Acrylic is one of the versatile plastic materials with extensive applications in various industries. As it is, the acrylic molding method helps to produce different high-quality plastic parts with excellent properties.
This article explores the acrylic injection molding process. You’ll also learn about its typical applications and tips for successful acrylic molding operations. Let’s get to it!
What is Acrylic (PMMA) Injection Molding?
The acrylic (PMMA) injection molding method involves injecting molten acrylic into a mold cavity or chamber under high pressure to create products with desired shapes after cooling and solidification. It is ideal for producing different components, such as plastic auto parts, mobile phone screens, lamp holders, and aquariums.
PMMA exhibits remarkable physical properties similar to glass, including excellent transparency, flexural strength, and extensive tensile strength. As a result, it is an ideal substitute for glass in various applications.
The Process of Acrylic (PMMA) Injection Molding
The injection molding process is effective for making acrylic parts. However, the acrylic injection molding process may vary depending on mold design, intended product requirement, and equipment used. Below is a stepwise breakdown of the PMMA injection molding process.
Step 1: Design and Mold Preparation
The first stage of acrylic molding is drafting a design for the components or products using a computer-aided design. Then, you have to create a mold according to the prepared design. Usually, the mold comprises two halves: a cavity and a core. These components define the shape and properties of the end product.
Step 2: Material Preparation
The acrylic resin used for injection molding takes the form of granules or small pellets. You must dry the acrylic resin to extract any moisture content present. Moisture in acrylic can adversely affect the quality of molded products.
Step 3: Injection Molding Machine Setup
You have to mount the mold on the injection molding machine. Ensure the machine is programmed with the proper parameters, including injection speed, temperature, and pressure depending on the specific acrylic type.
Step 4: Injection Process
The acrylic resin is heated and melted as it is fed to the machine’s hopper. Then, inject the molten acrylic into the mold cavity with a nozzle and sprue aid under high pressure. The pressure fills the mold wholly and uniformly.
Step 5: Cooling and Solidification
Once the mold cavity has been filled with molten acrylic, the molten begins to cool gradually and solidify within the mold. You can utilize water channels or other cooling mechanisms integrated with the mold to improve the cooling process. However, it would help to note that the complexity and thickness of acrylic molds often determine the cooling time.
Step 6: Mold Opening and Ejection
Open the mold to separate the two halves after the acrylic molded part has solidified. Use an ejection mechanism like an air blast or ejector pins to eject the part from the mold cavity successfully. Then, you can prepare the part for further post-processing operations and clean the mold for the next cycle.
Finishing Operations for Injection Molded Acrylic Parts
Product designers often resort to further post-production processing in the PMMA injection molding process to achieve the preferred finish and improve the functionality of acrylic molds. These include:
Trimming
You can cut excess material or flash, which may be on molded parts after molding. Trimming usually involves using cutting tools or computer-controlled trimming tools to create the product’s desired shape and size.
Sanding or Polishing
It would help if you subjected acrylic molds to further processing, such as sanding and polishing, to eradicate flaws such as tool marks, rough edges, or scratches. With abrasive materials or polishing compounds, you can achieve a smooth, glossy physical appearance.
Surface Treatment
There are various surface treatment techniques for enhancing the physical appearance and functionality of acrylic molded parts. These include:
- Clear Coating: It involves applying a clear protective layer that offers additional protection and enhancement for the molded part’s scratch resistance and durability.
- Vapor Polishing: Exposing the acrylic molds to a vaporized solution like acetone smoothens their surface, removing any flaws.
- Flame Polishing: This technique involves exposing the product’s surface to a controlled flame. The flame melts the surface of the acrylic part slowly, giving it a polished finish.
Painting
You can further paint or decorate injection molded acrylic parts to achieve the preferred aesthetic appearance or colors. These effects can be achieved through dip coating, spray painting, or applying special coatings such as textured or metallic finishes.
Parts Assembly
Part assembly may become a necessary finish operation in acrylic injection molding if your final products require a series of parts or additional features. It usually involves connecting or joining various acrylic parts with mechanical fasteners, adhesives, or ultrasonic welding.
Advantages of Acrylic Molding
Acrylic molding helps to produce products with excellent visual appeal, cost-effectiveness, and durability. As a result, acrylic is a widely embraced thermoplastic in various product manufacturing. Below are some of these advantages:
High Light Transmission and Optical Clarity
Acrylic molded components offer brilliant optical clarity because PMMA plastics possess precisely 92% light transmission. Moreover, polishing acrylic molds eradicates surface flaws, improves surface smoothness, and improves refractive index consistency, resulting in greater light transmission and optical clarity.
The 92% light transmission and about 1.5 refractive index of acrylic allow it to offer clarity similar to glass and water. Therefore, manufacturers commonly use PMMA in different optical-related applications like car lighting lenses or vehicle windshields.
Abrasive Resistance
Acrylics are a better alternative to glass because they offer a glass-like appearance. In addition, the resistance level is similar to that of glass and greater than that of other plastics, such as polyethylene and polypropylene. As a result, they exhibit a clear glossy appearance for a longer period of their usage lifespan. Additionally, acrylic molds don’t lose their shiny feature during and after processing.
High Color Flexibility
You can combine acrylic plastic with different resins to produce various colored products. Also, you can further improve specific properties of the molded parts using additives. These additives modify the common properties of light diffusion, optical effects, UV light filtering, impact resistance, and chemical resistance.
