What Is Plastic Shrinkage — and Why Does It Define Part Quality?
Plastic Shrinkage is the reduction in volume that a polymer experiences when it solidifies from the molten state in the mold cavity to ambient temperature when the part is removed from the mold. It is reported as a percentage of the mold size. A difference of 0.1% shrinkage over a 200 mm part is equivalent to a dimensional error of 0.2 mm — a difference that can cause failure in precision components when it comes to assembly.
Dimensional non-conformance due to a calculation error in shrinkage compensation represents 18–22% of the total first-article failures in injection molding processes, industry data show. When engineers face tolerances as tight as ±0.05 mm or even tighter, as is the case in medical, automotive and electronics applications, it is an inescapable requirement to select the right material and understand its shrinkage.
Shrinkage management is ingrained in the process of product development throughout the entire process from DFM (Design for Manufacturability) review, mold-flow simulation, in-process cavity pressure monitoring, to post-mold dimensional verification by CMM (Coordinate Measuring Machine).
Plastic Shrinkage Chart — ABS, PC, PA66, POM & PP at a Glance
The following table summarizes the shrinkage ranges and important processing parameters for the most commonly-used engineering thermoplastics in precision injection molding. All values are unfilled (neat) grades and will be greatly affected by glass or mineral fillers and introduce anisotropic behavior.
| Material | Shrinkage Range (%) | Typical Value (%) | Mold Temp (°C) | Melt Temp (°C) |
| ABS | 0.4 – 0.7 | 0.5 | 50–80 | 230–260 |
| PC | 0.5 – 0.7 | 0.6 | 80–120 | 270–320 |
| PA66 | 0.8 – 2.0 | 1.5 | 60–100 | 260–290 |
| POM | 1.8 – 2.5 | 2.0 | 60–90 | 185–215 |
| PP | 1.0 – 2.5 | 1.8 | 20–60 | 220–260 |
Source: Compiled from material datasheets (BASF, DuPont, Sabic) and SSPrecision internal process databases. Values apply to neat (unfilled) grades with nominal wall thickness of 2–3 mm.
Engineering Plastics Injection Molding — Material-by-Material Breakdown
ABS Shrink Rate: Low Shrinkage for High-Cosmetic Applications
Acrylonitrile Butadiene Styrene (ABS) is an amorphous thermoplastic that has a very low and well-defined shrinkage range of 0.4-0.7%. It does not crystallize, so the dimensional changes during cooling are not volumetric, which means that the dimensional changes are highly consistent from one production to the next.
Consumer Electronics Housing is a Tier-1 electronics company which works with SSPrecision to create a smartphone housing of ABS for the smartphone industry. Tolerances: ±0.08 mm for flatness. Thanks to a mold temperature of 65°C, melt temperature of 245°C and packing pressure of 85 MPa, SSPrecision was able to measure a shrinkage of 0.52% with a Cpk of 1.84 over 50,000 production cycles. No cosmetic rejection in the first 3 months of production.
- ABS Shrink Rate range: 0.4–0.7% (unfilled)
- Ideal mold temperature: 50–80°C; higher temperatures improve surface gloss
- Moisture sensitivity: moderate — pre-dry at 80°C for 4 hours before molding
- Typical gate style: pin, edge, or submarine gate
PC Molding Data: Optical Clarity Meets Dimensional Precision
Polycarbonate (PC) has a low shrinkage (0.5-0.7%) and is highly impact resistant and optically clear, it is used for lenses, light guides and safety shields. PC is extremely sensitive to moisture; insufficient drying (suggested: 120°C, 4 hours) results in a deterioration of molecular weight, splay and silver streaks that can change shrinkage by up to 0.15%.
PC Molding Data insight: PC parts molded at PC Molding Data at SSPrecision are molded with all-electric machines (Fanuc ROBOSHOT series) that are able to provide the exact velocity control needed to fill thin wall PC parts (0.8 – 1.2 mm) without jetting and burn marks. The use of conformal cooling channels created by 5-axis CNC creates a uniform wall thickness within ±0.05 mm, which is a 30% reduction in the cooling time compared to that of the conventional straight-line cooling.
