Acrylic Molding: PMMA Injection Molding Design & Processing
Acrylic, also known as Poly(methyl methacrylate) or PMMA, is a polymer that is frequently used in injection molding. It is renowned for its clarity and amazing resistance to breaking. Additionally, acrylic is a great substitute for conventional glass materials because it combines strength and lightness. It is useful for exterior applications such as windows and enclosures due to its resistance to moisture and ultraviolet (UV) radiation. PMMA is translucent by nature, but it can be easily colored or made opaque for particular applications.
Next, we look into the essential elements of carrying out a successful acrylic molding project as well as the material qualities of PMMA and acrylic molding processes.
Continue reading to delve deeper into this versatile material.
Benefits of Acrylic Molding
If you are contemplating the use of acrylic for your injection-molded components, here are key advantages to consider when assessing this popular polymer.
Energy Efficiency
Plastic injection molding is an energy-intensive process. Acrylic stands out as an energy-efficient choice due to its lower processing temperature, reducing overall energy consumption. This efficiency translates to shorter cycle times and cost savings, as less energy is required for heating and cooling.
Weather Resistance
Injection-molded acrylic exhibits innate resistance to sunlight and remains unaffected by exposure to rainwater. This makes PMMA an ideal option for outdoor applications demanding material clarity and long-term environmental durability.
Dimensional Stability
Maintaining precise dimensions is crucial for injection-molded parts. Acrylic’s outstanding dimensional stability makes it a top choice, especially for high-volume production runs where tool wear and processing conditions can impact consistency.
Chemical Resistance
Injection-molded acrylic demonstrates resistance to a variety of chemicals, including detergents, diluted acids, alkalis, and aliphatic hydrocarbons. This versatility makes acrylic molding products well-suited for various industrial and transportation applications.
Optical Transparency
With light transmittance ranging between 91% to 93%, acrylic excels in applications requiring clear and lightweight materials. Its remarkably low haze value further enhances optical clarity, surpassing other polymers in this regard.
Recyclability
Acrylic is entirely recyclable, making it an eco-friendly alternative to glass. Once virgin acrylic has served its primary purpose, it can be repurposed in other products instead of being discarded in landfills.
Opting for acrylic molding provides a range of advantages, encompassing energy efficiency, durability in diverse environments, and exceptional optical properties, while also promoting sustainability through recyclability.
DFM Analysis (Design for Manufacturability)
Sungplastic offers a complimentary DFM analysis along with your quote, providing critical insights into potential issues or risks for tool manufacturing and injection molding. Address any identified issues by updating the CAD design to avoid costly tooling modifications. This stage is crucial because adjustments made during the design phase are significantly more efficient and economical than modifications made post-tooling.
Additionally, Sungplastic will inform you about how tooling and injection molding will impact your plastic parts, considering factors such as:
Gate location (ensuring it doesn’t affect the cosmetic surface)
Parting line and split line location (evaluating their aesthetic impact)
Ejection mark placement (inside and non-disruptive to functionality)
Knit line appearance (where flow fronts meet)
Other risks like tolerances and deformation
Guidelines for Designing Acrylic Molding
When it comes to acrylic molding, adhering to standard design guidelines is essential to minimize defects and address potential processing challenges. The expertise of Sungplastic’s injection molding professionals can assist you in this process, and we also provide a valuable injection molding design guide for reference.
Here are specific design considerations for acrylic molding. These design considerations are instrumental in ensuring the successful and efficient production of acrylic molding parts.
Wall Thickness
Maintain a consistent wall thickness for injection-molded acrylic parts within the range of 0.025 to 0.150 inches (0.635 to 3.81 mm). Avoid abrupt and substantial changes in adjacent wall thicknesses.
Radii
To enhance moldability and reduce stress concentrations, avoid sharp corners, as acrylic is particularly sensitive to them. Use a minimum radius equal to 25% of the wall thickness. For maximum strength, a radius equal to 60% of the wall thickness is recommended.
