What materials should be selected for manufacturing extrusion blow moulds?
Blow moulds require less material and have a wider selection.The selection of materials for blow moulds should take into account the factors such as thermal conductivity, strength, abrasion resistance, corrosion resistance, polishing, cost, and used plastics and production batches.For example, for plastics that produce corrosive volatiles (such as PVC, polyacrylonitrile, polyformaldehyde, etc.), corrosion resistant materials are used to make molds or to coat the mold cavity with corrosion-resistant metals.There are several kinds of materials for manufacturing blow moulds.
(1) aluminum.Aluminum is an early and widely used material for extrusion blow moulds.Aluminum has good thermal conductivity, good machinability and ductility, low density, low hardness and easy wear.Aluminum alloys will wear better.Cast aluminum is less ductile, so billet inlays are made of steel or copper beryllium
Alloy manufacturing.The service life of aluminum mold is about 100,000 to 200,000 times.Aluminum is porous and sometimes infiltrates small amounts of plastic melt, which affects the appearance of the blow molding products.This can be solved by applying sealant to the mold cavity, but this will reduce the heat transfer performance between the product and the mold wall.
(2) copper base alloy.Cu-beryllium alloy is a kind of material commonly used in blow moulds. It has good thermal conductivity, hardness, wear resistance, corrosion resistance and mechanical toughness.The main disadvantages are high cost, poor mechanical processing performance (machining time is about a third longer than aluminum), and the density is about three times that of aluminum, which further increases the cost (copper beryllium per unit volume is about six times that of aluminum).Copper beryllium alloy can be heat treated to increase hardness (up to HRC40).Copper beryllium alloy can be machined, cast and hot extrusion to make molds.The hardness of copper beryllium alloy containing high beryllium (e.g., 1.8%~2.0%) is higher, and mechanical processing method should be adopted.To make irregular moulds, use copper beryllium alloys with a lower beryllium content, such as 1.65%.The copper beryllium alloy is mostly used in manufacturing billet inlays and is used in conjunction with aluminum moulds.Sometimes (especially for corrosive plastics) the whole set of blow moulds are made of copper beryllium alloy.For example, the copper beryllium alloy will not be corroded by the hydrogen chloride produced by PVC processing. It will also prevent water from scaling in the cooling channel and reduce heat transfer efficiency.The copper beryllium alloy mold is easy to repair by welding or inlaying.Beryllium is harmful to human health. Inhaling beryllium dust is harmful to lung. It is also suspected to be a carcinogen.In addition to the copper beryllium alloy, Ni/Si/CU, Cr/ CU and aluminum/bronze alloys are also available for manufacturing blow moulds. The thermal conductivity of the first two alloys is about 2 and 3 times that of the copper beryllium alloy, respectively.
(3) the steel.Steel is mainly used in the blow molding of PVC and engineering plastics. This is because of the extremely high hardness, wear resistance and toughness of steel.The main disadvantage of steel is poor thermal conductivity, which should be compensated by cooling system design and cooling fluid temperature and flow state.Mold of corrosive plastics (such as PVC) shall be made of stainless steel.Steel molds can be manufactured by machining, cold extrusion, casting or welding (for large moulds).The service life of steel mould can reach 10 million times, so when the production quantity of blow molding products is large, steel is a kind of preferred material.Steel (such as common tool steel) is also used in the manufacture of blow mold parts to withstand wear, such as billet inlay, pull rod, guide column, guide sleeve and formwork etc. These parts require hardening of the steel.But generally speaking, steel is used less in manufacturing blow moulds.
(4) other materials.
Zinc alloy.It has good thermal conductivity and low cost. It can be used for casting large moulds or moulds with irregular shape.Zinc alloy is less hard than aluminum alloy, so steel or copper beryllium alloy is used to make billet inlay.Zinc can also be alloy with aluminum or copper, which has better dimensional stability but less corrosion resistance.
Zinc nickel copper alloy.Also can be used as blow mold material, its thermal conductivity between copper beryllium alloy and aluminum alloy, however, under similar thermal conductivity, its hardness is lower than copper by alloy.
Resin (e.g. polyacrylate, epoxy).Can be used for casting low-cost test molds, molds with few production times or sample molds.They can be filled with metal powder or glass fiber to improve dimensional stability and thermal conductivity.