Complex Design
Injection molded PMMA allows easy and effective production of intricate designs than glass. PMMA melts at lower temperatures and offers good flow properties in the molten state. Its low viscosity allows the material to fill the mold evenly and completely to make intricate features accurately. Besides, acrylic low shrinkage properties ensure high dimensional stability and precision in the molded parts.
Limitations of Acrylic Molding
Although acrylic molding offers a wide range of benefits, some setbacks hinder the feasibility of acrylic molding for specific applications. Below are some of these limitations:
Chemical Sensitivity
Acrylic molds are highly sensitive to chemicals and solvents like acetone, particular oil, and acids. Even though acrylic material shows remarkable chemical resistance to common chemicals like diluted acids, detergents, and inorganic chemicals, it softens or degrades when exposed to incompatible chemicals.
Tooling and Mold Costs
The precision tooling and molds required for acrylic molding cost more to design and create. The process requires highly intricate and precise mold designs to achieve the complex geometries and precision the technique offers. As a result, this tooling and mold cost can hinder small-scale production runs.
Thermal Sensitivity
PMMA possesses lower heat resistance, making it deform or tender at elevated temperatures. As a result, it is not suitable for applications with thermal stability requirements or constant exposure to high temperatures. Therefore, you should always consider the temperature limitation of acrylic to avoid deformation.
Design Considerations for Acrylic Injection Molding
It is advisable to evaluate each design based on the preferred requirements of the intended products. It would be best to consider variables such as suitable acrylic grade, mold gate location, ventilation, and other critical factors. Below are some of the engineering considerations for a successful PMMA injection molding process:
Wall Thickness
It is crucial to ensure the mold maintains a uniform wall thickness that encourages the even flow of molten acrylic resin during injection molding parts production. It is advisable to avoid sharp variations in thickness as they cause issues such as warping, sink marks, or incomplete mold filling.
For instance, housing intended for electronic devices should possess an even wall thickness to avoid deformation and ensure structural integrity.
Use Fillets and Radii for Corners
It is advisable to generously utilize corner radii instead of sharp corners to lower stress concentrations. Sharp corners can lead to weak points in parts, increasing the risks of the parts breaking or cracking.
For example, adding fillets to a display case’s edges and sharp corners enhances its overall durability while mitigating the risk of cracking.
Surface Finishing
Although acrylic molds possess excellent transparency, it is essential to specify whether the preferred surface finish is smooth, high-gloss, or textured for enhanced grip. For example, a diffused or frosted surface finish improves light dispersion in lighting fixtures.
Gate Placement
Position the mold gate properly to facilitate an even circulation of molten acrylic resin within the mold cavity. Strategic gate placement mitigates aesthetic defects like flow lines or visible gate marks on the final products.
Draft Angles
It would be best to use draft angles in designing vertical surfaces. It allows smooth ejection of acrylic molded parts from the mold. The draft angles facilitate the smooth release of the part, mitigating the risk of distortion and damage.
Automotive Applications of Acrylic Molded Parts
PMMA prototypes and auto parts are in high demand in the automotive industry due to their cost-effectiveness, excellent transparency, lightweight feature, and impact resistance. Below are the typical examples of acrylic molded parts in the automotive industry:
Interior Lighting
Since acrylic permits the diffusion and transmission of light, it creates a pleasant and visually appealing interior appointment. PMMA molded products apply to interior lighting applications like light guides and ambient lighting panels.
Exterior Trim and Grilles
The design flexibility and resistance to environmental factors make acrylic molded parts suitable for making exterior trim elements and grilles. Using acrylics for automotive injection molding allows you to produce complex patterns and textures.
Headlamp Lenses and Tail Lamp Covers
Engineers in the automotive industry commonly use acrylic to produce headlamp lenses and tail lamp covers. It offers the preferred light transmission and adequate illumination needed for headlamp lenses. At the same time, it provides the transparency and color vibrancy required for tail lights. However, acrylics allow you to design lenses with complex shapes to add different lighting features.
Side Mirror Casings
The impact resistance and lightweight property of acrylics make it suitable for making side mirror casings. As such, it allows the creation of aerodynamic and fancy mirror designs.
Futuristic Concept or Design
Acrylic PMMA is an ideal material that different automakers use to showcase new vehicle concepts to get the attention of their customers. The futuristic look and material properties of PMMA it suitable for this application.
Conclusion
The acrylic injection molding process is reliable for producing high-quality plastic products for different applications. However, you must understand the workings of the process, its pros and cons, and design considerations to achieve the best results.
AutoProtoWay is the ideal one-stop shop for your acrylic molding and parts needs. Our professional injection molding technologies allow us to produce different automotive lighting products with complex shapes and designs that meet required quality standards. We also have the expertise to handle various challenges during the injection molding operation. Contact us today, and let professionals handle your next project!
FAQs
What are the common practices that ensure acrylic mold parts last longer?
There are several ways to ensure the longevity of your acrylic parts. These include keeping them cleaned and trimmed always. More importantly, don’t clean them with abrasive cleaners or harsh chemicals.
Does acrylic decompose over time?
Acrylic is not a biodegradable material because the constituting elements used to make acrylics are artificial. Thus, it is a synthetic product and doesn’t decompose over time.
Do ultraviolet rays damage acrylic injection molding parts?
Acrylic parts exhibit remarkable resilience to Ultraviolet rays and similar weather conditions, making them suitable for outdoor uses. Some acrylic grades can block about 98% of high levels of UV rays.
