Our "Understanding Plastic Property Datasheets" series continues its in-depth exploration of processing performance. Previously, we discussed how MFR/MVR solves "can it be molded?" problem. However, in actual production, root cause of product dimensional deviations, surface shrinkage marks, warpage, and assembly jams almost always points to one indicator—molding shrinkage.
It is percentage of volume/size shrinkage that occurs when plastic cools and solidifies from a molten state. It directly determines product accuracy, mold design, assembly compatibility, and is a core reference for mold designers. Today, we'll use a combination of text and graphics to thoroughly explain definition, types, testing, and material selection/mold adjustment techniques of molding shrinkage, addressing dimensional and appearance issues at their root!

Molding shrinkage rate refers to percentage difference between dimensions of finished product and dimensions of mold cavity after molten plastic has cooled and solidified within mold cavity during injection molding/extrusion processes. Unit is **%** (percentage).
Core Calculation Formula: Molding Shrinkage Rate S = (Mold Cavity Dimension - Actual Product Dimension) / Mold Cavity Dimension * 100%. In simple terms: If mold cavity is designed to be 100mm, and actual product dimension after cooling is 99.6mm, then molding shrinkage rate is 0.4%. A higher value indicates more significant plastic shrinkage during cooling, requiring more "enlargement compensation" in mold.
Key Characteristics: Thermal expansion and contraction + crystallization shrinkage are core reasons for shrinkage in plastics.
Thermal expansion and contraction: Common to all plastics. Melt volume is large at high temperatures (180~300℃), and shrinks when cooled to room temperature (25℃).
Crystallization shrinkage: Unique to crystalline/semi-crystalline plastics (PP, PE, PA). Upon cooling, molecules align from a disordered melt into ordered crystals, resulting in additional volume shrinkage. Therefore, shrinkage rate of crystalline plastics is much greater than that of amorphous plastics (PC, ABS, PS).

 

Molding shrinkage rate of plastics is not a single value. Shrinkage rates vary depending on direction and wall thickness. Industry classifies it into two categories based on "shrinkage direction" and "actual application." Mold design must consider both; neither is dispensable!
1. Linear Shrinkage Rate: Most commonly used, core basis for mold design. Linear shrinkage rate refers to dimensional shrinkage rate of plastic in a single direction (length, width, height). It is the most frequently listed value in material property table and a direct reference for mold designers to "compensate" for cavity dimensions.
Labeling Format: Material property tables often indicate a shrinkage rate range (e.g., PP: 1.0%~2.0%), as processing technology, wall thickness, and temperature will affect actual shrinkage.
Core Application: Mold cavity size = product design size / (1 - shrinkage rate). For example, if product design is 100mm and shrinkage rate is 0.5%, mold cavity needs to be 100.5mm (to compensate for shrinkage).
2. Anisotropic Shrinkage Rate: Key to solving warpage deformation. Anisotropic shrinkage rate refers to difference in shrinkage rate between plastic in mold flow direction (MD) and perpendicular to flow direction (TD). Crystalline plastics exhibit more pronounced anisotropic shrinkage because molecules align along flow direction during cooling.
Key Indicator: Shrinkage rate in flow direction > Shrinkage rate perpendicular to flow direction. The larger difference, the more prone product is to warping, bending, and twisting.
Core Application: Mold design needs to optimize gate location and runner layout to reduce shrinkage difference between flow direction and perpendicular direction. For example, symmetrical gates can effectively reduce anisotropic shrinkage.

 

Molding shrinkage rate is tested using a nationally standardized method (GB/T 15585-2008, corresponding to ISO 294-4). Test sample is a standard rectangular strip (or circular piece), simulating actual injection molding process. Testing procedure is simple, results are accurate and referable. Core process consists of three steps:
1. Prepare a standard mold according to national standards (precisely calibrated cavity dimensions, e.g., 100mm × 20mm × 4mm);
2. Inject the strip according to material standard processing technology (temperature, pressure, holding time), and place it at room temperature (23℃) for 24 hours (until plastic completely cools and solidifies, and shrinkage stabilizes);
3. Measure actual dimensions of strip using high-precision calipers (0.01mm accuracy), substitute measurements into formula to calculate shrinkage rate, record values in flow direction and perpendicular to flow direction to obtain linear shrinkage rate and anisotropic shrinkage rate.
Key Reminder: Shrinkage rate in material property table is a reference value under standard processes. In actual production, processing parameters (holding pressure, temperature), product wall thickness, and mold structure will affect actual shrinkage rate. Fine-tuning mold after trial molding is necessary.

Core rule of molding shrinkage rate: Crystalline plastics > Semi-crystalline plastics > Amorphous plastics; Modified materials (glass fiber/mineral fillers) will significantly reduce shrinkage rate (filler acts as a "skeleton," inhibiting molecular shrinkage). This table summarizes molding shrinkage rates (linear shrinkage rate, national standard value) of the most commonly used plastics in cups/jet pots/small kitchen appliances for direct reference in mold design!

