The inner and outer walls of the part are formed simultaneously and integrally, but interior and exterior designs are essentially independent so we review them separately. As the design develops, the designer should begin thinking about the interaction of the plastic and the mould that will produce the part. The interior surface of double-wall blow moulded parts is normally formed by a mould core. Since the mould core must fit inside the cavity, there should be no question it meets the same core blow ratio = W>2D overall size requirement as the cavity.

The half of the parison that is draping over the mould core is already beginning to set as the air is injected into the parison. As in the cavity, the plastic begins to stretch to fit the mould contour. Almost no flow occurs. Unlike the cavity, some different rules apply.

BLOW RATIO

After the material has been selected, the part design needs to be examined to determine if the blow ratio, or the ratio of depth-to-width in the mould cavities, is acceptable. A poor blow ratio can cause the inability to maintain minimum wall thickness, thin spots in the corners or deep draw areas, unnecessary weight increase, part shrink, and warpage. This is why blow ratio is one of the most important design constraints in blow moulding.

Determining the blow ratio will help determine localized thinning (areas of the part that might get thin during the blow moulding process). Keep in mind, when a part design has multiple blow ratio conditions, each drawing area can affect the other. Once you’ve determined the blow ratio, adjustments can be made to the thickness of the parison as it is being extruded, enabling it to have different thicknesses at specific points along the tube to compensate.

Calculating the blow ratio on your own is a daunting task. There are dozens of formulas online, each suggesting a different way of calculating a different scenario. Even if you find a formula that looks applicable, variables such as material and part requirements may render your findings not applicable. This is why we highly encourage you to involve your supplier from the start. Some suppliers, such as Regency Plastics, have software that simulates the blow moulding process so problem areas can be tackled before any tooling is built.

Blow Ratio – W>D 

As the mould halves close on the parison, the core presses against the parison and forces it into the cavity until the pinch-off is sealed around the perimeter of the part. The highest point on the core forms the deepest depression inside the part.

If the double-wall part design has a dividing wall between two compartments, this wall is formed by stretching the plastic into a groove in the mould core. As the plastic begins stretching into a groove, it begins to thin. If the groove is too deep, the plastic quickly reaches the point where it thins until the internal air blows out through the wall to the outside of the part. No part will form.

Because of this, there is one simple yet absolute rule, which governs the design of the ribs or divisions between compartments. The depth (D) of the groove between core sections must not exceed the width (W) of the rib W>D. This rule also applies to other structural shapes. For example, a 1” tall, round post in the centre of a tray would have to be 1” or more in diameter.

If the part design requires a mould parting line that steps to various levels for the part to function properly, then the core must have a positive draft on these steps at the pinch-off to match the pinch-off on the cavity element of tooling. Varying pinch-off levels can change the W-D relationships of nearby pockets or ribs. All of the levels within a part must pass the W>D requirement in each direction.

Sidewalls & Draft 

When the mould closes, half of the parison is draped over the mould core to form the interior of the part. As the plastic cools, it shrinks onto the metal mass of the mould core. A positive draft is needed on all sides of the mould core to remove the plastic part after it has shrunk. The more generous the draft, the easier the part can be removed from the mould. Apart from a 5° positive draft on all sides of the core can be removed with the assistance of ejector pins. Parts with lesser draft can also be removed with ejector pin assistance but as the draft on the core decreases, the risk of damaging the part during ejection increases.

If a core design requires a no-draft or back-draft section, a positive draft should be provided on the opposite side of the core, if possible. Snap-fits and small undercuts can be fine-tuned to allow ejection.

With core-cavity moulds, the parison becomes fixed at two levels, the top of the core and the pinch-off. When the part is blown, the fixed plastic walls stretch (no flow) to meet the sidewall of the core. A deep core with a little draft and a sharp corner will produce a thin, weak-walled part. Draft, corner radii and chamfer-angles can help eliminate thin walls

Shrinkage & Warpage

Overall the shrinkage of the interior will match the shrinkage of the exterior. But, the shrinkage of an interior shape is restricted by the metal core used to form the shape. Minor mould size adjustments may be needed to meet specific dimensions.

Interior part design must consider the potential for warpage. Warp will be caused by variations in wall thickness and material distribution during cooling. Both W>2D overall sizing and W>D localized draw ratios need to be followed throughout the part design to prevent warpage

Cooling

Frequently, the metal mass of the core is greater than the cavity and will require a greater cooling capacity. Targeting waterlines for optimal heat extraction can be critical to the success of the part.

Venting

Any location where air can be trapped between the parison and the mould wall is a location for a vent. Deep cores can trap large volumes of air and the blowing speed can require a larger venting capacity for the trapped air to escape. If there is doubt, it is better to include a vent than to discover the problem at production.

TO BE CONTINUED ON BELOW LIINK

PART 2 ON EXTERIOR SURFACE (MOLD CAVITY) DESIGN: http://cmppin.com/blog/exterior-surface-mold-cavity-design/