How to Specify Molds for Silicone

International Silicone Conference 2010

Presented by Rick Finnie
M.R. Mold & Engineering

How to Specify Molds for Silicone

Good communication between the mold buyer and the mold maker is imperative. It is vitally important that the buyer furnish the mold makers with all the necessary specifications to build a mold that fits all the pertinent requirements of the buyer’s facilities and operating procedures. These are items such as mold type, number of cavities, and automation requirements. It is critical that each mold maker receive identical information from which to quote. Without this communication, the quotations will not be realistic and will require renegotiation. A practical method of communication is a Mold Request for Quotation (RFQ) form, or simply a mold data sheet. (see Exhibits A & B) The buyer completes the form in such a way as to convey to the mold makers the complete scope of the tool build. Communication can be in any form. Regardless of the method, the quotation requirements need to be distributed in the same way, with the same details, to the selected vendors chosen to quote the mold.

3D part models are an excellent way of conveying the size and shape of the molded part. However, 2D drawings are still required. 2D drawings contain information that can’t be seen in a part model. 2D drawings have information such as material, tolerances, surface finish, engraving, and special notes.

The Society of Plastics Industry (SPI) has established standards for mold classification. While these standards are obviously for the plastics industry, the standards are useful for the silicone industry as well. The following classifications are based on the number of cycles the mold is expected to run. When a buyer selects a classification, the mold builders will immediately understand the scope of the project.


Cycles: One million or more
Description: Built for extremely high production. This is the highest priced mold and is made with only the
highest quality materials.

Cycles: No exceeding one million
Description: Medium to high production mold, good for abrasive materials and/or parts requiring cost
tolerances. This is a high quality, fairly high priced mold.


Cycles: Under 500,000
Description: Medium production mold. This is a very popular mold for low to medium production needs.
Most common price range.


Cycles: Under 100,000
Description: Low production mold. Used only for limited production preferably with non-abrasive materials.
Low to moderate price range.


Cycles: Not exceeding 500
Description: Prototype only. This mold will be constructed in the least expensive manner possible to produce
a very limited quantity of prototype parts.

Buyers need to assess the entire scope of the project. Specific questions need to be answered.

• What material will be used?
• Will the production process be high or low volume?
• Will inserts be needed?
• Are there any special requirements such as secondary operations?
• Does the tool need to be automated?

Other significant items to address are:

• What, if any, are the critical tolerances?
• Are there critical wall thicknesses?
• Is there a critical sealing surface?
• How critical is flash?
• What is the parting line mismatch tolerance?
• Is a surface finish required?

When deciding what type of mold needs to be built, volume is the key. Are you making a thousand parts, ten thousand parts, or millions of parts? It is conceivable that if expected yearly volumes are so high, more than one production mold might need to be built. The number of cavities within the mold is based on the volume of production, your targeted part price and the clamp tonnage of your molding machine.

The need for prototyping should always be assessed. There are at least 3 reasons for building prototype tooling.

• Test the part design, material, surface finish, etc.
• Test the method of manufacture, parting line locations, gate locations, etc.
• Test for automation - where do parts stick, part removal, method of automation, etc.

Prototype molds should mimic the production mold as much as possible.

When specifying a mold the buyer should determine which direction their company will take when approaching the project and what molding process will best fit the specifics. Based on the prospective volume, an SPI classification is given to the mold. Consider the simplicity or complexity of the part. Can the mold run full automatic or do the parts need to be removed manually by an operator.

When considering the method of manufacture, the parting line locations and gate locations are critical. Consider the part design to determine what half of the mold the part will stay in. The gate location(s) must be in a place that allows the part to fill properly and air to escape through venting. With complex parts, a flow analysis might be considered. This will eliminate unwanted defects. Gate and parting line locations need to be designed in such a manner that the last place to fill will be at a parting line. (The types of gates and placement of parting lines will be discussed later in this paper.) Also when considering the method of manufacture, consider how the part will be removed from the mold. Can the process be done fully automatic with a mold sweep, an air blow or a robot; or will an operator remove the parts manually?

