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?
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:
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:
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:
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:
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:
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:
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): 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?
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 polymer | Explicit name of the polymer | Min Value (%) | Max Value (%) |
|---|---|---|---|
| ABS | Acrylonitrile-Butadiene Styrene | 0.700 | 1.600 |
| ABS High Impact | Acrylonitrile-Butadiene Styrene High Impact | 0.400 | 0.900 |
| ABS/PC | Acrylonitrile-Butadiene Styrene/Polycarbonate | 0.500 | 0.700 |
| Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (High Flow) | Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (High Flow) | 0.800 | 1.000 |
| Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow) | Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow) | 0.800 | 1.000 |
| ASA | Acrylonitrile Styrene Acrylate | 0.400 | 0.700 |
| ASA/PC | Acrylonitrile Styrene Acrylate/Polycarbonate | 0.300 | 0.700 |
| ASA/PVC | Acrylonitrile Styrene Acrylate/Polyvinyl Chloride | 0.300 | 0.700 |
| CA – Cellulose Acetate | Cellulose Acetate | 0.300 | 1.000 |
| CAB – Cellulose Acetate Butyrate | Cellulose Acetate Butyrate | 0.200 | 0.900 |
| CP – Cellulose Proprionate | Cellulose Proprionate | 0.100 | 0.900 |
| CPVC – Chlorinated Polyvinyl Chloride | CPVC – Chlorinated Polyvinyl Chloride | 0.300 | 0.700 |
| ETFE | Ethylene Tetrafluoroethylene | 3.000 | 4.000 |
| EVA | Ethylene Vinyl Acetate | 0.400 | 3.500 |
| FEP | Fluorinated Ethylene Propylene | 3.000 | 6.000 |
| HDPE – High Density Polyethylene | HDPE – High Density Polyethylene | 1.500 | 4.000 |
| HIPS – High Impact Polystyrene | HIPS – High Impact Polystyrene | 0.200 | 0.800 |
| LCP | Liquid Crystal Polymer | 0.100 | 0.600 |
| LCP CF | Liquid Crystal Polymer carbon fiber | 0.100 | 0.500 |
| LCP GF | Liquid Crystal Polymer glass fiber | 0.100 | 0.400 |
| LDPE – Low Density Polyethylene | LDPE – Low Density Polyethylene | 2.000 | 4.000 |
| LLDPE – Linear Low Density Polyethylene | LLDPE – Linear Low Density Polyethylene | 2.000 | 2.500 |
| MABS | Transparent Acrylonitrile Butadiene Styrene | 0.400 | 0.700 |
| PA 11 30% Glass fiber reinforced | Polyamide 11 30% Glass fiber reinforced | 0.500 | 0.500 |
| PA 11 conductive | Polyamide 11 conductive | 0.700 | 2.000 |
| PA 11 flexible | Polyamide 11 flexible | 1.400 | 1.800 |
| PA 11 rigid | Polyamide 11 rigid | 0.700 | 2.000 |
| PA 12 conductive | Polyamide 12 conductive | 0.700 | 2.000 |
| PA 12 rigid | Polyamide 12 rigid | 0.700 | 2.000 |
| PA 46 | Polyamide 46 | 1.500 | 2.000 |
| PA 46 30% GF | Polyamide 46 30% glass fiber | 0.300 | 1.300 |
| PA 6 | Polyamide 6 | 0.500 | 1.500 |
| PAI | Polyamide-Imide | 0.600 | 1.000 |
| PAI 30% GF | Polyamide-Imide 30% glass fiber | 0.100 | 0.300 |
| PAI low friction | Polyamide-Imide low friction | 0.100 | 0.500 |
| PAN | Polyacrylonitrile | 0.200 | 0.500 |
| PAR | Polyarylate | 0.900 | 1.200 |
| PARA 30-60% GF | Polyarylamide 30-60% glass fiber | 0.100 | 0.400 |
| PBT | Polybutylene Terephthalate | 0.500 | 2.200 |
| PBT 30% GF | Polybutylene Terephthalate 30% glass fiber | 0.200 | 1.000 |
| PC high heat | Polycarbonate high heat | 0.700 | 1.000 |
| PC/PBT | Polycarbonate/Polybutylene Terephthalate blend | 0.600 | 1.100 |
| PCTFE | Polymonochlorotrifluoroethylene | 0.500 | 1.500 |
| PE 30% GF | Polyethylene 30% glass fiber | 0.200 | 0.600 |
| PEEK | Polyetheretherketone | 1.200 | 1.500 |
| PEEK 30% CF | Polyetheretherketone 30% carbon fiber | 0.000 | 0.500 |
| PEEK 30% GF | Polyetheretherketone 30% glass fiber | 0.400 | 0.800 |