- PC shrinkage: 0.5–0.7%; near-isotropic due to amorphous structure
- Melt temperature: 270–320°C — do not exceed to prevent yellowing
- Applications: automotive headlamp lenses, face shields, electrical connectors
PA66 Shrinkage: Managing Semi-Crystalline Complexity
Polyamide 66 (PA66 / Nylon 66) is a semi-crystalline engineering thermoplastic with the largest shrinkage range of the five materials discussed here: 0.8-2.0% neat, 0.5-1.0% with 30% glass fiber (PA66-GF30). The shrinkage is also directional: flow-direction shrinkage is normally 40-60 % less than transverse shrinkage in fiber-reinforced grades, and the gate location must be carefully designed.
Dimensional growth also affects PA66 after molding: In normal atmospheric conditions, PA66 can absorb up to 2.5% moisture by weight, and will swell by 0.1–0.3% in 24–72 hours. This “wet” condition is taken into account by the SSPrecision engineering team when determining functional tolerances for PA66 structural brackets for automotive applications.
- Drying critical: 85°C for 8 hours; moisture content must be below 0.2% before molding
- Glass-fiber grades (PA66-GF30) reduce shrinkage and warpage significantly
- Mold temperature: 60–100°C — higher temperatures increase crystallinity and surface hardness
POM Shrinkage: The Precision Polymer with High Crystallinity
Known for its outstanding stiffness, low friction and fatigue resistance, POM (acetal) has one of the highest crystallinity levels of all non-reinforced engineering plastics, resulting in shrinkage values of 1.8-2.5%. Minimizing differential shrinkage and warpage in POM parts requires a uniform wall thickness and balanced runner system.
To achieve this, SSPrecision medical micro-gear case requires process control that other non-specialist precision molders simply don’t attain, such as mold temperature of 80°C (±2°C), packing time precision of ±0.1 seconds, and gate freeze-off detection via cavity pressure sensors.
- POM shrinkage: 1.8–2.5%; homopolymer POM shrinks slightly more than copolymer
- No pre-drying required, but storage below 50% RH is recommended
- POM degrades rapidly above 230°C — strict melt temperature control is mandatory
PP Shrinkage: Balancing Economy with Warpage Risk
The second most produced polymer by volume (80 million metric tons produced globally in 2023) is polypropylene (PP), which is known to be chemically resistant, of a low density (0.90-0.91 g/cm³), and can be produced with living hinge properties. It has a large shrinkage range (1.0-2.5%) and a high orientation-dependent behavior, however, which makes it one of the harder materials to accurately size.
Random copolymer PP (PP-R) usually has a lower shrinkage (1.0-1.5%) than homopolymer PP (1.5-2.5%) and is used by SSPrecision in applications such as packaging where the dimensional stability of the product over the multi-cavity tooling is important and the thickness of the wall is low. SSPrecision achieved part-to-part weight variation of less than 0.08 g (0.5%), on a 100,000 shot production run, for a 32-cavity PP closure mold.
- PP shrinkage: 1.0–2.5% — homopolymer > copolymer > mineral-filled grades
- Warpage risk is highest in flat, thin-walled PP parts — balanced cooling is essential
- No pre-drying needed; PP is non-hygroscopic
PC Molding Data & Processing Guide for All Five Engineering Plastics
The proper selection of processing parameters is as important as material selection. The table below is a consolidated processing / application guide based on SSPrecision’s production database with more than 2400 active mold programs.