Draft Angle
Similar to other injection-molded plastics, acrylic parts should include a draft angle ranging from 0.5° to 1°. This angle is essential to ensure easy ejection of parts from the mold without causing damage. In cases of highly polished surfaces, such as optically clear moldings, a greater draft angle may be necessary.
Part Tolerance
Injection-molded parts come with two primary types of part tolerances: commercial and fine. For acrylic molding parts under 160 mm, commercial tolerances typically range from 0.1 to 0.325 mm. Fine tolerances, which achieve greater precision, fall within the range of 0.045 to 0.145 mm and are suitable for injection-molded acrylic parts equal to or smaller than 100 mm.
Acrylic Material Properties
Similar to other injection-molded plastics, acrylic’s defining material properties can be outlined. Among these properties, mold shrinkage and water absorption take on special significance due to their connection with the drying and processing procedures. Additionally, mechanical attributes like tensile and flexural strength play a vital role in the context of part design.
PMMA Physical Properties:
Density (g/cm3): 1.13-1.19
Shrinkage Rate (%): 0.4-0.61
Rockwell Hardness (R): 71-102
PMMA Mechanical Properties:
Tensile Strength at Yield (PSI): 6,390-10,700
Elongation (%): 3.0-12(Break)
Flexural Modulus (PSI): 247,000-509,000
Flexural Strength (PSI): 6,770-18,900
PMMA Injection Molding:
Drying Temperature: 75-91°C
Drying Time: 3.4-5.1 hrs
Melt Temperature: 225-272°C
Mold Temperature: 59.4-81.1°C
Acrylic Material Processing
Proper processing of acrylic, like other types of injection-molded plastics, is vital to prevent part defects and related issues. Here are the key factors to consider in acrylic molding.
Moisture
Acrylic absorbs moisture over time, and while it can be used in damp environments, it’s crucial to thoroughly dry acrylic pellets before processing. Failure to do so can result in molded parts exhibiting cloudiness, bubbles, or surface defects. The standard drying period for PMMA ranges from 3.4 to 5.1 hours.
Temperature Control
Overheating acrylic can lead to thermal decomposition, characterized by brown or burn marks on molded parts. This can occur due to problems with temperature control or screw speed settings on the molding machine.
Injection Pressure
Acrylic’s viscosity often necessitates higher injection pressures, although lower pressures may suffice for smaller, simpler, or thicker products. Excessive pressure can lead to parts sticking and increased production costs, while inadequate pressure can result in out-of-tolerance parts.
Injection Speed
Injection speed is a critical processing consideration for acrylic. Excessive speed can cause plastic burning or thermal decomposition, leading to defects like weld lines, poor finishes, or discoloration. Conversely, a slow injection speed may result in incomplete mold filling.
Shrinkage
All plastics experience some degree of shrinkage during injection molding. Acrylic exhibits shrinkage rates ranging from 0.4% to 0.61%, which, while lower than other plastics, remains a significant factor. Shrinkage primarily occurs within the mold but can continue after ejection.
To ensure the dimensional accuracy of acrylic molding parts, injection molds must be designed to accommodate the anticipated shrinkage of the plastic. Various factors, including injection pressure, melt and mold temperatures, and holding pressure, can influence shrinkage. When collaborating with Sungplastic, you’ll have access to our skilled engineers and partners with extensive expertise in acrylic molding, including design for manufacturing (DFM) feedback.
Acrylic Injection Molding from Sungplastic
As a professional plastic molding manufacturer, we are equipped to assist you with the design of custom acrylic molding parts, including prototyping, and production, support color match and various surface finishing, small parts and large parts manufacturing, batch production and mass production.
Additionally, PMMA CNC machining, vacuum forming, PMMA sheets extrusion, PMMA products assembly and other services are all available.
If your projects are related to acrylic molding or other processing techniques, please contact us. If you want to study more about clear parts injection molding, kindly read our LED Lens manufacturing cases…
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