Acrylic is also called polymethyl methacrylate (PMMA). Due to the defects , such as low surface hardness, easy scratching, low impact resistance and poor molding fluidity, the modification of PMMA appears alot. Such as copolymerization of methyl methacrylate with styrene and butadiene, mixing of PMMA with PC, etc.
The flow of PMMA is worse than that of PS and ABS. The melt viscosity is sensitive to temperature changes. In the molding process, the melt viscosity is changed mainly from the injection temperature.
PMMA is an amorphous polymer with a melting temperature of more than 160 ° C and a decomposition temperature of 270 ° C.
PMMA has a certain degree of water absorption, which reaches 0.3-0.4%, while injection molding must have a humidity below 0.1%, usually 0.04%.
The presence of water causes bubbles, streaks, and reduced transparency in the melt.
Therefore, drying treatment is required.
The drying temperature is 80-90 DEG C with time more than 3 hours.
Recycled materials can be used 100% in some cases, and the actual quantity depends on the quality requirements, usually over 30%.
Recycled materials should avoid foreign materials, otherwise transparency and the properties of finished products will be affected.
Injection Molding Machines
PMMA has no special requirements for injection molding machines. Because of its high melt viscosity, it needs a deeper spiral groove and a nozzle with larger diameter hole.
If the strength of the product is required to be high, low-temperature plasticization shall be adopted with screws of larger length-diameter ratio.
In addition, PMMA must be stored in a drying hopper.
Mold and Gate Design
The temperature of the mold can be 60-80℃, the diameter of the main runner should match the internal taper, and the best angle is 5-7.
If the PMMA molded product with 4 mm wall thickness is to be injected, the draft angle should be 7, the diameter of the main runner should reach 8-10 mm, and the overall length of the gate should not exceed 50 mm.
For products with wall thickness less than 4 mm, the diameter of runner shall be 6-8 mm.
For products with wall thickness greater than 4 mm, the runner diameter should be 8-12 mm. The depth of side gate, tab gate should be 0.7-0.9 T(T is the wall thickness of the product), and the diameter of pin point gate should be 0.8-2 mm; Smaller size should be selected for low viscosity.
Vent slots are within 0.05 depth and 6 mm wide, and the demoulding angle is between 30′-1, cavity side: 35′-1 30.
Shoot to air : from 210℃ to 270℃, depending on the information provided by the supplier.
Re-track the backseat, make the injection machine nozzle away from the main runner bushing, and then manually do plasticizing injection molding, which is the way to shoot to air.
Fast shoot is available, but to avoid high internal stress, multi-stage injection, such as slow-fast-slow, should be used. Slow injection speed is used for thick parts.
If the temperature is 260℃, the residence time should not exceed 10 minutes at most, and if the temperature is 270℃, the residence time should not exceed 8 minutes.
How to solve PMMA injection crack?
Acrylic molded parts are usually subject to warpage, deformation, bubbles, cracks, sinks, flash, weld lines, burns, silver streaks, spray lines and other defects during injection molding.
What solution should we adopt when warpage occurs?
Cracking is a common defect in plastic molding, and the main reason is due to stress deformation.
There are mainly residual stress, external stress and stress deformation caused by external environment.
The residual stress is mainly caused by the following three factors, i.e. over filling, ejection and metal insert.
Residual stress due to over filling, the solution can be mainly started from the following aspects:
Due to the minimum pressure loss of the sprue gate, if cracks are mainly generated near the gate, the multi-point distribution of point gates, side gates and tab gates can be considered .
Ensuring that the resin does not decompose or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity, and at the same time it can reduce the injection pressure to reduce the stress.
In general, stress is easy to occur when the mold temperature is low, and the temperature should be appropriately increased. However, when the injection speed is higher, even if the mold temperature is lower, the possibility of stress can be reduced.
Too long injection and pressure holding time will also lead to stress, and it is better to shorten it appropriately or switch pressure holding for Th times.
Amorphous resins, such AS PS resin, ABS resin, PMMA resin, etc. are more prone to residual stress than crystalline resins, such as polyethylene, polyoxymethylene, etc.
When demoulding , due to small draft angle and rough mold core surface, the ejection force is too large, resulting in stress and sometimes even whitening or cracking around the ejector pins. The reason can be determined by carefully observing the location of cracks.
When metal parts are embedded during injection molding, stress is most likely to occur, and cracking is likely to occur after a period of time, which is extremely harmful.
This is mainly because the thermal expansion coefficients of metal and resin are very different, resulting in stress, and with the time, the stress exceeds the strength of the gradually deteriorated resin material, resulting in cracks.
In order to prevent the cracking caused by this, as a rule of thumb, polystyrene are basically not suitable for metal inserts molding, and the metal inserts have the least influence on nylon. As the thermal expansion coefficient of glass fiber reinforced resin material is small, it is more suitable for inserts. In addition, preheating the metal insert before molding also has good effect.
The external stress here is mainly caused by stress concentration due to unreasonable design, especially at sharp corners. R/7 “-0.5-0.7 is preferable.
Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials will degrade physical properties and produce cracks.