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Basic Interview Questions on Plastic Part Design

In this article we will discuss some of the commonly asked interview question with their answers on Plastic part design. Generally questions are asked based on guidelines need to be followed during plastic part design and what will be the effect if guidelines are not followed. 

 


What are the types of plastic based on their chemical behaviour?

Plastic can be divided in two major categories:

 Thermoset / Thermosetting Plastics: After cooling and hardened, these plastics retain their shapes and cannot return to their original form. They are hard and durable. Some of thermosetting plastic include polyurethanes, polyesters, and epoxy resin.

Thermoplastics: This type of plastic can be soften upon heating and return to their original form. They are less rigid than thermoset. They can be easily molded and extruded into films, fibers and packaging. Some of thermoplastics includes ABS, poly-carbonate (PC), polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

 


 

What is the difference between thermoset and thermoplastics?

Thermoplastics

  • Cross linked structure
  • Can not be reused once molded
  • Chemical changes after molding
  • Hard and Brittle
  • Available in powder Foam

Thermoset

  • Needle Structure
  • Can be reused after moulding
  • no change in chemical structure after moulding
  • Hard but not brittle
  • Available in granular foam

 


What is injection moulding?

Injection moulding is commonly used thermoplastic (ABS, PC, PE, PS, Nylon) plastic manufacturing process to achieve high production rate

During injection moulding molten thermoplastic is injected into a mould and a high pressure is applied the desired Plastic shape is obtained inside the mold. After solidification of finished part it is injected out of mould.

You can learn in detail about injection moulding here !!

 


What are Basic Injection Moulding defects?

Some of the common found injection moulding defects are: short shot, flow marks, flow lines, sink marks, warpage, voids, flash, knit lines, burn marks etc

You can learn in details about injection moulding defects here 

 


What insert / over-moulding?

Over-moulding is the injection moulding process where one material (usually a TPE or plastic) is moulded onto a second material (plastic or metal). If material selection is done properly, the over moulded parts will form a strong bond with the plastic. Primers or adhesives are not required to achieve an optimum bond between the two materials. Sometime primers are used to increase the bonding strength

You can learn more in details about over-moulding here !!

 


What are the advantages of plastics over metals?

  • Higher strength to weight ratio as compared with metal
  • Lighter weight and improved ease of use
  • Reduced manufacturing costs for high production rate
  • Shorter lead time after tool manufacturing
  • Increased corrosion resistance
  • Consolidation of multiple metal parts into a single plastic part
  • Toughness compared to metal parts
  • Greater product structural strength

Check Out more advantages of plastics here !!

 


What are the disadvantages of plastics over metals?

  • High initial tooling Cost
  • Part design restrictions
  • Low production rate will be very costly
  • High lead time for mould manufacturing
  • Prototyping Cost is very high

 


What are the types of material used in injection mould making?

Basically 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

Check out in details about injection mould making steels here !! 

 


What are the types of injection mould?

There are two main types of injection molds: cold runner and hot runner.

Cold Runner Moulds:  In cold runner mold, the runner is cooled and ejected with the part. Two major types of cold runner molds: two plate and three plate molds are available.

Hot runner mould: In hot runner mold, the runner is situated internally in the mold and kept a temperature above the melting point of the plastic.

Check out more about the types of injection moulds here !!

 


What is Runner 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 in injection moulds? 

  • Sprue gate: Used for large components, the gate mark is visible in component and no runner is required. e.g.: bucket molding (backside cylindrical gate mark visible and can be felt).
  • Edge gate: Most suitable for square, rectangular components
  • Ring gate: Most suitable for cylindrical components to eliminate weld line defect
  • Diaphragm gate: Most suitable for hollow, cylindrical components
  • Tab gate: Most suitable for solid, thick components
  • Submarine gate: Used when auto de-gating is required to reduce cycle time
  • Reverse taper sprue gate (Pin gate): Generally used in three plate molds.

 


What are ejector Pins in injection mould?

Ejector Pins are used to take out part from mould. Ejector pins apply a force to eject a part from the mold, Since in some cases they can leave marks. Generally, ejector pins are located in the B-side mold half, the side in which the part will stay when the mold opens. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, allowing the mold to close and be refilled.

 


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 & applications of ABS, CA, HDPE, LDPE, PA, PC, PP, PS, PVC?

 


What are fillers and additives in injection moulding?

 


What is parting plane?

 


What are the types of sliders used in injection moulding?

 


Define drooling?

 


What is maximum and minimum thickness of plastics that can be injection moulded?

 


What are cooling media used in injection molding?

 


What is clamping force in injection molding?

 


What are the main elements of an injection mould?

 


Difference in single and two plate mold?

 


What are the types mould finishes?

 


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