Polymethyl methacrylate (PMMA) or acrylic is a well-established thermoplastic. It has come to be well known in the plastic industry. It is a good choice for clear plastic products. Often as a replacement for glass. At first sight, a clear PMMA product resembles glass. You’d have to know a bit more than average to differentiate between PMMA, Polycarbonate, and glass. Where polycarbonate is too expensive PMMA serves as a good alternative. This is true for parts where the properties of polycarbonate are not necessary. Examples of such are jewelry, plaques, aquariums, windows, and display pieces. The common term for glass type PMMA sheets is plexiglass. It is also used in high-end applications such as hip replacement and lenses for eyewear. PMMA is lightweight and inexpensive. With the growing concern over the potential hazard of bisphenol A in polycarbonate. PMMA is a safer option.
A German chemist called Otto Rohm discovered PMMA in the year 1901. This form is not so much like the commercial PMMA we know today. So further developments followed in later years. By 1943, PMMA technology had developed to a more processible form. And later into PMMA that could get cured at room temperature. Its earliest application was in dentistry. Later found use in orthopedics as thigh and hip replacements. These applications are down to the low toxicity, lightweight, and processibility of PMMA. It can get formed into the complex shapes required for the biological environment. As the technology to produce PMMA advanced, it became more available and cheaper. The applications have since extended to more commodity goods. Thus making PMMA a very versatile plastic whose application spreads across several industries. With the development of injection molding, it is even more versatile. Today PMMA gets injection molded into countless products. While you can injection mold PMMA into a countless number of products. You should also be aware of the challenges that exist in the injection molding of PMMA. This article looks at the possible problems that can occur in the injection molding of PMMA. It then gives a set of solutions to address these problems.
Images of PMMA products in different colors.
Injection Molding of PMMA
Acrylic polymerization occurs via the free radical chain polymerization of methyl acrylate monomer. The suspension polymerization method is often used. The starting material is usually PMMA beads or pellets. The material should be free of solvents by drying to avoid any problems of bubbles or trapped air. Injection molding of PMMA follows the same process as conventional injection molding. The plastic gets introduced in the pellet form through the hopper. This then gets melted and mixed by the screw rotating within a heated barrel. The molten plastic gets injected into a mold. It gets held under pressure until it cools and hardens. Cooling occurs through cooling fluid passed through cooling channels around the mold. Once the product forms it gets ejected. After the product gets released some post-processing may be necessary. This includes removing the gate and runners and any flashes. These get done using sharp tools to remove these excess plastic protrusions. In the typical case of injection molding, not much post-processing is necessary. The best case is to have a ready to use a product from the mold. This is the injection molding process in a nutshell. But when each stage gets analyzed in detail we see that the specific properties deviate. Each of the plastics calls for settings and procedures suited to that plastic. So although the injection molding of acrylic follows the conventional injection molding process. Some problems are more common or peculiar to injection molding of PMMA. These get discussed further down in the article. In doing so we also look at the solutions to each of these problems.
Process parameters for injection molding of PMMA
| Process parameters | Value |
| Feed : Compression: metering | 50% : 25% : 25% |
| L/D ratio of screw | 20:1 |
| Compression ratio | 3:1 |
| Nozzle | Short and wide |
| Mold surface | Polished and fine |
| Mold material | Tool steel |
| Orientation | Vertical or horizontal |
Properties of PMMA
| Property | Value |
| Tensile strength | 55 – 76 GPa |
| Elongation at break | 2% |
| Flexural strength | 83 to 117 GPa |
| Compressive strength | 76 to 131GPa |
| Melting point | 130 to 140oC |
| Service temperature | >80oC |
Advantages of PMMA
PMMA poses some advantages as an injection moldable plastic. When injection molding plastics you consider the processing as well as the application. So below looks at the advantage of the injection molding process and in use. These include the following:
Abrasion resistance
They are often used as alternatives to glass. So they must keep a glass-like appearance. Although they do not have the level of resistance as glass. When compared to other plastics they are better. PMMA offers better abrasion resistance than plastics like polypropylene or polyethylene. So for a larger proportion of their usage lifetime, they keep a clear glossy appearance. In the processing, this means that the material will not lose its gloss during production.
Optical Clarity
PMMA gives brilliant optical clarity. With a well-managed process with no imperfection, you get very clear products. It has a 92% light transmission and a refractive index up to 1.5. To compare, that of water is 1.3 and that of air is 1.0 while for crown glass is 1.5. So PMMA gives clarity a little below that of water and pretty much the same as glass.
PMMA eye protection.
Lighter Alternative to Glass
One of the limitations of glass is that it is heavy. This is what leads to the replacement of the glass with alternatives like PMMA. Silica-based glass has a density of 2.5 g/cm3 this is much heavier than that of PMMA which is less than 1.22 g/cm3. To compare water has a density of 1g/cm3. This means PMMA will sink in water but is not as heavy as silica-based glass. When considering this in the injection molding process. It means that there is that much less of a struggle against gravity.
Design Complexity
Compared to glass, it is easier to achieve complex designs using injection-molded PMMA. This is for a combination of reasons. For one thing, PMMA melts at a much lower temperature. So getting PMMA into a viscous melt form requires less input of energy. But glass will need a modified setup. Injection molding of glass fiber filled plastics poses a set of challenges. These we discussed in our previous blog posts on glass fiber injection molding. Combined with other properties like biocompatibility. The range of applications also extends to medical industries.
Colors and Pigments
Where needed, PMMA mixes well with resin for adding colors to the plastic. So PMMA can get made into transparent and translucent colored sheets and products. You might have noticed the white PMMA used in some mobile phone shops for display. You can also have large or small colored PMMA signs. Often made into life-size lettered structures. Some get made into illuminated letter signs.
