifts outside the window, they should not simply increase shot size. The first action should be to identify why the reading changed.

## Typical Cushion Starting Ranges by Part Type

The following table is a practical reference for process development and tool handover. Ranges may shift depending on machine, material, mold design, and hold pressure settings.

| Part or Process Type | Suggested Starting Cushion | Acceptable Variation Band | Key Control Point |
| --- | --- | --- | --- |
| Small precision components | 2–5 mm | ±1.0 mm or less | Prevent zero cushion and check ring leakage |
| General structural plastic parts | 3–8 mm | ±1.5 mm | Log cushion with shot size and transfer position |
| Thick-wall or large shot parts | 6–12 mm | ±2.0 mm | Watch residence time and material degradation |
| Glass-filled or abrasive materials | 4–8 mm | ±1.5 mm | Inspect screw and check ring wear more often |
| Hot runner mold | 5–10 mm | ±1.5 mm | Separate cushion from hot runner decompression settings |

These values should be treated as starting points. The correct cushion is the one that gives stable pressure transfer and acceptable part quality without excessive residence time.

## How Cushion Changes Reveal Process Problems

Cushion readings act like a process instrument. The direction and speed of change often point to the root cause before parts become visibly defective.

**Gradual cushion decrease over hours or days:** Often associated with check ring wear, screw or barrel wear, or a shift in material viscosity from a new lot. Inspect the non-return valve and compare current shot weight with the original setup.- **Sudden cushion drop to near zero:** Check whether the transfer position changed, the metering stroke was reduced, the nozzle froze, or material feed was interrupted. A blocked feed throat can produce the same effect.
- **Gradual cushion increase:** This may point to a transfer position drift, a controller zero reference change, or a nozzle seating issue. Compare the current screw position at transfer with the validated process sheet before changing shot size.
- **Erratic cushion reading:** Look for inconsistent material feed, unstable back pressure, barrel temperature fluctuation, or a damaged check ring that seats differently shot to shot.

## Cushion-Related Defect Detection

When a defect appears, cushion should be checked together with part weight and fill time. It should not be the only data point.

**Short shot near the end of fill:** If the cushion dropped below the lower limit or reached zero, the cavity may not have received the required melt volume or holding pressure.- **Sink marks at bosses or ribs:** A cushion that is too small may prevent sufficient packing pressure from reaching thick sections after fill.
- **Flash after a previously stable process:** Check whether cushion suddenly increased or transfer position shifted, which can change the effective packing profile.
- **Burn marks, black specks, or color shift:** An oversized cushion can increase material residence time and degrade melt in the barrel.
- **Dimensional variation across cavities:** For multi-cavity tools, unstable cushion may mean inconsistent pressure delivery to all cavities, especially if runner balance is marginal.

## What Buyers and Engineers Should Ask During Tool Transfer

When a mold moves from one molder to another, or from tool tryout to mass production, cushion is not always transferred correctly. Buyers and engineering teams should ask for more than a single parameter sheet.

How is cushion defined and zeroed on the target machine?- What is the validated cushion target, lower limit, and upper limit?
- Which process parameters are monitored together with cushion, such as shot size, transfer position, fill time, and peak pressure?
- What is the action plan when cushion drifts outside the control window?
- For transferred tools, will the process be re-qualified with a cushion study rather than simply copying machine values?
At HYNEU, process reviews treat cushion as part of a connected set of injection parameters. A supplier that cannot explain cushion control or only records pressure and temperature may be missing an important process stability signal.

## Final Process Control Checklist

Confirm machine zero reference and cushion definition before use.- Verify check ring, screw, barrel, and feed condition before setting cushion.
- Set shot size from part, runner, and process margin, not from guesswork.
- Use a fill-only trial to establish transfer position before tuning cushion.
- Record cushion over 10 to 20 consecutive shots and set upper and lower control limits.
- Monitor cushion during production together with shot weight and fill time.
- Investigate cushion drift before adjusting shot size or packing pressure.
- Re-qualify cushion after material lot changes, machine changes, or mold modifications.
For injection-molded plastic components and related structural parts, stable cushion control is one of the most practical ways to prevent dimensional drift and packing-related defects. The setting itself is simple, but the discipline around it is what separates stable production from repeated troubleshooting.

## Related Resources

- [Injection Molding Guide](https://www.hyneu.com/guides/injection-molding/)
- [Plastic Injection Molding](https://www.hyneu.com/capabilities/injection-molding/)
- [Injection Molding Buying Guides](https://www.hyneu.com/buying-guides/injection-molding/)
- [Mold Design & Tooling Guide](https://www.hyneu.com/guides/mold-design-tooling/)
- [Materials & Material Selection Guide](https://www.hyneu.com/guides/material-selection/)
- [Custom Manufacturing](https://www.hyneu.com/custom-manufacturing/)
- [Manufacturing Defects](https://www.hyneu.com/knowledge/manufacturing-defects/)
- [Injection Molding Q&A](https://www.hyneu.com/questions/injection-molding/)

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