Shrinkage Value of Plastics Material and Injection Molding

Shrinkage Value of Plastics Material and Injection Molding

Understanding shrinkage in plastics is crucial. This article dives into the concept, shedding light on the intricacies of volume contraction in molded materials.

But why focus on this topic? For both industry professionals and casual readers, gaining insights into the shrinkage value of plastic materials can significantly impact the quality of the final product in injection molding.

By reading this piece, you’ll equip yourself with essential knowledge, ensuring that molded parts meet the desired standards. Dive in and discover the significance of shrinkage in the world of plastics.

Shrinkage of Plastics Signifies Volume Contraction

When we speak of shrinkage in plastics, we’re talking about volume contraction. But what does this mean? Let’s explore this in simple terms.

Imagine blowing up a balloon. When you release the air, the balloon gets smaller. Similarly, plastic materials can also be reduced in size. This reduction is what we call ‘shrinkage.’

Now, think of a freshly molded plastic toy. Once out of the mold, it doesn’t stay the same size. It tends to get a bit smaller. This change isn’t due to magic. It’s because of shrinkage.

So why does this happen? When plastic is heated, it becomes soft and moldable. After shaping, as it cools, it solidifies. But as it cools, it contracts or shrinks in volume. This is a natural behavior of many materials, not just plastics.

For professionals in the plastic industry, understanding this shrinkage is crucial. It affects the final size and quality of the product. If not considered, you might end up with parts that don’t fit together or function well.

What is Shrinkage?

Shrinkage is a term many might relate to clothing or fabric, but it holds a different meaning in the plastics world. So, let’s break it down.

When we mold a plastic object, it starts in a liquid or semi-liquid form. This is because it’s heated. Once the desired shape is achieved, the plastic begins to cool. As it cools, something interesting happens: it shrinks or reduces in size.

This isn’t just about plastics becoming smaller. Shrinkage is about the volume contraction of plastic. In other words, if you had a chunk of plastic that was initially the size of a big apple, it might look more like a smaller apple after cooling and shrinking.

The reason for this behavior lies in the material’s nature. When heated, the molecules in the plastic become excited and move apart.

Cooling makes them relax and come closer together. This dance of molecules is what causes the plastic to shrink in size.

For someone making plastic products, this is a vital concept. Why? If you want a final product of a specific size, you need to account for this shrinkage. If not, you might end up with an item smaller than expected!

Read More: Best Ways to Prevent and Fix Sink Marks in Injection Molding

When Does Shrinkage Occur?

When Does Shrinkage Occur?

Shrinkage begins right after molding. During molding, plastics are heated to a molten state, making them flexible. This hot, fluid plastic fills up molds to make specific shapes.

The cooling process starts once the plastic is in the mold and shaped. It’s during this cooling phase that shrinkage kicks in. As the hot plastic cools down, it begins to contract.

Remember how things tend to come together when they cool? Think of hot steam turning to water droplets. Similarly, the molecules in the plastic move closer together as they cool. This movement is the primary reason behind shrinkage.

In simpler terms, it shrinks when the plastic goes from hot and flexible to cool and solid. So, the critical time for shrinkage is during the cooling phase post-molding.

What is the Significance of Shrinkage?

Shrinkage in plastics isn’t just a casual observation. It holds a great deal of importance, especially in the molding industry.

Let’s highlight the reasons why:

Quality Control:

Quality Control

Shrinkage affects the final size and shape of molded products. Properly accounting for shrinkage ensures the end product meets specified dimensions.

Functional Integrity:

If different product parts shrink unevenly, it can lead to warping or distortion. Consistent and predictable shrinkage ensures parts fit and function as intended.

Economic Implications:

Overlooking shrinkage might lead to product rejections or reworks, increasing costs. Manufacturers can minimize waste and save on resources by predicting and managing shrinkage.

