Pinch-off edge

This will also influence the selection of the mold steel. It is in each case useful to polish the shut-off edges in the demolding direction. The selection of a favorable shut-off edge for a certain molded part is essentially dependent on the experiences of each SMC processor and mold maker. However, it has to be ensured to avoid pinch-off edges (horizontal lands). 

Before the most suitable materials can be selected, the cost, product, and process requirements must be checked out. Table 1.6 lists some of these materials used in blow molds. Molds for large volumes (> 60 L) are manufactured using either cast zinc or aluminum. For molds with a volume > 1,000 1, aluminum should be used for weight reasons. The pinch-off edge areas are mostly made from steel (for casting molds or molds from forged aluminum) to make deflashing easier or to automate it and to increase the die life. In the last 5 years, the CNC-milled production blow mold (made from aluminum) has established itself. The aluminum industry has developed new materials for this application (Table 1.6). 

During mold closing, the pinch-off edges precut the product and the flash. The edges must he designed to achieve a good seam quality. The quality is dependent on the following parameters ... 

The shape of the pinch-off edge has a significant influence on the clamp force required to precut the flash. Table 1.7 gives typical values of the clamping force (in N/cm of edge), depending on the plastic used. 

Pinch-off edges are critical, in particular, for coextruded parts with wall structures of different materials, as well as for products made from Selar RB (DuPont). 

However, for all applications there are suitable geometries for the pinch-off edges with sufficient strength (Figure 1.153). The seams of coextruded products must ensure that the multi-material seam is properly layered and undisturbed [4]. 

FIGURE 1.153 Schematic illustration of various pinch-off edge geometries... 

I) Conventional pinch-off edge (II) coextrusion pinch-off edge (III) pinch-off edge for Selar RB (1) Flash pocket (2) cavity... 

Blow molds should be provided with stops on surfaces outside the cavity area to prevent damage to the pinch-off edges during setup or when clamping the mold without plastic. These stops protect the mold against excessive clamp force they also ensure parallel clamping and increase the mold life. 

Generally, in large volume production, pinch-off inserts are made of steel (P-20 or S-7) with the other portions of the blow mold produced from a nonfer-rous metal. The pinch-off edge should not be similar to a knife edge, or it will tend to act as a cutter and will yield a V groove where the tail or pinch-off area of the parison is forced to bond. Fig. 13.20 depicts a poor weld line due to the pinch-off having a knife edge. 

Many blow molded articles do not have an integral aperture that can accept a blow pin. In such cases, blowing air can be supplied through a blow needle that punctures the parison wall after the mold closes (Fig. 10.45). The needle can be advanced and retracted by means of an air or hydraulic cylinder and is located at a point close to a pinch-off edge. The parison is gripped... 

The knife-edge-cutter width of the pinch-off depends on the resin used, the wall thickness of the blown part, the size of the relief angle, the closing speed, and the time when blowing starts. As a general guide for small parts... 

Pinch-off n. (1) In blow molding, a raised edge around the cavity in the mold that seals off the part and separates the excess material as the mold closes around the parison. (2) In making tubular film, the paired rollers (pinch rolls) at the top of the tower that flatten the tube and confine the air in the bubble. 

Fig. 28. Poor weld line (weak) pinch-off had knife edge relief angle was either too large or too small.

Fig. 34. Design of welding edges and pinch-off pockets, s, welding edge width , opening angle of pinch-off pocket t, width of pinch-off pocket. Courtesy of Hoechst AG.

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