How To Buy an Injection Mold

One of the most significant cost drivers for the cost of an injection mold is how many cavities are in the mold.  For each cavity, one part is produced per machine cycle.  Molds can be constructed with 1-256 cavities, always in multiples of 2 to maintain balance of the plastic flowing into the cavities (i.e 1,2,4,8,16…256).  It is important for you as a buyer to understand what is most important for you.

Each time you double the number of cavities you get twice as many parts per hour so the non-material costs are cut in half.  However, the part still consumes the same amount of material.  There is definitely a diminishing return on continuing to add cavities to a mold past a certain point.

For ultra-high annual usage adding more cavities is about meeting demand.  Building a second mold might be a better route in many cases. Molds bet bigger as cavities are added and this requires much larger injection molding presses, power and overhead.  Running two 16 cavity molds in smaller presses might be more cost effective than running a 32 cavity mold in a much larger press.

Figures 3 and 4 show total cost of the mold and 2 years of production for a medium sized part. Please keep in mind when looking at the following data that every project is different, and an analysis must be done for each.  Optimum cavitation for an engineering material might be 8 cavities when the same part made from a commodity material might have an optimum cavitation of 4 cavities.   In figure 3 the optimal cavitation for 100,000 parts per year would be a 2 cavity mold.  However, in figure 4 with an EAU of 500,000 parts the optimal cavitation becomes 8 cavities.

The shape of these curves can change a lot based on how many parts you need a year, part size, material cost and type of mold.  But in every case, there is a point that yields the lowest overall cost for a given time period.

Often a customer will anticipate low volumes in year 1 with increasing volumes year over year.  In this case the optimum cavitation at the start of the project is very different then the optimum cavitation several years after the product launch.  One possible approach to lower up-front cost and minimize risk would be to build a prototype mold or low cavity mold to handle the low volume in the first year or two and then build a higher cavitation mold once volumes are established.  The piece price will be higher at the beginning with this approach.  However, if the volumes do not increase as anticipated you haven’t risked the price of a high-volume production mold.

Below is a description of each of the 5 SPI mold classes and what you can expect in a mold of each class.  These are largely driven by estimated annual usage and tolerances.

  Class 101 Tool – Cycles: One million or more

• Description: Built for extremely high production. This is the highest priced tool and is made with only the highest quality materials.
• Detailed tool design required.
• Tool structure components to have a minimum hardness of 28 Rc.
• Cavity and cores must be hardened to a minimum of 48 Rc range. All other details, such as sub-inserts, slides, heel blocks, gibs, wedge blocks, lifters, etc. should also be of hardened tool steels.
• Ejection should be guided.
• Slides must have wear plates.
• Temperature control provisions to be in cavities, cores and slide cores wherever possible.
• Over the life of a tool, corrosion in the cooling channels decreases cooling efficiency thus degrading part quality and increasing cycle time. It is therefore recommended that plates or inserts containing cooling channels be of a corrosive resistant material or run with corrosion inhibiting fluid.

Parting line locks are required on all tools.

Class 102 Tool – Cycles: Not exceeding one million

Description: Medium to high production tool, good for abrasive materials and/or parts requiring close tolerances. This is a high quality, fairly high-priced tool.

• Detailed tool design required.
• Tool structure components to have a minimum hardness of 28 Rc.
• Cavities and cores should be hardened to a minimum of 48 Rc range. All other functional details should be made and heat treated.
• Temperature control provisions to be directly in the cavities, cores, and slide cores wherever possible.
• Parting line locks are recommended for all tools.
• The following items may or may not be required depending on the ultimate production quantities anticipated. It is recommended that those items desired be made a firm requirement for quoting purposes.
• Corrosive Resistant Temperature Control Channels
• Plated Cavities

Class 103 Tool – Cycles: Under 500,000

• Description: Medium production tool.
• Detailed tool design recommended.
• Tool structure components must have a minimum hardness of 18 Rc.
• Cavity and cores must be 28 Rc or higher.
• All other extras are optional.

Class 104 Tool – Cycles: Under 100,000

• Description: Low production tool. Used only for limited production preferably with non-abrasive materials. Low to moderate price range.
• Tool design recommended.
• Tool structure components can be of mild steel or aluminum.
• Cavities can be of aluminum, mild steel or any other agreed upon metal.

Class 105 Tool – Cycles: Not exceeding 5000 (depending on material)

Description: Prototype only. Tool to be constructed in the least expensive manner possible to produce a very limited quantity of prototype parts.

All components are optional

From a mold cost standpoint, cold runner molds are much less expensive than hot runner molds with the same number of cavities.  Hot runner molds have the added complexity of the hot manifold and a nozzle body from the manifold to the cavity often referred to as a “drop”.  A cavity has a single drop in most cases but larger parts may require multiple drops per cavity.  This added complexity and increased mold size can increase the mold cost significantly.

However, from a part cost standpoint it is quite the opposite.  For cold runner molds the machine must process and melt a larger volume of plastic for each shot (part volume + runner volume).  This takes time and energy.  Sometimes the runner is thicker than the part so cooling time can also be increased with a cold runner mold.  These two factors can increase total cycle time for your part.   And you know the saying “time is money”.  Also with a cold runner mold there is often a robot that removes the runner each cycle and drops it into a grinder to be reused.  If the runner is too large compared to the part volume, not all the reground material can be reused, and you are paying for material that doesn’t go into your part.

In the case of a hot runner mold, less plastic must be melted and processed with each cycle, cooling times are shorter and overall cycle times are shorter.  These factors combine to reduce the overall cycle time and hence part cost.  Hot runner systems tend to be used on high volume projects were the decrease in part cost offsets the higher mold cost.

Steels used for mold building can be sorted into 3 broad categories.

Heat Treated Tool Steels:  These are the hardest of the commonly used tool steels for injection molds and after heat treatment have a Rockwell hardness of 48-52 on the Rockwell C scale (HRC).  The steel is rough machined, heat treated and then finish machining is completed to form final cavity geometry. H-13 is the most common, however there are many H-13 equivalents that have different designations.  If you see AISI H-13 or Gröditz 1.2344 they are the same thing and this can be verified by their hardness.  These are extremely durable, wear resistant, however much harder to machine.  An extremely good choice for glass filled plastics.

Corrosion Resistant Tool Steels: This is a class of stainless steels for mold building that are used specifically for polymers that can be acidic or attack standard mold steels like PVC, CPVC, PVDF, PTFE etc.  If you do have a corrosive material, don’t forget that all mold components that come in contact with the plastic or gas released during the molding cycle should be corrosion resistant.

Non-Heat Treated Tool Steels (or pre heat treated): The most common being P-20 with a hardness of 28-40 HRC.  P-20 is much easier to machine than H-13, however the lower hardness increases the speed at which the mold can wear, particularly with glass filled materials.  These are a great choice for low to medium volume production due to their overall durability and ease of machining.

There are dozens of other specialty mold steels that might be a good choice if you have specific requirements for surface finish, texture, optical polish or prototype molds.