| Material | Gate Type | Drying Req. | Warpage Risk | Common Applications |
| ABS | Pin / Edge | 80°C / 4h | Low | Consumer electronics, automotive trim, housings |
| PC | Edge / Sub | 120°C / 4h | Moderate | Safety lenses, lighting covers, medical devices |
| PA66 | Pin / Fan | 85°C / 8h | High | Gears, bearings, structural brackets, connectors |
| POM | Pin / Edge | Not required | Moderate | Precision gears, cams, valves, fuel system parts |
| PP | Sub / Pin | Not required | Moderate–High | Packaging, automotive bumpers, living hinges |
How SSPrecision Delivers Shrinkage-Controlled Precision at Scale
With its main business of engineering plastics injection molding in automotive, medical, industrial and consumer electronics, SSPrecision (ssprecision.com.cn) is an expert in precision injection molding. The process engineering philosophy of the company regards shrinkage control as a system and not a single variable and incorporates the following capabilities:
- Shrinkage compensation is done by mold flow simulation prior to the cutting of steel (Moldex3D / Moldflow)
- Cavity surface finish of Ra 0.4 µm or less from 5 axis CNC and EDM machining
- All-electric and servo-hydraulic molding machines (50–1,600 tons of clamping force)
- In-mold cavity pressure sensor for real time process monitoring and automated rejection
- Full dimensional inspection and reporting on each first article using CMM (Zeiss / Renishaw) dimensional inspection equipment
- ISO 9001:2015 and IATF 16949:2016 certified quality management systems
SSPrecision’s typical shrinkage compensation accuracy (the difference between the simulated and the actual measured shrinkage) is: ±0.05% for amorphous materials and ±0.10% for semi-crystalline materials, resulting in a first-article approval rate that exceeds 92% in all new programs.
SSPrecision Is a Trusted Partner for Die Manufacturing Cost Optimization
SSP Precision is an ISO 9001 & IATF 16949 certified manufacturer delivering end-to-end precision solutions, from design and prototyping to high‑volume production, for the automotive, medical, electronics, aerospace, and industrial sectors. We handle every stage in‑house – DFM engineering, rapid prototyping, CNC machining, EDM, grinding, and global logistics – to manufacture the tooling that makes your parts and the parts themselves.
What we build and supply:
- Stamping dies manufacturing and stamping die parts – high‑precision transfer stamping dies and progressive/compound dies for volume metal stamping.
- Injection molding and injection mold – custom injection molds for plastic components, including single‑, multi‑cavity, and over‑molding & insert‑molding tools that combine metal and plastic in one part.
- Specialty molded components – eco‑friendly green mold parts and microscopic medical micro‑molded parts.
- Precision metal and plastic end‑use parts – high‑volume serial production of precision products (metal stampings, plastic moldings) with full PPAP traceability.
- Tooling spare parts manufacturing & – tooling spare parts (punches, inserts, ejector pins) and precision robotics spare parts to keep your production running.
Frequently Asked Questions — Plastic Shrinkage & Engineering Plastics Injection Molding
Q1. What is a good plastic shrinkage rate for precision injection molding?
Materials that tend to shrink less than 0.7% like ABS (0.4-0.7%) or PC (0.5-0.7%) are preferable for high precision components. With controlled manufacturing parameters, SSPrecision is able to routinely hold tolerances of ±0.02 mm for these materials.
Q2. Why does PA66 have such a wide shrinkage range (0.8–2.0%)?
PA66 is hygroscopic, meaning that it absorbs moisture from the environment and this has a direct impact on its shrinkage. Glass-fiber reinforced PA66 (PA66-GF30) generally shrinks 0.5-1.0% in the flow direction and 1.0-2.0% transverse to the flow and it is important to take into account the gate location and fiber orientation during design.
Q3. How does mold temperature affect shrinkage?
Generally, the higher the mold temperature, the higher the crystallinity of the semi-crystalline polymers (PP, POM, PA66) and the higher the shrinkage. For amorphous materials such as ABS and PC, the increase of the mold temperature will not cause the magnitude of the shrinkage to increase greatly, but can effectively decrease the residual stress.
Q4. Can shrinkage data from a Plastic Shrinkage Chart be used directly in mold design?
A starting reference range is given in the shrinkage charts. The actual shrinkage will be influenced by part geometry, thickness of the wall, design of the gate, cooling rate and packing pressure. The experienced mold makers at SSPrecision use simulation tools (Moldflow/Moldex3D) in conjunction with the chart data to determine the cavity dimensions.
Q5. What role does injection pressure play in managing shrinkage?
During cooling, additional material is pushed into the cavity in order to fill the volume loss. Lack of packing is one of the major reasons for excessive shrinkage and sink marks, particularly in parts over 4 mm thick.
Q6. How does SSPrecision ensure shrinkage consistency across production runs?
Closed-loop process monitoring is employed by SSPrecision, which is the control of melt temperature, injection speed and cavity pressure as they occur. Using Statistical Process Control (SPC) charts, the anomalies are identified before they cause dimensional non-conformance and the Cpk values on critical features remain above 1.67.
