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March 6, 1999

4 Min Read
Long Tubular Parts Injected Through the Core

Editor's note: Today's mould construction is strongly supported by computer aided analysis and design. However, the mouldmaker's experience and skills are still significant contributions to successful moulding of high-quality parts. This series of articles will illustrate the significant design and construction details of moulds that solve certain problems for challenging parts. This month's article was adapted from Injection Molds: 108 Proven Designs, by Hans Gastrow, 1993.

The polypropylene toothpaste dispenser is a cylindrical, 146-mm-long article, essentially consisting of two tubular sections. One section has an internal diameter of 36 mm and is 26 mm long. The second part is 120 mm long with an internal diameter of 38 mm. A partition between the two sections is equipped with various functional components.

Gating Options

If one were to gate this moulding at a single point on the outside, the long core would deflect due to the unilateral entry of the melt; the article would not fill uniformly.

If a number of gates were distributed around the periphery, the core would not deflect, but the design would leave gate marks and mean a large percentage of wasted material in the runner.

Internal gating would be possible with a three-plate mould and a break-away pinpoint gate. However, an even greater material loss in the sprue would be unavoidable, as the specific shape of the partition allows gating only through the long tubular section.

How the Problem Was Solved

An externally heated, temperature-controlled hot runner nozzle can be employed here to advantage, with the melt being carried in a tube electrically heated to the required processing temperature at low voltage (3 to 5V) and effectively insulated against heat losses. As Figure 1 illustrates, it is thereby possible to employ a hollow core (9) equipped with a cooling spiral inside the long core for the tubular section of 38-mm internal diameter, which accommodates the 200-mm-long hot runner nozzle in a bore of 22-mm diameter.

This nozzle is equipped with a cone-shaped heated tip entering the gating point. Thus a small ring gate is created that ensures clean article separation. There is no interference between cooling system and hot runner nozzle; the article therefore cools rapidly.

The melt conveying system inside the manifold (Figure 2) is designed differently from that in the nozzle. The melt-carrying heating tube (a) is electrically insulated and surrounded by a supporting tube (f). This tube system is enclosed in a stationary layer of rigid material (d) and rests in a bore of the manifold plate (4). The "frozen" material layer (d) acts as heat insulation, so that the manifold plate (4) is allowed to make full surface contact with the two adjacent mould plates without requiring any other form of heat containment. A hot runner system so designed combines the advantages of externally heated hot runners (thermally homogeneous melt) with those of the internally heated systems (simple manifold construction, good thermal insulation).

The bores in the manifold plate (12) (Figures 1 and 3) have been arranged in two layers, one above the other. Thus the channels serving the eight cavities — grouped in two rows in the mould — can be of equal length, so that a natural balancing of the flow resistances can be achieved between the sprue bushing and the cavities.

Mould Temperature Control

The short cores (2) (Figure 1) are accommodated in the moving mould half. They have been equipped with an effective spiral cooling system (8). The mould cavities are formed by two cylindrical sleeves each (3, 4), around which spiral cooling grooves have been arranged. Even the stripper rings (16) have a grooved ring for cooling.

Part Release and Ejection

The mould opens at the parting line (I). Plates (5, 6) are retained by the latches mounted on the fixed half of the mould, so that the parts remain on the two cores (1, 2) to start with. They are displaced only in relation to the cavity (3, 4).

When the opening stroke has reached distance (S), the pair of plates (5, 6) stops, the tubes remain on the long core (1), and the cavities (3, 4) are further withdrawn. The short cores (2) are pulled, and the undercut at the left end of the articles is stripped.

One sliding core (11) each is housed in the center of the cores (2) with a displacement stroke (W). When the articles are released from the short cores (2), each core (11) travels with the tube for the distance of stroke (W). Having reached the end of the stroke, the core — which had formed the part's partition — is only then released from it. Finally, the stripper plate (7) pushes the parts off the long cores (1).

A cushioning device (13) has been fitted in the stripper plate (7). Its two ends, which protrude beyond the plate (7), enter bores in the cavity plate (14) and the core retaining plate (15) with a friction fit. This prevents the mould plates from chattering during mould closing. The core (1) returns the sliding core (11) to its starting position with the closing movement.

Contact Information
Günther hot runner Systems
Mr. Eckart Spork
Sachsenberger Straße 3
D-35066 Frankenberg
Phone: (49) 6451 5008-0
Fax: (49) 6451 5008-50

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