Table of contents

BAVIA ERRORS IN PLASTIC MASTERS

1.DEFINITION

     Molds for engineering plastics have very high precision and meticulousness. The precision of the mold is at the micron level. And the mold is closed by high clamping pressure during injection molding. However, the mold opens slightly and creates a gap when the injection pressure is too high. Burrs arise when the plastic enters that gap. Burrs arise mainly at the PL line, slide core, vents, etc.

     Bavia is a phenomenon in which plastic enters the middle of the parting surface (PL surface) of the mold and  causes an unwanted thin film on the product. Burrs arise when the surface of the mold part opens under the influence of plastic pressure or when the PL surface has a gap in the middle.

     When reading the description of Bavia as an accent wall that causes the plastic to touch the dividing line, it may be a bit difficult for the reader to relate. But when you associate it with the leftover nail powder in Taiyaki cake (Photo 2), it will be easy to imagine Dinh lines. With Taiyaki, it is a profit, but with plastic molded products, it will be an NG product

2. Cause of bavia error

2.1. Resin application is too high

     When the plastic pressure is too high, the mold opens and causes burrs. On the other hand, mold clamping force that is too small also causes burrs. Factors that create high plastic pressure include:
          – Injection speed is too high
          – Injection pressure is too high
          – Holding pressure is too high
          – V-P position transition is too slow
     In general, when product surface appearance is highly required, the holding pressure is very likely to be pushed too high, especially to fix dents, it is often set higher than the standard. The result is burrs.

2.2. The plastic flow is too high

     As the flowability of the plastic increases, the tendency for the plastic to enter the gap increases causing larger burrs. In general, as the resin temperature and mold temperature increase, the likelihood of burr defects becomes greater, and as the temperature decreases, the likelihood of burr defects becomes smaller.

2.3. Gap between mold parting surface

     Even for two-piece molds, when a defect is formed on the mold due to a mistake during injection molding or the like, burrs are sometimes generated at the defect site. When the core slide is used, special attention should be paid to the gap between the slide surfaces. Although the mold is made of steel and the mold clamping pressure is very large, the plastic pressure is also very large. Therefore, most molds are deformed during the forming process. In particular, mold deformation is evident in large injection molded products, where the presence of Pillar Support affects the formation of burrs (The absence of a Pillar causes larger deformation, larger gap, thus causing larger burrs.

2.4. For the case of PPS plastic

     PPS plastic has the disadvantage of easily forming burrs as the plastic's characteristics of high flowability with low displacement range. Therefore, when using PPS plastic, more attention should be paid to solutions to overcome burrs than other plastics. Mold accuracy is also required at a more stringent level than other plastics.

3. Anti-burr solution

3.1. Reduce injection pressure

     The following methods are used to reduce the plastic pressure:
          – Reduce spray speed
          – Lower injection pressure
          – Lower holding pressure
          – Create V-P transitions earlier.

3.2. Check mold clamping force

     Check if there are any errors in the installation conditions. This is a very important step.

3.3. Accelerate the solidification process of plastic

     Bavia are prevented by accelerating the solidification of the resin, such as reducing the Nozzle temperature or the mold temperature. (Both methods can be done simultaneously)

3.4. Facial analysis test

     Check whether there is any gap or defect in the parting surface. The actual deformation degree of the mold can be determined by the dial gauge. When the deformation is too large, the repair of the mold structure should be considered. In large injection molding products, the installation of a Support Pillar should be considered.

3.5. For PPS plastic

     For PPS, you may encounter a wide plastic sheet at the thin part such as gas vent. The maximum thickness for gas vent is around 10μm. Please increase the number of gas vents. The accuracy of Slide core and Ejector Pin is required to be higher than other plastics.

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