Hey, fellow engineers working on the front lines, have you ever encountered this frustrating situation:
Drawings are beautifully done, dimensions accurate to two decimal places; molds are perfectly made, fitting seamlessly. But as soon as product is injection molded and cools slightly, a quick measurement with calipers reveals—hey, why are dimensions outside tolerance range? Why is hole spacing off?
Rework, mold modification, process adjustment… a flurry of activity, yet dimensions still teeter on the edge of tolerance. What's behind this annoying "shrinkage"?
Don't worry, today we'll become "plastic detectives," thoroughly investigating this troublesome little devil, "shrinkage rate," so you can say goodbye to dimensional anxiety forever.

Calculation Formula: Shrinkage Rate (%) = (Mold Size - Part Size) / Mold Size × 100%
Plastic mold shrinkage principle diagram

 

Simply put, shrinkage rate is percentage of plastic volume that decreases as it cools from a liquid to a solid state.
This formula seems simple, but behind it lies a dramatic "volume shrinkage dance" as plastic transitions from a molten state to a solid state. It's not a simple matter of physical thermal expansion and contraction, but rather result of combined effects of material properties, mold structure, process adjustments, and product structure.
Understanding it is essential for every mold designer and injection molding engineer, and a necessary step towards becoming a master.

Shrinkage rate is not an isolated number; behind it stands a vast "family of influences." We can categorize them into four main types: material properties, mold structure, process adjustments, and product structure.
Crystalline plastics (such as PP, PE, PA): Molecular chains arrange themselves regularly upon cooling, forming a "crystalline structure," like soldiers lined up in orderly rows. This arrangement leads to a significant reduction in volume, resulting in a higher shrinkage rate (typically 1.5%~2.5%).
Amorphous plastics (such as PC, PS, ABS): Molecular chains remain disordered upon cooling, like a tangled mess. Volume change is relatively small, resulting in a lower shrinkage rate (typically 0.4%~0.8%).

 

This is fundamental factor determining shrinkage, just as DNA determines your height.
Core Difference: Crystalline vs. Amorphous
Practical Tip: If you find that PA6 parts are always smaller than expected, don't doubt yourself—this is normal! PA6 is a typical crystalline plastic with a shrinkage rate as high as 1.5%~2.0%.
Crystalline plastics (such as PP, PE, PA, POM): Molecular chains are arranged in an orderly manner, like rows of soldiers. Upon cooling, molecular chains transform from a disordered molten state to an ordered, tightly packed crystalline state, a process accompanied by a significant volume reduction. Therefore, crystalline plastics typically have a high shrinkage rate. For example, PP can shrink by 1.6% to 2.0%.
Amorphous plastics (such as PC, PS, ABS): Molecular chains are disordered, like a tangled mess. From molten to solid, there is no crystallization process; it's simply "frozen," resulting in minimal volume change. Therefore, amorphous plastics typically have a very low shrinkage rate. For example, pure ABS has a shrinkage rate of only 0.5% to 0.7%.
Key Operations: Modification and Filling
Want to reduce shrinkage? The most direct method is to add a "rigid skeleton." Adding glass fiber (GF), carbon fiber (CF), or mineral fillers (talc, calcium carbonate, etc.) to plastic matrix is like adding steel bars and stones to cement, or stuffing straw into a mud wall. These rigid fillers themselves do not shrink and restrict movement of polymer chains, thereby greatly reducing the overall shrinkage rate of material.
Consider this striking example:
Shrinkage rate of pure Nylon 6 (PA6): 1.0% ~ 2.0%
Shrinkage rate of PA6 with 30% glass fiber added (PA6-GF30): drastically reduced to 0.3% ~ 0.5%
See, that's magic of blending modification!
B. Mold Design
If material is "flesh," then mold is the "skeleton." Design of skeleton directly affects final shape of material.
Gate and Runner: A gate that is too small or a runner that is too narrow is like giving a strong man a straw to drink water—making it difficult to replenish material. Before plastic part solidifies, melt cannot effectively fill in, resulting in significant pressure loss and naturally leading to higher shrinkage.
Cooling System: Design of cooling water system is paramount. Uneven cooling, with one side cooling faster than the other, will cause enormous stress inside plastic part, leading not only to inconsistent shrinkage but also to the even more troublesome problem of warping.
C. Processing Conditions
Even with good materials and molds, without proper process parameters for "tuning," everything is in vain.
Temperature Paradox: Higher Mold Temperature, Greater Shrinkage?
For amorphous plastics, increasing mold temperature prolongs flowability, facilitates pressure transmission, and reduces shrinkage. However, for crystalline plastics, opposite is true!
Increasing mold temperature gives molecular chains more time and more energy to "queue up" and form perfect crystals, resulting in higher crystallinity and more dramatic volume shrinkage. Therefore, you'll find that for materials like PP and PA, increasing mold temperature actually leads to increased shrinkage.
Magic of Pressure
Higher injection pressure and holding pressure, along with longer holding time, means more force is used to "stuff" more melt into mold cavity, compacting gaps between molecules and effectively resisting shrinkage. Therefore, increasing pressure and time is one of the most common methods to reduce shrinkage.
D. Part Design
Design of product itself also contributes to shrinkage.
Wall Thickness Effect: The thicker wall of a plastic part, the slower central layer cools, the longer shrinkage time, and the greater the total shrinkage. This is why sink marks are prone to appear in thick-walled areas.
Structural Resistance: Complex structures such as holes, inserts, and reinforcing ribs in plastic parts can hinder free shrinkage of plastic. Shrinkage in these areas will be less than in unobstructed areas, thus generating stress within product and even leading to cracking.

Myth 1: "Shrinkage rate is a fixed value."
Truth: Shrinkage rate is a range and has directionality. Orientation of plastic melt in flow direction will result in different shrinkage rates in parallel flow direction (MD) and perpendicular flow direction (TD). This difference is particularly pronounced for materials with added fibers. So, don't design molds with a fixed value!
Myth 2: "Higher mold temperature means more stable dimensions."
Truth: This is only half true. As mentioned earlier, for crystalline plastics, high mold temperature means high shrinkage. It's crucial to judge based on material type; generalizations are ineffective.
Misconception 3: "Choosing right material guarantees success."
Truth: This is a systemic issue. Even top-quality materials paired with poor molds or flawed processes will result in defective products. Materials, molds, processes, and design are an inseparable trinity.

Having discussed so much, how exactly should we proceed? Here's a set of control secrets from source to end.
Design (Fundamental Solution): Maintain uniform wall thickness in plastic parts, avoiding excessively thick areas.
Use rounded transitions instead of sharp corners to reduce stress concentration.
Material Selection (Addressing Root Cause): Prioritize amorphous plastics or filled/modified plastic grades with low shrinkage rates, while meeting performance requirements.
Mold Design (Building a Healthy Nest): Design gate location and size appropriately to ensure smooth pressure transmission and sufficient material filling.
A meticulously designed cooling circuit ensures uniform and efficient cooling throughout mold cavity.
Process Technology (Refined Craftsmanship): Utilizing three key levers of pressure, temperature, and time.
For products requiring high precision, optimal combination of process parameters is scientifically determined using Design of Experiments (DOE) method.

Having read this far, you should have a completely new understanding of plastic shrinkage. It's no longer a mysterious "black box," but rather a series of traceable and controllable engineering laws.
Finally, please remember this:
Plastic shrinkage is not a simple physical phenomenon, but result of a game between materials, molds, processes, and design. Understanding rules of this game is key to becoming a winner.
Hopefully, this article has helped you clear up your confusion and enable you to accurately control the dimensions of every part in your future work, becoming a true "master of dimension control."