By measuring the dimensions of the molded parts before and after the process, you can calculate the shrinkage. This method involves accurately measuring the length, width, and thickness of the parts.
Injection molding is a widely used manufacturing process for creating plastic products. One crucial aspect of this process is shrinkage, which affects the final dimensions and quality of the molded parts.
What is Injection Molding Shrinkage?
Injection molding shrinkage refers to the reduction in volume and size of the plastic material as it solidifies after being injected into the mold cavity. This occurs due to the change in temperature and pressure during the cooling process.
When does shrinkage occur?
Shrinkage occurs during the cooling phase of the injection molding process. As the plastic material begins to cool and solidify, it contracts, leading to a reduction in volume and size.
What happens if the molded parts shrink unequally?
If parts shrink unequally, it can lead to various issues such as warping, distortion, and poor dimensional accuracy. Uneven shrinkage can cause the parts to have internal stress, which may result in cracks or other defects.
What is the shrinkage calculation formula?
The shrinkage formula is typically expressed as:
Shrinkage = (Initial volume – final volume) / initial volume * 100%.
What are the methods to calculate mold shrinkage?
Use of a plastic shrinkage chart
A plastic shrinkage chart provides information about the shrinkage values of different plastics. These charts are based on the material properties and can be used to estimate the shrinkage of a specific plastic.
Use a shrinkage calculator
There are online shrinkage calculators available that can help you calculate the shrinkage based on the specific parameters of your injection molding process.
Measure the dimensions of the molded parts
By measuring the dimensions of the molded parts before and after the process, you can calculate the shrinkage. This method involves accurately measuring the length, width, and thickness of the parts.
Calculate the shrinkage
Once you have the initial and final dimensions of the parts, you can calculate the shrinkage using the formula mentioned above.
How can I minimize injection mold shrinkage?
Optimize the mold design
Uniform wall thickness: Ensure that the walls of the mold have a consistent thickness. A uniform wall thickness helps in even cooling and reduces the likelihood of uneven shrinkage. For example, if one part of the mold has a significantly thicker wall than others, it may cool slower and cause the part to shrink more in that area.
Flow channels design: Design the flow channels in the mold to ensure proper filling. A well – designed flow channel can prevent areas from being under – filled or over – filled, which can lead to shrinkage.
Ventilation: Provide proper ventilation in the mold to allow air to escape during the injection process. This helps prevent air traps that can cause shrinkage.
Adjust the injection process parameters
Temperature control: Maintain a consistent temperature during the injection process. A higher temperature can cause the plastic to expand more, and then shrink more during cooling. By carefully controlling the temperature, you can reduce the amount of shrinkage. For example, using a mold temperature control system to keep the mold at a stable temperature.
Pressure control: Apply appropriate pressure during injection. If the pressure is too high, it can cause the plastic to compress and then shrink more during cooling. On the other hand, if the pressure is too low, the part may not be fully filled, leading to shrinkage.
Injection speed: Adjust the injection speed to ensure proper filling of the mold cavity. A slow injection speed may result in uneven filling, while a fast injection speed can cause air to be trapped in the mold.
Select the right plastic material
Material properties: Different plastics have different shrinkage characteristics. For example, some plastics have a higher coefficient of thermal expansion, which means they will shrink more when cooled. Choose a plastic material with lower shrinkage properties.
Additives: Some additives can be used to reduce shrinkage. For example, fillers such as glass fiber can be added to the plastic to increase its strength and reduce shrinkage.
Conduct post - processing
Heat treatment: After the injection molding process, heat treatment can be used to reduce shrinkage. This involves heating the molded parts to a specific temperature and then slowly cooling them.
Surface treatment: Applying a surface treatment such as coating or plating can help reduce shrinkage. This can also improve the appearance and durability of the molded parts.
What are the shrinkage values (%) of various plastics?