Material Selection:

Material Selection

Different plastics have varied shrinkage rates. Knowing the significance of shrinkage helps choose the suitable plastic for specific applications.

Safety Concerns:

Uneven shrinkage can result in weak product points, leading to potential breakages or malfunctions. Properly addressing shrinkage ensures safer, more reliable products.

What Happens if the Molded Parts Shrink Unequally?

Unequal shrinkage in molded plastics can lead to various issues. It’s crucial to ensure consistent cooling to avoid these complications.

Here are the potential problems:

1. Warping:


Uneven shrinkage can cause parts to twist or bend out of shape. Such deformities can render the molded product useless or aesthetically unpleasing.

2. Internal Stresses:

Different rates of shrinkage within a product can introduce internal stresses. These stresses might lead to cracks or breaks in the product over time.

3. Weak Structural Integrity:

3. Weak Structural Integrity

Parts that shrink inconsistently can compromise the product’s overall strength. This can be problematic, especially for items that need to withstand external forces.

4. Assembly Issues:

If a product is made of multiple molded parts, uneven shrinkage can result in pieces not fitting together correctly.

5. Aesthetic Problems:

For products where appearance matters, uneven shrinkage can lead to visible defects.

6. Functional Failures:

Functional Failures

Devices or parts that rely on precise measurements can malfunction with uneven shrinkage. This can lead to operational failures, making the product unsafe or unreliable.

Check Out: Injection Molding Flash: Top 9 Causes and How to Solve Them

What Are the Causes of Variation in Molded Parts or Shrinkage?

Shrinkage in molded parts isn’t random. Several factors can influence how much a plastic part might shrink.

Let’s explore these causes:

Material Type:

Material Type

Different plastics have distinct molecular structures. These structures can react differently to cooling, causing varying shrinkage rates.

Molding Temperature:

The heat applied during molding can influence shrinkage. Too much or too little heat can result in uneven cooling and varied shrinkage.

Cooling Rate:

How fast a molded part cools is crucial. Rapid cooling might cause more shrinkage than slower, controlled cooling.

Mold Design:

Mold Design

The design of the mold, including its thickness and shape, can affect how a part shrinks. Uneven mold thickness can lead to inconsistent cooling and varied shrinkage.

Pressure in the Mold:

The amount of pressure applied when injecting plastic into the mold plays a role. Inconsistent pressure can result in uneven material distribution and different shrinkage rates.

Ambient Conditions:

The environment where molding takes place can impact shrinkage. High humidity or varying room temperatures might influence the cooling process.

Which Polymers Have High or Low Shrinkage?

Understanding which polymers exhibit high or low shrinkage is vital for those in the molding industry. Different projects might require materials with specific shrinkage characteristics.

Here’s a breakdown:

Polymers With High Shrinkage:

Polyvinyl Chloride (PVC)

  • Polyvinyl Chloride (PVC): PVC, commonly used for pipes and cable insulation, can exhibit high shrinkage upon cooling.
  • Polypropylene (PP): PP, seen in packaging and automotive parts, shrinks quite a bit after molding, especially when not filled with other materials.
  • High-Density Polyethylene (HDPE): This polymer, used in containers and pipes, also has a notable shrinkage rate, especially when molded in thicker sections.

Polymers With Low Shrinkage:

  • Polystyrene (PS): Often used in packaging and disposable tableware, polystyrene tends to have lower shrinkage, making it suitable for precision items.
  • Acrylonitrile Butadiene Styrene (ABS): Found in toys, computer cases, and various consumer goods, ABS boasts a lower shrinkage rate, which benefits tight-tolerance products.
  • Polymethyl Methacrylate (PMMA): Also known as acrylic or Plexiglas, PMMA is used for windows or displays and typically shows minimal shrinkage upon cooling.

What Are the Methods to Determine Shrinkage?

What Are the Methods to Determine Shrinkage?

To ensure the precision and quality of molded plastic products, it’s crucial to measure shrinkage accurately.

