Building Your New Plastic Injection Mold

Your company is about to invest in a mold to manufacture a plastic part. That is, you’re ready to write the purchase order once you decide which molder has quoted you the best value for your dollar. You want the lowest price, the lowest mold maintenance, AND the highest product quality. Right? Yet, how can you tell what you’re really buying when one supplier quoted $3000 and another quoted $13,000? They both saw the same blueprint! It’s probably true that both molds could produce good quality plastic parts.

Injection Mold – What Steel to USE?

Buyers should consider two scenarios: quick machining steel/low production quantity versus slower machining steel/high production-high quality. Quoting molds is often an “assumption game.” The more the customer can define what they want, the fewer variables enter into the quote. The customer’s goal is to get apples-to-apples quotes from each supplier, and this isn’t always easy.
One variable in quoting the mold is the quality of steel to be used. Total Solution Plastic typically quotes high quality tool steel unless the customer specifies otherwise. We do, however, build tooling from all grades of material such as prehard steel (P-20), hardened tool steel (S-7,H-13,O-1,A-2,etc.), or stainless steel. When dealing with Asian suppliers, it is also important to understand the country of origin of the steel. Even though it meets the industry standard, doesn’t mean all steel is the same.
Prehard: P-20
P-20 is the most popular and versatile mold steel. It is used for all sizes of machine-cut molds, and does not require post machining heat treatment. No heat treatment eliminates the finish machining and grinding operations which are necessary because of the dimensional changes which occur in heat treatment.
Prehard can be chrome-plated or nickel-plated for wear and corrosion resistance, if needed. However, we typically do not recommend P-20 steel if the production quantity is high (+500,000) or reinforced plastics are to be used. The trade-off is usually less mold builder labor ($) versus longevity of the mold (time).
Hardened Tool Steel: H-13, S-7
This steel renders a fine combination of toughness and strength. The steel is mostly furnished in the annealed condition and must then be heat treated after the mold is finished. The steel hardens to about Rockwell C 46 – C 54. Hardened tool steel is used when a steel is needed for molding reinforced plastics, or when a large production quantity is anticipated. Hardened steel also takes an excellent polish, therefore it is used for clear parts such as lenses.
Stainless Steel: 420
Stainless steel is frequently used when corrosion or rusting may be a problem. Rusting of waterlines or the mold due to condensation can be very costly because of the extra maintenance and frequent polishing. Stainless 420 must be heat treated. It has outstanding dimensional stability in heat treatment and therefore, the mold needs very little finishing and correcting after heat treatment. This steel also takes a high luster polish.
Three variables enter into the calculation of a mold’s price: the material to be used, the estimate of mold maker hours, and the labor/overhead rate.
Price = Material + (Hours X Rate)
The steel quoted will determine the material price and mold builder hours required. Although the steel or material cost typically amount to only 10% of the mold’s price, the amount of time required to machine the steel is a major part of the cost. The prehard steel may be a very cost effective route if for example, the plastic to be molded is polyethylene, the quality of the plastic part is not “gold-plated”, and the life of the mold is small, such as 10,000 pieces.
If, on the other hand, the plastic part is 30% glass-filled nylon, specifies no parting line flash, and will run one million parts, Total Solution Plastic will quote the tool steel you request.
The challenge for all buyers is to define which steel is being quoted and whether that steel will be satisfactory for the project’s objectives.