Metric hex socket cap screws, property class 12.9, supplied to your size and quantity. The standard fastener for mold plate stacks, where a small head has to develop a lot of clamp force in a counterbore. Send the sizes and quantities, or a BOM, and we quote in writing, normally within one business day.

What property class 12.9 actually means

The two numbers are a specification, not a grade name. The first (12) is the nominal tensile strength in hundreds of MPa — 1200 MPa. The second (9) is the ratio of yield to tensile, 0.9, so yield is about 1080 MPa. That is roughly twice the strength of a common 8.8 screw of the same size, which is why mold plates can be held by screws that fit in a modest counterbore.

The trade-off is worth knowing: higher strength classes are less ductile and more notch-sensitive. A 12.9 screw tolerates less abuse than an 8.8 — a damaged thread, a burr under the head, or a socket rounded out by the wrong key are all more likely to end in a broken screw rather than a stripped one. And a screw broken flush in a mold plate is an expensive afternoon.

The three things that break these in practice

• Seating on a burr or a chip. The head must bear on a clean, flat, square counterbore. A chip under the head means the preload you think you applied is not there, and the joint moves — then the screw fatigues.
• The wrong hex key. An imperial key in a metric socket, or a worn key, rounds the socket. On 12.9 the socket walls are hard and will chip rather than deform.
• Reusing screws that have been fully tightened many times. These are usually torqued into the elastic range and can be reused, but a screw that has been overtightened has yielded and will not develop the same preload again. If a joint keeps loosening, replace the screws before re-torquing them again.

Finish, and where it matters

Black oxide is the normal supply condition and offers modest corrosion resistance. Zinc plating is available, but be aware that plating processes can introduce hydrogen embrittlement risk in high-strength fasteners unless properly baked — if you need plated 12.9 screws, tell us and we will confirm the process and the baking treatment in writing rather than assuming it.

What to send us

• Thread size and pitch, and whether it is coarse or fine
• Under-head length and thread length
• Head diameter, head height and socket size, or the standard you work to (DIN 912 / ISO 4762 and similar)
• Finish required, and whether plating with baking is needed
• Certificates required — material, mechanical properties, or a batch certificate
• Quantity per size, packaging, and destination

Sending a mixed BOM for mold maintenance is fine — several sizes and small quantities in one quotation is normal for us.

Documentation and confidentiality

Property class 12.9 is confirmed in the formal quotation together with the dimensional standard, coating and certificate availability — it is not assumed from a product title. Technical files are used to evaluate and fulfill your inquiry or order. They may be shared only with relevant manufacturing, inspection, logistics or service partners as needed. Send only files you are authorized to share. See our Privacy Policy. NDA on request from rfq@privleymold.com.