PMMA illuminated letter sign
Low Shrinkage
PMMA has a shrinkage value of between 0.2 and 0.8%. This is low compared to other plastics that can achieve close to 10% shrinkage. PMMA also shrinks in an isotropic manner. This results in better product dimensional stability.
Regrind and Recycle
PMMA is a thermoplastic so it is recyclable. This can be during the production process or after its useful lifespan. As long as you stay below the degradation temperature and limit the number of heating cycles.
Limitations of PMMA
While PMMA is a great option for clear glass-like products, some limitations exist. It is important to be aware of this in the product design as well as process design. Knowing these earlier prevent a lot of headaches further down.
Lower impact resistance
PMMA has a lower impact resistance compared to other replacement for clear glass. When choosing PMMA for any application, consider the conditions it gets used in. PMMA used in windows, lenses, and screens are not put under such high impact forces. Unlike other applications like a guard or helmet shields.
Low heat resistance
The low glass transition temperature makes it easier to injection mold PMMA. But it does raise a disadvantage in its application. PMMA has a glass transition temperature that varies between 85 to 150oC. Exact value depending on the grade. Unlike glass that can withstand a temperature range of over 1000 degrees celsius. PMMA gets limited to applications below around 80oC. One of the ways to identify if a glassy material is PMMA is to expose it to boiling water. PMMA wouls soften under this condition while silica-based glass will remain rigid. This means that PMMA cannot be heat sterilized. This is particularly important in applications such as food and medicine. This means other methods of sterilization gets used for PMMA. These are more expensive compared to heat.
Mechanical Properties
Properties like flexural strength, tensile, and compressive strength are compared to polycarbonate. So have this in mind when considering PMMA for your intended product. The impact strength of PMMA is around 6.5KJ/m2MPa. That of polycarbonate can be 5 times or more than this.
Chemical Resistance
PMMA is not so resistant to solvents. This limits its use in applications such as used for liquids and chemical storage. It also means it is prone to chemical attack and cannot get exposed to strong acids or alkali. Acrylic tends to be hygroscopic so will absorb moisture when exposed.
Possible problems in PMMA injection Molding and how to Troubleshoot.
See a range of possible problems in injection molding of PMMA listed in the table below. These get arranged in alphabetical order making it easy to trace the problem you need to get solved. Other problems that are general to injection molding are also possible with PMMA.
Table of the list of possible problems and solutions.
| Problem | Troubleshoot |
| Blister formation | Screw rotating too fast. Decrease rpmMake back pressure higherIncrease temperature in the moldChange gate positionReduce pellets particle sizeEnsure the plastic is well dried |
| Brittle part | The temperature in the mold too low, raise mold temperatureAdd more cooling channels or use better onesProper drying to prevent porousness or holesIncrease the size of the gateUse thicker walls but not too thick |
| Bubbles | Improve venting (size, number, or location of ventsRaise the injection pressure.Avoid moisture in the feed. Dry pellets at around 80 degrees for 4 hours before injection molding.Ensure nozzle and sprue size get suited to the purpose. |
| Burnt part | Vents may get clogged or not well doneCheck temperature control system and heatersChange gate locationCheck injection speed and pressureAdjust the screw rpm, might be shearing too much or not mixing well enough.Reduce the residence time of the plastic in the barrel.Use a screw that has a lower compression ratioIncrease the size of the gate and nozzle |
| Cracks/crazing on the surface | Use higher barrel and mold temperature.Raise the injection speedSet the injection pressure to match other settings |
| Dark Specks | Contamination in the feed Reduceegradation of additives by reducing concentration or lowering the temperatureInjection pressure too high, reduce itPolymer degradation due to high temperature Improper venting leading to trapped air causing overheatingCheck for burnt part stuck to screw or barrelReduce filler content to stop the degradation |
| Dirty looking part | Remove dust from the environment, close windows, and use air conditioningEnsure raw materials and pellets are clean and sealed in storageEnsure hopper gets cleaned wellClean oven before use for dryingClean and polish mold Remove all traces from previous runs. Especially when using other types of plastics.Ensure no oil residues or metal powder and particles get into the barrel or mold. |
| Jetting | Increase melt temperature by increasing temperature in barrelIncrease temperature in the moldIncrease injection speed and pressure to allow better and faster mold fillingAllow longer cycle time to ensure proper meltingUse wider gates and runners |
| Short shot | Increase feed-in rateRaise injection pressure and speedIncrease the temperature of the barrel to reduce viscosity.Raise the backpressure to allow better compactingAllow longer cycle time to give more time for melting and mold filling.Use wider channels to increase the flow rate of melt into the mold |
| Silver Streak formation on the product surface | Ensure proper drying to remove all moisture Raise injection pressure. Lower the temperature in the barrelIncrease the speed of injectionUse lower injection speed and change the back pressure to match settingsFor parts with thin walls, use lower barrel temperature and higher mold temperatureUse shorter but wider channels (gate and runners) |
| Void areas in part | Shrinkage rate too fastRaise the temperature of the mold to allow a more gradual cooling rate.Avoid thick walls where possibleAvoid barrel temperature being higher than needed.Proper drying of feed |
Conclusion
Injection molding of PMMA occurs in the conventional injection molding machine. It is compatible with vertical or horizontal injection molding. It can also get used with inserts and made into complex shapes and forms. Injection molding of PMMA is prone to common injection molding problems. But there are some which are more peculiar to PMMA. The article covers a host of these. Solutions are also given to these listed problems. Whether you need professional advice on your injection molding project. Or a mold making service. www.cavitymold.com is here to help.
