Thermoset Compression Molding

Thermoset Compression Molding

Thermoset compression molding is historically the most common method of molding thermosetting resins; it is certainly the oldest, and many existing molds are of this type. Compression molds typically have a single cavity, although dual-cavity and even multi-cavity molds are possible.

A preheated preform is placed in the open mold

As the mold closes, the preform is compressed and starts to flow

The heated mold is held shut until the resin cures …

… then the mold is opened and the finished part ejected.

What characterizes compression molding is the method of charging the mold with resin: the plastic resin is placed directly in the bottom cavity of the open, heated mold, and then the top half of the mold is closed down on the resin under tremendous pressure, causing the resin to flow throughout the cavity until it completely fills the now-closed mold and assumes the shape of the finished part.

COMPRESSION MOLDING WORKS

The compression action itself generates heat, and in conjunction with the heat of the plastic mold, raises the temperature of the resin to the point where a chemical reaction called “crosslinking” takes place. It is this crosslinking that gives thermoset molded parts their characteristic strength, hardness and rigidity.

The heated mold is held closed for sufficient time to ensure that the crosslinking reaction is complete; this is called “curing,” and the length of time required for the cure cycle is almost entirely dependent on the wall thickness of the finished part. The thicker the part at its maximum cross section, the longer the curing time required for a given mold temperature.

Thermoset molding resins typically come in a powder or granulated form. Metering the correct amount of resin (called the “shot”) into the mold is accomplished in one of two ways. One is called “preforming” and the other is called “direct charging.”

PREFORMING
In preforming, the exact amount of raw resin required to completely fill the mold (plus a small excess percentage to allow for squeeze-out) is compressed into uniform “pills” — sometimes called “hockey pucks” in industry slang, because that’s what the larger ones look like. These preformed “pills” of resin are produced in batches prior to molding using machines called, appropriately enough, preformers. Preforms are made in various sizes, depending on the size of the mold.

PREHEATING
Just before use, these preformed pills are warmed up in a preheater — which is essentially an industrial-strength microwave oven — which softens the pill into a pliable, dough-like consistency so that the resin will flow more easily under compression. In the molding process, one of these preformed, preheated pills of resin is placed in the bottom cavity of the mold, and the mold is closed, compressing the softened resin into the shape of the mold. (Exceptions to preheating include Bulk Molding Compound (BMC) materials, which are typically formed in logs or used in bulk and are normally molded cold.)

DIRECT CHARGING
Direct charging is used in situations where the mold cavity is too small, too shallow, or too convoluted, to conveniently accept a preform. In these cases, a measured scoop of cold resin is distributed directly into the open mold cavity.

INSERTS
Metal inserts — electrical contacts, female threaded holes, male threaded studs, bushings, and the like — can be placed in the mold cavities, using holes or locator pins, prior to molding. Under compression, the plastic resin flows around these inserts, molding them into the finished piece. Even though thermoset plastics are machinable to a degree, the judicious use of molded-in metal inserts can often eliminate costly secondary operations and add value to the finished parts.

Metal inserts can be molded into the bottom surface, the top surface, or both surfaces of the finished parts. They can even extend completely through the piece. In cases where the inserts are especially small, or cannot be properly supported within the mold, they can be epoxied in as a secondary operation.

THERMOSET COMPRESSION MOLDING:
ADVANTAGES and DISADVANTAGES

In general, thermoset compression molding is well suited for larger pieces of relatively simple design with large, well-anchored inserts. A properly designed and well maintained compression mold will produce very little scrap, and through the use of preheated preforms, total cycle times can rival those of injection molding, plastic molding company

On the PLUS side …

  • Less expensive molds. Compression molds are generally less expensive than other types, both in initial cost and in maintenance.
  • Tighter tolerances, made possible by relatively small shrinkage. Because of the way direct compression packs the preheated preform into the mold, the result is a denser piece that tends to cure solid to the exact dimensions of the mold cavity, without “relaxing back” to create shrinkage. (Exception: dimensions across the parting line are harder to control.)
  • Less scrap can lower material costs.
  • Low-volume jobs are more economical, due to the simplier process and reduced start-up times. Compression jobs can be cycled in and out of the workflow more quickly, and on shorter notice.

On the MINUS side …

  • Heavier flash than other methods is possible. Deflashing is a secondary operation that may add to both labor costs and scrap loss.
  • Labor costs vs. cure time trade-offs. Labor costs can be reduced by using BMC materials and other cold molding techniques which forego preheated preforms, but the cure cycles will be longer.
  • Parting line thicknesses can vary making dimensions across the parting line more difficult to control tightly.
  • Metal inserts may be flashed, leading to more labor-intensive deflashing and clean up.
  • Deep, small diameter holes are difficult to mold reliably; in general, cored holes should be limited to 2.5 times their diameter in depth.