| PEI | Polyetherimide | 0.700 | 0.800 |
| PEI 30% GF | Polyetherimide 30% glass fiber | 0.200 | 0.400 |
| PEI mineral filled | Polyetherimide mineral filled | 0.500 | 0.700 |
| PEKK– Low cristallinity grade | Polyetherketoneketone– Low cristallinity grade | 0.004 | 0.005 |
| PESU | Polyethersulfone | 0.600 | 0.700 |
| PESU 10-30% GF | Polyethersulfone 10-30% glass fiber | 0.200 | 0.300 |
| PET | Polyethylene Terephtalate | 0.200 | 3.000 |
| PET 30% GF | Polyethylene Terephtalate 30% glass fiber | 0.200 | 1.000 |
| PET G | Polyethylene Terephtalate Glycol | 0.200 | 1.000 |
| PE-UHMW | Polyethylene -Ultra High Molecular Weight | 4.000 | 4.000 |
| PFA | Perfluoroalkoxy | 3.000 | 5.000 |
| PHB – Polyhydroxybutyrate | Polyhydroxybutyrate | 1.200 | 1.600 |
| PI | Polyimide | 0.200 | 1.200 |
| PLA-injection molding | Polylactide-injection molding | 0.300 | 0.500 |
| PMMA | Polymethylmethacrylate (Acrylic) | 0.200 | 0.800 |
| PMP | Polymethylpentene | 1.600 | 2.100 |
| PMP 30% GF | Polymethylpentene 30% glass fiber | 0.300 | 1.200 |
| Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 40 % Filler by Weight | Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 40 % Filler by Weight | 0.300 | 0.300 |
| Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight | Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight | 0.300 | 0.300 |
| Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight | Polyamide 66 (Nylon 66)/Glass Fiber, Long, 40 % Filler by Weight | 0.300 | 0.300 |
| Polyamide 66 (Nylon 66)/Glass Fiber, Long, 50 % Filler by Weight | Polyamide 66 (Nylon 66)/Glass Fiber, Long, 50 % Filler by Weight | 0.300 | 0.300 |
| Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 30 % Filler by Weight | Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 30 % Filler by Weight | 0.400 | 0.400 |
| Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight | Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight | 0.300 | 0.300 |
| Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight | Polypropylene Homopolymer (PP Homopolymer)/Glass Fiber, Long, 40 % Filler by Weight | 0.300 | 0.300 |
| POM | Polyoxymethylene (acetal) | 1.800 | 2.500 |
| POM impact modified | Polyoxymethylene (acetal) impact modified | 1.000 | 2.500 |
| PP 10-20% GF | Polypropylene 10-20% glass fiber | 0.300 | 1.000 |
| PP copo | Polypropylene copolymer | 2.000 | 3.000 |
| PP homo | Polypropylene homopolymer | 1.000 | 3.000 |
| PP impact modified | Polypropylene impact modified | 2.000 | 3.000 |
| PPA | Polyphthalamide | 1.500 | 2.200 |
| PPA – 30% mineral | Polyphthalamide– 30% mineral | 1.000 | 1.200 |
| PPA – 33% glass fiber | Polyphthalamide – 33% glass fiber | 0.500 | 0.700 |
| PPE | Polyphenylene Ether | 0.500 | 0.800 |
| PPE 30% GF | Polyphenylene Ether 30% glass fiber | 0.100 | 0.400 |
| PPS | Polyphenylene Sulfide | 0.600 | 1.400 |
| PS 30 % GF | Polystyrene 30% glass fiber | 0.200 | 0.200 |
| PS crystal | Polystyrene crystal | 0.100 | 0.700 |
| PS high heat | Polystyrene high heat | 0.200 | 0.700 |
| PSU | Polysulfone | 0.700 | 0.700 |
| PSU 30% GF | Polysulfone 30% glass fiber | 0.100 | 0.600 |
| PTFE | Polytetrafluoroethylene | 3.000 | 6.000 |
| PTFE 25% GF | Polytetrafluoroethylene 25% glass fiber | 1.800 | 2.000 |
| PVC 20% GF | Polyvinyl Chloride 20% glass fiber | 0.100 | 0.200 |
| PVC rigid | Polyvinyl Chloride rigid | 0.100 | 0.600 |
| PVDC | Polyvinylidene Chloride | 0.500 | 2.500 |
| PVDF | Polyvinylidene Fluoride | 2.000 | 4.000 |
| SAN | Styrene Acrylonitrile | 0.300 | 0.700 |
| SAN 20% GF | Styrene Acrylonitrile 20% glass fiber | 0.100 | 0.300 |
| SMA | Styrene Maleic Anhydride | 0.400 | 0.800 |
| SMA 20% GF | Styrene Maleic Anhydride 20% glass fiber | 0.200 | 0.300 |
| TPS-Injection General Purpose | Thermoplastic Starch GP | 0.600 | 1.500 |
| XLPE – Crosslinked Polyethylene | XLPE – Crosslinked Polyethylene | 0.700 | 5.000 |
