Design of Molded Parts

What are molded parts?

Mold elements that make up the cavity of an injection mold are collectively referred to as molded parts, sometimes known as internal mold inserts. The cavity is the area utilized to fill with plastic melt and form plastic components during mold production. Fixed mold inserts, moveable mold inserts, and cores are examples of internal mold inserts. The molded parts also feature push-out components, an inclined push rod, and a lateral core-pulling mechanism, among other things.

High-quality mold steel is typically used when forming items. This is done to make processing and maintenance easier, lower the cost of making molds, and maintain the mold’s strength, rigidity, and wear resistance so that it can continue to produce molds for the specified amount of time.

Basic requirements for molded parts

• Be sufficiently rigid and strong

The molding chamber needs to be strong and rigid enough to withstand the high-pressure action of the high-temperature melt during the injection molding process. Insufficient rigidity will result in excessive elastic deformation, which will cause the cavity to expand outward and cause the plastic component to become trapped in the fixed mold or produce a flash on the separating surface. Insufficient cavity strength will induce plastic deformation or even rupture. Increasing the location of the taper is the typical method for increasing stiffness in produced pieces.

• Able to find plastic parts that are molded and satisfy the specifications

These specifications cover things like shape and look as well as dimensional accuracy, surface roughness, mechanical and chemical qualities, etc.

• The produced plastic parts can be directly used for assembly, avoiding the post-processing of molded plastic parts.

All holes, self-tapping screw columns, buckles, inserts, and other structures are formed in the cavity as much as possible at 1 time.

•  Trustworthy forming and high effectiveness

The molded plastic parts are quickly cooled, the ejection mechanism is quick and dependable, and the runners and gates are simple to remove. There is also a fast-filling gating system.

• Low manufacturing cost

The structure is simple, reliable, and practical, shortening the production time and reducing the production cost.

• Simple to maintain and repair

For difficult-to-process and damaged parts, the inlaid structure should be taken into account to enable speedy replacement following damage. The mold must be designed with future maintenance in mind because it is a manufacturing tool with a lengthy lifespan.

• Materials

It is hard enough and wears resistant enough to withstand the friction caused by material flow. In general, the inner mold material should be harder than 35 HRC, but for mass-produced molds and molds injected with glass fiber-reinforced plastics, it’s sometimes necessary for the inner mold inserts to be harder than 50 HRC.

The material can be polished effectively. Ra, the surface roughness, must typically be less than 0.4um. The surface of the cavity must be mirror-polished and the surface roughness Ra must be less than 0.2 micrometers for molds used to make transparent plastic parts.

a successful cutting process. Grinding should be used to process imports and precisely fitted pieces, and turning or milling should be used to process general parts as much as feasible.

Material selection requirements for mold steel for Molded parts

1. Good machining performance. Choosing a steel grade that is easy to cut and can obtain high-precision parts after processing is necessary.

2. Excellent polishing performance. The working surface of injection molded parts usually needs to be polished to achieve a mirror surface, Ra≤o.05μm. Steel hardness is required. It is advisable to be at HRC35~40. Hard surfaces can make polishing difficult. The microstructure of the steel should be uniform and dense, with few impurities, no defects, and pinpoints.

3. Good wear resistance and fatigue resistance. The injection mold cavity is washed by high-pressure plastic melt and subjected to alternating stress of cold and hot temperatures. General high-carbon alloy steel can obtain high hardness through heat treatment, but its poor toughness makes it easy to form surface cracks, so it is unsuitable for use. The selected steel type should enable the injection mold to reduce the number of polishing and repairing molds, maintain the dimensional accuracy of the cavity for a long time, and reach the service life of the planned mass production.

4. Corrosion resistance. For some plastic varieties, such as polyvinyl chloride and flame-retardant plastics, it is necessary to consider the selection of steel types with corrosion resistance.

Design content for molded parts and general procedures

1. Find out how many mold cavities there are.

2. Identify the plastic part’s separation surface and parting line.

3. Establish the cavity’s ranking.

4. When lateral core-pulling is necessary, create a lateral core-pulling mechanism.

5. Establish the draft angle and the molding size of the core cavity.

6. Determine how the molded parts will be assembled and fixed.