Basic Interview Questions On Plastic Part Design

You shall carefully read job description before the start of interview. Always ask yourself, what skill set interviewer is looking for? Try to gain those skills. This article covers some of the commonly asked Plastic Part Design interview question with answers. 

Certainly Interviewer will ask questions on plastic part design guidelines, material selection, Injection molding defects and manufacturing process.

What are the types of plastic and what is the difference between them ?

According to chemical behavior, plastic are divided in two categories:

  1. Thermoset / Thermosetting Plastics
  2. Thermoplastics

Thermoset plastics cannot return to their original shape after heating. While Theroplastics can retain their original shape. 

Thermoset Plastics are hard and durable in nature. While Thermoplastics are softer and less rigid.

Polyurethanes, polyesters and epoxy resin are thermosetting plastic. 

ABS, polycarbonate (PC), polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) are thermoplastics.

What is injection moulding?

Injection molding is thermoplastic (ABS, PC, PE, PS, Nylon) manufacturing process to achieve high production rate.

During injection moulding molten thermoplastic is injected into a mold and a high pressure is applied. 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?

Short shot, Flow marks, Flow lines, Sink marks, Warpage, Voids, Flash, Knit lines and Burn marks are commonly found injection moulding defects. 

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.

What are the material used to make injection molding tool?

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 used 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 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 the common design rules in Plastic design?

Check out in detail about plastic part design guidelines here !!

What is trade name of Perspex, Nylon, Derlin, Teflon?

What is MFI in plastics?

What are the properties and applications of ABS, CA, HDPE, LDPE, PA, PC, PP, PS, PVC?

Why fillers and additives are used in injection molding?

What is parting plane?

What are the types of sliders?

Define drooling?

What is maximum and minimum thickness of injection molded plastics?

What are cooling media used in injection molding?

What is clamping force in injection molding?

What are the main elements of an injection mould?

What is difference between single and two plate mold?

What are the types mould finishes?

Conclusion :

To sum up, here we discussed commonly asked interview questions on plastic part design for injection molding. Interviewer will also cross question to judge you. One should understand basics of plastic part design.

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