Basic Interview Questions On Plastic Part Design

This article covers some of the commonly asked interview question with their answers on Plastic part design for injection molding. For the position of plastic part design engineer, interview questions are asked based on plastic part design guidelines, material selection, Injection molding defects and manufacturing process.

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

Plastic can be divided in two major categories:

Thermoset / Thermosetting Plastics: After heating and then cooling (e.g. molding), thermoset 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 & thermoplastics ?

Thermoplastics: Thermoplastic has a cross linked structure and they can not be reused once molded. They are hard and brittle in nature and also available in powder Foam

Thermoset Plastics : Thermoset plastics have needle structure therefore can be reused after moulding. There is no change in chemical structure after molding. Thermoset plastics are hard but not brittle in nature and available in granular foam.

What is injection moulding?

Injection molding is commonly used 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?

Some of the common found injection molding defects are:

  • Short shot
  • Flow marks
  • Flow lines
  • Sink marks
  • Warpage
  • Voids
  • Flash
  • Knit lines
  • Burn marks

What is insert / over-moulding?

Over-molding 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 and adhesives are also used to increase the bonding strength.

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
  • Higher toughness compared to metal parts
  • Greater product structural strength

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 tooling.
  • Prototyping Cost is very high.

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

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 mould?

Two types of injection molds : cold runner and hot runner are used.

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

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 on component and no runner is required. for example 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 eject the part from mould. Since ejector pins leave marks on moulded part. Therefore they are located on the B-class surfaces.

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 molding?

Define drooling?

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

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?

Conclusion :

To sum up, before interview one should revise complete plastics part design process starting from material selection to part manufacturing.

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