There are several tried-and-tested methods for this purpose:

1. Differential Scanning Calorimetry (DSC):

This method gauges the heat flow in and out of a polymer. DSC can determine the temperature at which the polymer contracts by monitoring heat changes during cooling.

2. Shrinkage Ruler Method:

Here, a special ruler with pre-defined markings is placed in the mold. After molding, the ruler is measured to see the differences, revealing the shrinkage percentage.

3. Volumetric Displacement:

This involves measuring the volume of a molded part before and after cooling. The volume change directly represents the amount of shrinkage.

4. Computer-Aided Engineering (CAE) Simulations:

Modern technology allows for virtual simulations. CAE can predict how a specific polymer will shrink under certain conditions, helping in the design phase.

Learn More: A Design Guide on Optimizing Parting Lines in Injection Molding

What Are the Shrinkage Values of Various Plastics?

Each plastic material exhibits different shrinkage characteristics. Molders need to be aware of these values to ensure the accuracy of final products.

Here’s a concise overview:

Name of the polymerExplicit name of the polymerMin Value (%)Max Value (%)
ABSAcrylonitrile-Butadiene Styrene0.7001.600
ABS High ImpactAcrylonitrile-Butadiene Styrene High Impact0.4000.900
ABS/PCAcrylonitrile-Butadiene Styrene/Polycarbonate0.5000.700
Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (High Flow)Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (High Flow)0.8001.000
Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow)Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow)0.8001.000
ASAAcrylonitrile Styrene Acrylate0.4000.700
ASA/PCAcrylonitrile Styrene Acrylate/Polycarbonate0.3000.700
ASA/PVCAcrylonitrile Styrene Acrylate/Polyvinyl Chloride0.3000.700
CA – Cellulose AcetateCellulose Acetate0.3001.000
CAB – Cellulose Acetate ButyrateCellulose Acetate Butyrate0.2000.900
CP – Cellulose ProprionateCellulose Proprionate0.1000.900
CPVC – Chlorinated Polyvinyl ChlorideCPVC – Chlorinated Polyvinyl Chloride0.3000.700
ETFEEthylene Tetrafluoroethylene3.0004.000
EVAEthylene Vinyl Acetate0.4003.500
FEPFluorinated Ethylene Propylene3.0006.000
HDPE – High Density PolyethyleneHDPE – High Density Polyethylene1.5004.000
HIPS – High Impact PolystyreneHIPS – High Impact Polystyrene0.2000.800
LCPLiquid Crystal Polymer0.1000.600
LCP CFLiquid Crystal Polymer carbon fiber0.1000.500
LCP GFLiquid Crystal Polymer glass fiber0.1000.400
LDPE – Low Density PolyethyleneLDPE – Low Density Polyethylene2.0004.000
LLDPE – Linear Low Density PolyethyleneLLDPE – Linear Low Density Polyethylene2.0002.500
MABSTransparent Acrylonitrile Butadiene Styrene0.4000.700
PA 11 30% Glass fiber reinforcedPolyamide 11 30% Glass fiber reinforced0.5000.500
PA 11 conductivePolyamide 11 conductive0.7002.000
PA 11 flexiblePolyamide 11 flexible1.4001.800
PA 11 rigidPolyamide 11 rigid0.7002.000
PA 12 conductivePolyamide 12 conductive0.7002.000
PA 12 rigidPolyamide 12 rigid0.7002.000
PA 46Polyamide 461.5002.000
PA 46 30% GFPolyamide 46 30% glass fiber0.3001.300
PA 6Polyamide 60.5001.500
PAI 30% GFPolyamide-Imide 30% glass fiber0.1000.300
PAI low frictionPolyamide-Imide low friction0.1000.500
PARA 30-60% GFPolyarylamide 30-60% glass fiber0.1000.400
PBTPolybutylene Terephthalate0.5002.200