There are many types of molds to consider. The simplest and least expensive mold to build is a compression mold. This type of mold might be used for simplistic part designs and low volume production. A transfer mold with pot and piston allows the mold to fill the part where needed and fill multiple cavities without preparing multiple pieces of rubber. Transfer with cold pot has the same benefits of the pot and piston mold except for the fact that the pot is cold and it reduces waste. When using a bottomless pot transfer mold, the pot and pistons can be removed and reused on another mold, thus saving you money. A transfer press is a molding machine that has a hydraulic ram for transferring material into the mold. An injection mold is the preferred method of molding. It’s cleaner, more efficient way to mold product. With an injection mold you have more control over the molding process and the ability to automate the mold. The cold runner injection mold is the ideal method for any high volume production job. With a cold runner, you eliminate the need to remove the runner from the mold and therefore, eliminate the waste of the runner system.

Choose the appropriate size molding machine. It is of the utmost importance that the machine size is conveyed in the RFQ. In order to determine what size machine is needed for a project, multiply the square inches of the molding area by 2-4 tons, for a suggested clamp tonnage of the machine. All information on custom mounting and/or universal bases needs to be communicated in the RFQ. If you intend to purchase a machine for a particular project, stating the make and model in the RFQ is very helpful. You might also consider a “turn-key” application when purchasing a molding machine. Having the machine shipped to the mold maker for set up and initial mold trials saves valuable time when the equipment arrives at your facility ready for production.

There are a number of different types of steel available for the purpose of building molds. Royalloy is a mold base steel. It’s based on 420SS (pre-hard 30RC) and is the preferred base material. Royalloy steel machines well, is stable and does not rust. #2 base steel (4130 pre-hard, 30RC) would be a second choice for mold bases. P-20 pre-hard cavity steel (32-35RC) is commonly used for prototype, low and medium volume production molds. 420SS (52-54RC) is commonly used for cavity and core material. It is the most common steel used for high volume production molds. S-7 shock resistant steel (54-56RC) is used in extreme applications for its robustness. M-2 steel (60-62RC) is typically used for core pins. For molds that require a high volume of production and abrasion resistant qualities, 440SS (56-58RC) and Elmax (56-58RC) are required.

Another key element in the quoting process is the mold style, or type of mold, the mold maker will build. The 2 plate design is the simplest of all and would be typical of simple parts such as o-rings. A 3 plate mold with a runner system is designed for a part that requires a pin point gate. This type of mold has a parting line for the runner system and a parting line for the part. The runner system is needed in order to feed multiple pin point gates. This runner system requires a parting line so that you can remove the runner from the mold.  Sometimes the part itself requires multiple parting lines. The multiple parting lines allow for part removal and allow air to escape from the cavity. For this scenario a 3 plate mold needs to be built. A combination of both 3 plate designs, the 4 plate design and runner system is needed when the part itself requires 2 parting lines and a third parting line is required for the runner system. Parting line selection is critical in some product and procedures. They determine where the air can escape and where the part will remain when the mold opens. Parting lines will form a seam on the molded part so care has to be taken in sealing applications.

There are a number of ways to handle the center plate of a mold. The following are a few examples:
  • the center plate Hydraulic mechanisms,
  • as part frame mounted to the machine is being operated by the of the molding machine are to hold the center plate.
  • knockouts of the molding used here to control the center machine plate.
The correct selection of gate style and gate location is often critical. Gates determine where the material will enter the cavity and occasionally leave a minor blemish on the part. Care must be taken in sealing and cosmetic situations. Additionally, the gate determines where the last place to fill will be located. The last place to fill is the area where air can become entrapped. A parting line needs to be present at this location to allow the air to escape through a vent.


An edge gate is the simplest and least expensive, The pinpoint gate is superior to the edge, however, is not always the best location to fill the gate in that you can inject the material at part. Typically edge gates fill from the parting line exactly the location you want it to enter which is where you need venting. the mold, driving the air out to the parting line.


A sub gate gives you the advantage of filling the The advantage of using a ring gate is that the part away from the parting line. A sub gate material fills around the perimeter of the part automatically shears off when the mold opens. and tears off cleanly, leaving little or no remnant of the gate on the part. This type of gate is typically used to fill cylindrical parts uniformly.


