The design of a simple injection mold


An injection type mold is a mold into which a plastics material is introduced from an exterior heating, or plasticizing, cylinder.

Such a mold operates at a constant temperature; cool, or slightly warm.

Injection molds are principally used for thermoplastic materials, which set to shape on cooling.

The essential parts are the cavities, cavity retainer plates, cavity backing plates, ejector mechanism, adaptor plates to fit the press, and the system of sprue, runners and gates for introducing the material into the cavities.

The mold shown below is a simple ejector-pin type mold for molding the ink bottle cap . Two views of the mold are shown, one a sectional elevation, and the other a view facing the injection half.

The parting line is shown (detail 12). The cavities are arranged in a circle, and thus are equidistant from the sprue at the center.

Six cavity blocks (detail 18) are hobbed from mold steel, of low carbon content.

After nobbing, they are machined on diameter and length, with grinding stock allowed. They are then carburized, hardened, ground and polished. The core blocks (detail 23) are turned from alloy mold steel, and are hardened, ground, and polished.

Both cavity and core blocks are shouldered at the back end to be held in the retainer plates. ‘Polishing is confined to the molding portions, and grinding is done only to the fitting portions.

All six cores are ground to the same length, and the cavity blocks are treated likewise.

The cavity blocks are doweled or peened at the back to keep them from turning.

This is to keep the gate in line with the runner. The gate is ground in the cavity block after assembly, and is made large enough by trial to allow the cavity to fill properly.

In this case, the cores do not have to be doweled as they are round, and turning in the retainer plate will do no harm.

plastic mold structure
plastic mold structure

Retainer Plates

The cavity retainer plate (detail 13) is made of alloy mold steel, but may be left soft to avoid possibility of distortion in hardening.

Water lines (detail lit are drilled, and the inlet and outlet (details 30 and 31) are tapped for pipe fittings. The plate is tapped for 8 retainer screws (detail 14).

The guide pin bushings (detail 20) are mounted in this plate.

They are hardened and ground, with a retaining shoulder at the back.

The cavity retainer plate is ground to the same thickness as the cavity blocks, and then a relief (detail 12) is turned outside that area required for closing off the runners and landing the ejector return pins (detail 7).

The land which is left is as small in area as possible to increase the unit pressure holding the mold closed.

The alloy steel plate, though unhardened, is tough enough to withstand some battering and crushing as the mold closes.

A hole is bored in the cavity retainer plate center to receive the sprue bushing. plate, and the hole for the cold slug and sprue ejector pin (detail 26) is bored in the center of the plate.

This hole is undercut slightly to hold the cold slug in place until the mold opens, and thus pulls out the sprue slug.

The core retainer plate (detail 10) is made of alloy mold steel and left soft as is its mating plate. It is also drilled for water lines in the same relative position.

It is tapped for 4 retainer screws (detail 8), which hold it to the backing plate (detail 9).

Clearance holes are provided for three ejector return pins (detail 7).

The ejector pins (detail 24) are fitted for a short distance back from the molding edge of the plate, and then the holes are opened a little to make clearance and to make the lapping easier.

The three guide pins (detail 21) are mounted in this plate. They are hardened and ground, with a retaining shoulder at the back.

Six runners (detail 17) are cut in the core retainer is just that—the first material to enter the mold.

It is liable to be too cold to flow or weld properly, so it is allowed to bypass into the space provided, and the hotter material following is in better condition to mold.

The same relief is provided on the core retainer plate as on the mating plate.

It is also ground to the proper height.

The two retainer plates are bored together in order to assure proper alignment of cavities and cores.

Holes for the guide pin bushings are also bored while the plates are clamped together.

Backing Plates

Backing plates are used to hold the cavity and core blocks in the retainer plates.

They also hold the guide pins and bushings.

In this mold, one special backing plate is provided (detail 9) and the cavities are backed up by the injection adaptor plate.

These plates may be hardened, if the mold is large, or if the projected cavity area is large.

This is to prevent bending and to prevent the blocks from sinking under the molding pressure.

In less severe cases, the backing plates may be made of alloy steel and left soft.

For light service, they may be made of cold rolled or machine steel.

In any event, they should be ground to make a good fit.

If the core retainer backing plate is to be hardened, holes for ejector pins and retaining screws must be finished before hardening.

Plain holes should have sufficient clearance for free operation of the pins. Tapped holes can be provided for by clearance in the drilled holes in the mating parts.

Good workmanship calls for fairly good fit even in clearance holes, although the same accuracy is not required as in the cavity proper.

It should not be necessary to increase the size and weight of a mold in order to provide a lot of extra clearance space for careless workmanship.

This applies particularly to holes which come close to water lines.

Ejector Mechanism

The ejector mechanism consists of casting ejector pins, sprue ejector pin, or pins, ejector return pins, pin retainer plate, pin assembly plate, ejector box, and ejector rod.

Ejector pins are made of drill rod, upset at the retaining end and preferably straight, with no turning required. They should be hardened.

In this mold, 3 ejector pins (detail 24) are used for each casting.

One sprue ejector pin (detail 26) is used. Three ejector return pins (detail 7) are used.

They push the ejector assembly back to the position as shown when the mold closes.

The pins are held in the retainer plate (detail 27) which is screwed to the assembly plate (detail 28). These plates should be flat, but not necessarily ground, or hard.

The ejector assembly moves back and forth in the ejector box; the travel is shown at detail 25.

When the press opens the ejector rod (detail 1) hits a stop and pushes the assembly forward to the limit of the travel. The travel is sufficient to push the piece off the core.

When the mold closes, the return pins (detail 7) are projecting from the retainer plate (detail 10) the amount of the travel.

They hit the face of the retainer plate (detail 13) and push the assembly back to the molding position.

The ejector assembly can hang on the pins, and the plates (details 27 and 28) can clear the sides of the box (detail 6). In this mold, the ejector box is square, and four spacers are used to form the box.

The screws (detail 5) are used to hold the spacers to the adaptor plate (detail 3). This is for convenience in assembling.

The ejector box may be made round and bored out from a solid block to include the adaptor plate, if the mold is not too large.

The Adaptor Plates

The ejection end adaptor plate (detail 3) in this mold may be made of cold rolled steel.

It has a clearance hole in the center for the ejector rod. A circular mounting boss (detail 2) is turned on the back.

One type of press platen has an arrangement for clamping the adaptor plates, and the clamping slot is shown at detail 4.

The plate is drilled and tapped for the retainer screws (details 5 and 29) as shown.

The injection end adaptor plate (detail 19) serves as a cavity backing plate as well, and should be of alloy steel for severe service.

It has a mounting boss (detail 15) to fit the press platen.

It is bored out to allow the injection nozzle to come in contact with the sprue bushing (detail 16).

Holes are drilled for the retainer screws (detail 14).

Both adaptor plates should be ground to assure parallel mounting in the press.

Sprue, Runners, and Gates

The material is introduced into the mold through the sprue; the tapered hole in the sprue bushing is shown as detail 16.

From the sprue, it flows through the six runners (detail 17) and the gates, shown by a small notch in t h e cavity block at the end of the runner.

The sprue bushing is made of alloy mold steel and is hardened.

At the head end, a small depression is machined to fit the particular type of nozzle preferred by the molder.