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PMMA Acrylic Injection Molding Process

Date: Jul 29 2021 | By:LC Rapid
Tag:Injection Molding,

PMMA is commonly known as plexiglass, acrylic, etc. The chemical name is polymethyl methacrylate. Due to the shortcomings of PMMA surface hardness, easy scratching, low impact resistance, poor molding fluidity, etc., the modification of PMMA has appeared one after another, such as the copolymerization of methyl methacrylate with styrene and butadiene, the blending of PMMA and PC, etc. 372 plexiglass is obtained by copolymerization of methyl methacrylate and styrene. If a small amount of nitrile rubber (about 50%) is added to 372 plexiglass, 373 plexiglass will be produced.

The flow of PMMA is worse than that of PS and ABS, and the melt viscosity is more sensitive to temperature changes. In the injection molding process, the melt viscosity is mainly changed from the injection temperature. PMMA is an amorphous polymer with a melting temperature greater than 160°C and a decomposition temperature of 270°C.


Ⅰ. Acrylic injection molding process

1. Treatment of plastics

PMMA has a certain degree of water absorption, with a water absorption rate of 0.3-0.4%, and injection molding must have a humidity below 0.1%, usually 0.04%. The presence of water causes bubbles, gas lines, and reduced transparency in the melt. SoPMMA needs to be dried. The drying temperature is 80-90℃, and the time is more than 3 hours. Recycled materials can be used 100% in some cases, the actual amount depends on the quality requirements, usually more than 30%. Recycled materials should avoid pollution, otherwise it will affect the transparency and the properties of the finished product.

2. Selection of injection molding machine

PMMA has no special requirements for injection molding machines. Because of its high melt viscosity, a deeper screw groove and a larger diameter nozzle hole are required. If the strength of the product is higher, a screw with a larger length to diameter ratio should be used for low-temperature plasticization. In addition, PMMA must be stored in a dry hopper.

3. Mold and gate design

The mold temperature can be 60℃-80℃, the diameter of the sprue should match the inner taper, the best angle is 5° to 7°. If you want to inject 4mm or more products, the angle should be 7°, the diameter of the sprue can reach 8 to 10mm, and the overall length of the gate should not exceed 50mm. For products with a wall thickness of less than 4mm, the diameter of the runner should be 6-8mm.

For products with wall thickness greater than 4mm, the runner diameter should be 8-12mm diagonal, fan-shaped and tabular gate depth should be 0.7 to 0.9t (t is the thickness of the product wall), and the diameter of the needle gate should be 0.8 to 2mm; smaller size should be used for low viscosity.

Common vent holes have a depth of less than 0.05 and a width of 6mm. The demolding angle is 30'-1°, and the cavity part is 35'-1°30°.

4. Melting temperature

It can be measured by air injection method: ranging from 210°C to 270°C, depending on the information provided by the supplier. Withdraw the back seat, make the nozzle of the injection molding machine leave the sprue bushing, and then manually perform plasticization injection molding, which is air injection molding.

5. Injection temperature

Fast injection can be used, but avoid high internal stress, and it is advisable to use multi-stage injection, such as slow-fast-slow, etc. When injecting thick parts, use slow speed.

6. Detention time

If the temperature is 260°C, the residence time should not exceed 10 minutes, and if the temperature is 270°C, the residence time should not exceed 8 minutes.

Ⅱ. What should I do if the acrylic product warps during injection molding process?

Acrylic products generally have defects such as warpage, deformation, bubbles, cracks, wrinkles, pitting, shrinkage pits, overflow, weld marks, burns, silver lines, and jet lines during injection molding. What kind of solutions should we adopt when warping occurs?

Cracking is a common defect in plastic products, and the main cause is stress and deformation. There are mainly residual stress, external stress and stress deformation caused by the external environment.

The residual stress is mainly caused by the following three conditions, namely, overfilling, ejection from the mold, and metal inserts. As the cracks produced in the case of overfilling, the solution can be mainly in the following aspects:

Since the pressure loss of the sprue is the smallest, if the cracks are mainly generated near the sprue, you can consider switching to the multi-point distribution point gate, side gate and shank gate method.

On the premise of ensuring that the resin does not decompose or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity. At the same time, it can also reduce the injection pressure and stress.

Under normal circumstances, the mold temperature is easy to produce stress when the mold temperature is low, so the temperature should be increased appropriately. But when the injection molding speed is high, even if the mold temperature is lower, the stress generation can be reduced.

Too long injection and pressure holding time will also produce stress. It is better to shorten it appropriately or perform Th times of holding pressure switching.

Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, etc. are more likely to produce residual stress than crystalline resins such as polyethylene and polyoxymethylene, so attention should be paid.

When demolding and pushing out, due to the small demolding inclination, the mold rubber and the roughness of the punch, the pushing force is too large, causing stress and sometimes even whitening or cracking around the pushing rod. As long as you carefully observe the location of the cracks, you can determine the cause.

In inserting metal parts during injection molding, stress and cracks are likely to occur after a period of time, which is extremely harmful. This is mainly due to the large difference between the thermal expansion coefficients of metal and resin, which produces stress. And over time, the stress exceeds the strength of the gradually degraded resin material to cause cracks.

In order to prevent the resulting cracks, as a rule of thumb, general-purpose polystyrene with a wall thickness of 7" and the outer diameter of the embedded metal part is basically not suitable for adding inserts, and the inserts have the least impact on nylon. The resin material of glass fiber reinforcemen has a small thermal expansion coefficient, which is more suitable for inserts. In addition, preheating the metal inserts before injection molding also has a better effect. 

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