Design & Engineering

The custom-pak blow Moulding design guide provides you with basic design tools for making engineered blow Moulded parts. This guide focuses on the extrusion blow Moulding process. No two designs are alike, so the Mould and process must be adjusted to optimize each design. Software products can help predict Moulding characteristics and our engineers are here to help make your product great. Our design assistance is confidential and free.

• Blow-Moulding Process
• Materials
• Capturing the Parison
• Exterior Cavity-Mould Design
• Interior Core-Mould Design
• Air Space
• Creating Structure
• Finishing

Blow-Moulding Process

Blow Moulding is a simple five-step sequence.

The first step involves mixing, melting and pushing plastic (extrusion) to form it into a tube called a parison that will be used to make the part.

Materials

Material selection is a critical aspect of design and should involve serious study of:

• The plastic resin properties
• The material cost
• The processing properties
• Your finished-part objectives

Capturing the Parison

In order to design a blow-Moulded product, you must understand the interaction between the molten plastic parison and the Mould. If you’ve blown a bubble from bubble gum you can understand blow Moulding. The plastic material stretches like the gum and if it gets too thin it ruptures. Since the parison is extruded as a tube, it is easy to make a tube or bottle shaped part, not much stretching occurs. The two Mould halves open, the parison is inserted, the Mould halves close and the part is blown. The split between Mould halves is known as the parting line. There is often a knife like edge on the parting line around the part shape known as pinch-off.

If the part shape to be Moulded is changed from a tube into a flat panel type part, the parison tube must be flattened to make the panel. When this happens the circumference of the parison becomes the surface that needs to cover the width of the panel. So we try to have a large enough parison diameter that as it flattens, it can be captured by the entire perimeter of the panel at the pinch-off. If the parison does not extend to all areas of the pinch-off, it must stretch the rest of the way.

The soft plastic can stretch only a short distance before it begins thinning. Like the bubble gum, the first thin spot is weakest and it gets thinner fastest until it pops. If the plastic parison pops it is called a “blow-out” and results in no part formation at all.

As the complexity of the part progresses to double-wall shapes with side walls and inner contours, the parison must not only be captured at all points along the parting line, but it must also meet the material thickness needs for the variety of Moulding conditions specific to each area of the part. Many of the design criteria used to make a tray with Moulded inner shapes will be the same for designing a complex industrial part.

Exterior Surface (Mould Cavity) Design

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 visual exterior of many products is formed in one half of the Mould called a cavity.

Interior Surface (Mould Core) Design

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 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.

Air Space

It is the combination of closing the Mould on the parison and the expansion of air inside the parison that forms the part. The designer must leave sufficient space between the inner and outer part surfaces to permit adequate blowing of air into every square inch of the part. If the air passage inside the part is reduced or obstructed, the part will not form.

Creating Structure

The double walls in blow Moulded parts provide engineers with a tremendous opportunity to create structure within the plastic part. A properly designed double-wall part will be substantially stronger than a ribbed single-wall part of equal weight and can easily outperform metals in many applications. There are several ways to add strength to blow Moulded part designs.

Finishing

There are a limitless number of secondary operations that can be performed on a blow-Moulded part to meet the needs of the finished product. Drilling, sawing, milling, cnc routing, die cutting, punching, riveting, screwing, sonic, spin or heat welding, and surface treating are common operations. Nearly any secondary step can be performed economically if the right equipment is available. A surprising number of operations can be done in-Mould. Custom-Pak provides a huge variety of secondary operations equipment at no charge. There are many ways to get the appearance that you want in your blow Moulded parts. We can help you make sure your design will look great long after the consumer has made their purchase.

Selection Guide

Blow Moulding Selection Guide

EXTRUDER

EXTRUDER

SINGLE PARISON

MOULD CLAMPING UNIT

HYDRAULIC SYSTEM (optional)

Mechanical

Features

Optional

Video

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