Typical Applications

Thermoset compression moldings are all around you. From the knobs on your stove to the electrical boxes in a jumbo jet — if it’s plastic, hard, rigid, heat-scratch-and-chemical-resistant, an electrical insulator, and it’s got molded-in metal inserts, and it’s black (or shades of reddish brown) — chances are it’s a thermoset molded product.

  • Aerospace — an aircraft terminal housing is just one example where thermoset moldings provide the excellent dielectric properties, lightweight rigidity, and dimensional stability required by precision in-flight systems.
  • Household Appliances — a blower fan blade for a residential clothes dryer illustrates the ability of thermoset compression molded products to meet critical density and dimensional stability requirements.
  • Industrial Machinery — an adjustment wheel for an industrial milling machine displays the rugged workhorse qualities of a typical thermoset compression molding.

Other Molding Methods

Compare with Thermoset Transfer Molding, which is similar to compression molding, but with important differences.

Compare also with Thermoset Injection Molding, a familiar process in thermoplastics, now equally applicable to many thermoset materials.

Injection Molded Products

Injection Molded Products

Quashnick Tool works closely with customers to develop products which can be molded accurately and economically. Employing SPC, Six Sigma and working toward ISO certification, we supply our customers with injection molded parts that consistently exceed their dimensional, aesthetic and performance requirements. Our staff strives to meet our customers scheduling and production needs and we stand behind our products 100%. Please review the following molded part categories for more information and contact us if we can assist you with your next project.

With our fully equipped Production Plastic Molding department we are able to sample and troubleshoot problem molds, run hand-load and overmolded parts as well as multi-cavity, high production molds. As a custom molding facility we have experience working with a large variety of polymers ranging from standard commodity resins like Polypropylene to more challenging grades of engineering resins such as PEEK and LCP. Over the past 35 years, Quashnick Tool Corporation has been involved in hundreds of projects relating to each of the following fields:

  • Medical Disposables
  • Medical Devices
  • Automotive
  • Consumer Products
  • Electronics
  • Computer & Peripheral Devices
  • Connectors
  • Telecommunications
  • Space & Military
  • In-Mold Decorating
  • Micro Molding

Our emphasis, as directed by Mr. Quashnick himself, has always been quality and accuracy. This is reflected in both the Injection Molds we manufacture as well as the Molded Plastic Parts they produce.

Plastic Injection Molding for the Medical, Biotech and Hightech industries.

For 39 years Quashnick Tool Corporation has developed custom injection molding for the who’s who of technology companies in the greater San Francisco Bay Area.
Knowing what’s at stake when it comes to developing and maintaining quality plastic injection molds and the parts produced is the secret to their continued success. They realize that when companies begin new designs of molded materials, significant corporate R&D, engineering, manufacturing, administrative and support resources are at stake. Any redundant or rework efforts will add cost, delay time –to- market, and may upset medical device trials and qualification.


With this reality in mind they work with you to look as far ahead as possible in foreseeing and side-lining potential pitfalls in a successful molding program. Quality Pros think of it as it “FEMA” in action. They do this specifically by drawing from their 39 years of experience, merging extensive design and materials experience, with state of the art corporate management and sophisticated engineering and project management skills.
Whether you are looking to work with extremely tight time lines, wish to minimize development risks, want to keep manufacturing local in the greater Bay Area, or you are looking to save costs by building tools at a qualified QTC offshore partner, it pays to have a well versed and professional organization shepherd you through the process.
They’ve delivered success for several hundred companies in the greater Bay Area since 1971. They will deliver success for you.

Quashnick Tool Corporation was founded by State Certified Journeyman Tool Maker and past President, Terry Quashnick and his wife Jeanette in 1973. For the first eleven years they specialized in the design and construction of injection molds. In 1984 they added injection molding to the business.

Inducted into the Society of Plastic Engineers Hall of Fame in 1998.

In 2008, Terry assumed the role of CEO/CTO to focus on technical development when Ed Baker joined the firm as President and COO.

After 37 years of operation, QTC remains a custom manufacturer with two divisions and has built a team of versatile and experienced employees to meet any customer’s injection molding needs.

The tooling division specializes in the construction of complex plastic molds built to very high tolerances.  Technology is an important component at QTC and the company continues to invest in the latest machinery and metrology equipment to maintain our reputation for competitiveness and quality.  Also incorporated in the tooling division is one of the most advanced computer aided mold design facilities in Northern California, which utilizes the latest versions of Pro/Engineer, AutoCAD, SolidWorks and SURFCAM.

As the industry has changed, QTC has changed with it, with a focus on clean manufacturing and producing higher volumes of complex medical parts, in addition to custom work. The molding facility currently operates eleven state of the art injection molding presses ranging from 35 to 230 tons across the entire spectrum of resins from polypropylene to engineering resins.

Today, QTC operates using lean business processes, ISO level documentation, six sigma problem solving tools and a total team concept, to seamlessly integrate mold design, mold construction and plastic molding operations, and assure we deliver quality parts to our worldwide customer base.