 

Controlling molding shrinkage rate requires a combination of mold design, material selection, and production mold adjustment. Core goal is to "match product precision requirements, reduce shrinkage differences, and avoid warping/shrinkage marks/poor assembly." Following techniques are directly applicable to cup/kettle/small kitchen appliance production:
✅ Technique 1: Select Materials According to Product Precision Requirements
High-precision assemblies (e.g., clips, shafts, connecting parts): Select low-shrinkage materials (glass fiber reinforced PP/PC, PC, ABS), with a shrinkage rate ≤0.7%, to avoid dimensional deviations causing assembly jams;
Ordinary appearance parts (e.g., cup body, outer shell): Select medium/high shrinkage materials (pure PP/PC, PC, ABS, etc.). PP and HDPE are suitable materials, balancing cost and moldability. Avoid using high-shrinkage materials for precision parts, as dimensions will be difficult to control even after mold compensation.
✅ Tip 2: Three Key Points for Shrinkage Compensation in Mold Design
Cavity Size Compensation: Enlarge cavity according to average shrinkage rate of material (e.g., 1.5% for PP, 0.5% for ABS), fine-tune according to actual product dimensions after trial molding;
Reduce Anisotropic Shrinkage: Use symmetrical gates or multi-point gates to allow melt to flow evenly within cavity, reducing shrinkage difference in flow/vertical direction;
Optimize Cooling System: Evenly distribute mold cooling water channels to allow all parts of product to cool synchronously, avoiding uneven shrinkage caused by localized excessively fast/slow cooling.
✅ Tip 3: 4 Key Parameters for Precise Control of Actual Shrinkage in Mold Adjustment
Actual shrinkage rate of plastics can be fine-tuned through injection molding parameters. Core principles are "increasing holding pressure, extending holding time, and reducing cooling rate," specifically:
Increasing holding pressure/time: Replenishes mold cavity with melt, offsetting cooling shrinkage, reducing shrinkage marks and dimensional inconsistencies;
Appropriately increasing mold temperature: Allows melt to cool slowly, reducing crystallization shrinkage and anisotropic shrinkage, and minimizing warpage (applicable to crystalline plastics);
Reducing injection speed: Allows melt to fill mold cavity evenly, avoiding excessive molecular orientation and reducing shrinkage differences;
Extending cooling time: Allows product to cool fully within mold, ensuring shrinkage stabilizes before opening mold, preventing secondary shrinkage after demolding that could lead to dimensional deviations.
✅ Tip 4: Optimize Shrinkage Through Product Structure
Avoid abrupt changes in wall thickness: Thick-walled areas cool slowly and shrink significantly, while thin-walled areas cool quickly and shrink less, easily causing shrinkage marks and warping. Aim for uniform product wall thickness (1.5~2.5mm is recommended for cups and jugs);
Add process holes/reinforcing ribs to thick-walled areas: Distribute volume of thick walls, reduce localized shrinkage, and avoid surface shrinkage marks (e.g., add process holes to thick-walled areas at the bottom of cups and jugs).

❌ Misconception 1: Treating Shrinkage Rate in Material Property Sheet as a Fixed Value
Material property sheet provides reference values under standard processes. Actual shrinkage is affected by wall thickness, mold temperature, and holding pressure. Mold design must allow for fine-tuning; cavity dimensions cannot be directly determined based on a single value.
❌ Myth 2: Ignoring anisotropic shrinkage and focusing only on linear shrinkage
Focusing solely on shrinkage in one direction, without optimizing mold design for flow direction, will easily lead to warping and deformation of final product, especially for crystalline plastics (PP/PE). Difference in shrinkage between flow and vertical directions must be considered.
❌ Myth 3: Using high-shrinkage materials for precision assemblies
For example, using pure PP for buckles, with a shrinkage rate of 1.0%~2.0%, results in large dimensional deviations. Buckles will either be loose or jammed. ABS or fiberglass PP should be preferred.
❌ Myth 4: The higher holding pressure, the better
Excessive holding pressure will lead to flash, high internal stress, and easy cracking after demolding. A balance must be found between "counteracting shrinkage" and "no flash".
❌ Myth 5: Measuring dimensions immediately after demolding
Plastics continue to shrink at room temperature after demolding (especially crystalline plastics). Measurements should be taken after 24 hours to ensure accurate finished product dimensions; otherwise, mold compensation will be inaccurate.

Molding shrinkage rate is a core indicator connecting mold design, material selection, and production. It not only determines dimensional accuracy of product but also directly affects its appearance (shrinkage marks, warpage) and assemblability (jamming, looseness), making it crucial for "precision molding" in plastic processing.
Remember core logic: Select materials based on their shrinkage rate level according to precision; compensate for shrinkage and optimize flow in mold according to shrinkage rate; fine-tune holding pressure/mold temperature based on actual shrinkage. By coordinating these three aspects, you can fundamentally solve problems such as dimensional deviations, warpage, and shrinkage marks.