A gate around a core pin is typically used to fill round A valve gate can only be used with a cold runner
parts. It insures that the hole in the part is clean, system for which there is high volume injection free of flash and leaves no gate remnant. molding. The gate can be strategically placed anywhere on the part. There is no runner so there is no waste and leaves a very minimal gate remnant/blemish on the part.

There are many types of ejection methods in which to release the part from the mold. Below are several suggestions.


Cores can pop up using the ejection This is a form of automation where A mold sweep can be used for parts
on the molding machine. Parts are where air blasts are being used to that are symmetrical at the parting then manually removed from the to remove the part and ejectors line, such as o-rings. The mold sweep cores. are removing the small runner. removes parts from both parting lines simultaneously.


End of Arm tooling can be manufactured Frequently with silicone molds, custom robots for standard robots to remove the parts are attached to the mold to remove the parts from the mold.

The buyer must determine whether their molding machines have heated platens or if the mold has to have heat of its own. The preferred method would be internal heat inside the mold with a heater box and connector.

The buyer needs to convey mounting specifications within the RFQ so the mold maker has an understanding of how the mold will be mounted in the press. Below are some examples:
  • This vertical molding machine has mounting holes.  The mold has cutouts to access the mounting holes.  The mounting holes are standard on some molding machines.
  • This mold is held simply with clamps
  • The mounting holes in the mold are designed to align with the existing mounting holes in the molding machine.  This eliminates the need for clamps and adds a greater degree of safety to support the mold.  This is the preferred mounting method.
There are times additional requirements are needed to enhance the molds capabilities. For instance, critical parting line mismatch requires additional mold components to assure proper alignment.

• Parting line locks are used for alignment on high volume production molds.
• Taper locks are devices that engage at the final closing of the mold for proper alignment.
• Self-register cavity and core is typically done with round diaphragms.
• Vacuum is used to evacuate the air out of the mold prior to injection. Proper sealing of the mold to achieve high vacuum is critical.
• Tearbead\overflows are typically used in transfer and compression molds.
• Hand load inserts- typically on low volume parts there might be a need for hand load inserts.
• Cycle counter only available in cold runner systems. They aren’t capable of handling the heat of rubber molds.
• If the mold is to be tested by the mold maker prior to shipment it is important that this is noted on the quote.

Side Actions are sometimes required when there is an undercut feature in the part design. Side actions with silicone should be avoided, if possible. Side actions can be operated by air cylinders, hydraulic cylinders or angle pins. The buyer should indicate a preference which style of side action to use.

Information needs to be communicated to the mold makers within the RFQ in regards to finish and engraving. Cavity and core finishes are critical information. High polish and chemical etch textured finishes are expensive to apply. Steel selection can be critical with surface finish requirements. High polish will leave silicone with a clear shiny finish. However, silicone will not stick to polished surfaces. Silicone releases best from matte or textured finishes. (see Exhibits C & D) Refer to these exhibits for various surface finishes.

Many customers do a lot of business on simple universal bases. Universal bases can reduce the cost of prototype and low volume production tooling. A universal base can accept many different styles of tooling.

The due date of the quote needs to be communicated to the mold maker on the RFQ, especially if you need a quick turn-around. It is equally important to make the mold maker aware of any expedited delivery date your customer has requested for parts. Communicate ALL the information you have so a complete understanding of the project is had by all.

Prequalify the mold makers to whom you are sending RFQs to ensure that their capabilities meet yours. Be open to suggestions and concerns of the mold maker such as parting line location changes, venting, etc. The mold maker has the experience to anticipate problems and tell you how to avoid them prior to the mold build. Review the quote for accuracy. Verify that the mold maker plans to build the mold in a manner that matches your needs. At M.R. Mold we use our quote sheet as a check and balance. (see Exhibit B) Weigh each price and delivery presented. The lowest price isn’t always the way to go. If you have vendors of similar quality and capability, quotes should be competitive. Be wary of extreme high or extreme low quotes. Chances are the mold maker missed something or doesn’t understand the scope of the project. Award purchase orders only to qualified mold makers with experience in your type of tooling.

If you have any questions, please contact M.R. Mold & Engineering at (714)996-5511