Compression Molds and Their Types

 Compression Molds

A hand-operated mold type built for testing. (above) 1-upper half 2-Lower half 3-Split collar 4-Pressure plates b) sprue/runner d) Ejector hole

Types of Compression Molds

The design of compression molds draws on certain basic mold types. The basic type to use is chosen based on the part to be pressed. Each basic type has advantages and disadvantages. Today, the following basic types of compression mold are used:

1. Flash-type compression molds
2. Positive-type compression molds
3. Split (jaw) type compression molds
4. Jacketed-type compression molds

Let’s look at the most important characteristics of these compression mold types:

Flash-Type Compression Molds

In flash-type molds, as soon as the mold closes, excess material is squeezed out through the parting line between the upper and lower halves. The movement of the mold halves is limited by the land (bearing) surface at the parting line. This land surface’s width (B) is usually about 3 mm. The excess material squeezed out through the parting line forms flash.

This flash needs to be trimmed off afterward. The thickness of the flash that forms varies. The part’s height varies with the flash thickness. Because of this, in this basic type of mold, producing parts that need precise height dimensions is difficult. Another drawback is that the material charge starts escaping through the parting line as soon as pressing begins. Because of this, the pressing pressure can’t be held steady at a nominal value. For this reason, this type of mold is only used to produce flat parts from unfilled plastic materials like rubber, phenolic, and urea-formaldehyde (carbamide) molding compounds.

A flash-type mold construction (above) 1-molded part 2-Core pin 3-Lower mold half 4-Upper mold half 5-Mold base plate 6-Ejector pin 7-Collar carrying the pins 8-Ejector rod carrying the collar

For this purpose, a mold built in the flash type is shown in the figure above. The mold’s lower half (3) is fastened onto the base plate (5). The ejector pins (6) are fitted into a collar (7); the collar carrying the ejector pins (7) is moved by an ejector rod that passes through the base plate.

Another example of a flash-type mold. (above) 1-Lower mold half 2-base plate 3-ejector pin 4-bridge (carries the ejectors) 5-ejector rod (carries the bridge) 6-molded part 7-Core 8-Upper mold half

The tool construction shown in the figure above is also of the flash type. In flash-type molds, a sheet-metal template is placed between the lower mold half (3). This sheet template has projections that match the pressed part and act as ejectors. At the end of pressing, the sheet template is removed together with the pressed part, and the part is pushed off and separated from the sheet. Since the construction of flash-type molds built using such sheet templates is very cheap, they can be reliably used for parts that don’t require any particular precision in their outer shape — that is, parts that can be made in almost any form. This type of flash mold can also be used for testing molding compounds.

Positive-Type Compression Molds

The positive type is the most widely used mold type.
1-Punch 3-Ejector 4-Molded workpiece 5-Lower mold half

The diameter of the mold cavity is the same in both the lower and upper mold halves.

Design of Positive-Type Compression Molds

In positive-type molds, the entire charge of molding material stays inside the mold cavity once the mold closes. During this, the press punch travels a few millimeters (a few centimeters on large molds) into the lower mold half.

During this, the pressure needed to prevent the molding material from escaping is set. Essentially, at the top of the mold cavity there’s a feed chamber (loading well) with enough volume to hold the material needed for the part being molded.

The length of the section that guides the press punch’s linear motion is kept as short as possible. In addition, the punch’s edges are rounded to prevent flash from forming where the mold halves meet, and to create a gap the excess material can escape into.

1-Punch 2-Intermediate plate 3-Ejector  4-Molded workpiece 5-Lower mold half  a-Gap between the upper mold section and the cavity wall  c-Length of the guiding section of the upper mold half d-Height of the molded workpiece  g-Gap in the lower section of the mold

To make it easier to remove the pressed part from the mold, the cavity’s diameter is made slightly larger toward the top. As can be seen from examining the figures, especially for molds where the part’s height dimension matters, the lower mold section is expensive to produce. For this reason, many designers choose to use an intermediate plate to form the mold cavity, as shown in the figure above.

Even so, in many cases, a construction is chosen where the mold cavity’s diameter matches the workpiece’s actual diameter. The drawback of this construction is that the exact amount of plastic material needed to produce a flawless part shape must be precisely calculated.

1- Punch 2-Lower mold half 3-Counter-punch

Determining the exact amount of material needed is difficult in practice. For this reason, the construction shown in the figure above is chosen instead. The mold cavity’s volume is set to accept a certain material surplus as well. During pressing, the excess material overflows into the gap between the upper punch and the lower mold half. (This gap dimension, a, varies between 1.5 and 3 mm.)

Split (Jaw) Type Compression Molds

Split (jaw) type mold constructions are preferred in cases where the part can’t easily be removed from the mold. In cases where the workpiece has grooves, holes, holes running in the direction of motion, or metal insert parts, it’s not possible to remove the workpiece by simply pushing it out.

1-Left insert jaw  2-Core  3-Punch  4-right insert jaw  5-jacket (cavity sleeve)  6-plate

Operating System of Split (Jaw) Type Compression Molds

To remove the molded workpiece, both insert jaws (1, 4) need to be pulled out and removed. For this, the mold needs to be cooled beforehand.

The schematic image above shows a sample application. The mold is essentially made up of the jacket (5), the insert jaws (1, 4), the punch (3), and the core (2). The jacket is usually fitted with a heating or cooling system. To make it easier to remove the part after pressing, the fitting surfaces are made conical. (Taper 1:5). In the simplest arrangement, the insert jaws sit movably inside the jacket. Sometimes, the jaws are instead hinged to the lower mold half; this way, when the punch and ejector move upward, the jaws open by themselves under their own weight.

Operating System: Once pressing is finished, the jaw assembly (1) is pushed upward with the help of a threaded pin (2) and a rod. This brings the lower edge of the workpiece clear of the mold. At this point, the punch (3) is withdrawn, and the pressed workpiece is pulled off and removed from part no. 1.

A split (jaw) type compression mold with a vertical parting surface (above)

Operating System: The separating disc (b) is made with its thickest section in the middle, to improve separation of the plastic mass. The disc is then removed. The insert jaws are positioned so they guide against each other. This is achieved with guide pins and a common helical spring that holds the insert jaws together.