Problems encountered in the processing of acrylic products

    Acrylic Products Processing Phenomenon and Solution for Injection Molding Defects Acrylic Products Processing During injection molding, warp, deformation, bubbles, cracks, wrinkles, and defects such as pitting, shrinkage, overflow, weld lines, burns, silver lines, and jet lines are common. What kind of solutions should we adopt when these defects occur? Below we have a long history of experience in sharing the experience with our customers.

I. Warpage, deformation Warpage and deformation of injection products are very difficult problems. The main problem should be solved from the mold design, and the adjustment effect of the molding conditions is very limited. The causes and solutions for warpage and deformation can be referred to the following items:

1) When deformation is caused by residual stress caused by molding conditions, the injection pressure can be lowered, the mold temperature can be increased and the mold temperature can be uniformed and the resin can be improved. The temperature or the annealing method is used to eliminate the stress.

2) When the stress is deformed due to poor mold release, it can be solved by increasing the number or area of the push rod and setting the draft angle.

3) The cooling method and extended cooling time can be adjusted because the cooling method is not suitable and the cooling is not uniform or the cooling time is insufficient. For example, a cooling circuit can be placed as close as possible to the deformation.

4) For the deformation caused by the molding shrinkage, it is necessary to correct the design of the mold. Among them, it is more important to pay attention to the consistency of the wall thickness of the product. Sometimes, in the case of a last resort, the mold must be trimmed in the opposite direction by measuring the deformation of the product. A resin with a large shrinkage ratio is generally a crystalline resin (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin) than a non-crystalline resin (such as acrylic resin, polyvinyl chloride, polystyrene, ABS resin). And AS resin, etc.) have large deformation. In addition, since the glass fiber reinforced resin has fiber orientation, the deformation is also large.

Second, according to the cause of the bubble, the solution to the problem is as follows:

1) When the wall thickness of the product is large, the outer surface is cooled faster than the center. Therefore, as the cooling progresses, the resin at the center portion expands toward the surface while shrinking, causing insufficient filling at the center portion. This condition is called a vacuum bubble. The main solutions are:

a) According to the wall thickness, determine the reasonable gate and sprue size. Generally, the height of the gate should be 50% to 60% of the wall thickness of the product.

b) Until the gate is sealed, leave a certain amount of supplemental injection.

C) The injection time should be slightly longer than the gate closure time.

d) Reduce the injection speed and increase the injection pressure.

e) Use a material with a high melt viscosity grade.

2) The bubbles caused by the generation of volatile gases are mainly solved by:

a) Full pre-drying. b) Reducing the temperature of the resin to avoid decomposition gas.

3) The bubbles caused by poor fluidity can be solved by increasing the temperature of the resin and the mold and increasing the injection speed.

Third, cracks Cracks are a common defect in plastic products, the main reason is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by the external environment.

(-) Cracks caused by residual stress Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. As a crack generated in the case of overfilling, the solution can be mainly started in the following aspects:

(1) Since the pressure loss of the sprue is the smallest, if the crack is mainly generated in In the vicinity of the sprue, consider using a multi-point distribution point gate, side gate and shank gate method.

(2) Under the premise of ensuring that the resin does not decompose or deteriorate, the proper increase in the resin temperature can lower the melt viscosity and improve the fluidity, and at the same time, the injection pressure can be lowered to reduce the stress.

(3) Under normal circumstances, stress is easy to occur when the mold temperature is low, and the temperature should be appropriately increased. However, when the injection speed is high, even if the mold temperature is lower, the stress can be reduced.

(4) If the injection and holding time are too long, stress will also be generated, and the effect of appropriately shortening or performing the Th holding pressure switching is better.

(5) Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, etc., which are more likely to cause residual stress than crystalline resins such as polyethylene and polyoxymethylene, should be noted.

When demolding is introduced, the ejection force is too large, stress is generated due to the small draft of the mold release, and the mold-type glue and the punch are rough, and sometimes whitening or cracking occurs even around the push-out rod. Just look carefully at the location of the crack and determine the cause.

When inserting metal parts while injection molding, it is most prone to stress, and it is easy to crack after a period of time, which is extremely harmful. This is mainly due to the fact that the thermal expansion coefficients of the metal and the resin are in great difference to generate 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 an experience, the wall thickness 7" and the outer diameter of the embedded metal piece

general-purpose polystyrene is basically unsuitable for the insert, and the insert is for the nylon The influence is minimal. Because the glass fiber reinforced resin material has a small thermal expansion coefficient, it is suitable for inserts.

In addition, preheating the metal insert before molding also has a good effect.

(2) Cracks caused by external stress The external stress here is mainly due to unreasonable design, resulting in stress concentration, especially at sharp corners. R/7" is 0.5 to 0.7.

(3) Cracks caused by the external environment Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials can deteriorate physical properties and cause cracks.