You can find lot of injection molding parts around you. With the advancement in plastic technologies and advantages of plastic parts. Lot of industries are converting their metal parts into Injection molded plastic parts. Therefore injection molding field has lot of scope. This article covers commonly asked Plastic Part Design interview questions with answers.
What are the types of plastic ?
According to chemical behavior, plastic are divided in two categories:
1. Thermoset plastics cannot return to their original shape after heating, They are hard and durable in nature. e.g. Polyurethanes, polyesters and epoxy resin e,t,c
2. Theroplastics can retain their original shape after heating. They are softer and less rigid in nature. e.g ABS, polycarbonate (PC), polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) e,t,c
What is injection moulding?
Injection molding is a plastic manufacturing process to achieve high production rate.
During injection moulding molten thermoplastic is injected into a mold and high pressure is applied. As a result desired plastic shape is achieved inside the mold. After solidification of molten plastic, finished part it is injected out of mold.
What are common Injection Moulding defects?
Commonly found injection moulding defects are Short shot, Flow marks, Flow lines, Sink Marks, Warpage, Voids, Flash, Knit lines and Burn marks. You can check in detail about injection molding defects and how to control them here.
What is insert / over-moulding?
Over-molding is a type of injection moulding process where one material is moulded over another material.
Material selection is very important to achieve strong bonding between over molded parts. Primers and adhesives are also used to increase the bonding strength.
What are the advantages of plastics compared to metals?
- Higher strength to weight ratio
- Lighter weight and improved ease of use.
- Low manufacturing costs for high production rate.
- Increased corrosion resistance
- Consolidation of multiple metal parts into a single plastic part
- Higher toughness compared to metal parts
- High structural strength
What are the disadvantages of plastics compared to metals?
- High initial tooling Cost.
- Part design restrictions
- Low production rate will be very costly
- High lead time for tooling.
- Prototyping Cost is very high.
Mostly two types of steels are used in injection mould making
- Heat Treating Tool Steels: H13 steel, Stavax tool steel, Ramax tool steel, S7 tool steel, stainless steel etc
- Pre-Hardened Tool Steels: P20, NAK 50, NAK 55
What are the types of injection molds ?
Two types of injection molds : cold runner and hot runner are used.
Cold Runner Molds: Runner is cooled and ejected with the part. Types of cold runner molds are: two plate and three plate molds.
Hot runner mold: Runner is situated internally in the mold and kept a temperature above the melting point of the plastic.
Why Runner is required in injection molds?
A runner is the channel in the mold that conveys the plastic from the barrel of the injection molding machine to the part.
What are the advantages of injection molding compared to other plastic manufacturing process?
Advantages of Injection Molding
- High Efficiency
- Complex Part Design
- Design Flexibility
- Ability to use multiple Plastic Materials
- Ability to use multiple colors
- Reduced Waste
- Dimensional Accuracy
What are the types of gates used in injection moulds?
- Sprue gate: Used for large components. In this, gate mark is visible on component and runner is not required. For example bucket molding.
- Edge gate: Suitable for square and rectangular components
- Ring gate: Suitable for cylindrical components to eliminate weld line defect.
- Diaphragm gate: Suitable for hollow and cylindrical components.
- Tab Gate: Suitable for solid, thick components
- Submarine Gate: Used for auto de-gating to reduce cycle time
- Reverse taper sprue Gate (Pin Gate): Used in three plate molds.
What are ejector Pins?
Ejector Pins are used to eject the part from mould. They are located on the B-class surfaces because they leave marks on molded part.
What are design guidelines for injection molding plastic part design?
To sum up, lot of opportunities are available for mechanical engineers in plastic industry. If you are fresher or switching to injection molding field. You can start learning plastic part design guidelines, material selection, Injection molding defects and manufacturing process.
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