| Polymer Name | Min Value (%) | Max Value (%) |
| ABS - Acrylonitrile Butadiene Styrene | 10.0 | 50.0 |
| ABS Flame Retardant | 2.0 | 80.0 |
| ABS High Heat | 2.0 | 100.0 |
| ABS High Impact | 2.0 | 100.0 |
| ABS/PC Blend 20% Glass Fiber | 1.90 | 2.10 |
| ABS/PC Flame Retardant | 50.0 | 90.0 |
| ASA - Acrylonitrile Styrene Acrylate | 15.0 | 40.0 |
| HDPE - High Density Polyethylene | 500.00 | 700.00 |
| HIPS - High Impact Polystyrene | 10.00 | 65.00 |
| HIPS Flame Retardant V0 | 10.00 | 50.00 |
| LCP - Liquid Crystal Polymer | 1.00 | 3.00 |
| LCP Carbon Fiber-reinforced | 1.00 | 1.00 |
| LCP Glass Fiber-reinforced | 1.00 | 3.00 |
| LCP Mineral-filled | 2.00 | 5.50 |
| LDPE - Low Density Polyethylene | 200.00 | 600.00 |
| LLDPE - Linear Low Density Polyethylene | 300.00 | 900.00 |
| PA 11, Conductive | 186.00 | 186.00 |
| PA 11, Flexible | 225.00 | 405.00 |
| PA 11, Rigid | 225.00 | 355.0 |
| PA 12, Fiber-reinforced | 4.00 | 8.00 |
| PA 12, Flexible | 300.00 | 340.00 |
| PA 12, Glass Filled | 30.00 | 40.00 |
| PA 12, Rigid | 250.00 | 390.00 |
| PA 46 - Polyamide 46 | 160.00 | 300.00 |
| PA 46, 30% Glass Fiber | 11.00 | 15.00 |
| PA 6 - Polyamide 6 | 200.00 | 300.00 |
| PA 6-10 - Polyamide 6-10 | 150.00 | 300.00 |
| PA 66 - Polyamide 6-6 | 150.00 | 300.00 |
| PA 66, 30% Glass Fiber | 2.00 | 2.20 |
| PA 66, 30% Mineral filled | 2.00 | 45.00 |
| PA 66, Impact Modified, 15-30% Glass Fiber | 3.00 | 10.00 |
| PAI - Polyamide-Imide | 3.00 | 15.00 |
| PAI, Low Friction | 7.00 | 9.00 |
| PBT - Polybutylene Terephthalate | 5.00 | 300.00 |
| PBT, 30% Glass Fiber | 2.00 | 3.00 |
| PC (Polycarbonate) 20-40% Glass Fiber | 2.00 | 4.00 |
| PC (Polycarbonate) 20-40% Glass Fiber Flame Retardant | 2.00 | 4.00 |
| PC - Polycarbonate, high heat | 50.00 | 120.00 |
| PC/PBT blend, Glass Filled | 2.00 | 4.00 |
| PE - Polyethylene 30% Glass Fiber | 1.500 | 2.500 |
| PEEK - Polyetheretherketone | 30.00 | 150.00 |
| PEEK 30% Carbon Fiber-reinforced | 1.00 | 3.00 |
| PEEK 30% Glass Fiber-reinforced | 2.00 | 3.00 |
| PEI - Polyetherimide | 59.00 | 60.00 |
| PEI, 30% Glass Fiber-reinforced | 3.00 | 3.00 |
| PEI, Mineral Filled | 6.00 | 6.00 |
| PEKK (Polyetherketoneketone), Low Cristallinity Grade | 80.00 | 80.00 |
| PET - Polyethylene Terephthalate | 30.00 | 70.00 |
| PET, 30% Glass Fiber-reinforced | 2.00 | 7.00 |
| PET, 30/35% Glass Fiber-reinforced, Impact Modified | 6.00 | 6.00 |
| PLA - Polylactide | 5.00 | 7.00 |
| PLA, High Heat Films | 179.00 | 181.00 |
| PLA,injection molding | 2.00 | 3.00 |
| PMMA - Polymethylmethacrylate/Acrylic | 2.00 | 10.00 |
| PMMA (Acrylic) High Heat | 2.00 | 10.00 |
| PMMA (Acrylic) Impact Modified | 4.00 | 70.00 |
| POM - Polyoxymethylene (Acetal) | 15.00 | 75.00 |
| POM (Acetal) Impact Modified | 60.00 | 200.00 |
| POM (Acetal) Low Friction | 10.00 | 70.00 |
| POM (Acetal) Mineral Filled | 5.00 | 55.00 |
| PP - Polypropylene 10-20% Glass Fiber | 3.00 | 4.00 |
| PP, 10-40% Mineral Filled | 30.00 | 50.00 |
| PP, 10-40% Talc Filled | 20.00 | 30.00 |
| PP, 30-40% Glass Fiber-reinforced | 2.00 | 3.00 |
| PP (Polypropylene) Copolymer | 200.00 | 500.00 |
| PP, Impact Modified | 200.00 | 700.00 |
| PPA - Polyphthalamide | 2.60 | 30.00 |
| PPA – 30% Mineral-filled | 1.10 | 1.30 |
| PPA, 33% Glass Fiber-reinforced | 2.00 | 2.20 |
| PPA, 45% Glass Fiber-reinforced | 227.00 | 229.00 |
| PPE - Polyphenylene Ether | 45.00 | 60.00 |
| PPE, 30% Glass Fiber-reinforced | 3.00 | 3.00 |
| PPE, Flame Retardant | 30.00 | 50.00 |
| PPE, Impact Modified | 40.00 | 60.00 |
| PPE, Mineral Filled | 20.00 | 40.00 |
| PPS - Polyphenylene Sulfide | 1.00 | 4.00 |
| PPS, 20-30% Glass Fiber-reinforced | 1.00 | 2.00 |
| PPS, 40% Glass Fiber-reinforced | 1.00 | 2.00 |
| PPS, Conductive | 0.50 | 3.00 |
| PS (Polystyrene) 30% glass fiber | 1.00 | 1.50 |
| PS (Polystyrene) Crystal | 1.00 | 4.00 |
| PS, High Heat | 1.00 | 4.00 |
| PSU - Polysulfone | 50.00 | 100.00 |
| PTFE - Polytetrafluoroethylene | 200.00 | 400.00 |
| PVC, Plasticized | 100.00 | 400.00 |
| PVC, Plasticized Filled | 200.00 | 500.00 |
| PVC Rigid | 25.00 | 80.00 |
| TPS, Injection General Purpose | 25.00 | 135.00 |
| TPS, Water Resistant | 2.00 | 2.00 |