PBT 30% GFPolybutylene Terephthalate 30% glass fiber0.2001.000
PC high heatPolycarbonate high heat0.7001.000
PC/PBTPolycarbonate/Polybutylene Terephthalate blend0.6001.100
PE 30% GFPolyethylene 30% glass fiber0.2000.600
PEEK 30% CFPolyetheretherketone 30% carbon fiber0.0000.500
PEEK 30% GFPolyetheretherketone 30% glass fiber0.4000.800
PEI 30% GFPolyetherimide 30% glass fiber0.2000.400
PEI mineral filledPolyetherimide mineral filled0.5000.700
PEKK– Low cristallinity gradePolyetherketoneketone– Low cristallinity grade0.0040.005
PESU 10-30% GFPolyethersulfone 10-30% glass fiber0.2000.300
PETPolyethylene Terephtalate0.2003.000
PET 30% GFPolyethylene Terephtalate 30% glass fiber0.2001.000
PET GPolyethylene Terephtalate Glycol0.2001.000
PE-UHMWPolyethylene -Ultra High Molecular Weight4.0004.000
PHB – PolyhydroxybutyratePolyhydroxybutyrate1.2001.600
PLA-injection moldingPolylactide-injection molding0.3000.500
PMMAPolymethylmethacrylate (Acrylic)0.2000.800
PMP 30% GFPolymethylpentene 30% glass fiber0.3001.200
Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 40 % Filler by WeightPolyamide 66 (Nylon 66)/Carbon Fiber, Long, 40 % Filler by Weight0.3000.300
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by WeightPolyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight0.3000.300
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by WeightPolyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight0.3000.300
Polyamide 66 (Nylon 66)/Glass Fiber, Long, 50 % Filler by WeightPolyamide 66 (Nylon 66)/Glass Fiber, Long, 50 % Filler by Weight0.3000.300
Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 30 % Filler by WeightPolypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 30 % Filler by Weight0.4000.400
Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by WeightPolypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight0.3000.300
Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by WeightPolypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight0.3000.300
POMPolyoxymethylene (acetal)1.8002.500
POM impact modifiedPolyoxymethylene (acetal) impact modified1.0002.500
PP 10-20% GFPolypropylene 10-20% glass fiber0.3001.000
PP copoPolypropylene copolymer2.0003.000
PP homoPolypropylene homopolymer1.0003.000
PP impact modifiedPolypropylene impact modified2.0003.000
PPA – 30% mineralPolyphthalamide– 30% mineral1.0001.200
PPA – 33% glass fiberPolyphthalamide – 33% glass fiber0.5000.700
PPEPolyphenylene Ether0.5000.800
PPE 30% GFPolyphenylene Ether 30% glass fiber0.1000.400
PPSPolyphenylene Sulfide0.6001.400
PS 30 % GFPolystyrene 30% glass fiber0.2000.200
PS crystalPolystyrene crystal0.1000.700
PS high heatPolystyrene high heat0.2000.700
PSU 30% GFPolysulfone 30% glass fiber0.1000.600
PTFE 25% GFPolytetrafluoroethylene 25% glass fiber1.8002.000
PVC 20% GFPolyvinyl Chloride 20% glass fiber0.1000.200
PVC rigidPolyvinyl Chloride rigid0.1000.600
PVDCPolyvinylidene Chloride0.5002.500
PVDFPolyvinylidene Fluoride2.0004.000
SANStyrene Acrylonitrile0.3000.700
SAN 20% GFStyrene Acrylonitrile 20% glass fiber0.1000.300
SMAStyrene Maleic Anhydride0.4000.800
SMA 20% GFStyrene Maleic Anhydride 20% glass fiber0.2000.300
TPS-Injection General PurposeThermoplastic Starch GP0.6001.500
XLPE – Crosslinked PolyethyleneXLPE – Crosslinked Polyethylene0.7005.000
Table . Shrinkage Values of Various Plastics
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